Strain rate sensitivity of a high strain rate superplastic TiNp/2014 Al composite

The effects of deformation temperature and strain in hot rolling deformation on strain rate sensitivity of the TiN_p/2014 Al composite were studied by tensile tests conducted out at 773, 798, 818 and 838 K with the strain rates from 1.7 ×10^?3 to 1.7 × 10⁰ s^?1. It is shown that the curves of m value of the TiN_p/2014Al composite deformed at different temperatures can be divided into two stages with the variation of strain rate, and the critical strain rates are 10^?1 s^?1. The optimum deformation temperature of the TiN_p/2014 Al composite is near incipient melting temperature of 816 K and the optimum strain rate is a little higher than the critical strain rate. The effect of deformation temperature on strain rate sensitivity is relative to liquid phase helper accommodation. The effect of strain in hot rolling deformation on strain rate sensitivity attributes to change of microstructure and deformation mechanism.     

Dependence of tensile deformation behavior of TWIP steels on stacking fault energy,temperature and strain rate

Three experimental high manganese twinning induced plasticity (TWIP) steels were produced based on thermodynamic stacking fault energy (SFE) calculations, following the thermodynamic modeling approach originally proposed by Olson and Cohen (Metall Trans 7A (1976) 1897). At room temperature, the SFE γ_SFE of the three materials varied from 20.5 to 42 mJ m^?2. In order to study the correlation between the SFE and the mechanical behavior of the TWIP steels, as manifested by the propensity of the material to deformation-induced phase transformations or twinning, tensile tests were performed at temperatures ?50 °C ? T ? 80 °C using strain rates varying between 10^?3 s^?1 and 1250 s^?1. The mechanical behavior of TWIP steels reveals clear temperature dependence, related to the prevailing deformation/strain hardening mechanism, i.e., dislocation slip, deformation twinning or ε-martensite transformation. At high strain rates an increase in temperature due to adiabatic deformation heating also contributes to the SFE, shifting γ_SFE either towards or away from the optimum value for twinning.     

Effect of strain rate on microstructure of polycrystalline oxygen-free high conductivity copper severely deformed at liquid nitrogen temperature

The microstructure of polycrystalline oxygen-free high conductivity copper subjected to severe uniaxial single compression at liquid nitrogen temperature and strain rates ranging from 10^?2 to 10⁵ s^?1 is characterized using transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. A difference in strain rate leads to a change in the density, character and arrangement of dislocations, as well as the size and configuration of dislocations cells/(sub)grains in the deformed sample. A threshold strain rate of 10³ s^?1 is identified for the formation of localized deformation bands, which characterizes heterogeneity of deformation at high strain rates. These bands are composed of grains that are significantly smaller than those outside them, as well as those obtained at strain rates lower than 10³ s^?1. Under particular conditions, grains as small as several nanometers can be generated in the vicinity of these bands, through the activation of rotational dynamic recrystallization. Amorphization is identified as a deformation mechanism in structures consisting of grains smaller than ～13 nm, and this offers an explanation for the “inverse Hall–Petch effect”. A model that illustrates the initiation and propagation of an amorphous phase during deformation is proposed. Deformed samples exhibit the tendency of an increase in strength with the value of the Zener–Hollomon parameter, which captures strain rate and temperature rise during deformation. This study suggests that a strain rate in the order of 10² s^?1 should be adopted in severe plastic deformation techniques to produce nanometer-sized grains.     

Characterization of hot deformation behavior of Haynes230 by using processing maps

The hot deformation characteristics of Haynes230 has been investigated in the temperature range 1050–1250 °C and strain rate range 0.001–10 s^?1 using hot compression tests. Power dissipation map for hot working are developed on the basis of the Dynamic Materials Model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring in the temperature range of 1200–1250 °C and in the strain rate range of 0.001–0.03 s^?1, which associated with grain coarsening; the other occurring in the temperature range of 1100–1200 °C and strain rate range of 0.001–0.01 s^?1, which are the optimum condition for hot working of this material. The average apparent activation energy for hot deformation is calculated to be 449 kJ/mol. The material undergoes flow instabilities at temperatures of 1050–1100 °C and at strain rates of 1–10 s^?1, as predicted by the continuum instability criterion. The manifestations of the instabilities have been observed to be adiabatic shear bands which are confirmed by optical observation.     

Strain rate sensitivity of nanocrystalline Au films at room temperature

The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 μm free-standing microscale tension specimens tested over eight decades of strain rate, between 6 × 10^?6 and 20 s^?1. The elastic modulus was independent of the strain rate, 66 ± 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575–895 MPa and 675–940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 10^?1 s^?1. The activation volumes for the two film thicknesses were 4.5 and 8.1 b³, at strain rates smaller than 10^?4 s^?1 and 12.5 and 14.6 b³ at strain rates higher than 10^?4 s^?1, while the strain rate sensitivity factor and the ultimate tensile strain increased below 10^?4 s^?1. The latter trends indicated that the strain rate regime 10^?5–10^?4 s^?1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5–6 h) and significant primary creep with initial strain rate of the order of 10^?7 s^?1.     

Impact mechanical response and microstructural evolution of Ti alloy under various temperatures

A compressive split-Hopkinson pressure bar and transmission electron microscope (TEM) are used to investigate the mechanical behaviour and microstructural evolution of a Ti alloy (Ti–1.1Mo–5.2Zr–2.9Al–0.35Fe–0.05N–0.20 O–0.02H in wt.%) deformed at strain rates ranging from 8 × 10² s^?1 to 8 × 10³ s^?1 and temperatures between 25 °C and 900 °C. In general, the results indicate that the mechanical behaviour and microstructural evolution of the alloy are highly sensitive to both the strain rate and the temperature conditions. The flow stress curves are found to include both a work-hardening region and a work-softening region. The strain rate sensitivity parameter, β, increases with increasing strain and strain rate, but decreases with increasing temperature. The activation energy varies inversely with the flow stress, and has a low value at high deformation strain rates or low temperatures. The microstructural observations reveal that the strengthening effect evident in the deformed alloy is a result primarily of dislocations and the formation of α phase. The dislocation density increases with increasing strain rate, but decreases with increasing temperature. Additionally, the square root of the dislocation density varies linearly with the flow stress. Correlating the mechanical properties of the current Ti alloy with the TEM observations, it is concluded that the precipitation of α phase dominates the fracture strain. TEM observations reveal that the amount of α phase increases with increasing temperature below the β transus temperature. The maximum amount of α phase is formed at a temperature of 700 °C and results in the minimum fracture strain under the current loading conditions.     

Distribution characterization of boundary misorientation angle of 7050 aluminum alloy after high-temperature compression

Compression tests of 7050 aluminum alloy have been conducted at different temperatures (340, 380, 420, and 460 °C) and different strain rates of 0.1, 1, 10, and 100 s^?1. The microstructure characteristics of the alloy after deformation are investigated using OM, electron backscatter diffraction (EBSD) technique and TEM. Results show that the volume fraction of recrystallized grains and the average misorientation angle increase with the increase of deformation temperature with the strain rate of 0.1 s^?1. When the 7050 aluminum alloys were deformed at 460 °C, the volume fraction of recrystallized grains and average misorientation angle decrease with increasing strain rate. The primary softening mechanism of the 7050 aluminum alloy deformed at 340, 380, and 420 °C with the strain rate of 0.1 s^?1 is dynamic recovery. Dynamic recrystallization is the main softening mechanism of the alloy deformed at 460 °C and different strain rates. The softening mechanism of the alloy is not sensitive to strain rate.     

Deformation behavior of isothermally forged Ti–5Al–2Sn–2Zr–4Mo–4Cr powder compact

The isothermal deformation behavior of hot isostatic pressed (HIPed) Ti–5Al–2Sn–2Zr–4Mo–4Cr(Ti-17) powder compact was investigated by compression testing in the temperature range of 810–920 °C and constant strain rate range of 0.001–1 s^?1. The true stress–true strain curves of the powder compact exhibit flow oscillation and flow softening phenomenon in both beta field and beta + alpha field. The flow softening behavior is related to the globularization of the primary acicular microstructure and deformation heating. The apparent activation energy for deformation in beta field is estimated to be 149 kJ mol^?1, indicating that the deformation is controlled by diffusion. The high apparent activation energy of 537 kJ mol^?1 for deformation in beta + alpha field may be related to the dynamic recrystallization of the primary acicular microstructure. Constitutive equations with the form of Arrhenius-type hyperbolic-sine relationship are proposed to delineate the peak flow stress as a function of the strain rate and the temperature for isothermal forging HIPed Ti-17 powder compact.     

Superplastic deformation of a heat-resistant Al–Cu–Mg–Ag–Mn alloy

The superplastic behavior and deformation mechanism of a heat-resistant Al–Cu–Mg–Ag–Mn alloy prepared by ingot metallurgy was investigated by using optical microscopy, scanning electron microscopy and transmission electron microscopy. It is shown that the Al–Cu–Mg–Ag–Mn alloy shows good superplastic properties at temperatures higher than 450 °C and strain rates lower than 10^?2 s^?1. A maximum elongation-to-failure of 320% was observed at 500 °C and 5 × 10^?4 s^?1, where the corresponding strain rate sensitivity index m is 0.58 and the flow stress σ is 5.7 MPa. Microstructure studies revealed that the observed superplastic behavior resulted from severe grain elongation rather than grain boundary sliding. It is suggested that this phenomenon may provide a new concept for developing superplastic materials.     

Inelastic deformation of nanocrystalline Au thin films as a function of temperature and strain rate

The mechanical behavior of nanocrystalline Au thin films with average grain size of 64 nm was investigated at strain rates 10^?5–10 s^?1, and temperatures between 298 and 383 K. The yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by ～100% within the range of applied strain rates, while it decreased by as much as 50% in the given temperature range at each strain rate. The ductility and activation volume trends pointed to two distinct regimes of plastic deformation: namely, creep-driven and dislocation-mediated plasticity, with the transition occurring at increasing strain rate for increasing temperature. The activation volume for creep-influenced deformation increased monotonically from 6.4b³ to 29.5b³ between 298 and 383 K, signifying grain boundary (GB) diffusion processes and dislocation-mediated creep, respectively. Dislocation climb, as an accommodation mechanism for GB sliding, provided an explanation for the increased activation volume at higher temperatures. The activation volumes calculated at high strain rates decreased from 19.7b³ to 11.4b³ between 298 and 383 K. A model for thermally activated dislocation depinning was applied to explain this abnormal decreasing trend in the activation volume, resulting in activation energy of 1.2 eV.     

Mechanical behavior and microstructural evolution of a Mg AZ31 sheet at dynamic strain rates

The mechanical behavior of an AZ31 Mg sheet has been investigated at high strain rate (10³ s^?1) and compared with that observed at low rates (10^?3 s^?1). Dynamic tests were carried out using a Hopkinson bar at temperatures between 25 and 400 °C. Tensile tests were carried out along the rolling and transverse directions and compression tests along the rolling and the normal directions in both strain rate ranges. The tension–compression yield asymmetry as well as the yield and flow stress in-plane and out-of-plane anisotropies were investigated. The microstructure of the initial and tested samples was examined by electron backscatter diffraction. The dynamic mechanical behavior is characterized by the following observations. At high temperatures the yield asymmetry and the yield anisotropies remain present and twinning is highly active. The rate of decrease in the critical resolved shear stress of non-basal systems with temperature is smaller than at quasi-static rates. Rotational recrystallization mechanisms are activated.     

The optimal determination of forging process parameters for Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with thick lamellar microstructure in two phase field based on P-map

The deformation behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with thick lamellar α microstructure is investigated by using the Processing-map (P-map). The results show that the P-map can predict the regime of flow instability and reveal deformation mechanisms well. Through analyzing P-maps and observing the microstructure evolution of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in forging process, the phenomena of flow instability are found to occur at the temperature and strain rate ranges of (750–880 °C, 0.005–10.0 s^?1) and (880–950 °C, 0.17–10.0 s^?1), which include macrocracks, adiabatic shear bands and prior β boundary cavities. The preferable temperature and strain rate for hot working of the Ti-alloy are (790–900 °C, 0.001–0.003 s^?1) and (900–950 °C, 0.001–0.017 s^?1). In these two deformation domains, the globularization of α lamellae occurs, and the combination of the globularization of α lamellae and α + β → β phase transformation happen, respectively. For forging of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in α + β phase field, the optimum temperature can be selected from the temperature range of 850–950 °C and the optimum stain rate is 0.001 s^?1 based on the volume fraction of α phase for obtaining the needed properties of forgings in design of forging processes.     

Thermal stability and strength of deformation microstructures in pure copper

The plastic flow field produced by machining is utilized to access a range of deformation parameters in pure copper: strains of 1–7, strain rates of 1–1000 s^?1 and temperatures as low as 77 K. The strength and stability of the severe plastic deformation microstructures including cellular, elongated, equiaxed and twinned types are characterized. Unique combinations of strengthening and stability are identified in the case of heavily twinned microstructures. These observations offer insights for improving the stability of both single-phase and multicomponent ultrafine-grained alloys.     

Tensile behavior of a free-standing Pt-aluminide (PtAl) bond coat

The tensile behavior of a high activity stand-alone Pt-aluminide (PtAl) bond coat was evaluated by the micro-tensile test method at various temperatures (room temperature to 1100 °C) and strain rates (10^?5 s^?1–10^?1 s^?1). At all strain rates, the stress–strain behavior of the stand-alone coating was significantly affected by the variation in temperature. The stress–strain response was linear, indicating brittle behavior, at temperatures below the brittle–ductile transition temperature (BDTT). The coating exhibited appreciable ductility (up to 2%) above the BDTT. The strength (both yield stress and ultimate tensile strength) of the coating decreased and its ductility increased with increasing temperature above the BDTT. The tensile behavior of the coating was sensitive to strain rate in the ductile regime, with its strength increasing with increasing strain rate at any given temperature. The BDTT of the coating was found to increase with increasing with increasing strain rate. The coating exhibited two distinct mechanisms of deformation above the BDTT. The transition temperature for the change of deformation mechanism also increased with increasing strain rate.     

Studies of shear band velocity using spatially and temporally resolved measurements of strain during quasistatic compression of a bulk metallic glass

We have made measurements of the temporal and spatial features of the evolution of strain during the serrated flow of Pd₄₀Ni₄₀P₂₀ bulk metallic glass tested under quasistatic, room temperature, uniaxial compression. Strain and load data were acquired at rates of up to 400 kHz using strain gages affixed to all four sides of the specimen and a piezoelectric load cell located near the specimen. Calculation of the displacement rate requires an assumption about the nature of the shear displacement. If one assumes that the entire shear plane displaces simultaneously, the displacement rate is approximately 0.002 m s^–1. If instead one assumes that the displacement occurs as a localized propagating front, the velocity of the front is approximately 2.8 m s^?1. In either case, the velocity is orders of magnitude less than the shear wave speed (～2000 m s^?1). The significance of these measurements for estimates of heating in shear bands is discussed.     

Constitutive modeling and processing map for elevated temperature flow behaviors of a powder metallurgy titanium aluminide alloy

The flow behaviors of PM titanium aluminide alloy were studied by isothermal compression simulation test. The apparent activation energy of deformation was calculated to be 313.53 kJ mol^?1 and a constitutive equation had been established to describe the flow behavior. Processing map was developed at a strain of 0.7. With an increase of strain, two domains can be found: dynamic recrystallization and superplastic deformation, which are further confirmed by microstructural observations. The dynamic recrystallization occurs extensively at 1000 °C and 10^?3 s^?1, with a peak efficiency of 50%, and the superplastic deformation occurs at 1100 °C and 10^?3 s^?1, with a peak efficiency of 60%. At a strain rate higher than 10^?1 s^?1, the alloy exhibits flow instability.     

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

Hot compression tests on samples of the TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) titanium alloy have been done within the temperatures of 750–950 °C and strain rate ranges of 0.1–10 s^?1 to 40–60% height reduction. The experimental results show that the flow stress behavior can be described by an exponential law for the deformation conditions. The hot deformation activation energy (Q) derived from the experimental data is 538 kJ mol^?1 with a strain rate sensitivity exponent (m) of 0.107. Optical microstructure evidence shows that dynamic recrystallization (DRX) takes place during the deformation process. Moreover, only α DRX grains are founded in the titanium alloys. The influences of hot working parameters on the flow stress behavior and microstructural features of TC11 alloy, especially on the type of phase present, the morphologies of the α phase, grain size and DRX are analyzed. The optimum parameters for hot working of TC11 alloy are developed.     

Microstructural evolution and grain boundary sliding in a superplastic magnesium AZ31 alloy

Tensile experiments on a fine-grained single-phase Mg–Zn–Al alloy (AZ31) at 673 K revealed superplastic behavior with an elongation to failure of 475% at 1 × 10^?4 s^?1 and non-superplastic behavior with an elongation to failure of 160% at 1 × 10^?2 s^?1; the corresponding strain rate sensitivities under these conditions were ～0.5 and ～0.2, respectively. Measurements indicated that the grain boundary sliding (GBS) contribution to strain ξ was ～30% under non-superplastic conditions; there was also a significant sharpening in texture during such deformation. Under superplastic conditions, ξ was ～50% at both low and high elongations of ～20% and 120%; the initial texture became more random under such conditions. In non-superplastic conditions, deformation occurred under steady-state conditions without grain growth before significant flow localization whereas, under superplastic conditions, there was grain growth during the early stages of deformation, leading to strain hardening. The grains retained equiaxed shapes under all experimental conditions. Superplastic deformation is attributed to GBS, while non-superplastic deformation is attributed to intragranular dislocation creep with some contribution from GBS. The retention of equiaxed grain shapes during dislocation creep is consistent with a model based on local recovery related to the disturbance of triple junctions.     

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹.