Strain Rate Sensitivity Coefficient Research Articles

Experimental and numerical analysis of dynamic splitting mechanical properties and crack propagation for frozen sandstone

To reveal the dynamic tensile properties and crack growth law of frozen sandstone under impact loading, dynamic splitting tests (−30–20 °C) with different strain rates were carried out using a split Hopkinson pressure bar (SHPB), and the crack growth process of the specimens was captured using a high-speed camera. The study focused on the analysis of dynamic tensile mechanical properties, energy dissipation, crack growth, and failure modes of frozen sandstone. Numerical simulation analyses were also performed using LS-DYNA. The results indicated that: (1) The dynamic tensile strength has an obvious strain rate enhancement effect, and the strain rate sensitivity coefficient increased with a decrease in temperature. The Logistic function can characterize the relationship between the dynamic strain rate enhancement factor and the strain rate of frozen sandstone. (2) In the dynamic Brazilian disk (BD) test, the energy absorption rate showed a transition between the strain rates of 40 s−1 and 50 s−1, and there were two main failure modes of the specimen. The Y-direction secondary crack in mode II was caused by tensile stress in the Z-direction. (3) In the dynamic BD test, the central main crack (CMC) did not occur at the central position first, and its Z-direction propagation rate was linear with the strain rate. In the dynamic flattened Brazilian disk (FBD) test, the condition of CMC initiation was that the platform loading angle (2α) was greater than 10°. The dynamic tensile strength increased with an increase in 2α, and the correction coefficient was introduced in the calculation. This study can serve as a theoretical reference and basis for the study of the dynamic tensile properties of rock-like materials.

Effect of Sc, Hf, and Yb Additions on Superplasticity of a Fine-Grained Al-0.4%Zr Alloy

This research was undertaken to study the way deformation behaves in ultrafine-grained (UFG)-conducting Al-Zr alloys doped with Sc, Hf, and Yb. All in all, eight alloys were studied with zirconium partially replaced by Sc, Hf, and/or Yb. Doping elements (X = Zr, Sc, Hf, Yb) in the alloys totaled 0.4 wt.%. The choice of doping elements was conditioned by the possible precipitation of Al3X particles with L12 structure in the course of annealing these alloys. Such particles provide higher thermal stability of a nonequilibrium UFG microstructure. Initial coarse-grained samples were obtained by induction casting. A UFG microstructure in the alloys was formed by equal-channel angular pressing (ECAP) at 225 °C. Superplasticity tests were carried out at temperatures ranging from 300 to 500 °C and strain rates varying between 3.3 × 10−4 and 3.3 × 10−1 s−1. The highest values of elongation to failure are observed in Sc-doped alloys. A UFG Al-0.2%Zr-0.1%Sc-0.1%Hf alloy has maximum ductility: at 450 °C and a strain rate of 3.3 × 10−3 s−1, relative elongation to failure reaches 765%. At the onset of superplasticity, stress (σ)–strain (ε) curves are characterized by a stage of homogeneous (uniform) strain and a long stage of localized plastic flow. The dependence of homogeneous (uniform) strain (εeq) on test temperature in UFG Sc-doped alloys is increasing uniformly, which is not the case for other UFG alloys, with εeq(T) dependence peaking at 350–400 °C. The strain rate sensitivity coefficient of flow stress m is small and does not exceed 0.26–0.3 at 400–500 °C. In UFG alloys containing no Sc, the m coefficient is observed to go down to 0.12–0.18 at 500 °C. It has been suggested that lower m values are driven by intensive grain growth and pore formation in large Al3X particles, which develop specifically at an ingot crystallization stage.

Влияние состояния границ зерен на эффект пластификации в ультрамелкозернистом сплаве Al-0.4Zr

The influence of small additional deformation by cold rolling (CR) on the microstructure and mechanical properties of ultrafine-grained (UFG) Al–0.4Zr alloy structured by high pressure torsion (HPT) has been studied. The results are compared with the application of small additional deformation by HPT, which, after low-temperature annealing, leads to a substantial increase in ductility (plasticization effect) with small decrease in strength. It is shown that, in contrast to the additional deformation of HPT, the deformation of CR after intermediate low-temperature annealing leads to a sharp drop in plasticity to ~2%, while the strength increases to ~275 MPa. The key role of the nonequilibrium state of grain boundaries in the manifestation of the plasticization effect in the UFG Al–0.4Zr alloy is revealed. A new approach is proposed for simultaneously increasing the strength and ductility of the UFG Al–0.4Zr alloy due to a small additional deformation by CR without intermediate annealing. As a result of this approach, a significant increase in strength by ~30% (ultimate tensile strength ~223 MPa) was achieved with a simultaneous increase in ductility up to ~26%, which is associated with an increase in the strain hardening rate due to an increase in the density of lattice dislocations in the UFG structure with nonequilibrium grain boundaries. The strain rate sensitivity and strain hardening coefficients have been determined for the UFG Al–0.4Zr alloy in various states.

Influence of decreased temperature on the plasticization effect in high-strength Al-Cu-Zr alloy

The influence of deformation temperature of tensile test on the mechanical properties of ultrafine grained (UFG) Al-Cu-Zr alloy in three different states: after high pressure torsion (HPT), subsequent low-temperature annealing as well as after annealing and a small additional deformation has been studied for the first time. The UFG structure was formed by HPT processing. The temperature dependences of the yield stress and elongation to failure under uniaxial tension in the temperature range of 77–293 K were obtained for all three states. The effect of increasing ductility from ∼3–5 to 11 % (the plasticization effect) at room temperature while maintaining a high strength of ∼465 MPa was observed after deformation-heat treatment (DHT), consisting of annealing and additional deformation. The plasticization effect decreases monotonically with decreasing temperature of tensile test and completely disappears at ∼223 K. In the temperature range 223–293 K, the temperature sensitivity of the yield stress and the value of activation energy (Q) of plastic flow differ in all three states, which is related to the changes in the grain boundary (GB) structure (dislocation density, size and shape of Al2Cu nanoprecipitates). In the temperature range of 77–223 K, the temperature sensitivity of the yield stress does not depend on the state of GBs. The plasticization effect at room temperature is preserved when the strain rate is changed from 10−4 to 10−3 s−1 and sharply decreases as strain rate is further increased. The strain rate sensitivity coefficient (m) for the UFG Al-1.47Cu-0.34Zr (wt%) alloy was determined in the states before and after DHT. The results and relevant mechanisms are analyzed by using the resulting m valuе and Q values as well as the observations of specific features of GB structure.

Influence of Fe on the microstructure, superplasticity and room-temperature mechanical properties of Ti–4Al–3Mo–1V-0.1B alloy

Decreasing the superplastic forming temperature of Ti alloys is currently an important issue. This study investigated the impact of modifying the Ti–Al–Mo–V alloy with different percentages of Fe (0–2 wt.%) on the microstructure, superplasticity and post-forming mechanical properties. The results revealed that during superplastic deformation, the increase in Fe content facilitated the recrystallisation and fragmentation of the phases and grain boundary sliding due to the accelerating diffusivity by Fe. In contrast, the presence of Fe increased the susceptibility to grain growth. A high strain rate sensitivity coefficient m (0.45–0.5) and maximum elongation to failure (500–1000%) were achieved for these alloys at a constant strain rate of 1 × 10−3 s−1 and in a low temperature range (625–775 °C). Alloying the investigated alloy with Fe increased the post-forming room-temperature tensile strength by 90–220 MPa and decreased the ductility by 1–2%. Alloying with 0.5% Fe provided a good combination of the superplastic and room-temperature mechanical properties for the studied alloys.

Effect of cryorolling on the microstructure and high-temperature mechanical properties of AA5083 sheets

To explore the effect of cryorolling on the high-temperature mechanical properties of aluminium alloys, AA5083 sheets were rolled at 83, 173, and 298 K, and the rolled sheets were then subjected to high-temperature tensile testing at 748 K. The elongation to failure of the 83 K cryorolled samples reached 150% at a strain rate of 1 × 10−3 s−1, which was higher than those of the 173 K cryorolled samples (92%) and 298 K rolled samples (80%). Under this condition, the strain rate sensitivity coefficient of the 83 K rolled samples was 0.36, which reached the standard of superplasticity. The enhancement of plasticity of the 83 K cryorolled samples was attributed to the finer grains, the smaller second phase, and the subgrains alleviating more stress concentration. The main deformation mechanisms during the high-temperature tensile deformation were grain-boundary sliding and dislocation slip. Compared to the 298 K rolled sample, the grain size of the 83 K cryorolled sample was smaller, which was suitable for the grain-boundary sliding mechanism. In addition, the uniform distribution of high-density dislocations prevented the dislocation slip mechanism but reduced the stress concentration and the nucleation and growth of cavities at high-temperature deformation.

Plastic deformation behavior and constitutive modeling of Cu-50Ta alloy during hot compression

This paper presents a study on plastic deformation behavior of Cu–50Ta alloy at temperatures of 286–473 K and strain-rate of 0.01–6200 s−1. The effects of temperature, strain-rate, and strain on the yield strength, flow stress, and strain-rate sensitivity coefficient were determined. A phenomenological model was established to predict variation of the strain-rate sensitivity coefficient for Cu–50Ta alloy under dynamic compression. A Johnson–Cook constitutive model was established to predict the equivalent stress–equivalent plastic strain relationship under extreme deformation (high temperature and strain-rate). The results showed that the plastic deformation behavior of Cu–50Ta alloy was affected by temperature, strain-rate, and strain. The material exhibited obvious strain-rate strengthening and thermal softening. As the strain-rate increased, the yield strength logarithmically increased. At a temperature of 286 K, the strain-rate increased from 0.01 s−1 to 6200 s−1, and the yield strength increased from 543.75 MPa to 881.13 MPa. In addition, the yield strength linearly decreased as the deformation temperature increased. Under conditions of dynamic deformation, the variation of strain-rate sensitivity coefficient could be expressed as a function of strain-rate and strain. The phenomenological model accurately described the variation of the strain-rate sensitivity coefficient of Cu–50Ta under dynamic deformation conditions. The Johnson–Cook constitutive parameters, calibrated by experimental data, described the plastic deformation behavior of the alloy under high-velocity impact.

Influence of Strain Rate on the Effect of Plasticization of High-Strength Ultrafine-Grained Al-Cu-Zr Alloy

The influence of the strain rate on the plasticization effect (PE) of ultrafine-grained (UFG) Al-1.47Cu-0.34Zr (wt%) alloy structured by high pressure torsion. A significant increase in plasticity (more than 2 times) while maintaining a high level of strength (ultimate tensile strength ~465 MPa) in the UFG alloy was achieved as a result of additional deformation-heat treatment (DHT), consisting of low-temperature annealing and a small additional deformation. It is shown that the PE after DHT is retained when the strain rate changes from 10-4 to 10-3 s-1 and decreases by half with its further increase to 10-2 s-1. The values of the strain-rate sensitivity coefficient for the UFG Al-1.47Cu-0.34Zr (wt%) alloy were determined in both states before and after DHT. Possible reasons of the PE suppression at high strain rates (≥10-2 s-1) are discussed. Keywords: aluminum alloys, severe plastic deformation, ultrafine-grained structure, strength, plasticity, strain-rate sensitivity.

Effect of strain rate and sample thickness on mechanical properties of spray deposited and extruded Al–Zn–Mg–Cu alloy

The microstructure of spray deposited and extruded Al–Zn–Mg–Cu alloy after regression re-aging (RRA) treatment was investigated by transmission electron microscope and electron back-scatter diffraction. The influence of strain rate and sample thickness on the mechanical properties were evaluated. The results showed that the main precipitate phases in the RRA Al–Zn–Mg–Cu alloy included short rod-shaped η′, coarse rod-shaped η and G.P. zones with nanoscale dimensions. The yield and ultimate tensile strength of RRA alloys were determined by sample thickness and strain rate to regulate the mass fraction of precipitate phases and the restraint of internal grains by sample surfaces, the number of activated slip systems and dislocation density. The RRA Al–Zn–Mg–Cu alloy exhibited a mixed-mode fracture due to the nanoscale precipitate phases restraining grain boundary sliding, the precipitate-free zones and high angle grain boundary inducing crack nucleation and propagation, and the post-necking elongation producing cleavage planes. The strain hardening is conducted by deformation texture. The strain hardening exponent n and the strain rate sensitivity coefficient m were determined by the sample thickness. The deformation texture intensity of RRA alloy was related to the strain rate controlled by texture components. The high strain rate improved the content of Cu texture {112}<-1-11> to elevate the mechanical properties of RRA alloy.

Study on the Formability and Texture evolution of AA6061 Alloy Processed by Repetitive Corrugation and Straightening.

The enhanced mechanical properties obtained by refining the grain size down to the ultrafine-grained (UFG) regime have attracted considerable attention in recent years. The severe plastic deformation (SPD) techniques allow obtaining ultrafine-grained materials. Different SPD techniques permit processing sheet shape materials such as repetitive corrugation and straightening (RCS) and accumulative roll bonding (ARB). In this study, the formability of an AA 6061-T6 processed by RCS was evaluated. The forming limit diagrams (FLD) were obtained by Nakazima tests of samples in initial condition (T6 state) and after one and two RCS cycles. The FLD curves showed that the forming capacity decreased from the first RCS cycle. Likewise, uniaxial tensile tests at different temperatures and strain rates were conducted to analyze the effect of the RCS process on the strain rate sensitivity. They showed a relatively high strain rate sensitivity coefficient in the samples after one and two RCS cycles, which indicates an improvement of i) the capacity of the material to delay the onset of the necking and ii) the formability at increasing temperatures. Finally, texture analysis was carried out employing X-ray diffraction, calculating the orientation distribution functions (ODFs). The initial texture showed a predominant cube texture component, whereas, for further RCS cycles, a weakening of the cube texture and an increment of the S texture component were observed.

Size-induced twinning in InSb semiconductor during room temperature deformation

Room-temperature deformation mechanism of InSb micro-pillars has been investigated via a multi-scale experimental approach, where micro-pillars of 2 µm and 5 µm in diameter were first fabricated by focused ion beam (FIB) milling and in situ deformed in the FIB-SEM by micro-compression using a nano-indenter equipped with a flat tip. Strain rate jumps have been performed to determine the strain rate sensitivity coefficient and the related activation volume. The activation volume is found to be of the order of 3–5 b3, considering that plasticity is mediated by Shockley partial dislocations. Transmission electron microscopy (TEM) thin foils were extracted from deformed micro-pillars via the FIB lift-out technique: TEM analysis reveals the presence of nano-twins as major mechanism of plastic deformation, involving Shockley partial dislocations. The presence of twins was never reported in previous studies on the plasticity of bulk InSb: this deformation mechanism is discussed in the context of the plasticity of small-scale samples.

Coupling Effect of Strain Rate and Freeze-Thaw Temperature on Dynamic Mechanical Properties and Fractal Characteristic of Saturated Yellow Sandstone

Strain rate is not only an important influence factor for deformation property but also an important parameter for analyzing the dynamic mechanical behavior of rock material. In this study, the dynamic compressive mechanical properties of saturated yellow sandstone at four strain rates and six freeze-thaw temperatures are investigated by using the SHPB test system. The coupling effect of strain rate and freeze-thaw temperatures on the mechanical parameters of rock material are discussed in detail, and the relationship formula of peak strain and dissipated energy with strain rate and freeze-thaw temperature are also established. Finally, the fractal dimension characteristic of fracture specimens with the strain rate and temperature are analyzed by using the fractal dimension method. The research results indicate that (1) with the increase of strain rate, the increase speed of peak strain, peak strength, and dissipated energy at medium strain rate level was obviously higher than that at high strain rate level, indicating that the strain rate effect weakened at high strain rate. (2) Freeze-thaw temperature can improve the brittleness-ductile transformation rate of saturated specimens. (3) According to the strain rate sensitivity coefficient, at room temperature, the strain rate effects on peak strain and peak strength are weakest, while at -20°C ~ -30°C, they are most significant. In addition, the strain rate effect on dissipated energy is significant at room temperature, while weakest at -30°C. (4) The fractal dimension gradually increases with strain rate increasing or freeze-thaw temperature decreasing, indicating that the freeze-thaw environment has a positive function for increasing the damage and fracture degree of specimens for saturated specimen. Our research results can provide an extremely important theoretical basis for the dynamic disaster prevention and structural design of rock engineering in cold regions.

Strain rate dependent deformation behavior of BCC-structured Ti29Zr24Nb23Hf24 high entropy alloy at elevated temperatures

The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti29Zr24Nb23Hf24 (at%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700° to 1100 °C and 10−3 to 10 s−1, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (SRS) coefficient is ~3 times that of BCC alloy Nb-1Zr and even ~3.5 times that of pure Nb. This high SRS is attributed to the high Peierls stress of the HEA, which is about twice the Peierls stress of pure Nb. On the other hand, the flow stress exhibits a tendency from strain softening to strain hardening with the increase of strain rate especially at the relatively low temperatures. This behavior is explained by a change in dislocation motion from climbing to multiple slip with the increase of strain rate. Taking the specimen subjected to 800 ºC for example, dislocation walls formed at the early stage of deformation and at low strain rate of 10−3 s−1. In this case there is sufficient time to activate dislocations climb, which results in discontinuous dynamic recrystallization, and strain softening. However, when the strain rate amounts to 1 s−1, thermally activated processes such as dislocation climb are too sluggish. As a consequence, multiple slip systems are activated, and the dislocation interactions lead to the evolution of deformation bands, leading to strain hardening.

Combined effects of elevated temperatures and high strain rates on compressive performance of S30408 austenitic stainless steel

This paper presents an experimental investigation on the dynamic mechanical performance of S30408 austenitic stainless steel (ASS) under elevated temperatures, which is essential for determining the behaviour of structures made with this type of steel subjected to the coupled fire and impact/explosion. For this purpose, the quasi-static and dynamic compression tests using Split Hopkinson Pressure Bar (SHPB) were conducted under temperatures of 20–600 °C and strain rates from 0.001 to 3000 s−1. In addition, the corresponding microstructures of tested samples were observed. The stress–strain responses, strain rate and temperature effects as well as the microstructural evolutions were analyzed. Test results show that the stress–strain responses are sensitive to the strain rate and temperature. The strain-rate sensitivity coefficient increases as the strain rate and temperature rise. The microstructural observation reveals that the grain dimension declines with an increment of strain rate or a decreasing temperature. Finally, the dynamic compressive stress–strain models for S30408 ASS under 20–600 °C were suggested on the basis of the Johnson-Cook (J-C) model and have been proved to give a reasonable prediction.

The parameter optimization and mechanical property of the honeycomb structure for Ti2AlNb based alloy

In this paper, the parameter optimization and the mechanical property of the honeycomb structure fabricated by superplastic forming/diffusion bonding (SPF/DB) process for Ti 2 AlNb based alloy were investigated. The uni-axial tensile test was carried out at different temperatures and strain rates. The results showed that the maximum elongation was 236 % at 970℃/0.0003s −1 and the strain rate sensitivity coefficient was 0.3. The mechanical model of the face sheet and core sheet forming was established and the forming process of the structure was analyzed by the finite element method(FEM). The FEM results showed that the grooves on the surface of the honeycomb structure gradually faded and disappeared with the width of the bonding joints increasing. And the honeycomb structure with the bonding width of 10mm was flexible and successfully fabricated. Finally, the thickness distribution of the structure was measured and it was consistent with the FEM results. The obvious microstructure evolution resulted in the reduction of tensile property. The compressive property of this structure in experiment and in FEM was investigated. The compressive strength of experimental test was 7.7MPa and that of the FEM was 6.6MPa, respectively.

Warm Tensile Deformation and Fracture Behavior of AZ31 Magnesium Alloy Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP), as a promising severe plastic deformation method suitable for processing sheet metals, was applied to AZ31 magnesium alloy sheets, and ultra-fine grained structure was achieved after two passes of CGP at 473 K. A bimodal structure was also developed due to partial dynamic recrystallization (DRX) and deformation homogeneity. Tensile tests at temperatures from room temperature to 523 K and strain rates from 1 × 10−4 to 1 × 10−1 s−1 were conducted to the processed alloy sheets, and their tensile deformation and fracture behavior were investigated in this work. The maximum elongation to failure of 38.7% is achieved at 1 × 10−4 s−1 and 473 K, and the yield strength and tensile strength are 37.0 and 43.0 MPa, respectively. The strain hardening ability increases gradually with increasing strain rate at elevated temperatures, and its dependence on strain rate is more significant at lower temperatures. The strain rate sensitivity coefficient gradually increases with increasing temperature, and the relatively high values of ~ 0.17 and ~ 0.14 are obtained at 473 and 523 K, respectively, indicating the absence of superplastic behavior. The fracture morphology shows that with increasing temperature, the fracture mode changes from brittle fracture to ductile fracture, which is closely related to the microstructural evolution during tensile deformation. The grain coarsening at 523 K may result in the slight decreases of elongation to failure and strain rate sensitivity coefficient as well as the recovery of strain hardening exponent. The apparent activation energy at 423-523 K is estimated to be 68.8-105.5 kJ/mol. Conclusively, DRX and grain growth should be the dominant mechanism accounting for the warm tensile deformation of AZ31 magnesium alloy sheets processed by CGP, while grain boundary sliding mechanism may contribute little, due to the developed bimodal structure during CGP and the relatively low temperatures for the tensile tests.

Experimental study of the superplastic deformation mechanisms of high-strength aluminum-based alloy

The role of different deformation mechanisms and the contributing factors behind them needs to be clearly defined to develop materials exhibiting high strain rate superplasticity. It is still not fully understood to what extent the deformation conditions and microstructural parameters affect the mechanisms of superplastic deformation. A novel Al–3.9Zn–4.1Mg–0.8Cu–2.8Ni–0.25Zr alloy is employed to determine what effects the testing conditions, elemental and phase composition and evolution of grain and sub-grain structure make towards deformation mechanisms. To analyze the deformation behavior and microstructure evolution, uniaxial tensile tests are performed following two deformation regimes with the different constant strain rates and temperatures: (1) 2 × 10-3 s-1, 480°С and (2) 2 × 10-2 s-1, 440°С. The elongation to failure exceeds 650% and the strain rate sensitivity coefficient m is near 0.5 at both deformation regimes. Electron microscopy and focused ion beam techniques are employed to analyze mechanisms associated with grain boundary sliding and intragranular strain by using the samples in a stable flow. Surface grids indicated that superplastic flow is accompanied by the formation of striated regions on the surface, whereas grain boundary sliding involves grain neighbor switching and grain rotation. Intragranular deformation with increased dislocation density and dynamic recrystallization are also observed. Weaker intergranular and intense intragranular deformation are revealed at high strain rate deformation regime. Solute effect and the presence of two types of secondary phases, nanoprecipitates of L12-Al3Zr and micron-sized eutectic Al3Ni, are discussed in comparison to conventional AA7475 alloy in different aspects of grain growth, dynamic recrystallization and strain-rate sensitivity.

Influence of heat treatment on the structure evolution and creep deformation behavior of a precipitation hardened B2-(Ni, Fe)Al alloy

The effect of quenching and aging on structural evolution of the Ni27Fe26Al32Cr10Co5 alloy hardened by coherent α-Fe(Cr) precipitates was studied by scanning electron microscopy (SEM) and high-resolution electron microscopy (HRTEM). Heat treatment of the alloy was shown to optimize the morphology and size of hardening α precipitates and give rise to particles of the intermetallic σ-FeCr phase along the grain boundaries.The mechanical properties of the alloy were measured in the temperature range of 273–1573 K. Temperature dependences for the offset yield strength at different strain rates and the Young's modulus values were obtained. Quenching followed by age hardening increased the resistance to viscoplastic strain. The strain rate sensitivity coefficient was evaluated in the temperature range of 873–1573 K. The elastic limit and yield stress values reached under external tensile stress at 973 and 1073 K were determined.The analyzed B2-(Ni, Fe)Al-based alloy was subjected to deformation at stresses of 100, 150, 200, and 300 MPa in the temperature range of 773–1200 K. The steady-state creep strain rate was approximated by the Arrhenius creep equation (stress power being 8.3 and the effective creep activation energy being 335 kJ/mol). Threshold stresses equal to 94, 70, and 33 MPa were observed at 873, 973, and 1073 K, respectively. Calculations of critical stresses for the potential mechanisms controlling creep strain showed that the threshold stresses are caused by local and general dislocation climb over coherent α precipitates. The HRTEM studies of the dislocation substructure of the alloy after high-temperature creep testing confirmed that the creep strain mechanism was controlled by dislocation climb.

Construction of Constitutive Model and Strain-Rate Sensitivity Coefficient Distribution Map of Ti2AlNb

The hot deformation behavior of Ti–22Al–25Nb was studied by the high temperature compression over a range of temperatures (950–1050 °C) and strain rates (0.001–10 s−1) in this paper. The work-hardening (WH) and softening deformation behaviors of Ti–22Al–25Nb were analyzed. Obvious linear decreasing regimes of WH rate curves can be found before the dynamic recrystallization (DRX) onset, which indicates WH + DRV (dynamic recovery) stage. And WH rate decreased significantly with strain rate reduced and temperature elevated. A physically-based constitutive model was established, which can well predict the flow behavior of Ti–22Al–25Nb. Additional, strain-rate sensitivity coefficient distribution map was established. The higher values of m appeared at the low strain rate. When the strain rate exceeded 0.1 s−1, the values of m were lower than 0.25.

Formability prediction of Ti6Al4V titanium alloy sheet deformed at room temperature and 600 °C

Formability predictions of Ti6Al4V titanium alloy sheets deformed at room temperature and 600 °C, using D-Bressan’s shear stress rupture criterion and the critical strain gradient macroscopic modelling are presented and discussed in relation to limit strain results obtained experimentally. Ti6Al4V Forming Limit Strain Curves were predicted and compared with experimental curves at 25 °C and 600 °C and strain rate 0.1 s−1. The analytical models were calibrated by means of tensile tests performed on samples cut at 0°, 45° and 90° to the rolling direction at different temperatures and strain rates to obtain the Lankford coefficients and material strain and strain rate hardening behavior. The applied critical shear stress rupture criterion showed to give a fairly good fit with experimental limit strains outcomes, proving that the shear stress rupture nature of specimens deformed at room and elevated temperatures were well reproduced, despite a not-null strain rate sensitivity coefficient at 600 °C. Fracture occurrence by fast shear stress mechanism through thickness direction was corroborated by experimental fractograph observations close to fracture surfaces. It was shown that predicted limiting major true strain of fracture by shear stress curve is governed by normal anisotropy, strain hardening exponent, pre-strain and normalized critical shear stress parameter, which depends on temperature and strain rate whereas the strain hardening exponent depends largely on temperature. In contrast, the critical strain gradient modelling for onset of localized necking showed poor correlation with the experimental limit strains. Best fit of the critical shear stress criterion with experimental limit strain curves was given by specimens deformed near plane strain or FLCo. Hence, a common feature of various strain-rate independent metals, annealed or cold rolled, is that the main fracture mechanism of thin sheet metals can be considered as fast shear stress through sheet thickness without a visible localized necking in biaxial stretching.