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This study used laser additive manufacturing technology to fabricate austenitic–martensitic stainless steel functionally graded materials (ASS-MSS FGM). The microstructure and mechanical properties of ASS-MSS FGM were characterized, and the surface integrity after milling experiments of the ASS-MSS FGM was analyzed. The microstructure of the ASS-MSS FGM exhibited a dense subgrain structure with diverse subgrain types and no fixed growth direction. The microhardness of the ASS-MSS FGM increased from 283 HV to 530 HV due to the increased proportion of the martensitic phase. The ASS-MSS FGM tensile specimens fractured in the austenitic stainless steel region. The tensile strength is 896 MPa, which is increased by 121 MPa compared to ASS. The fracture elongation is 18%, which is increased by 140% compared to MSS. The milling experiments revealed that the degree of work hardening decreased by 8.1% along the building direction of the ASS-MSS FGM. Compared with the austenitic and martensitic layers, the degree and depth of work hardening of the gradient layer were in the transitional zone between them. The microstructure of ASS-MSS FGM after the milling mainly exhibited three deformation phenomena: grain squeezed deformation, grain broken dislodgement, and grain sheared fracture. [ABSTRACT FROM AUTHOR]

In this work, the texture evolution, mechanical properties, and formability of cold-rolled lightweight steel annealed at 500, 600, 700, and 800 °C for 30 min were investigated. The phases in the annealed specimens mainly included ferrite, austenite, and martensite. The retained austenite showed a trend of increasing and then decreasing with the increase in annealing temperature. At the annealing temperature of 700 °C, the retained austenite content reached the maximum value of 53.0%, resulting in a large amount of martensitic TRIP effect during the subsequent deformation. So the steel obtained the optimal mechanical properties with the ultimate tensile strength of 1613 MPa, ultimate elongation of 30.2%, and largest product of strength and ductility of 48.7GPa%. In addition, the annealing temperature of 700 °C was conducive to the stamping texture of {111} < 110> orientation, as well as higher strength, and the plastic strain ratio (r), the strain hardening index (n), and the flexural strength ratio (ReL/Rm) were 1.25, 0.55, and 0.41, respectively, the material obtained excellent formability. [ABSTRACT FROM AUTHOR]

This work involves additively manufactured Ti-6Al-4V alloys, which are widely used in automobile, biomedical, and aircraft components for a comparison of the microstructure–properties relationship between electron beam melted (EBM) and selective laser melted (SLM) alloys after hot isostatic pressing treatment. We carried out microstructural, mechanical, and electrochemical measurements on both alloys. They showed comparable α and β phase contents with slightly higher lattice parameters in the EBM sample compared to the SLM. The EBM sample showed higher yield strength and uniform elongation due to the activation of multistage defects-driven strengthening and strain hardening mechanisms. Cracking during the tensile test nucleated mainly at the α phase near high-mechanical mismatch α/β interfaces. This mechanism was consistent with the reported generation of hetero-deformation-induced strengthening and strain hardening. Both alloys showed similar electrochemical behavior, but the SLM sample was more susceptible to corrosion than the EBM alloy. [ABSTRACT FROM AUTHOR]

In this study, changes in the properties of pure aluminum (trace elements: Mg, Si, Ti, and Fe) castings with inserted aluminum nitride (AlN) substrates caused by thermal cycling similar to that of in-use power modules were examined. The hardness of the late-solidified part of the casting with AlN inserts after thermal cycling (500 cycles) was considerably higher than that before cycling; notably, this increase in hardness was not observed in castings without AlN inserts. In contrast, the electrical conductivity, density, and crystal grain size of the castings remained almost constant irrespective of thermal cycling and the presence of AlN inserts. The microstructures of these castings revealed no notable precipitate phase. Electron backscattering diffraction analysis indicated that the transgranular crystal orientations of the castings with AlN inserts before and after thermal cycling were uniform and nonuniform, respectively, suggesting that thermal cycling increased the number of dislocations. Therefore, work hardening due to repetitive plastic deformation produced by the difference between the thermal expansions of pure Al and AlN during thermal cycling is key to increasing the hardness of castings. [ABSTRACT FROM AUTHOR]

Surface mechanical grinding treatment (SMGT) is a promising surface strengthening technique by exerting gradient microstructure, phase transformation, and work hardening on the surface layer. However, due to the large strain and high strain rate, it is difficult to evaluate the microstructure attributes of the SMGT-processed surface. In the present work, an analytical model is developed to correlate the SMGT processing parameters to the surface microstructure, grain size, martensite content, and microhardness. Firstly, the processing force and distributions of stress and strain are identified by developing a multi-physics framework for SMGT process. Afterward, the surface grain refinement induced by dynamic recrystallization (DRX) is modeled by cellular automata (CA) method. The grain size along with the depth of cross section is predicted by Johnson–Mehl–Avrami–Kolmogorov (JMAK) DRX model. The martensite content is evaluated according to the strain-induced martensitic transformation kinetics. Ultimately, accounting for both microstructure alteration and phase transformation, the microhardness variation is predicted. To verify the results derived by analytical model, series of confirmatory experiments were carried out on 316L austenitic stainless steel. The SEM was utilized to observe the surface microstructure of the processed samples. The martensitic transformation was characterized by EBSD and XRD methods. The microhardness measurements were taken on an automatic Vickers hardness tester. It was obtained from the results that the surface microstructure, gradient grain size, phase content, and microhardness distribution derived from predictive model were consistent well with the experimental measurements. The confirmed simulation model is further utilized to understand how the process factors, i.e., ball size and penetration depth, influence the evolution of microstructure. It is found that the finer grain structure, higher martensite content, and microhardness can be obtained by adopting smaller ball and larger penetration depth. [ABSTRACT FROM AUTHOR]

The wear and corrosion resistance of 316L austenitic stainless steel after strain hardening were studied. 316L austenitic stainless steel was composed of a large amount of austenite and a small fraction of martensite. After strain hardening at room temperature, the contents of phases were not changed basically. With the increase of pre-strain deformation, the microhardness of the surface and cross-section slightly increased, and the microhardness of the sample with pre-strain deformation of 20% was the largest. When the pre-strain deformation was greater than 5%, the surface microhardness was greater than the cross-sectional microhardness. The dynamic average friction coefficients of the stainless steel after pre-strain deformation of 0%, 5%, 10%, and 20% were 0.67, 0.57, 0.2, and 0.12, respectively. The friction coefficient of the samples gradually decreased with the increase of strain hardening. The wear resistance of 316L stainless steel was improved after pre-strain deformation. The steel with pre-strain deformation of 20% had a small abrasive scratch depth and low friction coefficient, which exhibited good wear resistance. The corrosion resistance of the samples after strain hardening deteriorated for the initial state without immersion due to the existence of defects. The corrosion resistance of the sample with pre-strain deformation of 5% after soaking for 3 days was the best, compared with the other as-strained samples. [ABSTRACT FROM AUTHOR]

The present study introduces a novel method of high-density pulsed electric current (HDPEC) as a substitute for the conventional annealing heat treatment process to relieve strain hardening in 6061 aluminum alloy (A6061) during the manufacturing process. The study investigates the effects of different HDPEC treatments on the strain-hardening relief of cold-rolled A6061, with a comparison to the traditional annealing heat treatment. The results reveal that the HDPEC-treated samples demonstrate a remarkable reduction of approximately 50% in strength and a considerable increase of approximately 200% in ductility, indicating complete strain-hardening relief of cold-rolled A6061. Consequently, the HDPEC treatment is faster and more efficient than the traditional annealing heat treatment. Furthermore, the HDPEC-treated samples display equivalent mechanical properties as the untreated ones after the final precipitation heat treatment, indicating that the HDPEC treatment has no detrimental effect on the materials. The microstructural characterization demonstrates that the HDPEC-induced microstructural modification through dislocation elimination and grain recovery leads to the strain-hardening relief of cold-rolled A6061. These findings suggest that the HDPEC treatment can even replace the hot-forming process of A6061, contributing to low-cost and high-efficient manufacturing. [ABSTRACT FROM AUTHOR]

The work presented in this paper focuses on modeling the welding process to develop a numerical model able to perform a good prediction of welding distortions. The model is developed for a butt welded joint using S235 steel as the base metal and an electrode (AWS E6013) as the filler metal. To assess accuracy, numerical and experimental results are compared. The present work makes it possible to identify the main factors influencing the accuracy of the numerical model, which must be taken into account to obtain satisfactory results. To carry out this analysis, the effect of the mechanical properties of deposited metal and the effect of the deposition process were taken into account. The effect of the variation of the mechanical properties of the filler metal on the distortions is illustrated. The model was developed by using APDL language, and the birth and death technique is used to model the deposition process. Distortion results obtained by numerical models approach properly the experimental measurement. Further analysis of the numerical data reveals considerable fluctuation of the obtained results by modifying the models used to describe the plastic behavior and the work hardening process. Regarding this strong correlation, numerical modeling of the welding process needs a vigilant identification of the work hardening mode appropriate for the used materials. [ABSTRACT FROM AUTHOR]

Cold spray additively manufactured (CSAM) aluminum 6061 components are characterized by heterogeneous compressive residual stresses induced during manufacturing. This heterogeneity is further compounded by spatial variations in microstructures and mechanical properties, leading to poor inter-particle (intersplat) bonding and significant marring of overall component performance. Thermal post-processing is a keenly researched method for recovering mechanical toughness by enhancing intersplat bonding and altering highly concentrated residual stress distributions. The current work incorporates a modified microscale–mesoscale material model into a viscoplastic self-consistent simulation framework to capture material response in the as-sprayed and post-processed states. The updated model incorporates physically informed parameters emphasizing residual stresses measured experimentally through X-ray diffraction. The model calibrated using experimental tests and published literature was able to predict the stress–strain response of CSAM parts at post-heat-treated conditions. Results of the parametric study showed the significance of intersplat boundary effects on the overall yield and strain hardening of the CSAM parts. Without any information on the processing conditions of CSAM parts, the modified plasticity model predicted the deformation response using information gathered from microstructure characterization. [ABSTRACT FROM AUTHOR]

Autofrettage processes allow engineers to reduce the thickness of thick-walled cylinders or components in high-pressure applications without sacrificing strength, life, or safety. However, during the autofrettage process, residual stresses will be generated due to plastic deformation. The complex tube material behavior is dominated by the Bauschinger effect. A better understanding and accurate prediction of the residual stress field is critical, which will enable better piping design strategies to minimize deformation and stresses under operating conditions. This study aims to predict and analyze residual stresses resulting from hydraulic re-autofrettage of a swage-autofrettaged thick-walled cylinder by computer modeling. A case study was performed on a thick-walled cylinder of A723 alloy with a radial interference of 2.5%. In order to investigate the effect of the chosen material constitutive representation, results based on the true material constitutive model were compared with the simplified prevalent material model of bi-linear kinematic strain hardening. Computer implementation for the true material was via a user-developed subroutine that incorporates the complex Bauschinger effect. The results indicate that an accurate material constitutive representation is crucial for better and more accurate prediction and understanding of residual stresses induced by autofrettage processes. Computer modeling based on the true material constitutive representation will likely prove to be a powerful tool for the design of autofrettage processes in general and thick-walled cylinders in particular. [ABSTRACT FROM AUTHOR]

In this study, GCr15 bearing steel with an ultrahigh strength was subjected to surface laser shock processing (LSP). The effects of the laser pulse energy (LPE) on the microstructure, microhardness, residual stress and tribological properties were studied systematically. The results indicated that the martensitic laths and carbide particles underwent severe plastic deformation and were refined into nanostructures. The grain refinement was aggravated with the increase of the LPE. A grain size of 50 nm was obtained on the sample's surface by LSP at 7 J. Many dislocation tangles and deformation twins were formed, which led to an increase in the microhardness. Furthermore, LSP induced compressive residual stress (CRS) with a high amplitude on the sample's surface. As the pulse energy of LSP rose, the impact layer of GCr15 steel deepened. The wear rate and the friction coefficient reduced, and the wear resistance increased as the LPE increased. The tribological properties of GCr15 steel were enhanced by the grain refinement and working hardening, and by the formation of a high CRS layer. [ABSTRACT FROM AUTHOR]

In this paper, the effect of ambient temperature, sliding speed and normal load on friction coefficient and wear mechanism were investigated through end-face testing to simulate the influence of process parameters during hot forming–quenching integrated process. With the increasing ambient temperature, the wear mechanism changes from abrasive wear to adhesive wear and oxidation wear. When the ambient temperature reaches 400 °C, the delamination wear characteristics occur due to the crack propagation caused by thermal stress. The friction coefficient and wear rate increase with the increase in the normal load. When the normal load is 1.8 and 3 kN, because of the generation of a large amount of heat and the increase in the contact area, the peeling morphology appears, and the adhesive wear and oxidation wear are further enhanced with the increase in the temperature. The enhancement of work hardening weakened the adhesion effect due to the high sliding speed. The friction coefficient decreases with the increase in the sliding speed. In addition, the wear rate at 10 r/min and 50 r/min sliding speed are 29.6 and 29.2 mm3/m, respectively. [ABSTRACT FROM AUTHOR]

The hot compression test of CrFeCoNiAl0.1 high entropy alloy at temperature range of 950-1100 °C and strain rates range of 0.001–1 s−1 was carried out by Gleeble−3500 thermal compression simulator. The characteristics of microstructure evolution of the studied alloy during hot deformation were studied by means of OM and EBSD. The Arrhenius-type constitutive equation was constructed, and the hot deformation activation energy was calculated to be 307.77 K J mol−1. The proportion of low angle grain boundaries is 10.2% at the condition of 1050 °C/0.01 s−1, and the complete dynamic recrystallization (DRX) occurs. The characteristic parameters are determined according to the relationship between the work hardening rate and the true stress, and the mathematical relationship between the critical conditions for DRX and Z parameters is obtained. The critical stress for DRX decreases with increasing temperatures and decreasing strain rates. The kinetic model of DRX is constructed by the Avrami equation. Higher temperature and lower strain rate were beneficial to the occurrence of DRX. Based on microstructure evolution, the DRX volume fraction model of CrFeCoNiAl0.1 high entropy alloy was verified, and the experimental results are close to the predicted results. The DRX volume fraction model can correctly predict the DRX behavior of CrFeCoNiAl0.1 high entropy alloy. [ABSTRACT FROM AUTHOR]

This study investigated the anisotropy of the tensile properties in a duplex stainless steel of 24Cr-5Ni-0.18N based on in situ neutron diffraction experiments. The 24Cr-5Ni-0.18N steel showed a better balance of tensile strength (TS) and uniform elongation (U.El) compared with 329J4L and 329J1 steels. The Lankford value (r-value) of the 24Cr-5Ni-0.18N steel was comparable to other duplex stainless steels while showing a larger TS. Regarding the anisotropy of the mechanical properties, the results for a test specimen oriented at 45° showed a low yield strength (YS) and TS, but a better U.El and r-value. The hard phase of ferrite (α) played an essential role in the YS and TS. The austenite (γ) phase, which exhibited a large difference in the lattice strain related to the work hardening, contributed to the U.El, and its effect was further enhanced by the volume fraction of γ. The r-value increased with an increase in the lattice strain of {200}γ or a decrease in that of {200}α in the transverse direction. The lattice strain in the loading and transverse directions obtained by in situ neutron diffraction experiments also helped clarify the excellent r-values of duplex stainless steels. [ABSTRACT FROM AUTHOR]

The effect of modes of deformation (unidirectional, two-step and multi-step-cross rolling) by cold rolling on microstructure, texture, and mechanical properties of the Ni-16Cr and Ni-16Mo alloys has been discussed in this work. The overall texture intensities of both the alloys are always strong in unidirectional mode of cold rolling after 90% deformation as compared to two-step and multi-step cross rolling. There is a strong bearing between the overall texture intensity and intensity of the Brass ({0 1 1}〈2 1 1) component of 90% deformed sheets irrespective of the modes of deformation and alloy system. The evolution of {111}-fibers is an important phenomenon during cold rolling of both the alloys which appear to be stable in a particular range of overall intensity of ODF (Orientation Distribution Function) and this range varies with modes and extent of deformation. The alloy Ni-16Mo exhibits better mechanical properties than the alloy Ni-16Cr in 90% deformed condition. The measured values of relative resolved shear stress corresponding to the strongest texture present in the 90% deformed sheets well explain the variation in uniform elongation with change in modes of deformation. Both the alloys in highly (90%) deformed condition display two-slope behavior and follow Ludwigson (with negative deviation at low strain) and Voce relationships. The instantaneous work hardening curves of both the alloys show continuous decrease of work hardening till it attains saturation in the last stage of plastic deformation before necking. The linear portion of this last stage has been explained using Kock-Mecking-Estrin analysis and correlated with overall uniform elongation. [ABSTRACT FROM AUTHOR]

In the present work, Al 2014 alloy was subjected to multidirectional cryoforging (MDCF) and subsequently to annealing in the temperature range from 150 to 350 °C to investigate its influence on strain hardening behavior, macroscopic fracture mechanisms, and fracture toughness. The mechanical properties are correlated with the microstructural evolution after deformation and post-deformation annealing with the help of optical microscopy and transmission electron microscope (TEM) studies. Moreover, the process and annealed alloy's fracture mechanism is established using macrograph analysis and Scanned electron microscope (SEM) studies. The study reveals that the strain hardening ability, fracture mechanisms, and fracture toughness of the deformed and the post-annealed samples are influenced by the shear banding, combined recovery /recrystallization process, and evolution of second phase precipitates during annealing treatment at various temperatures. On investigating the tensile and fracture toughness properties, it was observed that these are retained up to the annealing temperature of 250 °C. In contrast, a significant drop in these properties was observed above this temperature. The macroscopic fracture mechanism of the multidirectional cryoforged (MDCFed) sample was dominated by shear fracture, while upon annealing, it gradually transformed to shear plus necking. [ABSTRACT FROM AUTHOR]

Incoloy 800 is a Fe-based super alloy used primarily in high-temperature applications like boilerplates due to its ability to maintain its austenitic phase. Samples of 100 × 20 × 5 mm were manufactured in vertical orientation using selective laser melting (SLM) and were annealed at 980 °C for 15 min. Annealed samples were cold rolled at room temperature, in multiple passes to 20, 40, 60 and 80% reduction in thickness, perpendicular to the building direction. The aim of this study is to understand the strain hardening effect upon rolling and also to know the effect of cold rolling on texture, microstructure and hardness after rolling the annealed alloy. Microstructural features were observed under optical microscope and scanning electron microscope while the texture measurement was done using X-ray goniometer. Typical fish scale morphology was observed in the transverse plane of as-built condition, which was diminished in annealed samples. SEM study shows fine grains along with traces of lamellar cell morphology. Hardness increased with an increase in percentage reduction. Austenite (γ) phase was detected as the only phase in XRD analysis. The bulk texture of annealed sample revealed strong copper texture until 40% rolling, and thereafter, it reduced. Goss texture component showed a similar trend, while brass texture increased up to 60% rolling condition and thereafter diminished. [ABSTRACT FROM AUTHOR]

Incoloy 800 is an iron-based Ni-Cr super-austenitic stainless steel that finds applications involving exposure to high temperatures and corrosive environments. This study aims to investigate the effect of cold rolling on the microstructure, texture and hardness of Incoloy 800 fabricated by selective laser melting. Rectangular specimens of 100 × 20 × 5 mm were printed in the vertical build orientation. The as-built alloy was subjected to unidirectional cold rolling along the long transverse plane to achieve thickness reductions of 20, 40, 60 and 80%. Optical and scanning electron microscopy were used to observe the microstructure. EDS was used to find the elemental composition. An x-ray goniometer was used to measure the bulk texture. Vickers hardness test was performed to measure the hardness. The optical micrographs of the as-built sample revealed a laser scan track pattern on the top surface, whereas the short and long transverse sections revealed typical fish-scale morphology with overlapping melt pool boundaries. Melt pool widening was observed upon increasing the rolling percentage. Scanning electron microscope revealed three types of grain morphology–cellular, elongated cellular and columnar. X-ray diffraction revealed the presence of austenite (γ) phase. The as-built alloy showed cube, goss and copper texture components. The sample rolled to 80% reduction showed strong brass and a weakened goss component. The 80% cold-rolled sample showed 42 and 116% higher hardness than the as-built condition and as-cast condition, respectively, due to strain hardening. [ABSTRACT FROM AUTHOR]

A combined experimental and modeling study of strain hardening and backstresses of an iron-based Metal Matrix Composite MMC (Fe-TiB2) is presented. New quantitative data related to the kinematic hardening of this industrially challenging composite have been obtained via reverse shear tests up to large plastic stains. As the data show an increasing contribution of the kinematic hardening with both TiB2 reinforcement effect and solid solution in ferritic matrix, the relevance of existing phenomenological models of work hardening for automotive steels for the description of the behavior of this steel-based MMC has been tested. This model based on the extended KME hardening model validated by Bouaziz et al. (JMPT 106:5–67, 2000) for ferritic steels and a standard mixture laws between particles and matrix, succeeds in reproducing the overall stress–strain of this MMC from phase constituents from the use of fixed set of physically based parameters for ferrite and a small number of additional ones. Good agreement is found between the computed and measured backstresses. The model, despite its phenomenological character and although it is limited to a one-dimensional approach, provides a reliable estimation of particles fracture stress, suggesting thus its efficiency to predict and model steel-based materials with increasing microstructural complexity and contents. [ABSTRACT FROM AUTHOR]

Springback, buckling, and cracking are typical problems that take place during the stamping of titanium alloy sheets. Accuracy of material response characterization at moderate strain rates is an important factor for improving the adequacy of predictions of structures subjected to forming processes. In this paper, the deformation and fracture of alpha titanium alloys were studied using numerical simulations and the punch test at strain rates up to several hundred per second. Loading velocities from 0.5 to 10 m/s were realized during the spherical body penetration through a thin titanium plate. A viscoplastic constitutive model and damage model are used to describe the mechanical behavior of alpha titanium alloys at strain rates ranging from 10 to 103 s−1. Estimates of the residual stress of alpha titanium alloys are obtained using inverse simulation technique. It was found that amplitudes of residual stresses in CP-Ti after high speed punching are consistent with the increment of the yield stressas a result of strain hardening. Therefore, a comprehensive understanding of plastic deformation localization is essential for analyzing residual stresses in titanium alloys deformed at moderate strain rates. [ABSTRACT FROM AUTHOR]

We elucidate here the underlying reason for the differences in mechanical properties of cold-rolled lightweight medium manganese TRIP steel subjected to two unique heat treatments with the objective to optimize the mechanical properties. Furthermore, the relationship between the mechanical properties of the material and the stability of austenite was explored. After quenching and tempering (Q and T) treatment, tensile strength and total elongation of the experimental steel were 942 ± 11-1380 ± 22 MPa and 12.5 ± 1.5-52.5 ± 2.1%, respectively. While, the tensile strength and total elongation of experimental steel after quenching and partitioning (Q and P) treatment were in the range of 890 ± 10-1470 ± 24 MPa and 5 ± 0.8-47.4 ± 1.7%, respectively. The primary reason for the difference in mechanical properties is that the volume fraction of austenite in Q and T steel was higher, and the degree of TRIP effect was significantly greater. Interestingly, when the volume fraction of austenite was similar, the elongation of steel after Q and P treatment was greater. Short-time heat treatment results in non-uniform distribution of Mn in austenite grain. The Mn content at the austenite grain boundary regions is higher than that inside the grain, which leads to low stability of austenite. Austenite transforms in the short time during deformation, which cannot provide strong TRIP effect, resulting in low elongation. However, the diffusion time of Mn in the steel heat treated by Q and P is relatively long, the diffusion distribution of Mn in austenite is more uniform, and the stability of austenite is enhanced, which can delay the emergence of TRIP effect during deformation, and provides strong work hardening ability and increases elongation. [ABSTRACT FROM AUTHOR]

The utilization of lightweight materials such as AA5754 aluminum alloys in the inner body panel parts of vehicles has been significantly important for automotive manufacturers to minimize the high fuel consumption by reducing the overall weight. In this study, the flow behavior of AA5754 sheet metal has been discussed by conducting uniaxial tensile tests at five different temperatures (RT, 200, 250, 300, 350 °C) and three strain rates (0.001, 0.01, 0.05 s−1). Additionally, the capability of Fields and Backofen (F&B) and Voce hardening rules in describing the flow behavior of AA5754 at different temperatures and strain rates has been investigated by conducting uniaxial tensile tests in finite element analysis. It has been found that the main deformation mechanisms for the AA5754 are the strain hardening mechanism up to 250 °C, strain hardening and dynamic recovery mechanisms at 300 °C, dynamic recrystallization and strain hardening mechanisms at 350 °C. While the F&B hardening rule has been able to successfully capture the flow behavior of AA5754 up to 250 °C with a 14.36% error, its capability has significantly reduced after 250 °C due to its incapability of describing the effects of dynamic recovery and recrystallization. Voce hardening model has been better able to describe the flow behavior of AA5754 at all the temperature levels than F&B model due to its saturation behavior. [ABSTRACT FROM AUTHOR]

Al-Si alloys of hypoeutectic, near-eutectic, and hypereutectic compositions, prepared from commercial pure aluminum and silicon, were investigated to study their tensile and compressive properties. The microstructures in as-cast stage were found to be refined after subsequent remelting and casting for two, five, and ten cycles. The proportion of eutectic phase and silicon particles increased with silicon content and influenced the nature of stress–strain curves. Both tensile and compressive stress–strain curves exhibited increase in flow stress with increasing strain, which could be expressed by Hollomon type relationships, but the tensile deformation occurs at lower flow stress than that in compression. While tensile specimens showed limited elongation to failure, the compression ones exhibited larger strains, up to 30% without failure. The tension–compression flow asymmetry was noted to vary as a function of the number of remelting cycles and of the silicon content in the alloy. The work hardening rate showed only stage III behavior in tension but both stage III and IV behavior in compression. The flow stress, including the yield strength, in compression was found to increase with decrease in grain size. However, in tensile deformation, instead of grain size effect, the yield strength was noted to increase with the decrease in inter-particle spacing. In both these cases, the Hall–Petch type relationships were found to be obeyed. [ABSTRACT FROM AUTHOR]

The evolution of microstructure, texture, and mechanical properties of AZ31 (Mg-3Al-1Zn, wt.%) and Mg-0.6Gd (wt.%) alloys was investigated and compared after high-pressure torsion at room temperature through the equivalent strain range of εeq = 0.6 − 287.5. The results demonstrated that the grain refinement behavior is different for these two alloys. For the AZ31 alloy, dynamic recrystallization (DRX) was restricted leading to a gradual and continuous formation of ultrafine grains with a mean grain size of ~ 0.3 µm through the entire strain range and the development of deviated B, C1, and C2 texture fibers. For the Mg-0.6Gd alloy, the DRX was very fast and a rapid ultrafine grain microstructure with a mean grain size of ~ 0.7 µm was developed at a strain range of εeq = 0.6 − 5.7 and this remained stable with a relatively stable B-fiber over the strain range εeq = 5.7 − 287.5. The evolution of microhardness in the AZ31 alloy indicated a strain hardening without recovery while that of the Mg-0.6Gd alloy showed a strain hardening with recovery. The differences between the AZ31 and Mg-0.6Gd alloys are discussed based on a comprehensive characterization of twinning, dislocation density, the initial microstructure and the presence of second phases. [ABSTRACT FROM AUTHOR]

In this present study, element birth and death technique is used in simulation to compute thermal history as in transient temperature distribution and thermal cycle curve to find cooling rates in the heat-affected zones and residual stresses in 8-mm-thick AISI304 steel weldment. The peak values of residual stresses in the weldment were observed as high as (269 and 289 MPa in SQBWJ and 258 and 260 MPa in VGBWJ) which is significantly higher than the yield stress (σy) value of AISI304 at room temperature due to higher rate of strain hardening and larger coefficient of thermal expansion. The predicted results of each criterion of residual stresses lead to finding the location for cause of premature failure of AISI304 weldment. All the aspects of failure investigation are important in answering the questions raised for the failure and to avoid any future miss-happening. [ABSTRACT FROM AUTHOR]

A kind of bulk metallic glass composites (BMGCs) with ex situ tungsten (W) particles and in situ dendrites were designed and successfully prepared through infiltrating Ti40.9Zr30.4Nb4.2Cu7Ni1.7Be15.8 (at.%) melt into the W preforms at different temperatures of 1103-1243 K. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements show the dendrites change from single bcc phase to the mixture crystals along with the significant increase in the volume fraction due to the more W dissolution when the casting temperature increases over 1183 K. With such the microstructural evolution, the mechanical properties change significantly from high strength and ductility along with work hardening capacity to the reduced strength and ductility along with work softening capacity. The samples prepared at 1123 K exhibit the optimal mechanical properties including the yield strength of ~ 1670 MPa, the maximum fracture strength of ~ 2080 MPa and the plastic strain of ~ 26%, which is attribute to the cooperative deformation mechanism between W particles and in situ BMGC matrix. These findings deepen the understanding of the cooperative deformation of composites containing the multiple phases and shed light on design BMGCs for specific application. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration has considerable influence on the plastic deformation and resulting fracture responses of metallic alloys. This manuscript critically aims at quantification and assessment of two-dimensional ductile dimple geometry of Ti-6Al-4V alloy generated through the variation of ultrasonic vibration with different powers during tensile experiments. The analyzed dimple geometry statistics as a function of ultrasonic power are completely based on a published article. The alteration of microstructural states, leading to different dimple appearances after inducing different ultrasonic vibrations during tensile deformation, have been extensively quantified/assessed, compared and interpreted with respective mechanical responses of the alloy as a function of ultrasonic power. Moreover, the change in strain hardening path patterns through the variation of ultrasonic power has clearly reflected on the dimple statistics/dimensions on their respective tensile fractographs. This novel 'dimple geometry-ultrasonic power-tensile response' interpretation allows the use of two-dimensional ductile dimple fracture appearances in a quantitative way. The remarkable manifestation of ultrasonic vibration on tensile responses of Ti-6Al-4V alloy and hence the change in fracture appearances have been convincingly revealed. [ABSTRACT FROM AUTHOR]

The special characteristics of the laser deposition manufacturing (LDM) process for forming titanium alloys (Ti6Al4V) lead to significant anisotropy in the mechanical properties of the alloy. The existence of anisotropy leads to different processing efficiency and processing performance in different processing directions. In this study, the cutting force, cutting temperature, and surface integrity of the additive titanium alloy machined by LDM at five angles of 0, 22.5, 45, 67.5, and 90° to the deposition direction were comparatively studied. Tensile properties, cutting force, cutting temperature, surface topography, surface roughness, and work hardening were used as evaluation indicators to observe the surface layer of the material after end milling under the same milling parameters. The results showed that the smallest grain size at the machining angle of 22.5° resulted in slightly higher cutting forces, cutting temperatures, and surface roughness than the other four groups. Meanwhile, large surface defects such as scratches and plowing were observed on the surface of the specimens at 22.5°, with a higher degree of machining hardening and deeper hardening layer depth. It proves that the anisotropy of the additive titanium alloy itself will have a greater impact on the cutting force, cutting temperature and surface integrity during its machining. [ABSTRACT FROM AUTHOR]

The excellent strain hardening behavior of high-manganese steel (HMnS) makes it widely used under strong impact and heavy wear conditions. However, the rapid hardening ability of HMnS needs to be improved under low stress. In this paper, laser remelting technology (LRT) was proposed to improve the hardening ability of HMnS under low stress. Firstly, the surface of HMnS was treated by LRT, and its surface and internal microstructure were characterized by a 3D profiler, an x-ray diffractometer, a scanning electron microscope (SEM) and an energy-dispersive spectrometer. Then, the microhardness and wear test were carried out by using a Vickers hardness tester and a wear tester, and the sub-surface microstructure and surface wear scars were tested by SEM and 3D profiler to study the wear performance of HMnS before and after laser remelting. Finally, the hardening effects and wear mechanism of laser-remelted high-manganese steel (LR-HMnS) were revealed. It was found that heterogeneities of LR-HMnS were composed of columnar structures, equiaxed structures and segregation region. These heterogeneities were conducive to improving the hardening ability of HMnS under low stress conditions, which could improve its wear resistance. [ABSTRACT FROM AUTHOR]

The mechanical properties of Inconel 625, which is a traditional superalloy, have attracted attention in terms of improving its potential. Herein, the tensile behavior of the gradient structure of the Inconel 625 were investigated. The 600-μm-deep gradient nanotwinned (GNT) structure that was obtained featured three layers, and the formation of the structure resulted in a gradient increase in hardness from 2.3 to 7.7 GPa. Compared with the pristine sample, the GNT sample featured a yield strength of 1040 MPa and good ductility. With regulation of the volume fraction of the GNT layer, the synergies of GNT and coarse-grained (CG) structure provide an increase in yield strength (632 MPa) with rather ductile behavior (36%). The synergetic strengthening of GNT and CG structure changes the yield strength and work hardening behavior of Inconel 625 alloy. The increase in yield strength is mainly attributed to the strengthening of GNT surface fine grains and the synergistic strengthening of twins and GNT-CG structure. The high plasticity is mainly attributed to the CG structure and high proportion of nanotwin and twinning in the alloy. This work provides insights on the mechanisms of strengthening of mechanical properties for Inconel 625. [ABSTRACT FROM AUTHOR]

Microstructural evolution and the mechanical properties of recently developed lightweight AA2050 Al-Cu-Li alloy have been presented. A processing route of multi-directional forging (MDF) at 170 °C followed by artificial aging at 150 °C was employed. Systematic EBSD analysis revealed significant grain refinement with grain size reducing from 74.3 ± 12 to 22.1 ± 2.8 µm after 12 passes of MDF. Transformation of deformation bands into subgrains with dynamic recrystallization has led to grain refinement. TEM results show the presence of large dislocation clusters and deformation bands in MDF processed samples with a large number of fine precipitates in peak aged MDF processed samples. XRD analysis shows variation in peak intensities and occurrence of peak shifts due to induced lattice strain upon MDF. A substantial increase in microhardness and strength was observed with a minor trade-off with ductility after 12th MDF pass. Further, enhancement in strengths and microhardness were observed in post-MDF aged samples. Experimental results show the combined effect of strain hardening, grain size reduction, and precipitate hardening which influence the material strength. A combination of MDF and artificial aging has shown great potential to enhance the strength and ductility of AA2050. [ABSTRACT FROM AUTHOR]

Samples based on 316L stainless steel were prepared by conventional manufacturing process (CM) and laser powder bed fusion (L-PBF). Surface morphology changes under air oxidation in the temperature range 600-900 °C were carried out. Tensile tests were carried out in the temperature range of 700-900 °C for strain rates between 0.001 and 0.1 s−1. The materials showed good oxidation resistance up to 700 °C. The CM and L-PBF material had a high mass gain instability and similar microstructures developed under high temperatures were found in both alloys. Increased temperature increases Cr concertation in the L-PBF material up to 40 at.% at 800 °C and a rich Fe based oxide is formed at 900 °C. Slightly thicker oxide scales were formed in the CM than in the L-PBF material.The hot tensile tests reveal that a fast work hardening occurs for all hot tensile tested samples up to a strain of approximately 0.025. Low temperatures and high strain rates within the investigated range promote a second work hardening regime, while a plateau in the flow stress is observed at high temperatures and low strain rates. The highest yield stress and peak stress values are reached at 700 °C. The yield stress is nearly independent of the strain rate at 700 °C. It decreases with a decrease in strain rate for 800 and 900 °C, and it decreases with an increase in temperature. The elongation till fracture varies from 10 to 22%, and it is strongly influenced by defects inherent of the L-PBF process. [ABSTRACT FROM AUTHOR]

The inhomogeneous microstructures and mechanical properties in thickness direction are clearly present for extra-thick ultra-high-strength steel, due to the cross section effect. In this work, multi-level structure and its effect on strength and yield ratio for extra-thick steel are investigated, and the relationship between the microstructures and strength was discussed by the Hall-Petch formula. The results show that the size of prior austenite grain, martensite packet, block and lath are all gradually increasing from the surface to the center, and corresponding strength and yield ratio are decreasing. Simultaneously, the high-angle boundary of the martensite block and the low angle boundaries of the martensite lath have a stacking effect on the dislocation movement, and they act as the "effective grain size" to control the strength and yield ratio. Furthermore, the strain hardening rate is high and decreases rapidly in the first stage in the work hardening behavior, whereas, it is low and reaches a dynamic equilibrium in the second stage. [ABSTRACT FROM AUTHOR]

Surface machining including cutting, rolling and grinding was employed to machine 304 austenitic stainless steels, aiming at obtaining high hardening layer and corrosion resistance. The rolling sample exhibited an improved surface quality as indicated by the lowest Sa of 992 nm and less amounts of defects compared with cutting and grinding samples. The main defects were tearing surface for the rolling sample, whereas side flows, tearing surface and plowing groove, severe scratch mark were observed for the cutting and grinding samples. Only single austenite (γ-Fe) was detected among the tested samples. Residual compressive stress (430 MPa) appeared in rolling sample, with respect to residual tensile stress in cutting sample (445 MPa) and grinding sample (432 MPa). The degree of work hardening of the surface for cutting, rolling and grinding sample was 172, 188 and 175%, respectively, and the rolling sample displayed the highest degree of work hardening of the surface mainly resulting from the high residual compressive stress. The rolling sample displayed the highest corrosion resistance among the tested samples as indicated by the lowest corrosion current density and corrosion rate, the highest pitting potential and the largest charge transfer resistance in comparison with the cutting and grinding samples. The high corrosion resistance of the rolling sample can be attributed to the fact that the samples at the condition of residual compressive stress and less micro-defects might form more compact passive film, which possessed better resistance to breakdown from the corrosive medium. The rolling sample fabricated in the present study shows a great potential for application of nuclear power plants due to the excellent work hardening of the surface behaviors and corrosion property. [ABSTRACT FROM AUTHOR]

Warm forming has been shown to be an effective way to increase the formability of various aluminum sheet alloys. If applied to precipitation hardenable alloys, aging and/or coarsening may occur during heating and/or forming processes. As such, there is a potential to leverage the warm forming process to artificially age precipitation hardenable alloys. In this study, the aging characteristics of a pre-aged Al-Mg-Si-Cu alloy (designated AA6013-PA and comprising a 4 h, 100 °C pre-aging cycle following solutionization) were examined using: (i) room temperature tensile and (ii) bendability experiments, as well as (iii) elevated temperature tensile and formability experiments in the temperature range of 220-280 °C. In the pre-aged condition, the alloy exhibited very high work hardening, elongation and bendability. It was found that T6-like properties can be obtained with short duration secondary aging of the AA6013-PA starting temper (410 s at 235 °C), followed by a paint bake cycle (30 min at 177 °C). In such heavily aged conditions, the bendability is found to be moderate and consistent with a T6 temper. Bendability of the pre-aged alloy is shown to be enhanced by reducing the extent of aging, which may be beneficial for applications requiring energy absorption during crash, for example. In the pre-aged condition, AA6013 displayed a 55% improvement in room temperature formability, relative to the T6 condition, for near plane-strain loading using a Nakazima punch, although neither temper exhibited formability improvements through warm forming. The aging kinetics of AA6013-PA were determined from mechanical test data and used to calibrate an aging kinetics model. A strong obstacle model was used to relate precipitation state to yield strength, for artificial aging in the range 220-280 °C. The incremental strength increase due to exposure to an automotive paint cure cycle was also modeled and found to yield accurate results. Warm deformation, as well as room temperature deformation, were shown to enhance the paint bake response of the alloy, reducing the time to the peak aged condition by up to 14% for artificial aging at 177 °C. [ABSTRACT FROM AUTHOR]

The present study conducted the isothermal compression of 2024/6061 aluminum alloy bimetallic casting blank using a Gleeble-3500 thermal simulator in the temperature range of 300-450 °C and strain rate range of 0.01-5 s−1 to clarify the interface bonding mechanism. The effect of temperature and strain rate on the stress was studied, and the strain-compensated constitutive model was established. The effects of element diffusion on the interface bonding were clarified. It is found that the stresses decrease with increasing compression temperature and decreasing strain rate. The stress softening behavior is attributed to the combined effect of strain hardening and dynamic softening. The material constants in constitutive model considering compensation of strain are calculated, and the modified constitutive model reflects a relative low predictability of the interface bonding behavior. The sub-grains with low-angle grain boundaries are transformed into the recrystallized grains with high-angle grain boundaries due to the DRX in the bonding interface. The interface recrystallized grains and microstructural reconstitution are the main interface bonding behavior. The diffusion of interface elements is accelerated and the width of bonding layer increases with increasing compression temperature and decreasing strain rate. The continuous distribution of element diffusion in the bonding interface is enhanced. The interface bonding mechanism of the bimetallic blank is mainly characterized by a metallurgical bonding effect. [ABSTRACT FROM AUTHOR]

In this study, hot extrusions at 460, 470, 480, 490, and 500 °C were carried out on 0.5 wt.% graphene nanoplatelet-reinforced aluminum (0.5 wt.% GNPs /Al) composite prepared by powder metallurgy and hot pressing. The microstructure and mechanical properties of hot extruded composite were investigated. The reinforcement effect of hot extrusion on GNPs/Al composite is the synthetic actions of work hardening, dynamic recovery and dynamic recrystallization of matrix grains, and the dispersion, delamination and structure of the GNPs in the composite due to the different fluidity of the matrix at different temperatures. As a result, the comprehensive mechanical properties of the composite extruded at 490 °C are the best. The TEM image exhibits that, at the optimal extrusion temperature of 490 °C, GNPs are uniformly distributed in the aluminum matrix, mainly dispersed at the grain boundaries, where some Al4C3 and Al2O3 exhibit pinning effects to form fine interface between the GNPs and the matrix. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the rolling process's impact on the formability and mechanical characteristics of the AA 1050/Mg AZ31B bilayer sheet manufactured by explosive welding. Different thicknesses of composite bilayer sheets can be achieved by the work hardening of the explosive-welded Al/Mg composite sheet. The most important point is the simultaneous rolling process's impact on the formability and mechanical characteristics of the bilayer sheet. In this research, Al/Mg bilayer sheet was first prepared by an explosive welding technique. Then, it was cooled down to room temperature in the furnace for 24 h after being annealed for 2 h at 250 °C. The rolling process was performed with reduction ratios of 10 and 20%. The strength, elongation, stress–strain diagram, and forming limit diagram, as well as the microstructural examinations, are compared in the three cases of unrolled bilayer sheets and rolled bilayer sheets with reduction ratios of 10 and 20%. The results show an increase in strength and a decrease in elongation after the rolling process of the bilayer sheets. Moreover, the strength also rises with an increase in the reduction ratio. Besides the strength enhancement, the sheet hardness is also improved due to the rolling process. The hardness improvement in the interface can be associated with forming of brittle intermetallic compounds. The rolling process also causes a drop in the forming limit diagram. Moreover, the formability is decreased by increasing the reduction ratio. The respective reduction of 25 and 49% are observed in FLD0 values for the reduction ratios of 10 and 20% as compared to the unrolled bilayer sheet. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration (UV)-assisted technology has been widely investigated in metal forming in the past decade. However, the research on ultrasonic-assisted forming of Ti2AlNb alloy has not been reported. To study the flow behavior of Ti2AlNb alloy during ultrasonic-assisted forming and set up the constitutive model that can accurately describe its flow behavior, the room temperature compression test of Ti2AlNb alloy with ultrasonic amplitude range of 0-31 µm and strain rate of 0.001-0.125 s−1 were carried out. Considering the influence of yield strength, strain hardening and strain rate hardening effect, a modified Johnson–Cook (J–C) constitutive model is proposed, which can accurately describe the flow behavior of Ti2AlNb alloy during ultrasonic-assisted compression. The results show that under the studied deformation conditions, the yield strength and strain hardening coefficient of Ti2AlNb alloy decrease with the increase of ultrasonic the amplitude. When the amplitude increases to 31 µm. the yield strength decreases by 42%. The ultrasound provides larger dislocation motion energy, increases dislocation activity, and alleviates dislocation stacking, thus reduce the yield strength. The strain hardening exponent and strain rate hardening coefficient increase with the increase of amplitude. The results, obtained from the quantitative analysis of the optimized J–C model, show that the prediction results are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Nanoindentation testing technology can effectively and conveniently evaluate the mechanical properties of materials at the micro- and nano-scales. However, the nanoindentation test is a nonlinear mechanical process, which is difficult to determine the elastic–plastic constitutive parameters of the indentation material via analytical solution. It is important to mastering these constitutive parameters for the application of the materials at micro and nano scales. In this study, a reverse analysis method for determining elastic–plastic constitutive properties of metallic materials is proposed via the dimensional analysis and finite element (FE) simulation. The dimensionless function including the yield stress (σy) and strain hardening exponent (n) is firstly constructed via the dimensional analysis method. After assuming a reasonable the n value, the constitutive properties are determined via constructing the dimensionless function to solve the σy value and the FE simulations are carried out. By adjusting the n value until the simulated unloading curve is agreement with the experimental curve we can determines the n of the materials. Next the σy of the materials is obtained accurately and solely via the dimensionless function and modified simulation. Finally, nanoindentation experiments were conducted with sintered nano-silver to verify the accuracy and effectiveness of the method. [ABSTRACT FROM AUTHOR]

Metastable β titanium alloy usually exhibits limited work hardening and high yield-to-strength ratio which is detrimental to aerospace application. In this paper, a systematic research on the weak work hardening and microstructure evolution of metastable β titanium alloy Ti-5Al-4Zr-7Mo-8V was performed. Dislocation density, microstructure evolution and slip behavior under different tensile strains were investigated by XRD, EBSD and TEM. The results indicated that the maximum work-hardening rate after yielding point was only ~ 638 MPa which was lower than the true stress. The total dislocation density increased slightly with the increase in the plastic deformation from 2.7 × 1014 to 8.3 × 1014 m−2. The density of geometrically necessary dislocations increased from 2.5 × 1014 m−2 at 3% tensile strain to 4.9 × 1014 m−2 at fracture. By using a Taylor-type model and Kocks–Mecking equation, the origin of the limited work hardening was analyzed at a macroscopic level, which is attributed to the slow propagation of dislocation density. In addition, characterization of slip behavior and dislocation configuration was performed to reveal the limited work hardening at microscopic level. Long straight planer slip initiated on (110) plane with b = 1 / 2 1 1 ¯ 1 . Then, multi-slip was generated on (110), (101) and (1 1 ¯ 2) planes during further deformation which displayed long straight crossing configuration. Planer slip facilitated dislocations to glide out of the grain and reduced the possibility of dislocation interaction which was the microstructure reason of the limited work hardening. [ABSTRACT FROM AUTHOR]

The isothermal compression tests were conducted to study the hot compression behavior of Al-Zn-Mg-Sc alloy with high Zn concentration using a Gleeble-3500 thermal simulator in the temperature range of 300-450 °C and strain rate range of 0.01-5 s−1. The stress softening behavior was analyzed, and the constitutive model was established to describe the hot compression behavior of the high Zn alloy. The microstructure evolution mechanism was clarified by optical microscopy and electron backscatter diffraction. The results indicate that the true stress rapidly increases to the peak value with the increasing accumulative true strain, and finally reaches to a steady state. As the strain rate increases and temperature decreases, the peak stress increases, and the isothermal compression behavior is regarded as a competing process of work hardening and flow softening. The softening mechanism is highly affected by the compression temperature and strain rate. The Al-Zn-Mg-Sc alloy with high Zn is characterized by the dynamic recovery at the low temperature and high strain rate. And both the dynamic recovery and dynamic recrystallization dominate at the high temperature and low strain rate. The relatively stronger ⟨ 111 ⟩ orientation plays a key role in developing the recrystallization grains. As the compression temperature increases and strain rate decreases, many sub-grains with low-angle grain boundaries are transformed into recrystallized grains with high-angle grain boundaries. The main softening mechanism is also transformed from dynamic recovery to dynamic recrystallization. [ABSTRACT FROM AUTHOR]

In the present work, the effects of 475 °C embrittlement on the tensile response of two different duplex stainless steel alloys namely 2507 SDSS and 2101 LDX aged for different times at a temperature of 475 °C were investigated. For this, a power law such as Hollomon's equation was applied in the plastic zone to model the true stress–strain behavior. Additionally, a difference approximation was conducted on the experimental data to obtain the strain hardening rate of the different aged specimens. The Hollomon's analysis of the experimental data shows that the 210 LDX alloy requires up to three sets of hardening exponents n and constants K to fit the model to the experimental data in the plastic region of stress–strain curves. Contrarily, the alloy 2057 SDSS exhibits a hardening transition from three to one set of n and K fitting constants as the aging time increases. Also, the strain hardening rate of the 2507 SDSS is higher than that of the 2101 LDX. SEM and EDX mapping of the microstructure indicated that in the holding time range between 100 and 1000 h, the 2507 SDSS showed the presence of precipitated particles characteristic of the G-phase whereas in the 2101 LDX no G-phase was observed in the same time range. EBSD analysis of grain material misorientation shows that the austenite phase shows higher elastic strains than the ferrite phase whereas the ferrite phase displays higher kernel average misorientation. [ABSTRACT FROM AUTHOR]

In the present study, the effect of pre-strain process on the mechanical properties, microstructure, fatigue life, and fracture mode of 304 austenitic stainless steel was discussed. Six different pre-strain states ranging from the original state to the 35% pre-strain state were investigated. The mechanical properties of different pre-strain state samples were tested first. A bilinear relationship between mechanical properties and pre-strain was established. Both yield strength and tensile strength of 304 stainless steel increase with increasing pre-strain, but the yield strength obviously increases more than the tensile strength. Combined with the observation of the microstructure, a critical pre-strain point was proposed. When the pre-strain is lower than the critical value, only work hardening occurs. When it is greater than the critical value, the mechanical properties are affected by the combined effects of work hardening and martensitic transformation. There is also a critical pre-strain point in the fatigue life of 304 stainless steel, as shown in the fatigue life test results of different pre-strain specimens. The inherent essence of fatigue life improvement of pre-strained material is the comprehensive effect of material phase transformation and plastic deformation strengthening. [ABSTRACT FROM AUTHOR]

In this work, a suitable constitutive model was selected, and its parameters were determined by experiments to investigate the relationship between flow stress and strain during ultrasonic surface burnishing process (USBP). The quasi-static and dynamic mechanical responses of 18CrNiMo7-6 alloy steel were obtained by tensile test and split Hopkinson pressure bar test, respectively. A modified Johnson–Cook (J–C) constitutive model of 18CrNiMo7-6 alloy steel under USBP was established based on test data. This modified J–C model considers the Voce hardening model to describe the strain hardening term of 18CrNiMo7-6 alloy steel. A nonlinear correction was applied to the strain rate strengthening factor by considering the coupling relationship between strain and strain rate strengthening factor. The modified J–C model exhibited better accuracy and was more suitable for predicting the stress flow behavior of 18CrNiMo7-6 alloy steel during USBP through error assessment between the original model and the modified J–C model. [ABSTRACT FROM AUTHOR]

This paper focuses on the shear fracture behaviors of U71Mn rail steel at high strain rates using a shear-modified Gurson–Tvergaard–Needleman model. Two hat-shaped specimens with different shapes (shape I, shape II) are tested by the split Hopkinson pressure bar setup at different strain rates. Besides, the plastic behaviors of U71Mn are also investigated using cylindrical specimens. It can be seen from the flow stress–strain curves of the cylindrical specimens; the plastic behaviors of U71Mn at high strain rates can be divided into two stages by a critical point. Before the critical point, strain hardening dominates the deformation. However, after the critical point, thermal softening dominates. To describe this difference, a modified Zerilli–Armstrong model is proposed. Furthermore, this modified model is chosen to be the hardening law of the shear-modified Gurson–Tvergaard–Needleman model. The parameters of the shear-modified Gurson–Tvergaard–Needleman model for U71Mn are calibrated by trial and error. Subsequently, the shear fracture processes and the fractography of the two hat-shaped specimens are observed by numerical simulations and scanning electron microscope, respectively. The results show that compared with the shape II hat-shaped specimen, the shape I hat-shaped specimens lead to an easier fracture and have more voids in the fracture surface. Moreover, the effect of stress triaxiality on the evolution of the void volume fraction is analyzed. According to the experimental and simulated results, it can be concluded that under the same equivalent plastic strain, the higher the stress triaxiality, the more the number of voids. [ABSTRACT FROM AUTHOR]

The relative contribution of aging hardenability to the overall dependence of tensile properties on microporosity variation in Al-6Si casting alloys was investigated in terms of the defect susceptibility concept for the quality index of tensile property. The tensile properties of as-cast and T6-treated Al-6Si-xMg alloys fabricated by the addition of 0.3-1.5 wt.% Mg were evaluated. Furthermore, the melt hydrogen concentration, melt fluidity, and scanning electron microscopy (SEM) fractographic porosity were measured. The increase in microporosity due to the decrease in melt fluidity with Mg addition was a fundamental factor in reducing the tensile strength and elongation, resulting in a lower quality index value than the nominal level in the base condition with no Mg addition. The increase in the strength coefficient and the decrease in the strain hardening exponent by Mg addition and T6 treatment were related to the increase in ultimate tensile strength (UTS) and decrease in elongation, respectively. Moreover, the defect susceptibility coefficient of the quality index based on microporosity variation decreased during T6 treatment with Mg addition. The quality index under the defect-free condition demonstrated a maximum value when 0.3 wt.% Mg was added, and its nominal value decreased gradually over a specific range of Mg addition, resulting in an increase in the fractographic porosity with a decrease in melt fluidity. The constitutive prediction of the quality index indicates the strength coefficient and strain hardening exponent determine the maximum quality index and defect susceptibility coefficient values, respectively. [ABSTRACT FROM AUTHOR]

The high-temperature deformation behavior of Ti-6.5Al-2Sn-4Zr-4Mo-1W-0.2Si alloy with lamellar starting structure was investigated by carrying a series of isothermal compression tests at temperatures of 850-1030 °C and strain rates of 0.001-10 s−1 on the Gleeble-3500 simulator. Strain-rate sensitivity exponent and deformation activation energy have been analyzed associated with the dynamic softening behaviors. Meanwhile, different identification approaches like power dissipation distribution and work-hardening derivative have been adopted to identify the softening behaviors corresponding to different deformation parameters. Then constitutive models based on the dislocation evolution have been employed to characterize the flow curves of Ti-6.5Al-2Sn-4Zr-4Mo-1W-0.2Si alloy. Finally, microstructure observation has been carried out to verify the dynamic softening behaviors occurred under different deformation conditions. [ABSTRACT FROM AUTHOR]

This study enhances the strength and ductility of (Ce + Yb)-modified Al-Si-Mg-Cu-Cr casting alloy by integrating hot deformation and heat treatment. In addition, the synergistic effects of hot deformation and heat treatment on the microstructure and mechanical behavior were thoroughly investigated. As-solutionized alloys were subjected to hot deformation examinations at temperatures ranging from 250 to 450 °C and strain rates varying between 0.0005-0.0125 s−1. The results revealed that the flow stress increased as the temperature decreased, and the strain rate increased. The maximum ultimate tensile strength (UTS), yield strength (YS) and elongation (El) of as-solutionized, hot deformed at 250 °C, strain rate of 0.0025 s−1 and aged alloy were 353, 296 MPa and 11.4%, respectively, higher by 79.1, 193.1 and 96.6% than the as-cast alloy. The interactions between Ce- and/or Yb-rich precipitates, the high-density dislocations induced by hot deformation and the creation of substructures such as subgrains are primarily responsible for the increase in strength. Finally, the high strength of the (Ce + Yb)-modified alloy after hot deformation and heat treatment is primarily attributed to the synergistic strengthening of several factors through solid solution strengthening, precipitation strengthening, substructural strengthening and work hardening. [ABSTRACT FROM AUTHOR]

The particle-reinforced (ZrB2 + Al3Zr)/AA6016 composites with different contents were prepared by in-situ chemical reaction of the Al-KBF4-K2ZrF6 system. With the (ZrB2 + Al3Zr) particles introduced, the composites exhibit enhanced mechanical properties and plastic instability. The ultimate tensile strength (UTS), yield strength (YS) and elongation of as-cast 6wt.% (ZrB2 + Al3Zr)/AA6016 composites reach 259 MPa, 102 MPa and 21.6%, respectively, which is significantly higher than the matrix with UTS-172 MPa, YS-70 MPa and EL -15.2%. The strain hardening rate curve shows that the presence of particles hinders the dislocation propagation process. According to the dynamical strain aging (DSA), the blocking effect of particles on dislocation motion is further strengthened with the increase of particle density, a larger span of upper and lower yield points appears. The moving range of active dislocations is reduced, which makes the time for fixed dislocations shorter. The existence of reinforcing particles indirectly affects the diffusion of solute atoms and the process of pinning dislocations during heat treatment. After T6 treatment, the type of serration was converted from (A + B) type to type B, and the serration frequency decreased with the increase in stretching rate. [ABSTRACT FROM AUTHOR]