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The development of aluminum alloys with superior mechanical properties and high strain rate superplasticity is an important prerequisite for advancing applications of superplastic blow-forming technology. This study focuses on the effect of Ce and Fe additions on the microstructural parameters and tensile properties of a superplastic Al-4.8%Mg-0.6%Mn-0.15%Cr (AA5083-type) alloy. The studied alloy exhibits a bimodal particle size distribution with coarse crystallization origin inclusions and fine secondary precipitates. Both the coarse and fine particles lead to grain refinement. The coarse Ce-, Fe-, and Mn-rich intermetallic particles of crystallization origin provide evidence of a particle-stimulated nucleation effect. The semi-coherent Mn-enriched compact-shaped dispersoids with an Ashby-Brown contrast, a quasicrystalline structure, and a mean size of 38 nm are precipitated after low-temperature homogenization and exhibit a strong Zener pinning effect. The proposed thermomechanical treatment results in the development of a grain size of 4 μm and an ultimate tensile strength of 340 MPa in the recrystallized sheet. The superplastic deformation behavior in a strain rate range of 1 × 10−2 to 1 × 10−1 s−1 and the associated strain-induced changes in the grain structure and mechanical properties of the developed alloy are presented and discussed.

A novel dual-phase-dominated fine-grained (5.5 μm) Mg-6.4Li-3.6Zn-0.37Al-0.36Y alloy was fabricated by multidirectional forging and rolling (MDFR); its mechanical properties and microstructural evolution were investigated. The ultimate tensile strength of 286 MPa and elongation of 31.8% were demonstrated in this alloy. The Portevin–Le Chatelier effect was observed in cold-rolled alloy. During MDF, the dominant grain refinement mechanisms of the α-Mg phase are mechanically fragmented grain refinement and deformation-induced in-situ dynamic recrystallization (DRX) grain refinement. However, the dominant grain refinement mechanism of the β-Li phase is deformation-induced in-situ DRX grain refinement. In later stage, thermally activated grain coarsening occurs in both phases. X-ray diffraction analyzes reveal the existence of α-Mg and β-Li phases, Al2Y, and MgLi2Al intermetallic compounds in the MDF state and the existence of α-Mg and β-Li phases, Al2Y, Al12Mg17, and MgLiZn intermetallic compounds in the cold-rolled state. Transmission electron microscopy analyzes show that high-density dislocation pile-up, dislocation tangle, subgrains, and MgLiZn particles exist in the cold-rolled alloy and raise the tensile strength. Texture analyzes reveal that strong {0001} basal texture or basal fiber of α-Mg phase exists, but the texture intensities in this alloy are not high. Annealing does not produce new texture component. A Hall-Petch model was established in this alloy: yield strength = 11.83 + 351.89 times minus square root of grain size. The relationship between critical resolved shear stress and resistance to deformation was modeled. Static recrystallization and grain coarsening during annealing were discussed.

An on-going problem in friction stir welded (FSW) joints used in the high-speed train sector is that the microstructure and mechanical properties can significantly vary in thick sections. Because inhomogeneous properties can reduce weld efficiency and degrade service performance, it is of some interest to understand how inhomogeneous properties can develop in FSW welds made from precipitation hardening alloys such as 7N01. In the current study, butt welds were made using 12 mm thick plates and then sectioned perpendicular to the weld line. Five 2.2 mm thick slices were cut from a section and used to measure tensile properties access the weld thickness. The microstructure was characterized using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) to measure variation in grain size and second phase particle distributions over the weld zone. Results showed that, with the exception of the top slice, yield strength (σy) and ultimate tensile strength (σUTS) obtained from the slices were fairly consistent and comparable to values from the full weld. Elongation (δ) was maximum at mid-thickness decreased significantly towards the crown. Although significantly reduced elongation was found at the top of the weld, and likely limits overall weldment ductility, elongation of the full FSW weld was improved over that of the base metal and can be attributed to enhanced post-necking straining. The finest grains and second phase particles were observed at mid-thickness. In comparison, the coarsest grains were observed at the top of the nugget. This microstructural variation can be understood by considering the temperature and strain field gradients that are generated in the plasticized zone. It is expected that the findings will help to promote a better understanding of post-weld microstructure development and mechanical properties of thick plates.

The flow stress and work-hardening of rapidly solidified, (RS), and conventionally cast, (IM), AM60 magnesium alloy has been investigated under conditions of uniaxial tension and compression at 4 K, 78 K and 298 K. Rapid solidification combined with plastic consolidation through hot extrusion permit to obtain the alloy with reduced grain size and refined intermetallic particles, which otherwise precipitate in the form of large aggregates in commercial alloys. RS alloy exhibit higher strength and better ductility than IM material. Enhancement of mechanical properties of RS alloy is discussed in terms of the features of material's microstructure. The analysis of work-hardening reveals that independently of deformation temperature and deformation mode, RS alloy shows a lower capacity of defects storage in the substructure. This behaviour is attributed to reduced non-basal slip and forest hardening and enhanced recovery of basal dislocations at grain boundaries in fine-grained RS alloy. Grain boundary sliding does not play an important role in the deformation process. Fracture of the alloys is particle-stimulated and occurs by nucleation and growth of voids around second phase particles. Refining particles by rapid solidification lead to the better ductility of RS alloy irrespective of deformation temperature.

Multi-pass friction stir processing (M-FSP) was used to produce large-scale AZ61 Mg plate with fine-grained structures. High-temperature tensile behavior of the M-FSP plate was investigated along different directions in a temperature range of 473–673 K at a strain rate of 1 × 10−2 s−1∼3 × 10−5 s−1, respectively. Microstructural evolution and texture change during high-temperature deformation were studied. The fine-grained plate with an average grain size of 7.8 ± 6.4 μm has been attained via M-FSP, while the microstructure is not homogeneous. The M-FSP plate is observed to exhibit tensile anisotropy at elevated temperatures. The highest elongation is obtained along the processing direction, with the maximum value of 211% achieved at 623 K and 3.3 × 10−4 s−1 due to the lower intensity of texture and higher value of Schmid factor with respect to this direction. Effects of grain structure inhomogeneity and texture on the high-temperature tensile behavior of the M-FSP plate are discussed.

Tensile properties of cold rolled AISI 316L stainless steel after full reversion of martensite to austenite, recrystallization of retained austenite, and grain growth were studied at 850, 950, and 1050 °C. At higher temperatures, it was found that the kinetics of the reversion and recrystallization processes enhance but coarser grain sizes will be obtained at the end of recrystallization. At 1050 °C, appreciable grain growth was observed after the completion of the recrystallization process, which was not the case for a low temperature of 850 °C. At the stage of full recrystallization, by decreasing the annealing temperature, the yield stress (YS) and the ultimate tensile strength (UTS) values increased and total elongation decreased, which was related to the grain size strengthening by the Hall-Petch law. However, the Hall-Petch slope for the UTS was found to be much smaller than that of YS, which reveals that YS has greater grain size dependency. The latter was ascribed to the improved work-hardening behavior and enhanced transformation-induced plasticity (TRIP) effect by coarsening of grain size. To obtain high-strength and ductile steel with tensile toughness higher than 300 MJ/m3 and yield ratio of ∼0.5, the average grain size of ∼3 μm was found to be desirable.

The aim of the paper is to study the combined effect of grain size and stress on the strain variation on cooling and heating of Ni50.2Ti49.8 alloy. To produce the samples with a 20–500 nm grain size, Ni50.2Ti49.8 alloy samples were subjected to high-pressure torsion (8 GPa, 3.5 turns) and subsequent heating. The strain variation in samples with different grain sizes was studied on cooling and heating under a tensile stress of 100–600 MPa. In the samples with 20 and 40 nm grain sizes, strain variation was found on cooling and heating if the stress was larger than some critical value, which depended on the grain size. In samples with 80–500 nm grain sizes, the strain variation on cooling under stress occurred in stages that were caused by different sequences of the martensitic transformation that depended on grain size and stress. Strain recovery always occurred in one stage and irrecoverable strain was found in all cases except for the sample with a 20 nm grain size. A reversible strain of 15% was found during cooling and heating under 400 MPa for the sample with a grain size of 130 nm. It was assumed that in this sample, the strain that appeared on cooling was caused by the forward transformation and deformation twinning that took place because plasticity was supressed in small grains. The strain that was induced by the deformation twinning was fully recoverable on subsequent heating and it gave the additional contribution to strain recovery.

In this study, the work-hardening behavior of Ag microwires with different grain sizes has been investigated, and the results show that the work-hardening rate of Ag microwires does not decrease with the increase of grain size, suggesting that the work-hardening capacity of Ag microwires is insensitive to grain size. Moreover, the microstructure evolution of the samples before and after tensile tests has been characterized using scanning electron microscope-backscattered electron (SEM-BSE), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). As a typical low stacking faults energy (SFE) material, stacking faults and twins are particularly easy to form in Ag microwires. Abundant stacking faults and twins, as well as minor lattice orientation adjustment in Ag microwires were observed and confirmed to play key roles on their grain-size insensitive work-hardening capacity. The current study provides insights for manufacturing Ag microwires for bonding process and other microelectronics applications.

Heterogeneous nanograined structures have been proposed to achieve unprecedented mechanical properties. The objective of the present work is to investigate fatigue crack growth behavior of harmonic structure designed austenitic stainless steels at room temperature. The harmonic structured SUS316L steels were produced by mechanical milling and subsequent hot isostatic pressing. Fatigue crack propagation tests were carried out under load ratio from 0.1 to 0.5. Results show that the change in fatigue crack growth rates of harmonic structured SUS316L steels is insignificant even as grain size decreases. The load ratio has little influence on the fatigue crack growth rates for harmonic structured SUS316L steels compared to their coarse-grained counterparts. It is worth mentioning that fatigue crack growth is impeded by embedded coarse grains, which results in fatigue crack deflection and secondary cracks. The continuously network shell (UFG structure) provides increased tensile strength, enhancing the fatigue crack nucleation resistance. Simultaneously, the embedded core (CG structure) restrains crack growth. Therefore, the harmonic structure design is proposed to be an effective method to achieve good balance of high strength and high ductility as well as good fatigue resistance.

Aluminum alloy 6061–5wt. %TiB2 in-situ metal matrix composite was produced by a salt-melt reaction between halide salts K2TiF6 and KBF4 inside AA6061 melt. The composites prepared were subjected to equal channel angular pressing (ECAP) up to 10 passes at ∼300 °C (0.45 Tm of AA6061). The improvement in strength and Vickers hardness due to grain refinement and redistribution of in-situ TiB2 particles was studied. The AA6061 (without TiB2) was also subjected to ECAP to identify the improvement arising from the presence of TiB2 particles and their redistribution in the AA6061-TiB2 composite. The microstructures of the AA6061 alloy and the composite were studied using Electron Back Scattered Diffraction (EBSD). The Vickers hardness and tensile strength of the alloy and the composite were evaluated as a function of the variables of the ECAP process. After 10 ECAP passes, the average grain size of AA6061 alloy had decreased from ∼87 ± 11 μm to ∼20 ± 2 μm. In contrast, the average grain size of the AA6061-TiB2 composite after 10 passes had decreased to ∼5 ± 2 μm from ∼37 ± 9 μm. There was an increase in the Vickers hardness of AA6061 alloy from ∼54 ± 1 HV to ∼64 ± 1 HV and the AA6061-TiB2 composite showed a hardness improvement from ∼57 ± 1 HV to ∼69 ± 1 HV. Compared with the as-received parent material, the yield strength of the alloy subjected to 10 ECAP passes had increased from ∼105 to ∼149 MPa while the ultimate tensile strength increased from ∼123 MPa to ∼153 MPa after 10 ECAP passes. The yield strength of the AA6061-TiB2 composite increased from ∼113 MPa (annealed) to ∼167 MPa after 10 ECAP passes while the ultimate tensile strength increased from ∼146 MPa to ∼177 MPa for the same treatment. EBSD results show that after 10 ECAP passes there is an increase in the percentage of high-angle grain boundaries (HAGBs) from 46% to 51% in the AA6061 alloy and an increase from 18% to 60% in the AA6061-TiB2 composite.

In this work, dissimilar rapid Rotary Friction Welding of WC-Co cermet to AISI 304 L austenitic stainless steel has been conducted using different friction times. The microstructural examination showed that the increase in friction time from 4s to 12s increases the grain size in both the heat affected zone and the thermo-mechanically affected zone and enlarges the extent of the fully dynamically recrystallized zone. EDS analysis revealed the existence of a Fe Cr W rich band along the WC-Co/AISI 304 L interface in the central region of the weld joint and its absence from the peripheral region. The formation of this band suggests the occurrence of a mutual inter-diffusion between the cermet and the steel which enhanced the metallurgical bonding of the interface. The mechanical behavior investigated by nano-indentation measurements and nano-scratch tests revealed that, regardless the friction time effect and considering the 304 L ASS side, the highest hardness (HIT) and the lowest Young's modulus (EIT) values were recorded in the fully dynamically recrystallized zone. Besides, the increase of friction time resulted in an increase of hardness and Young's modulus of each zone in the AISI 304 L steel side.

Laser metal deposition (LMD) is a common manufacturing technique of laser additive manufacturing (LAM) which belongs to coaxial powder feeding method. The LMDed SS304 specimens are fabricated by the same combination of the process parameters so that they have the similar properties. The microstructure is similar to that of the selective laser melted (SLMed) specimen. The grain sizes of the LMDed specimen are compared with those of the specimens manufactured by the traditional method and SLM. Through the calculation of Ni and Cr equivalents in the LMDed SS304 specimen and the analytical results of electron backscattered diffraction (EBSD) and energy dispersive spectroscope (EDS), the phase compositions of the LMDed SS304 specimen are evaluated. The hardness of the LMDed specimen is compared with that of the conventionally manufactured wrought SS304 and SLMed specimens. And due to the differences of the grain size and phase composition on the different directions, the hardness of the LMDed specimen is anisotropic. The static tensile properties (ultimate tensile strength (UTS), yield strength (σ0.2) and elongation (EL)) and fatigue strength (FS) of the LMDed specimen are compared with those of the specimens made by the traditional method and SLM. The S N curve is established by the multiple experiments. Scanning electron microscopy (SEM) is applied to observe the fracture morphology of the static tension and tension-compression fatigue tests. And EDS is applied to analyze the chemical compositions of the particles on the fracture surface. As expected, LMD has the higher static tensile properties and fatigue strength than those of the specimen manufactured by the traditional method.

Processing of dual phase steel with fine ferrite grains containing nanometric carbides and chain-like morphology of martensite from a 0.035C-0.005N-0.035Si-0.268Mn mild steel was realized via application of high cold rolling reductions, intercritical heat treatment, and room-temperature aging, which showed a yield stress (YS) of 450 MPa and ultimate tensile strength (UTS) of 700 MPa. It was revealed that significant enhancements in tensile properties can be achieved compared to the separate effects of intercritical annealing (YS of 130 MPa and UTS of 385 MPa), cold rolling and intercritical annealing (YS of 300 MPa and UTS of 600 MPa), and quench aging (YS of 275 MPa and UTS of 520 MPa). The results were rationalized based on the grain size strengthening, aging effect, and work-hardening behavior.

ZK60 Magnesium tubing has been friction extruded from as-cast billets and T5 conditioned bars using Shear Assisted Processing and Extrusion (ShAPE). Tubes having an outer diameter of 50.8 mm and wall thickness of 1.9 mm were extruded with >20 times less ram force compared to conventional extrusion due to the unique shearing conditions and tooling inherent to ShAPE. Microstructures of the as-cast billet and T5 bar feedstock materials were significantly different from each other in terms of grain size, texture, and second phase distribution; yet the resulting microstructures after ShAPE were remarkably similar. An average grain size of 4–5 μm, 20° tilt of basal texture away from the extrusion axis, and refined second phases having a uniform distribution were achieved independent of the feedstock material. Hardness for as-extruded and artificially aged tubes are presented with isotropic behavior explained by detailed microstructural analysis. This work suggests that bulk ZK60 magnesium alloys extrusions may be fabricated in a single step, with microstructures that are unobtainable with conventional extrusion.

In this article, we probe the strain partitioning between the microstructural features present in a continuously cooled carbide-free bainitic steel together with damage nucleation and propagation. These features mainly comprise of phases (bainitic ferrite, martensite, and blocky/thin film austenite), interfaces between them, grain size and grain morphology. A micro Digital Image Correlation (μ-DIC) technique in scanning electron microscope is used to quantify the strain distribution between these microstructural features. The results show a strong strain partitioning between martensite, bainitic ferrite and retained austenite that provides weak links in the microstructure and creates conditions for the crack initiation and propagation during deformation. Blocky austenite islands accommodate maximum local strains in the global strain range of 0–2.3% and undergo strain-induced austenite to martensite transformation governing the local strain evolution in the microstructure. However, the local strains are minimum in martensite regions during entire in-situ deformation stage. Narrow bainitic ferrite channels in between martensitic islands and martensite-bainitic ferrite interfaces are recognised as primary damage sites with high strain accumulation of 30 ± 2% and 20 ± 3% respectively, at a global strain of 9%. The inclination of these interfaces with the tensile direction also affects the strain accumulation and damage.

The effects of post-build heat-treatments on the microstructure, phase formation, recrystallization behavior, and mechanical properties of laser powder bed additively manufactured Inconel 718 superalloy were investigated. Several heat-treatment schedules were used, including simulated hot isostatic pressing (HIP), and variations of the standard double aging treatment with various soaking times and quench procedures. Their effects on the precipitation, grain morphology, grain size, texture sharpness and mechanical properties were all documented. For the as-built coupons, columnar grains with inter-dendritic micro-segregation were formed along the build direction (xz-plane) with equiaxed grains forming on perpendicular sections to the build direction (xy-plane). After prolonged soaking times during the simulated HIP process, the microstructure transitioned from heterogeneous columnar grains to homogenous recrystallized grains with MC-type carbide precipitates. This leads to changes of microhardness (281 HV2.0 to 171 HV2.0), Young's modulus (209 GPa–229 GPa) and texture intensity. However, aging treatments increased both hardness and Young's modulus possibly because of formation of γ″ and γ′ precipitates in the Ni-matrix and the small effective grain size. Phase analysis using XRD confirmed the evolution of the precipitate formation. The combination of additive manufacturing and post-build heat-treatments can result in optimized microstructures and mechanical properties for specific applications depending on part requirements and operating conditions.

The hydrogen embrittlement behavior of multilayer steel consisting of 7 alternating layers of martensitic (MART) steel and twinning-induced plasticity (TWIP) steel was investigated through variation in the ratio of the hard layer to the soft layer, and the size of the grains according to the autenization temperature. Multilayer steels with a high fraction of MART was predicted to be vulnerable to hydrogen embrittlement, but was found to be less susceptible to hydrogen embrittlement due to the presence of a TWIP layer, which is supported in the center of the multilayer steels. In addition, when the grain size is large, twinning in the TWIP layer and subdivision in the MART layer occur, and so hydrogen embrittlement is promoted due to the trapping of hydrogen. The effect of electrochemically-charged hydrogen on the multilayer steel during deformation was analyzed using electron backscatter diffraction in terms of the microstructure-mechanical property relationships.

Experiments were performed to analyze the microstructural evolution and mechanical behavior of commercial-purity copper (99.8%) processed by up to 48 cycles of multi-directional forging (MDF) using a low strain amplitude of ∼0.075 (total accumulated strain ε ≈ 10.8). Parabolic work-hardening concomitantly with increasing dislocation densities was observed up to ε ≈ 2, followed by a practically constant flow stress due to dynamic recovery. The average grain size was reduced from 30.5 μm in the annealed metal down to 4.1 μm for ε ≈ 7.2; the fraction of sub-micrometric grains reached ∼12% for ε ≈ 10.8. The microstructural changes were attributed to the fragmentation of the original grains by dislocation structures having low misorientation angles which gradually evolved into arrays of high-angle grain boundaries with increasing numbers of MDF cycles. The Cu samples subjected to 48 cycles of MDF displayed limited dynamic recrystallization, exhibiting basically dislocation cells and sub-grains with an average size of ∼0.6 μm. It is demonstrated that low strain amplitude MDF delays the kinetics of grain refinement in copper compared with MDF using higher strain amplitudes. Fil: Flausino, Paula Cibely Alves. Universidade Federal de Ouro Preto; Brasil. Universidade Federal de Minas Gerais; Brasil Fil: Nassif, Maria Elisa Landim. Universidade Federal de Minas Gerais; Brasil Fil: de Castro Bubani, Franco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universidad Nacional de Cuyo; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Pereira, Pedro Henrique R.. Universidade Federal de Minas Gerais; Brasil Fil: Aguilar, Maria Teresa Paulino. Universidade Federal de Minas Gerais; Brasil Fil: Cetlin, Paulo Roberto. Universidade Federal de Minas Gerais; Brasil

An integral modelling approach for understanding the strengthening mechanisms in Ti alloys is presented and applied to alloys undergoing deformation via dislocation slip. The model incorporates contributions from solid solution, grain boundary, dislocation forest and strain hardening. The metal forming and thermomechanical processing factors influence both grain size and the stored strain energy. The strain hardening of Ti-Fe-Sn-Nb alloys was modelled by considering dislocation accumulation and annihilation terms. By tailoring the contribution of each strengthening effect, the yield stress and plasticity of advanced Ti alloys can be optimised.

An ultrafine-grained (UFGed) low alloyed aluminum alloy (6061 Al alloy) with a special nano-sized spherical precipitate structure was achieved by friction stir processing (FSP) and post-FSP aging. The evolution of the grains and precipitates during post-FSP aging and their contribution to mechanical properties were investigated. Under a low rotation rate (200 rpm) with water cooling, the average grain size of the FSP sample was only 200 nm. The ultimate tensile strength of the UFG 6061Al alloy aged at 100 °C for 20 min achieved 573 MP with a high elongation of 17%. This strength was nearly 90% higher than that of 6061 Al-T6 alloy (∼300 MPa), and was equivalent to that of commercial 7075Al-T651 alloy (∼572 MPa, the typical high alloyed Al alloy). Such a good mechanical property was attributed to the good thermal stability and the accelerated precipitation of the FSP UFGed structure under the low-temperature and short-time aging conditions. Further, the contribution of various strengthening mechanisms to the total strength in post-FSP aged Al alloys was quantitatively evaluated. This work provides an effective strategy for obtaining low alloyed high-strength Al alloys, which is beneficial for reducing environmental burdens and improving long-term sustainability.

This paper presents a systematic study of the differences in the microstructures and micro-mechanical properties of various Cu/Fe wave interfaces prepared by explosive welding. These wave interfaces, fabricated by different parameters, have been divided into four categories according to variations in their microstructures and the element distribution: vortex region, transition layer, interface without metal bonding, and interface with defects. In the wave interfaces, nanocrystalline areas filled with nano-copper and nano-iron grains (50 nm), medium-sized grains (300 nm), coarse grains (1 μm) and finer nanocrystals (5 nm) or amorphous phases in wave interfaces. The variations in the grain size contribute to the inhomogeneity of hardness distribution in the first kind of interface, the vortex region. In the second kind of interface, the transition layer, both EDS and TEM measurements reveal a gradient in the element contents due to atomic diffusion. This transition layer possesses rather higher and more uniformly distributed nano-hardness (up to 15.75 GPa) because of the combined effects of fine grain strengthening and work hardening. The third kind of interface with an abrupt change in element distribution does not form metal bonds at the boundaries. Additionally, a comprehensive analysis on these four kinds of interfaces suggests that the second type of interface (transition layer) is the ideal one.

Using thermal-mechanical treatments, specimens with different grain sizes were produced for two Cr-Ni-based austenitic stainless steels with different stacking fault energies (analogues of AISI 316L and AISI 321 steels). The effect of strain-rate on the tensile deformation behavior and strength properties was evaluated for these steels. In given grain size interval, (3–73)μm for 316 steel and (0.2–32)μm for 321 steel, the yield strength σ 0.2 varies with grain size D in accordance with Hall-Petch relationship σ 0.2 = σ 0 + k H P D − 1 / 2 . The Hall-Petch coefficient kHP depends on steel composition (stacking fault energy) and possesses higher value for 321 steel as compared to 316 steel. Increase in strain-rate in the interval of 1.0 × 10−4 s−1 to 1.0 × 10−2 s−1 causes growth in stress σ0, but weakly changes coefficient kHP in Hall-Petch relationship: 322–327 MPa × m0.5 for 316 steel and 404–413 MPa × m0.5 for 321 steel. The strain-rate dependence of the constants in Hall-Petch relationship was discussed in terms of deformation mechanism and dislocation arrangement in both steels.

This paper studies the effects of high-pressure torsion (HPT) at ambient temperature on microstructural evolution and mechanical properties enhancement in pure copper. The aim is to introduce gradient microstructure, with various statistical distributions of grain size and grain orientations to examine their effect on strength and ductility. To this end, extruded cylindrical pure copper subjected to HPT for 1, 2, and 3-turns resulted in grain refinement down to the grain size of 500 nm. Combination of microhardness test and EBSD scans through the radial direction confirm the creation of a heterogeneous structure through the thickness and radial directions. The results demonstrate that increasing the shear strain leads to (1) ultra-fine grain (UFG) generation at deformed coarse-grain boundaries, (2) an increase in the fraction of recrystallized grains and high angle grain boundaries, and (3) a homogenous structure in the last step. A unique mixture has been obtained due to the particular shape of the anvils. The mixture included a chain of UFGs and coarse grains contain dislocations and subgrains. The highest level of gradient structure through the thickness was observed after 1-turn, which leads to the best combination of strength and ductility.

Multiphase microstructure with carbide-free bainite, martensite and a small amount of film-like retained austenite was obtained in a new Mn-Si-Cr-C steel, designed to promote bainite transformation. The microstructural constituents were studied by a combination of electron microscopy, x-ray diffraction, dilatometry and high temperature laser confocal microscopy. A critical analysis of experimental data and theoretical analysis underscored that the austenite stability is enhanced by bainitic transformation, during quenching and partitioning process, besides carbon partitioning. The nucleation of bainite divided the untransformed austenite and refined the grain size of retained austenite, benefiting the mechanical properties, especially low temperature toughness.

Load-controlled nanoscale deformation experiments were carried out in conjunction with post-mortem electron microscopy of nanoscale deformation region involving the use of focused-ion beam, to obtain nanomechanical insights on the strain rate sensitivity of ultrafine grained structure (UFG) and compare with the coarse-grained (CG) counterpart. The UFG steel had a grain size of ∼530 nm and spherical precipitates of ∼80 nm. Both yield strength (σy) and ultimate tensile strength (σUTS) increased significantly with increased strain rate from 0.0015 to 0.15 s−1, with consequent decrease in elongation-to-failure (ɛf) during tensile straining. Nanoscale deformation experiments indicated an increase of hardness from 3.02 ± 0.05 GPa to 3.36 ± 0.05 GPa on increasing the strain rate from 0.05 to 0.5 s−1 with positive strain rate sensitivity (SRS) of 0.025, which was larger than the CG counterpart and is in striking contrast to the observed phenomena in bcc metals. From the theoretical analysis, it is envisaged that the small activation volume of ∼25 b3 (b-Burgers vector) is a consequence of combination of UFG and nanosized precipitates that prevented the movement of dislocations leading to a peculiar hardening behavior, which was responsible for the abnormal SRS and activation volume of UFG ferritic steel.

Various paths of equal channel angular pressing (ECAP) and conventional post-deformation via forging and rolling were performed to improve the mechanical properties (particularly, the strength and hardness) of a copper-free silver alloy. The microstructure was investigated with optical and scanning electron microscopy. The primary grain size of approximately 130 μm was significantly refined to ultrafine-grained (UFG) conditions with mean grain sizes of 0.6 μm and 0.5 μm via ECAP and ECAP with forging and rolling, respectively. The remarkable improvement of the mechanical properties was confirmed via hardness measurements and tensile and compression tests. ECAP and subsequent forging and rolling increased the initial hardness and tensile yield strength by +234% and +355%, respectively. Furthermore, ECAP led to a much more homogeneous hardness distribution over the entire cross-sections of the bars. The thermal stability of the UFG material at room temperature, 100 °C, and 150 °C was maintained for at least 27 months, 1000 h and 4 h, respectively. Finally, the UFG state exhibited a better tarnishing resistance than the coarse-grained state. Thus, ECAP can overcome the most significant weaknesses of lean, silver-based alloys (i.e. poor mechanical and tarnishing properties), thereby creating opportunities for their wider use.

Dual phase (DP) steels are broadly deployed by the automotive industry to enhance strength and ductility for light weighting. As DPs micro-constituents’ grain size is frequently on the same order of sizes probed with nanoindentation, an inverse technique was combined with nanoindentation load-depth curves to extract the stress-strain behavior of ferrite and martensite in DP1180. Determining the appropriate depth to evaluate the properties of the micro-constituent phases enabled computation of the flow behavior of each individual phase. Post indentation microscopy correlated the microstructure to the spatially resolved indentation arrays. The post-indentation impression was characterized to eliminate indentation size effects and probe bluntness influences. The yield strength of ferrite, martensite and DP1180 were determined to be 370, 948 and 836 MPa respectively while the corresponding strain hardening exponents (n) were computed via an inverse method as 0.175, 0.025 and 0.11. Mechanical properties obtained from the indentation method exhibited good agreement with the experimentally measured macroscopic yield stress (922 MPa) and hardening exponent (0.11) of DP1180.

Whether selective laser melting (SLM) printed AlMgScZrMn alloy possesses more excellent performances than other processing routes was unknown. In the study, the microstructures and mechanical properties of AlMgScZrMn alloys fabricated by selective laser melting (SLM), spark plasma sintering (SPS) and cast were comparatively investigated. SLM printed sample shows columnar and equiaxed duplex grains with average grain size about 7 μm, which is coarser than SPS consolidated sample with finest equiaxed grain (4 μm) but smaller than casting sample with coarse equiaxed grain (25 μm). SLM printed sample shows the highest tensile strength (394 ± 5 MPa) and elongation (10.5 ± 1.2%) than SPS sample (231 ± 3 MPa, 0.59 ± 0.21%) and cast sample (188 ± 2.3 MPa, 3.7 ± 0.58%), although SPS sample has the finest equiaxed grains. For SPS sample, previous particle boundary (PPB) with oxide inclusion significantly deteriorates ductility. For cast sample, the coarse equiaxed grain with intergranular eutectic leads to inferior strength. In addition, the average dimension of Al3(Sc,Zr) particle in SLM sample is about 1–10 nm, which is much finer than SPS sample (0.5–0.8 μm) and cast sample (1–2 μm). The present study will provide a basis for the future industrial applications of the AlMgScZrMn alloy.

Mg-Sn-Mn-based alloys exhibit great application prospect because of their high mechanical properties and good creep resistance. The content of Mn in Mg-Sn-based alloys is commonly less than 1 wt%. In this study, the effect of Mn addition on the microstructure, mechanical properties and work-hardening behavior of Mg-1Sn (wt%) alloys has been investigated. The as-extruded alloys are mainly composed of α-Mg and Mn precipitates, which exist in TM11 and TM12. The average grain size sharply decreases with Mn addition. In addition, with increase in Mn content, the basal texture of the alloys has strengthened. The experimental results show that Mn addition significantly increases the tensile and compressive yield strengths, ultimate tensile strengths, and elongation of the as-extruded Mg-Sn-Mn alloys because of the good microstructures. Moreover, the decrease of number of tensile twins in compress leads to the yield asymmetry reduces with Mn addition. With increasing Mn content, the value of work-hardening exponent and capacity decreases, the work-hardening behavior in stage III significantly improved. The variation in microstructures, mechanical properties and work-hardening of Mg-Sn-Mn alloys have been analyzed.

The high temperature deformation behavior of a duplex ferrite based low density steel was investigated and the governing restoration mechanisms were determined. Toward this end, the isothermal hot compression tests in the temperature range of 800–1000 °C under the strain rate of 0.001–0.1 s−1 were executed. Interestingly, an appreciable ferrite grain refinement was achieved through thermomechanical processing and the ferrite phase was dynamically recrystallized through continuous mechanism at various temperature and strain rates. The ferrite grain size decreases from ∼900 μm (initial microstructure) down to ∼35 and 50 μm for the microstructure deformed at 800 °C/0.001s−1 and 1000 °C/0.001s−1, respectively. The occurrence of grain refinement within the austenite was also observed which increased the stability of austenite against thermally induced martensitic transformation during quenching. The deformation induced transformation of the austenite to ferrite was also characterized as one of the involved refinement mechanisms which was intensified under the higher strain rates at lower temperature of 800 °C. Considering the portion of the fine ferrite grain resulted from strain induced transformation, the grain size under the lower strain rate of 0.001s−1 decreases down to the ∼14 μm. Finally, the room temperature mechanical properties of the thermomechanically processed specimens was assessed through miniaturized shear punch testing method. It was found that the workability of deformed specimens was significantly improved owning to the simultaneous occurrence of dynamic recrystallization and deformation induced transformation.

Gradient nanograined Ti (GNG Ti) samples were produced by a surface rolling treatment. The mechanical properties were evaluated by microhardness and tensile tests. The results indicated that GNG Ti achieved an optimized combination of strength and ductility relative to the free-standing nano/ultrafine-grained Ti. The deformation and fracture mechanisms were studied using transmission electron microscope (TEM) and scanning electron microscope (SEM). As the grain size reduced from the coarse-grained (CG) matrix to the nano/ultrafine-grained (NG/UFG) layer, the transition from conventional plastic deformation mechanisms, such as dislocation activity and twinning deformation, to grain boundary-mediated deformation mechanisms, such as stress-inducing grain growth, was identified. At the early stage of tensile deformation, the microcracks were first initiated in the CG and deformed grain (DG) regions rather than in the NG/UFG region and the CG/DG interface had a deflection effect on crack propagation. After tensile failure, the fracture morphology showed a mixture of ductile dimples and shear band-like regions.

A novel repetitive continuous welding extrusion process was proposed to optimize the microstructure and mechanical properties of AZ31 magnesium alloy. Both strength and elongation were improved simultaneously after two passes extrusion processing owing to the effective grain refinement and basal texture modification by introducing additive severe shear deformation.

The mechanical and physical properties of component surfaces are determined by the evolution of microstructures during high speed machining (HSM). In order to investigete the crystalline structure and formation mechanism of the highly perturbed surface layer, a novel characterization technique, precession electron diffraction (PED), is applied for quantitative characterization of the nano-scaled microstructures of machined surfaces, including phase distribution, grain size, character of grain boundary, grain orientation distribution and geometrical necessary dislocation (GND) density distribution. The deformation conditions of machined surface are used to analyze the evolution of microstructure under the effects of high strain-rate during machining process. Nano-crystalline structures are observed under high strain-rate deformation in the highly perturbed layer of machined surfaces. The calculation results based on crystal orientation information show that high GND density up to 106/m2 is generated and deformed nano-twins boundary is 14% of total grain boundary, which demonstrate that both dislocation slip and twinning contribute to the plastic deformation during HSM. Composite stress state and rapid heating of HSM surface produce large numbers of nanoscale β phase precipitations, which has critical pinning effects on dislocation and grain boundary movement, and will inhibit grains recovery and growth until nanoscale grains are formed. As high strain rate combined with precipitation of second phase particles is the main factor for nanoscale grains formation, HSM could be considered as an effective method to control the deformation conditions through selecting proper machining parameters in order to acquire component surface with nanocrystalline structure.

In the current study, microstructural evolution and superplasticity of an extruded Mg–2wt% Gd sheet were studied after the constrained groove pressing (CGP) process. Microstructural observations by scanning electron microscopy and electron backscattered diffraction revealed that after 4 cycles of CGP, a rather homogeneous fine-grained microstructure with an average grain size of 4.3 μm, and a large fraction of high angle grain boundaries was obtained. By performing shear punch tests (SPT) at different temperatures and various shear strain rates, a peak strain rate sensitivity index (m-value) of 0.49 was obtained after 4 cycles of CGP process at 673 K, while peak m-values of 0.31 and 0.36 were obtained for the as-extruded and 2 cycle CGP process conditions, respectively. An m-value of 0.49 and an activation energy of 113 kJ/mol, obtained for the fine-grained material after 4 cycles of CGP, suggest that the dominant deformation mechanism in the superplastic regime is grain boundary sliding (GBS) controlled by grain boundary diffusion.

Equal channel angular pressing (ECAP) is a well-known process for developing an ultrafine-grained microstructure in metals and alloys. In this study, the hot deformation behavior of an AZ31B Mg alloy has been evaluated as a function of ECAP passes and processing routes. Specimens were subjected to three passes of ECAP using a die angle of 120° using processing routes BC and C. Isothermal hot compression testing at strain rate ranging from 0.01 to 10 s−1 and temperatures ranging from 250 to 400 °C were conducted and flow curves generated. The flow curve data generated were fitted to standard constitutive equations used to describe hot working behavior and the activation energies were determined. It was observed that the activation energies for hot deformation decreased after ECAP and this can be related to the grain refinement due to the ECAP process. Activation energy decreased from 161.6 kJ/mol in the annealed condition (grain size: 29 μm) to 145.3 kJ/mol for route 3BC (grain size: 4.7 μm) and 132.3 kJ/mol for route 3C (grain size: 4.9 μm). Equations for predicting the peak flow stress for various ECAP passes has been generated using the Zener Hollomon parameter and these are in agreement with the experimentally observed values.

In this study, the microstructure evolution of β-Mg17Al12 in Mg 9Al–0.5Zn–0.12Ag alloys aged at 175 °C for 18, 36, and 72 h was investigated. The results indicate that β-Mg17Al12 precipitates contain discontinuous precipitates and continuous precipitates. The interface between a discontinuous precipitation phase and the matrix appears with either a straight or a stepped morphology. Existing misorientation dislocations at a straight interface disappear with increasing aging time. For continuous precipitates, a [ 11 5 ¯ ]β//[ 2 1 ¯ 1 ¯ 0 ]α, ( 41 1 ¯ ) β//( 101 1 ¯ )α orientation relationship (OR) was observed, and a stereogram of this new OR was calculated, the results of which are discussed herein. As aging time is prolonged, the continuous precipitates grow, and their OR develops toward a common Burgers OR. The yield strengths of the alloys aged for 18 and 36 h exhibit no significant differences owing to a better strengthening effect achieved by the new OR in the 18-h aged samples than that of the Burgers and Potter OR. As the aging time approaches 72 h, a larger grain size reduces the yield strength of the alloy.

Bulk W-0.5 wt% ZrC (WZC) plates were fabricated using tungsten powders with different powder grain sizes of 0.2 μm, 0.5 μm and 2.8 μm, respectively. The different grain sizes resulted in completely different micro-structures and mechanical properties of as-rolled WZC plates. For the tungsten plate prepared from the initial powder with grain size of 0.5 μm (0.5WZC), more excellent mechanical properties were achieved than those of plates made of powders with grain sizes of 0.2 μm (0.2WZC) and 2.8 μm (2.8WZC). In the as-rolled 0.5WZC, the ductile-brittle transition temperature is as low as 150 °C and ultimate tensile strength is as high as 932 MPa at 150 °C, respectively. Moreover, the 0.5WZC plate has the most outstanding high-temperature stability with a recrystallization temperature of 1500 °C. Micro-structure analysis indicates that the second-phase particles are uniformly distributed in the 0.5WZC, but serious particle aggregation occur in the 2.8WZC. The possible micro-mechanisms have been proposed.

Many wrought Mg alloys, which are manufactured by hot forming processes, have an inhomogeneous grain structure with locally different grain sizes. In the present study, variations in twinning and annealing behaviors caused by the difference in grain size within a material were investigated by subjecting a rolled Mg alloy with a bimodal grain structure to precompression and subsequent annealing. Activation of {10-12} twinning during precompression was found to be more pronounced in the coarse-grain region (CGR) than in the fine-grain region (FGR) owing to a lower twinning stress in the former. As a result, the twin structure formed in the FGR was distinctly different from that in the CGR; the CGR had a larger twinned area, stronger twin texture, fewer twin boundaries, and weaker grain refinement effect than the FGR. During the subsequent annealing, the microstructure of the FGR was greatly altered through grain growth driven by strain-induced boundary migration on account of its high internal strain energy, whereas that of the CGR remained nearly unchanged owing to its relatively lower internal strain energy. Consequently, the microstructural differences between the FGR and CGR induced by the precompression almost disappeared due to the subsequent annealing treatment, which resulted in a homogeneous microstructure.

The impact of grain size on the work-hardening rate of two high-manganese steels with nearly identical stacking fault energy values of approximately 16.6 mJ m−2 (X60Mn17 alloy) and 16.5 mJ m−2 (X30MnAl17-1 alloy) was analyzed via macro-texture and micro-texture measurements as well as quasistatic tensile tests and phase analysis. Both alloys exhibited differences in twin volume fraction and volume fraction of the martensitic phase. The twin volume fraction increased with grain size. The X30MnAl17-1 alloy with Al and lower C content showed a lower overall twin volume fraction than the X60Mn17 alloy without Al. Additionally, the phase fractions of e-martensite and α′-martensite were very low in the X30MnAl17-1 alloy. The increased grain size and the subsequent increase in twin volume fraction led to a relative decrease of the volume fraction of grains with -orientation parallel to the tensile axis for both alloys.

The effect of austenitizing and tempering temperature on the microstructure and mechanical properties of 65Mn spring steel refined by Tempforming (tempering and deforming of a quenched steel) process was investigated. The martensite substructure of ultrafine grain is composed of high density dislocations and a few twins, but the quantity of twins increases in normal grain martensite steel. Super strength (2220 MPa) and good ductility (8.6%) are achieved at grain size of 5.6 μm when austenitized at 790 °C and tempered at 250 °C. However, contrasted 65Mn steel with normal grain (18.7 μm) displayed almost no any ductility when processed at identical temperature parameters. Accordingly, the tempforming preprocessing contributes to the decrease in grain size and formation of crystallographic texture, which is beneficial for eventual grain refinement after subjected to reheat treatment. By appropriate quenching and tempering treatment after tempforming process, 65Mn spring steel achieved ultrafine grain and super strength and good ductility.

An extruded ZK60 magnesium alloy with the nominal composition of Mg–6Zn–0.5Zr was processed by multi directional forging (MDF) for 2, 4, 6 and 8 passes at 473 K. Microstructural studies showed that the uniform microstructure of the as-extruded material transforms to a duplex structure consisting of fine recrystallized grains surrounding some patches of unrecrystallized grains after 2 and 4 passes of MDF. Further MDF passes, however, resulted in a homogeneous equiaxed structure in such a way that the as-extruded grain size of 12.7 μm was refined to 1.9 μm after 8 passes. Evaluation of crystallographic texture revealed that deformation by MDF changed the conventional fiber texture of the extruded condition to a new texture, in which the basal planes tend to align almost 45° to the transverse direction. Mechanical properties of the extruded and MDFed samples were studied by shear punch testing (SPT) at room temperature. The results indicated that the 182.6 MPa shear yield stress (SYS) and 184.9 MPa ultimate shear strength (USS) of the extruded condition were, respectively, reduced to 170.0 and 172.3 MPa after 2 passes of MDF. This drop in strength is attributed to the presence of coarse patches of unrecrystallized grains. Further pressing of the alloy was accompanied by a decrease in the volume fraction of coarse grains and an increase in the number density of fine grains. Textural softening that occurred at higher strain levels of the final MDF passes, however, offset the strengthening effect of these fine grains.

The current study aims at comparing the effect of three different process control agents (PCAs)– namely methanol (ME), ethylene glycol (EG) and stearic acid (SA)– on microstructure and mechanical properties of Al–4.4Cu–0.5Mg alloy (AA) produced by mechanical alloying of elemental powders and subsequent hot extrusion. The effect of PCAs on particle size with milling time is insignificant. The average particle size of the milled powders of all samples has reached a steady state after 3 h. Even though the grain size and its distribution in all the samples in upset-forged, hot extruded, solutionised and peak aged conditions are similar, the number density and volume fraction of nanosized θ’ precipitates are observed to be higher in the case of AA–ME sample as compared to the other samples. AA–EG samples exhibited highest hardness when compared to the other samples. This can be attributed to the higher oxygen content in AA–EG sample that results in the formation of oxides that got dispersed in the matrix. Valid tensile test data was obtained only from AA–ME sample. The failure of AA–EG and AA–SA samples can be attributed to the presence of microcracks and higher oxide content. It can be concluded that nature of PCA has significant effect on oxygen pick up, size, number density and volume fraction of precipitates and in producing defect free bulk sample.

The properties and microstructure of friction stir processed AA 7075-T651 (Al-Zn-Mg-Cu) Aluminium alloy were studied using various tool designs. Trials were conducted on AA 5005-H34 with the aim of determining the most suitable FSP tool design out of the considered three different pin designs. Fully recrystallised fine microstructure and a defect free processed zone was achieved through the use of some of the FSP pin designs. For FSP 5005-H34 aluminium alloy substantial grain refinement was detected. The initial 192 µm pancake-like microstructure in the AA 5005-H34 base material was refined to 10–20 µm in the processed regions. Similarly, the grain size of around 50 µm in the base material was decreased to 4–10 µm in the processed regions of the FSP of 7075-T651 aluminium alloy. The traverse speed has a greater influence on the microhardness and mechanical properties, compared to the tool rotational speed. Microhardness and mechanical properties were improved as the traverse speed increased, due to the reduction in the precipitate-free zones at the grain boundaries.

Microstructures and mechanical properties of pure Cu processed by equal-channel angular pressing (ECAP) at room temperature (RT) were investigated. Samples were subjected to eight passes using routes BC, A and X. A new route which is called route X was proposed in this paper. Route X has a specific rotation angle of the sample between consecutive passes to maintain the angle between the shear plane (SP) and grain elongation plane (EP) equal to the angle between adjacent faces of {111} slip plane in each pass. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were utilized to investigate the deformed microstructure. The ultrafine grained (UFG) bulk pure Cu was fabricated by three different routes for eight passes. The TEM and XRD results revealed that dislocation density and grain size are both lower using route X after eight passes. The results of the tensile tests at RT showed that route X has better strength and elongation than routes A and BC processed samples, which is attributed to the lower dislocation density and smaller grain size with mostly high angle grain boundaries using route X. Furthermore, the fracture mechanism of ECAPed samples was discussed.

Steels alloyed with high Ti addition levels present an interesting combination of high strength and formability, principally due to the high levels of precipitation hardening that can be attained. However, their mechanical properties can be highly sensitive to variations in the processing route. In this work, dilatometry tests were performed to study the effect of coiling conditions on the microstructures and hardening mechanisms of a reference Nb microalloyed steel (0.03%Nb) and two high Ti-Nb steels (0.05%Ti-0.03%Nb, 0.1%Ti-0.03%Nb). Coiling temperatures from 550 °C to 675 °C and cooling rates of 0.01 °C/s and 0.03 °C/s were considered. A significant increase in hardness was observed for the high Ti-Nb steel samples. While the grain size and dislocation hardening were similar for all steels, much higher precipitation strengthening values in the range of 69–163 MPa and 100–307 MPa were calculated for the 0.05%Ti and 0.1%Ti steels, respectively. As a consequence, high yield strength values (over 700 MPa) were estimated for coiling temperatures greater than 625 °C for the Ti10Nb3 steel. However, it was also observed that the mechanical behavior of this steel greatly depended on coiling conditions: maximum mechanical strength was achieved at 625–650 °C, while it decreased significantly for temperatures between 550 °C and 600 °C and at 675 °C. The small size and density of the precipitates detected with TEM support the large precipitation hardening effect calculated in these conditions.

In this paper, we proposed an inverse gradient nanograined structure (IGNS) to improve the mechanical behavior of an austenitic stainless steel. Using high strain cold rolling and subsequent ultra-high-frequency electromagnetic induction heating, we successfully fabricated an IGNS layer on AISI 316L stainless steel, within which the grain size exponentially decreases from the micrometer scale in the surface to the nanometer scale in the inner center. Microstructure evolution was systematically characterized by transmission electron microscopy. The uniaxial tensile test result shows a superior strength–ductility synergy in the IGNS sample. It obtained a yield strength of 842 MPa and a uniform elongation of 16%. A linear relationship between yield strength and uniform elongation was exhibited in IGNS samples with different volume fractions. Effects of the IGNS layer on strength and ductility were examined. Additionally, the effect of recrystallized grain distribution on mechanical properties was discussed.

The feasibility of engineering bimodal grain size distributions to achieve superior mechanical properties was explored in two face-centered cubic compositionally complex alloys, namely CrMnFeCoNi and its high-performance subvariant CrCoNi. Therefore both alloys were processed down to the nanocrystalline grain size regime by utilizing high-pressure torsion and subsequently annealed at intermediate temperatures. Especially the CrCoNi alloy is prone to formation of bimodal microstructures and for an annealing treatment at 500 °C for 100 h an excellent combination of ultra-high tensile strength, exceeding 1500 MPa, and decent ductility with elongations to failure of 10% could be achieved.

The creep behavior of nanocrystalline Cu with an average grain size of 25 nm was investigated by nanoindentation test at room temperature. Using the creep strain rate versus creep stress data obtained at different loading rates, the activation volume and strain rate sensitivity were determined obtained by cooperating the continuous stiffness measurement (CSM) technique. The results showed that the activation volume first increases and then decreases, and the strain rate sensitivity first decreases and then increases with increasing the creep stress. The experimental activation volume and strain rate sensitivity versus the creep stress data exhibit very good agreements with the theoretical values calculated by the previous models, respectively. The analysis based on the data of the activation volume and strain rate sensitivity revealed that at lower stress, the grain boundary activities dominate and lead to the lower creep strain rates; at higher stress, the dislocation activities dominate and lead to the higher creep strain rates. The analysis based on the data of the nanoindentation test also revealed that the use of the CSM technique can lead to the continuous creep strain rate versus creep stress data, which allows us to uncover the creep mechanisms over a wide range of the creep stress from the initial to steady stage.

In this study, the multi-pass high-pressure sliding (MP-HPS) process was applied for grain refinement of rods with an upsized dimension. To achieve a homogeneous microstructure throughout the cross section of the rod, the rod samples are rotated with 60° around the longitudinal axis (MP-HPS-R) or are shifted upward and downward from the mid-height on the cross-sectional plane (MP-HPS-S). A homogeneous microstructure with an average grain size of ~ 200 nm for an Al-3 wt%Mg-0.2 wt%Sc alloy and ~ 160 nm for Inconel 718 were developed through both of the MP-HPS processes. The rods of the Al-3 wt%Mg-0.2 wt%Sc alloy and Inconel 718 after MP-HPS processing exhibited superplasticity with total elongations more than 600% and 400%, respectively. A finite-element method (FEM) was used to simulate the magnitude of the strain including its distribution on the cross section of the rod through processing by MP-HPS-R. It was demonstrated that microstructural homogeneity was developed in the rod processed by the MP-HPS-R but inhomogeneity was left by the MP-HPS-S.

As one of the most mature titanium alloys, Ti6Al4V is widely used in many critical aerospace applications. However, the flow behavior of this alloy cannot be easily predicted using general computational models, mainly due to the existence of various microstructural morphology in titanium alloys and relatively complex deformation mechanisms during hot deformation. Principally, the variation in initial grain morphology and grain size/plate thickness significantly influences microstructural evolution during hot working, thus leading to dissimilar work hardening and related softening behaviors. In the current study, a microstructural based Estrin Mecking (EM) +Avrami model was developed and used to model the deformation behavior of Ti6Al4V alloy with different initial grain morphologies (i.e. equiaxed vs martensitic) during simulative hot compression testing in the α + β phase region. Herein, the effect of initial grain size was considered as a function of Hall-Petch strengthening, where experimental validation revealed very good accuracy on predicting the work hardening behavior, peak stress, peak strain and flow softening with varying grain morphologies. In addition, the current model was extended to predict the material flow behavior of Ti6Al4V alloy during bulk metallic deformation (i.e. forging) in 3D as an FEM based simulation tool.