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The deformation and failure mechanism of a multilayered thin film consisting of alternating soft Cu and hard TiN interlayers has been studied by in situ SEM compression of micro-pillars and finite element simulations. While the yielding of the multilayer is governed by the 'size-dependent' strength of Cu, the failure was found to occur by shearing of the columnar grains of TiN. At elevated temperatures of 200 and 400 °C, the yielding of the multilayers is governed by the stress-assisted diffusion of the Cu interlayers, which coalesce into microcrystals and grow into larger faceted crystals. [ABSTRACT FROM AUTHOR]

Fatigue specimens with four types of surface were assessed under three exposure conditions (no exposure, block exposure, individual exposure–oxidation at 700 °C for 10,000 h) to quantify the effects of surface roughness, stress concentration, oxidation and inner microstructural embrittlement on fatigue strength of a near lamellar γ-TiAl alloy Ti–44Al–4Nb–4Zr–0.2Si–1B. With the yield strength of σ 0.1 =621 MPa, S – N fatigue is found to be always conducted under a loading condition of σ max < σ 0.1 . Local plastic deformation is difficult to occur on the maximum-stressed surface. The surface quality with or without defects and residual stresses therefore becomes critical for fatigue performance. Introducing compressive-stressed layer by shot peening and removing tensile-stressed layer and defects by grinding-electropolishing can improve the fatigue strength significantly, and the latter is more capable than the former for the high strength alloy. It is found that the fatigue performances of all types of surface are deteriorated to some degree when subjected to block exposure, owing to exposure-induced embrittlement. On the other hand, exposure-induced fatigue strengthening occurs after individual exposure–oxidation. The relaxation of residual tensile stress and dissipation of bulk stress in warm-air environment are found to outweigh the negative effects of oxidation layer at surface and exposure-induced embrittlement inside specimen. [ABSTRACT FROM AUTHOR]

Dynamic recrystallisation of CuAgZr alloys within a composition range of (3–7)wt% and (0.05–0.3)wt% is studied as a function of alloy composition, temperature and strain. Dynamic recrystallisation was investigated using hot-compression and hot-rolling experiments at temperatures between 500°C and 850°C. For with 7wt% and 0.05wt% , an optimised hot-rolling temperature of 750°C was found and a mean grain size of was established at a true strain of 2.2. Similar grain size distributions were found for the extended range of alloy compositions while the active mechanism for dynamic recrystallisation changes from necklace towards a particle stimulated nucleation mechanism. This change is driven by the volume fraction of the ternary phase as these particles are identified to stimulate nucleation of dynamic recrystallisation in the samples with increased content. The final tapes exhibit an outstanding combination of ultimate tensile strength of 1GPa and an electrical conductivity of 70%IACS at a true strain of 4.8 of cold work being applied. [ABSTRACT FROM AUTHOR]

Abstract: In this study uniaxial tension tests are conducted over a temperature range of 293–800K and a strain rate range of 0.001–2500/s using a servohydraulic testing machine and an enhanced Hopkinson technique to gain understanding of the plastic flow behavior of a DH-36 steel. Test results reveal the strong dependencies of the flow stress on strain rate and temperature. Moreover, dynamic strain aging (DSA) occurs at temperatures above 450K, causing an increase in flow stress. As the strain rate increases, the magnitude of the DSA effect reduces while the temperature range over which the DSA induced material strengthening takes place shifts to higher temperatures. Based on the experimental data, a dislocation barrier model is modified to describe the material behavior and to include the DSA effect. Good agreements between the model predictions and the experimental results are obtained. [Copyright &y& Elsevier]

Abstract: Titanium alloys containing a majority volume fraction of the low temperature HCP α-phase have been known to display yield and creep strength asymmetries when deformed under uniaxial conditions at room temperature. This behavior has only been documented in materials alloyed with significant concentrations of aluminum (<∼10at%); however, little work has examined the phenomenon in commercial purity alloys. In this study, the yield strengths of two commercial purity titanium alloy sheet materials (Grade 1 and Grade 4) are examined under tensile and compressive loading conditions. Yield strength asymmetry has been observed and has been found to be dependent on alloy composition and crystallite distribution relative to the deformation axis. [Copyright &y& Elsevier]

Abstract: A high strength bulk nano-crystalline Al–1Fe–(0-1)Zr alloys were processed by mechanical alloying and spark plasma sintering. The high yield strength of ∼850MPa was achieved in an Al–1Fe–0.5Zr alloy with 19% strain in compression. The high strength was attributed to the nanosized Al–Fe grains and the dispersion of fine Al6Fe particles. High temperature compressive yield strength of 455MPa and elongation of 16.7% was achieved at 250°C. [Copyright &y& Elsevier]

Abstract: The effects of age-forming on microstructures and mechanical properties of 7050 Al alloy were investigated in this work. The alloy was subjected to age-forming as well as stress-free ageing at 160°C for 6, 12, 18 and 24h, and its microstructures were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It was shown that creep might lead to grain elongation during age-forming, and the applied stress induces the coarsening of precipitates in 7050 Al alloy. The texture in the alloy was also influenced by age-forming. Consequently, the differences in microstructures result in differences in mechanical properties of age-forming versus traditional stress-free ageing. The ultimate tensile strength of age-formed samples were slightly lower than that of stress-free aged samples, while the yield strength of age-formed samples were apparently lower than that of stress-free aged samples. Specifically, the elongation of samples age-formed displays apparently decrease. [Copyright &y& Elsevier]

Abstract: The effects of high pressure torsion (HPT) processing at different temperatures on flow stress and hardness distribution of a 2024 Al alloy were studied. Using in situ torque measurements versus equivalent strain the flow stress of the material followed three distinct deformation regions at room temperature. In the first region the strength increases with strain and after taking a maximum at an equivalent strain of ∼8, the strength decreases with further straining in the second region. The third region appears at an equivalent strain of ∼22 or higher as a steady state where the strength remains unchanged. Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) analysis were conducted on selected samples and influential effects of precipitates on flow stress and hardness were examined. It is proposed that restoration process in the second region of deformation and saturation region of deformation can significantly be affected by deformation temperature and aging precipitates. HPT deformation at −196°C did not show any decrease in second region of deformation instead the strength is increased exponentially until the strain of ∼35 where saturation region of deformation starts. Nano sized grains of 50–70nm were observed in saturation region of deformation with more than three folds increases in hardness. [Copyright &y& Elsevier]

Abstract: The composites consisting of amorphous matrix reinforced with crystalline dendrites offer extraordinary combinations of strength, stiffness, and toughness and can be processed in bulk. Hence, they have been receiving intense research interest, with a primary focus to study their mechanical properties. In this paper, the temperature and strain rate effects on the uniaxial compression response of a tailored bulk metallic glass (BMG) composite has been investigated. Experimental results show that at temperatures ranging between ambient to 500K and at all strain rates; the onset of plastic deformation in the composite is controlled by that in the dendrites. As the temperature is increased to the glass transition temperature of the matrix and beyond, flow in the amorphous matrix occurs readily and hence it dictates the composite''s response. The role of the constituent phases in controlling the deformation mechanism of the composite has been verified by assessing the strain rate sensitivity and the activation volume for deformation. The composite is rate sensitive at room temperature with values of strain rate sensitivity and activation volume being similar to that of the dendrites. At test temperatures near to the glass transition temperature, the composite however becomes rate-insensitive corresponding to that of the matrix phase. At low strain rates, serrated flow akin to that of dynamic strain ageing in crystalline alloys was observed and the serration magnitude decreases with increasing temperature. Initiation of the shear bands at the dendrite/matrix interface and propagation of them through the matrix ligaments until their arrest at another interface is the responsible mechanism for this. [Copyright &y& Elsevier]

Abstract: Tensile fracture behaviors and mechanisms of 2D C/SiC-BC x composite were investigated. Results showed that the 2D C/SiC-BC x composite exhibited similar tensile strength (about 250MPa), but different fracture mode and damage evolution compared to that of 2D C/SiC composite under similar monotonic tensile stress condition at room temperature. The initial modulus and fracture modulus of 2D C/SiC-BC x composite obtained under tensile cyclic loading at RT were about 98.6GPa and 54.5GPa, respectively. At high temperatures, the tensile strengths were lower than that at room temperature and behaved as a step-like stress–strain curve. [Copyright &y& Elsevier]

Abstract: High dense tantalum sintering components with a hardness gradient within individual grains were synthesized successfully by means of spark plasma sintering (SPS) of the nitrided tantalum particles at a pressure of 400MPa and a temperature of 950°C. High yield strength up to 1350MPa combined with 12% fracture strain was achieved in the compression tests, which can be attributed to the novel gradient composition inside the grain structure. [Copyright &y& Elsevier]

Abstract: A type of duplex aging heat treatment was developed to improve the mechanical properties at room temperature and elevated temperatures in a pre-strained Al–Cu–Mg–Ag alloy. In contrast to the conventional T8 temper at 165°C and 200°C, the hardening response of the alloy to aging was increased by duplex aging treatment, the ultimate tensile strength and yield strength of duplex aging temper were improved by approximately 3–7%, which was attributed to the fact that the recovery of dislocations occurred and the precipitation of θ′ phase was restrained effectively at high aging temperature, and more Ω precipitates were formed during secondary aging. [Copyright &y& Elsevier]

Abstract: The Bauschinger effect of three commercially produced medium carbon bar steels representing different microstructural classes with similar tensile strengths and substantially different yielding and work-hardening behaviors at low-strain was evaluated at room temperature and in situ at temperatures up to 361°C. The influence of deformation at dynamic strain aging temperatures as a means to produce a more stable dislocation structure was evaluated by measuring the resistance to strain reversal during in situ Bauschinger effect tests. It was shown that the three medium carbon steels exhibited substantial increases in strength at dynamic strain aging temperatures with the peak in flow stress occurring at a test temperature of 260°C for an engineering strain rate of 10−4 s−1. Compressive flow stress data following tensile plastic prestrain levels of 0.01, 0.02 and 0.03 increased with an increase in temperature to a range between 260°C and 309°C, the temperature range where dynamic strain aging was shown to be most effective. The increased resistance to flow on strain reversal at elevated temperature was attributed to the generation of more stable dislocation structures during prestrain. It is suggested that Bauschinger effect measurements can be used to assess the potential performance of materials in fatigue loading conditions and to identify temperature ranges for processing in applications that utilize non-uniform plastic deformation (e.g. shot peening, deep rolling, etc.) to induce controlled residual stress fields stabilized by the processing at temperatures where dynamic strain aging is active. [Copyright &y& Elsevier]

Abstract: To investigate the tensile properties and facture toughness of high-strength structural steel of Q420 under different conditions, tensile tests and crack tip opening displacement (CTOD) tests were conducted. The test results revealed that temperature significantly influenced the fracture toughness of structural steel and that fracture toughness was significantly reduced with the decrease in temperature, resulting in brittle fractures. Prestraining increased the yield stress and tensile strength of the structural steel. However, it significantly reduced the plasticity and fracture toughness, and further increased the probability of brittle fractures. As determined by FE-analysis, temperature and prestraining facilitated brittle fractures due to the activation of the resulting near crack tip stress fields. Based on the Weibull stress fracture criterion, the temperature and prestraining effects on the fracture toughness were predicted from fracture toughness results of the virgin material at room temperature by the local approach. The prediction was in good agreement with the experimental results. It certified that the critical Weibull stress obeys the two-parameter Weibull distribution in the local approach and the fracture behavior of the prestrained material at different temperatures can be characterized by the local approach. [Copyright &y& Elsevier]

Abstract: In this work, the role played by plastic straining on the FCC(γ)→HCP(ɛ) strain induced transformation (SIT) ɛ-martensite in a Co–Cr–Mo–0.05C alloy was investigated. It was found that alloy plastic deformation at room temperature promotes the development of ɛ-martensite giving rise to the formation of intragranular striations. Moreover, aging at 800°C of pre-strained specimens initially promotes SIT ɛ-martensite. This is followed by a time plateau (1h) delay after which isothermal ɛ-martensite develops. Apparently, prior plastic straining leads to increasing incubation times for the onset of the isothermal ɛ-martensite transformation. In addition, tensile specimens containing various amounts of isothermal ɛ-martensite were tested all the way to fracture and their respective stress–strain properties were experimentally determined. Comparisons between experimental and simulated flow curves using the rule of mixtures indicated some deviations in the simulations at volume fractions above 0.25 ɛ-martensite. In contrast, predictions of yield and tensile strength were in good agreement with the experimental outcome. In particular, linear trends were exhibited by both, the yield and tensile strength as a function of the volume fraction of ɛ-martensite. Finally, the alloy elongation remained nearly constant with the volume fraction of ɛ-martensite in the range of the 0.15

Abstract: This paper describes the mechanical behavior of the 6061-T6 aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding. A fast-heating device has been designed to control and apply thermal loadings on tensile specimens. Tensile tests show that the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases. This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy (TEM). [Copyright &y& Elsevier]

Abstract: The effect of plasma sprayed NiCr and NiCrAl coatings on the creep resistance of Nickel alloy 690 at temperature of 1033K was investigated. Experimental results showed that the coatings had a beneficial effect on the improvement of the creep resistance of substrate. However, there was almost no difference in the creep lives between the NiCr and NiCrAl coated specimens at a given stress level. The relation between the applied stress and time to rupture of the coated specimens can be estimated by using Larson-Miller equation. For the coated specimens tested at low applied stress levels, the product of the minimum creep rate and the time to rupture was a constant value. The θ projection method can be used to accurately characterize the creep behavior of the coated specimens. The variation of the creep strain along with time predicted by using θ projection method agreed well with the experimental results. [Copyright &y& Elsevier]

Abstract: The effect of 1, 2 and 3wt.% rare earth (RE) additions on the microstructure and creep resistance of the Mg–6Zn–3Cu alloy (ZC63) was investigated by impression creep tests in the temperature range 423–498K and under punching stresses in the range 150–700MPa for dwell times up to 3600s. The results showed that the creep strength of the base alloy was remarkably improved by RE addition. This was attributed to the grain refinement effect of RE elements, and formation of thermally stable Mg12RE, MgRE, and Cu2RE compounds which strengthen both matrix and grain boundaries during creep deformation. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.017< σ imp/G <0.037. Based on the steady-state power-law creep relationship, the stress exponents of about 4.4–6 and 7–12.7 were obtained at low and high stresses, respectively. The creep activation energies were found to be in the range 73–78 and 122–130kJ/mol, at low- and high-stress regimes, respectively. It is suggested that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide in the low-stress regime, and dislocation climb with some sort of back stress, similar to those noted in dispersion strengthening alloys, in the high-stress regime. [Copyright &y& Elsevier]

Abstract: Nano-grain evolution in an austenitic stainless steel (SUS 316) during multidirectional forging (MDF) was investigated at temperatures of 77K and 300K. The flow stress during MDF and the room-temperature hardness increased significantly with increasing cumulative strain. The initial grains were subdivided by mechanical twinning and martensitic transformation. The formation of packets, which are composed of lamellar-structured mechanical twins with a spacing of 10–300nm, enhanced grain fragmentation. The packet size ranged from 40nm to 100nm depending on the MDF temperature and the cumulative strain. Tensile tests at ambient temperatures revealed a maximum proof strength of 2.1GPa. While the proof strength increased with cumulative strain, the plastic strain at fracture was approximately 10% independent of the cumulative strain over ∑Δɛ =2.4. [Copyright &y& Elsevier]

Abstract: This work was prompted by the publication of Puls''s recent papers claiming that the Dutton–Puls model is valid enough to explain the stress and temperature dependences of the crack growth rate (CGR) in zirconium alloys. The first version of the Dutton–Puls model shows that the CGR has positive dependences on the concentration difference ΔC, hydrogen diffusivity D H, and the yield strength, and a negative dependence on the applied stress intensity factor K I, which is one of its critical defects. Thus, the Dutton–Puls model claiming that the temperature dependence of CGR is determined by D H C H turns out to be incorrect. Given that ΔC is independent of the stress, it is evident that the driving force for DHC is ΔC, not the stress gradient, corroborating the validity of Kim''s model. Furthermore, the predicted activation energy for CGR in a cold-worked Zr–2.5Nb tube disagrees with the measured one for the Zr–2.5Nb tube, showing that the Dutton–Puls model is too defective to explain the temperature dependence of CGR. It is demonstrated that the revised Dutton–Puls model also cannot explain the yield stress dependence of CGR. [Copyright &y& Elsevier]

Abstract: A 12mm thick Al85Y8Ni5Co2 plate was spray deposited on a 30mm thick copper substrate pre-heated to 383K. The deposit microstructure consists of an amorphous phase, 50–150nm fcc-Al grains, 0.2–0.7μm Al2Y and Al3Y intermetallic phases and some unidentified phases. The hardness of different microstructural features and the compressive strength of the deposit, after extrusion at 723K, were evaluated. The small size overspray particles as well as the deposit show a glass transition phenomenon. The total crystallization energy of the deposit is 121J/g compared to 83.9J/g for the small sized overspray powder, indicating its high metastability. A compressive strength of 925MPa and a deformation strain of 9% were achieved for the deposit after extrusion. The novel microstructural features in the deposit are attributed to the “chilling effect” on highly undercooled or partially solidified large-size droplets during deposition onto the pre-heated substrate, and the rapid heat extraction thereof due to a close contact at the deposit/substrate interface. [Copyright &y& Elsevier]

Abstract: Mechanical properties and deep drawability of Mg–Gd–Y–Zr alloy rolling sheet were systematically studied. For this purpose, uniaxial tensile tests were conducted to investigate mechanical properties, strain rate sensitivity parameter (m-value) and plastic strain ratio (R-value) of the sheet at elevated temperatures. The sheet exhibited a high strength at 513K and superplasticity above 623K. The m-value and R-value ranged from 0.07 to 0.32 and 0.9 to 1.4, respectively. Non-isothermal deep drawing experiments were performed to explore the influence of technical parameters on deep drawability. For comparison purpose, traditional AZ31 sheet was chosen for testing on the same device. Limiting drawing ratio (LDR) of Mg–Gd–Y–Zr sheet reached >5.2 that was much higher than that of AZ31 sheet. Textures of the initial sheet and the necking regions for post-deforming cylindrical cups were measured. It is suggested from texture analysis that the activation of pyramidal slip system accounted for the locally necking of the cup-wall. The excellent drawability of Mg–Gd–Y–Zr sheet was attributable to its relatively stronger temperature sensitivity of flow stress and was not connected with an exceptional R-value or m-value. [Copyright &y& Elsevier]