High Temperature Deformation Mechanisms Research Articles

A fundamental study of the high temperature deformation mechanisms of magnesium

Single crystals of Magnesium have been deformed in plane strain compression from room temperature to 450 °C. The deformed crystals have been analysed by EBSD to identify the operative twin variants and parent rotations for comparison with the expected systems. In parallel with these experiments, we have developed a crystal plasticity model based on Taylor principles and the Schmid law, to compute the activated systems together with the twin reorientations and the lattice rotations for large strains. The CRSS values are determined by an iterative procedure that best correlates the experimental data with the numerical simulations for each orientation. From the measured stress/strain data the CRSS for twinning and some slip systems are given over this wide temperature range.

High temperature deformability and microstructural evolution of Ti–47Al–2Cr–0.2Mo alloy

High temperature deformability and microstructural evolution of Ti–47Al–2Cr–0.2Mo alloy were studied by using a Gleeble-1500 simulated machine at temperature from 1050 °C to 1150 °C with strain rate from 2.5 s −1 to 7.5 s −1 and engineering compressive strain from 20% to 50%. The results indicate that the largest engineering compressive strain of the alloy increases with decreasing strain rate and increasing deformation temperature. It has been shown that the strain rate affects the largest engineering compressive strain of the alloy effectively. High temperature deformation mechanism of Ti–47Al–2Cr–0.2Mo alloy mainly refers to the bending and fracturing of the lamellar structures and dynamic recrystallization (DRX). Electron back scattered diffraction (EBSD) and transmission electron microscopic (TEM) results reveal the DRX mostly occurring at grains boundary and interlamellar spacing at a strain rate of 7.5 s −1 is much finer than that at a strain rate of 2.5 s −1 at 1150 °C.

Quantitative evaluation of high temperature deformation mechanisms: a specific microgrid extensometry technique coupled with EBSD analysis

A microgrid extensometry method has been developed and used to obtain information about intragranular and intergranular creep mechanisms. An oxide grid was deposited on a creep specimen using an electron lithography technique. This oxide grid offers high backscattered electron contrast and can withstand long duration creep tests under vacuum in the 700–850 °C range without degradation. Specific methods were used to measure in-plane displacements at the grid nodes or at the grain boundaries using correlation of grid images taken before and after the creep test. The local strain and grain boundary sliding (GBS) data were then calculated. Combined information about grain boundary crystallography and GBS has been obtained by superimposing the electron backscattered diffraction (EBSD) map on the deformation maps. To illustrate the potential of this set of processes, two examples of application on a nickel-base disc superalloy are presented. The first one concerns the influence of the creep temperature on the local strain and the GBS. The second application quantitatively shows the influence of grain boundary character on GBS of this material.

Large Deformability of Wrought Magnesium Alloys: Is Superplasticity Needed?

The deformability of wrought magnesium alloys at room temperature is limited and a way to overcome this limit is to carry out forming operations in warm or hot conditions. In the case of fine grained alloys, superplastic properties can be generally achieved but in this regime, the Mg alloys are sensitive to strain induced cavitation. However, large grained alloys can also exhibit quite large deformabilities when they are deformed at high temperature. This can be due to the fact that on one hand, the Mg alloys may quite easily dynamically recrystallize and on the other hand, that dislocation movements may be controlled by a solute drag effect leading to significant strain rate sensitivity parameters. These various mechanisms of deformation will depend on the composition, the mean grain size and the conditions of deformation (i.e. temperature and strain rate). In this work, the high temperature deformation mechanisms as well as the associated damage mechanisms of two wrought magnesium alloys are discussed.

Large Deformability of Wrought Magnesium Alloys: Is Superplasticity Needed?

The deformability of wrought magnesium alloys at room temperature is limited and a way to overcome this limit is to carry out forming operations in warm or hot conditions. In the case of fine grained alloys, superplastic properties can be generally achieved but in this regime, the Mg alloys are sensitive to strain induced cavitation. However, large grained alloys can also exhibit quite large deformabilities when they are deformed at high temperature. This can be due to the fact that on one hand, the Mg alloys may quite easily dynamically recrystallize and on the other hand, that dislocation movements may be controlled by a solute drag effect leading to significant strain rate sensitivity parameters. These various mechanisms of deformation will depend on the composition, the mean grain size and the conditions of deformation (i.e. temperature and strain rate). In this work, the high temperature deformation mechanisms as well as the associated damage mechanisms of two wrought magnesium alloys are discussed.

High Temperature Deformation Mechanisms and Processing Map for Hot Working of Cast-Homogenized Mg-3Sn-2Ca Alloy

The hot working behavior of Mg-3Sn-2Ca alloy has been investigated in the temperature range 300–500 oC and strain rate range 0.0003–10 s-1, with a view to evaluate the mechanisms and optimum parameters of hot working. For this purpose, a processing map has been developed on the basis of the flow stress data obtained from compression tests. The stress-strain curves exhibited steady state behavior at strain rates lower than 0.01 s-1 and at temperatures higher than 350 oC and flow softening occurred at higher strain rates. The processing map exhibited two dynamic recrystallization domains in the temperature and strain rate ranges: (1) 300–420 oC and 0.0003–0.003 s-1, and (2) 420–500 oC and 0.003–1.0 s-1, the latter one being useful for commercial hot working. Kinetic analysis yielded apparent activation energy values of 161 and 175 kJ/mole in domains (1) and (2) respectively. These values are higher than that for self-diffusion in magnesium suggesting that the large volume fraction of intermetallic particles CaMgSn present in the matrix generates considerable back stress. The processing map reveals a wide regime of flow instability which gets reduced with increase in temperature or decrease in strain rate.

INFLUENCE OF INITIAL MICROSTRUCTURE ON HOT WORKABILITY OF Ti-6Al-4V ALLOY

Hot workability of Ti -6 Al -4 V alloy with different initial microstructures was investigated by considering processing maps and the dynamic material deformation behavior. The emphasis has been focused on the effect of initial microstructure (equiaxed versus bimodal structure). Process maps were generated using the dynamic material model (DMM), unifying the relationships between constitutive deformation behavior, hot workability and microstructures evolution. Also, the flow instability was investigated using the various flow instability criteria and microstructural analysis. To establish the processing maps with different initial microstructures, high temperature compression tests were carried out at various temperatures and strain rates up to a true strain of 0.7. Microstructural changes occurring during the deformation were analyzed in terms of high temperature deformation mechanisms. Finally the useful instability criterion for predicting the forming defects was suggested through the compression test results with different temperatures and strain rates.

Analysis of high temperature deformation mechanism in ODS EUROFER97 alloy

Oxide dispersion in tempered martensitic EUROFER97 steel is an efficient approach to improve its strength. The oxide dispersion strengthened (ODS) EUROFER97 steel shows a good strength up to 600 °C, but degrades rapidly beyond that temperature. To understand the origin in the microstructure of this drop in strength in situ heating experiment in TEM was performed from room temperature to 1000 °C. Upon heating neither microstructure changes nor dislocation movement are observed up to 600 °C. Movement of dislocations are observed above 680 °C. Phase transformation to austenite starts at 840 °C. Yttria particles remain stable up to 1000 °C. Changes in mechanical properties thus do not relate to changes in yttria dispersion. It is attempted to relate these observations to the thermal activation parameters measured by the technique of conventional strain rate experiment, which allow to identify at a mesoscopic scale the microstructural mechanisms responsible for the degradation of ODS steel at high temperatures.

Effects of Temperature and Strain Rate on the High-Temperature Workability of Strip-Cast Mg-3Al-1Zn Alloy

In this study, high-temperature workability of a strip-cast Mg-3Al-1Zn (AZ31) alloy was investigated on the basis of the processing map and microstructural analysis. To obtain the processing map, a series of isothermal compression tests was carried out up to a strain of 0.5 at temperatures of 473 to 673 K and strain rates of 0.01 to 10 s−1. It was found that the maximum efficiency indicating the optimum processing condition was obtained at 573 K and 10 s−1, and the processing instability occurred around the region of 473 K and 0.01 s−1. The possible high-temperature deformation mechanisms were also discussed in relation to initial texture development and twin formation.

High temperature deformation behavior and mechanism of spray deposited Al-Fe-V-Si alloy

Al-8.5Fe-1.3V-1.7Si alloy was prepared by spray deposition and hot extrusion. The high temperature plastic deformation behavior of the spray deposited Al-8.5Fe-1.3V-1.7Si alloy was investigated in the strain rate range of 2.77 × 10−4−2.77 × 10−2s−1 and temperature range of 350–550 °C by Gleebe-1500 thermomechanical simulator. The mechanism of the high temperature plastic deformation of the alloys was studied by TEM associated with the analysis of Rosler-Artz physical constitutive relationship based on the model of dislocation detaching from dispersion particles. The results show that Al-Fe-V-Si alloy has low strain hardening coefficient, and even exhibits work softening. Stress exponent n and activation energy Q were calculated based on Zener-Hollomon relation and Rosler-Artz physical model respectively. The Rosler-Artz physical model can give a good prediction for the abnormal behavior of high temperature deformation of spray deposited Al-Fe-V-Si alloy, that is, n larger than 8 and Q higher than 142 kJ/mol. However, because of the highly refined microstructure, the high temperature deformation behavior of spray deposited Al-Fe-V-Si alloy deviates more or less from the law predicted by using Rosler-Artz physical model.

Effect of Al addition on the elevated temperature deformation behavior of Mg–Zn–Y alloy

The effect of the addition of Al on the high temperature deformation behavior of wrought Mg–4Zn–0.8Y alloy has been investigated. The addition of Al significantly improves the high temperature formability, i.e. increases elongation from 260% to 370% under a strain rate of 1 × 10 −4 s −1 at 300 °C. Strain rate sensitivity measured at different strain rates shows that the addition of Al induces the transition of high temperature deformation mechanism from dislocation creep mode to grain boundary sliding mode. Effect of alloying elements and microstructural evolution including the role of icosahedral-phase during deformation are discussed.

High temperature deformation mechanism and evolution of dislocation structure during creep of Ni-Base superalloy 713LC

Creep deformation of cast nickel base superalloy 713LC has been investigated in a temperature range of 723 to 982°C. The values of the stress exponent and activation energy for creep of the alloy vary with a combination of temperature and stress. Introduction of threshold stress for creep of the alloy provided an explanation of the high values of the stress exponent and the apparent activation energy. Microstructural evolution of the alloy with creep deformation has also been studied. The analysis of the creep mechanism has been supplemented by microstructural observations after deformation under various test conditions. The dislocation structure of the alloy at high temperature and low stress was different from that at low temperature and high stress. Shearing of γ′ particles by dislocation pairs was the dominant creep mechanism at low temperature and high stress whereas dislocation climb over γ′ particles was the rate controlling process of creep at high temperature and low stress.

2상 Ti-6Al-4V 합금, 준단상 Ti-6.85Al-1.6V 및 단상 Ti-7.0Al-1.5V 합금의 고온 변형거동에 관한 연구

The high-temperature deformation mechanisms of a <TEX>${\alpha}+{\beta}$</TEX> titanium alloy (Ti-6Al-4V), near-a titanium alloy (Ti-6.85Al-1.6V) and a single-phase a titanium alloy (Ti-7.0Al-1.5V) were deduced within the framework of inelastic-deformation theory. For this purpose, load relaxation tests were conducted on three alloys at temperatures ranging from 750 to <TEX>$950^{\circ}C$</TEX>. The stress-versus-strain rate curves of both alloys were well fitted with inelastic-deformation equations based on grain matrix deformation and grain-boundary sliding. The constitutive analysis revealed that the grain-boundary sliding resistance is higher in the near-<TEX>${\alpha}$</TEX> alloy than in the two-phase <TEX>${\alpha}+{\beta}$</TEX> alloy due to the difficulties in relaxing stress concentrations at the triple-junction region in the near-<TEX>${\alpha}$</TEX> alloy. In addition, the internal-strength parameter (<TEX>${\sigma}^*$</TEX>) of the near-<TEX>${\alpha}$</TEX> alloy was much higher than that of the <TEX>${\alpha}+{\beta}$</TEX> alloy, thus implying that dislocation emission/ slip transfer at <TEX>${\alpha}/{\alpha}$</TEX> boundaries is more difficult than at <TEX>${\alpha}/{\beta}$</TEX> boundaries.

Constitutive analysis of the high-temperature deformation mechanisms of Ti–6Al–4V and Ti–6.85Al–1.6V alloys

The high-temperature deformation mechanisms of a near-α titanium alloy (Ti–6.85Al–1.6V) and an α + β titanium alloy (Ti–6Al–4V) were deduced within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on the two alloys at temperatures ranging from 750 to 900 °C. The stress versus strain rate curves of both alloys were well fit with inelastic-deformation equations which consisted of grain-matrix deformation (GMD) and grain-boundary sliding (GBS). The constitutive analysis revealed that the grain-boundary sliding resistance is higher in the near-α alloy than in the two-phase α + β alloy due to the difficulty in relaxing stress concentrations at the triple-junction regions in the near-α alloy. In addition, the internal-strength parameter ( σ *) of the near-α alloy was much higher than that for the α + β alloy, thus implying that dislocation emission/slip transfer at α/α boundaries is more difficult than at α/β boundaries.

Deformation Behavior of Ti-6Al-4V and Ti-6.85Al-1.6V Alloy with a Globular Microstructure

The high temperature deformation mechanisms of two phase a+b alloy and a near-a alloy were investigated, and compared within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on the two alloys at temperatures of 750~900°C. The flow stress-vs.-strain rate curves for both alloys were well fit with inelastic deformation equations describing dislocation glide and grain boundary sliding. The amount of grain boundary sliding resistance was higher in the near-a alloy rather than the two phase a+b alloy due to difficulty in stress relaxation at triple junction region.

Microstructural Design for High-Strain-Rate Superplastic Oxide Ceramics

Factors limiting the strain rate available to superplastic deformation in oxide ceramics are discussed from existing knowledge about high-temperature plastic deformation and cavitation mechanisms. Simultaneously controlling these factors is essential for attaining high-strain-rate superplasticity (HSRS). This is shown in monolithic tetragonal zirconia and composite materials consisting of zirconia, α-alumina and a spinel phase: at strain rates higher than 10 - 2 s - 1 , tensile ductility reached 300-600% in the monolithic material and 600-2500% in the composite materials. Post-deformation microstructure indicates that certain secondary phases should be effective in suppressing cavitation damage and thereby enhancing HSRS.

Mechanisms of high temperature deformation in electrolytic copper in extended ranges of temperature and strain rate

Abstract The hot deformation behavior of electrolytic copper in isothermal compression has been studied in the temperature range 300–950 °C and strain rate range 0.001–100 s −1 with a view to evaluate the rate controlling mechanisms relevant to different ranges. The flow curves observed are typical of the occurrence of dynamic recrystallization (DRX) and have exhibited single or multiple peak in the flow stress before reaching steady state, depending on the temperature and strain rate. Three different windows of temperature and strain rate have been identified in which the kinetic rate equation is obeyed by the temperature and strain rate dependence of flow stress. Both the power-law and hyperbolic sine relation yielded similar results. In the strain rate range 0.001–1.0 s −1 and temperature range 400–600 °C, the stress exponent is 7.8 and the apparent activation energy is 159 kJ/mole, and these indicate that dislocation core diffusion is the rate controlling mechanism. In the strain rate range 0.001–1.0 s −1 and temperature range 700–950 °C, the stress exponent is 7.1 and the apparent activation energy is 198 kJ/mole which matches well with that for lattice self diffusion in copper. The switching of the rate controlling mechanisms at temperatures higher than 700 °C has been attributed to the “clogging” of the dislocation pipes due to the rapid increase in the solid solubility of oxygen at higher temperatures. In the high strain rate range 30–100 s −1 and temperature range 700–950 °C, the stress exponent is 3.5 and the apparent activation energy is 91 kJ/mole which matches with that for grain boundary diffusion in copper. The apparent activation energy in electrolytic copper in all the above temperature and strain rate windows increases with increasing oxygen content and this is attributed to the increase in the back stress due to the presence of copper oxide particles in the matrix.

Mechanisms of high temperature deformation in electrolytic copper in extended ranges of temperature and strain rate

The hot deformation behavior of electrolytic copper in isothermal compression has been studied in the temperature range 300–950 °C and strain rate range 0.001–100 s −1 with a view to evaluate the rate controlling mechanisms relevant to different ranges. The flow curves observed are typical of the occurrence of dynamic recrystallization (DRX) and have exhibited single or multiple peak in the flow stress before reaching steady state, depending on the temperature and strain rate. Three different windows of temperature and strain rate have been identified in which the kinetic rate equation is obeyed by the temperature and strain rate dependence of flow stress. Both the power-law and hyperbolic sine relation yielded similar results. In the strain rate range 0.001–1.0 s −1 and temperature range 400–600 °C, the stress exponent is 7.8 and the apparent activation energy is 159 kJ/mole, and these indicate that dislocation core diffusion is the rate controlling mechanism. In the strain rate range 0.001–1.0 s −1 and temperature range 700–950 °C, the stress exponent is 7.1 and the apparent activation energy is 198 kJ/mole which matches well with that for lattice self diffusion in copper. The switching of the rate controlling mechanisms at temperatures higher than 700 °C has been attributed to the “clogging” of the dislocation pipes due to the rapid increase in the solid solubility of oxygen at higher temperatures. In the high strain rate range 30–100 s −1 and temperature range 700–950 °C, the stress exponent is 3.5 and the apparent activation energy is 91 kJ/mole which matches with that for grain boundary diffusion in copper. The apparent activation energy in electrolytic copper in all the above temperature and strain rate windows increases with increasing oxygen content and this is attributed to the increase in the back stress due to the presence of copper oxide particles in the matrix.

Physical aspects of hot-working gamma-based titanium aluminides

Titanium aluminide alloys containing relatively large Nb additions and subjected to precipitation hardening exhibit attractive thermo-physical properties, which extend the service range of conventional TiAl alloys. In order to use these materials at their full potential, structural and chemical consolidation is required. To this end, the feasibility of wrought processing of high Nb containing alloys was systematically investigated. The major areas of the study involve: (i) high-temperature deformation mechanisms, (ii) primary ingot break-down, (iii) secondary processing, (iv) texture evolution, and (v) mechanical properties.

High-strain-rate superplastic behavior of equal-channel angular-pressed 5083 Al-0.2 wt pct Sc

Tensile tests were carried out at temperatures of 673 to 773 K and strain rates of 1×10−3 to 1 s−1 on an ultrafine-grained (UFG) 5083 Al alloy containing 0.2 wt pct Sc fabricated by equal-channel angular pressing, in order to examine its high-strain-rate superplastic characteristics. The mechanical data for the alloy at 723 and 773 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain-rate sensitivity was 0.25 to 0.3 in the low-( $$\dot \varepsilon $$ <5×10−3 s−1) and high- ( $$\dot \varepsilon $$ >5×10−2 s−1) strain-rate regions, and ∼0.5 in the intermediate-strain-rate region (5×10−3 s−1< $$\dot \varepsilon $$ <5 × 10−2 s−1). The maximum elongation to failure of ∼740 pct was obtained at 1×10−2 s−1 and 773 K. By contrast, no sigmoidal behavior was observed at 673 K. Instead, the strain-rate sensitivity of 0.3 was measured in both intermediate-and low-strain-rate regions, but it was about 0.25 in the high-strain-rate region. High-strain-rate superplasticity (HSRS) in the intermediate-strain-rate region at 723 and 773 K was dominated by grain-boundary sliding (GBS) associated with continuous recrystallization and preservation of fine recrystallized grains by second-phase particles. However, the activation energy for HSRS of the present alloy was lower than that predicted for any standard high-temperature deformation mechanism. The low activation energy was likely the result of the not-fully equilibrated microstructure due to the prior severe plastic deformation (SPD). For 673 K, the mechanical data and the microstructural examination revealed that viscous glide was a dominant deformation mechanism in the intermediate- and low-strain-rate regions. Deformation in the high-strain-rate region at all testing temperatures was attributed to dislocation breakaway from solute atmospheres.