Molecular dynamics study of shear and tensile deformation of bicrystalline aluminum

This paper describes shear and tensile deformation of bicrystalline aluminum by computer molecular dynamics. A bicrystal model with a [001] (310) =5 tilt grain boundary is used for simulations. The simulations show that the effect of temperature on both the shear and tensile deformation is represented by a Boltzmann factor exp (-Q/k _B T) and that the deformation is thermally activated in a typical manner. We found that the activation energy Q for the high temperature range, where T is higher than approximately 500 K to 600 K, is significantly larger than that for the low temperature range. This result shows that there are different deformation mechanisms between high and low temperatures. The activation energy difference is considered to be caused by a structural transition at the grain boundary.A preliminary report about the work on this paper was given at the 3rd World Congress on Computational Mechanics (Chiba, Japan) in August, 1994     

Superplastic flow in nanocrystalline and sub-microcrystalline yttria-stabilized tetragonal zirconia

Tensile deformation of nanocrystalline ZrO₂ + 5 mol% Y₂O₃ at temperatures in the range of 1283–1403 K is described. It is demonstrated (a) that steady state flow is possible at temperatures of the order of 0.42 T _m, where T _m is the absolute melting point, (b) that 70% engineering strain could be obtained at 1403 K (0.46 T _m), and (c) that significant grain boundary sliding was present during deformation. Static and dynamic grain growth as also a decrease in the relative density of the specimen with deformation could be observed. The present results as well as those of Owen and Chokshi concerning superplastic flow in sub-microcrystalline materials taken from literature could be accounted for quantitatively using the grain boundary sliding controlled flow model of Padmanabhan and Schlipf, originally proposed for microcrystalline superplastic alloys.     

High temperature deformation of aluminium bronze

Abstract

Hot compression tests were carried out on commercial Cu–8 wt-%Al alloy to test the effect of the deformation conditions on high temperature deformation characteristics and the final structure of the hot deformed material. Dynamic recrystallisation of the material was found to operate at deformation temperatures above ～900 K. Nucleation and growth of recrystallised grains were observed for specimens deformed at temperatures below ～1000 K. However, the flow stress peaks that usually mark the onset of dynamic recrystallisation were hardly seen on the stress–strain curves. During hot deformation of Cu–8 wt-%Al alloy above ～1000 K the interaction of →β phase transformation and deformation processes affected both the flow stress value and the structure of the material. In particular, post-deformation water quenching of the specimens resulted in martensitic transformation within pre-existing β grains. Moreover, local coherent iron particles were detected within β and neighbouring grains.     

Effect of test temperature on fracture toughness of modified 9Cr–1Mo steel

Abstract

The quasi-static fracture behaviour (J–R curves) of modified 9Cr–1Mo (P91) steel was studied. The J–R curves were established at 298, 653, 823 and 893 K, and fracture toughness J_0·2 at 0·2 mm of crack extension was determined. The value of ～J_0·2 at 653 K was lower compared to that at 298 K followed by increases in J_0·2 values at 823 and 893 K. The decrease in J_0·2 at 653 K can be attributed to the influence of dynamic strain aging. At 893 K, a significantly higher (more than 200%) J_0·2 was observed, since plastic deformation of the net section, rather than crack growth, occurred in this condition.     

Hot deformation behaviour of Cu-1.5Ti (wt-%) alloy

Abstract

A Cu-1.5Ti (wt-%) alloy was subjected to hot compression tests at temperatures ranging from 750 to 900°C and strain rates from 10⁰ s^-1 to 10^-3 s^-1. Flow softening was found to occur at all temperatures and strain rates studied. Deformation at 750°C and a relatively high strain rate (100 s^-1) resulted in grain refinement of the alloy with a grain size of ~25 μm. Room temperature hardness decreased with increasing deformation temperature, i.e. 145 HV10 after deforming at 750°C and 90 HV10 at 900°C. The higher values of hardness observed after deformation at 750°C are attributed to the fine grain size. A maximum value of 0.21 obtained for the strain rate sensitivity index m is not indicative of superplasticity in this alloy. Activation energy Q for the hot deformation process at 1173 K and strain rate 10^-3 s^-1 was determined to be 76 kJ mol^-1.     

The variation of strain rate sensitivity exponent and strain hardening exponent in isothermal compression of Ti–6Al–4V alloy

The deformation behavior in isothermal compression of Ti–6Al–4V alloy is investigated in the deformation temperatures ranging from 1093 K to 1303 K, the strain rates ranging from 0.001 s⁻¹ to 10.0 s⁻¹ at an interval of an order magnitude and the height reductions ranging from 20% to 60% at an interval of 10%. Based on the experimental results in isothermal compression of Ti–6Al–4V alloy, the effect of processing parameters and grain size of primary α phase on the strain rate sensitivity exponent m and the strain hardening exponent n is in depth analyzed. The strain rate sensitivity exponent m at a strain of 0.7 and strain rate of 0.001 s⁻¹ firstly tends to increase with the increasing of deformation temperature, and maximum m value is obtained at deformation temperature close to the beta-transus temperature, while at higher deformation temperature it drops to the smaller values. Moreover, the strain rate sensitivity exponent m decreases with the increasing of strain rate at the deformation temperatures below 1253 K, but the m values become maximal at a strain rate of 0.01 s⁻¹ and the deformation temperature above 1253 K. The strain rate affects the variation of strain rate sensitivity exponent with strain. Those phenomena can be explained reasonably based on the microstructural evolution. On the other hand, the strain hardening exponent n depends strongly on the strain rate at the strains of 0.5 and 0.7. The strain affects significantly the strain hardening exponent n due to the variation of grain size of primary α phase with strain, and the competition between thermal softening and work hardening.     

High-temperature deformation characteristics of Ti-15at.% Al alloy

The deformation characteristics of Ti-15 at.% Al alloy have been investigated by compression tests in the temperature range 873 to 1273 K (0.44 to 0.64T _m) and by extensive transmission electron microscopy. Two types of deformation patterns were identified depending on the temperature: at lower temperatures below about 1073 K, the yield stress of the sample showed inverse temperature dependence, and serrations were found on the flow curves, whereas the normal dependences of the yield stress on temperature and strain rate were found at higher temperatures above about 1073 K. Corresponding dislocation substructures were composed of coarse bands of localized slip at 1023 K, and of rather uniformly distributed dislocations at 1123 K, and sub-boundaries as well as free dislocations at 1273 K. The main operating mechanisms in these temperature regimes were assumed to be the co-operative movement of numerous dislocations under the condition of the dynamic strain ageing, viscous glide of dislocations and dynamic recovery, respectively.     

Influence of precipitate morphology on the high-temperature deformation of Ir-Nb alloys

The compression strength of an Ir-15at%Nb alloy at 1473 and 2073 K was investigated. A coherent two-phase fcc L1₂ structure was found in the Ir-15Nb alloy. The L1₂ precipitate morphology depended on the heat treatment. Cuboidal L1₂ precipitates with a size of 100 nm and a plate-like fcc phase inside (type A) were found after heat treatment at 1773 K. The plate-like fcc phase disappeared after heat treatment at 2073 K for 24 h, and only a cuboidal L1₂ phase remained (type B). Coarse rectangular L1₂ precipitates with a length of 400 nm (type C) were found after heat treatment at 2073 K for 168 h. The influence of L1₂ precipitate morphology on the high-temperature strength and dislocation structure was investigated after the compression test. A bypass mechanism in which dislocations spread in the narrow fcc phase was dominant in the type A and B structures during deformation at 1473 K. In the type C structure, bypassing of precipitates was found to be dominant. At 2073 K, deformation by a shearing mechanism was dominant in the type A and B structures, while deformation by a bypassing mechanism was dominant in the type C structure. When the precipitate size was large and the fcc channel width was wide in the type B structure, a bypassing mechanism was dominant. The deformation mechanisms are discussed in terms of the precipitate morphologies.     

Dynamic softening mechanism of 30 % SiCp/2024Al composite during hot deformation process

Using Gleeb‐1500D simulator, the isothermal compression tests of 30 % SiCp/2024Al (volume fraction) are conducted at a temperature range of 623 K ‐ 773 K and a strain rate range of 0.01 s^‐1 ‐ 10 s^‐1. The softening mechanism of composites during hot deformation has been proposed based on the Zener‐Hollomon parameter Z, deformation temperature T and microstructure analysis. Cross slip of dislocation plays a dominant role under the conditions of lnZ≥59.634 and T≤673 K. While, deformation mechanisms such as cross slip, climb of dislocation and unzipping of the three dimensional dislocation network play a joint role when lnZ≤61.933 and T≥623 K. Particularly, dynamic recrystallization occurred when lnZ≤55.669 and T≥723 K. The cross slip, climb and unzipping of dislocation and dynamic recrystallization are the main softening mechanisms. The role of the dynamic recrystallization mechanisms become more significant and the critical strain of dynamic recrystallization decrease with the decrease of lnZ. Dynamic recrystallization nucleation mechanisms are mainly constituted of the subgrain combination and the bulging of the grain boundary.     

Elevated-temperature strengthening in carbide (Al4C3)-dispersed aluminium

The effect of strain rate on the compressive flow behaviour of DISPAL 2 is investigated in the temperature range 473–823 K. The stress exponent, n, was 28 in the temperature range 473–623 K, while it increased to 59 above 673 K. The activation volume and energy for deformation were 70 b ³ and 100–200 kJ mol^–1, respectively, in the temperature range 473–623 K. In the higher range, 673–823 K, the observed activation volume of 300–500 b ³ and the activation energy of ~ 1086 kJ mol^–1 cannot be reconciled with any of the deformation mechanisms. A new model-based creep equation for dispersion-strengthened materials proposed by Rosler and Arzt has been applied to the flow data from 673–823 K. Its predictions are in agreement with the experimental data in the temperature range 673–723 K. The predictions of the model, however, differ from the experimental data at 773 and 823 K.     

Deformation characteristics and real structure around indents on {111}A-surfaces of II–VI solid solutions

The microhardness of II–VI compounds and their solid solutions has been measured at temperatures from 0.2 to 0.44T _m. Deformation effects around indents on {111}_A-surfaces have been studied by means of selective etching. Changes in the real structure, reflected in the hardness-temperature slope, are discussed in terms of different rate-controlling mechanisms, e.g. dislocation slipping and climbing, and of solid solution hardening effects. Deformation by indentation is characterized, for the investigated temperature range, by an activation energy ofQ0.1 eV in the case of (Cd, Zn) (Te, Se) and ofQ=0.12 ... 0.23 eV for Hg-containing compounds. A slightly different hardness between {111}_A- and {111}_B-faces occurs in Hg_0.32Cd_0.68Te at temperatures between 360 K and 500 K. Critical stresses for -dislocation motion, derived from dislocation configurations around the indents, yield values ranging from 12.5 MPa at 295 K to 1.2 MPa at 500 K. Micro-twinning is one of the deformation modes of (Cd, Zn) (Te, Se) below 400 K.     

Superplastic deformation and microstructure evolution in PM IN-100 superalloy

The superplastic deformation behaviour of PM IN-100 alloys consolidated by hot isostatic pressing (HIP) was investigated in compression tests at temperatures between 1323 and 1373K. The microstructural changes were observed using scanning electron microscopy. In the high strain rate region, grain refinement occurs due to dynamic recrystallization, resulting in the work softening type stress-strain curves. At low strain rates, grain growth occurs during deformation corresponding to work hardening. The strain rate sensitivity index,m, reaches a maximum value (m = 0.6) at the optimum strain rate which depends on the test temperature. The grain size dependence coefficient,p, was determined to be 2.0. The activation energy for deformation was 348kJ mol^–1. The rate-controlling mechanism of superplasticity in as HI Ped IN-100 seems to be the grain-boundary sliding controlled by volume diffusion rather than grain-boundary diffusion.     

High-temperature deformation behaviour of YBa2Cu3O7−x

High-temperature compression tests were performed in air for YBa₂Cu₃O_7–x polycrystals with grain sizes of 3 and 7 m at various strain rates between 1.3×10^–5 and 4×10^–4s^–1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol^–1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.     

Effects of the creep deformation on the fractal dimension of the grain boundaries in an austenite steel

The change in the fractal dimension of the grain boundaries during creep was investigated using an austenitic SUS304 steel at 973 K. The fractal dimension of the grain-boundary surface profile (the fractal dimension of the grain boundaries, D, 1 < D < 2) in the plane parallel to the tensile direction (in the parallel direction) and in the transverse direction, was examined on specimens deformed up to rupture (about 0.30 creep strain). Grain boundaries became serrated and the fractal dimension of the grain boundaries increased with increasing creep strain, because the density of slip lines which formed ledges and steps on grain boundaries increased as the creep strain increased. The increase in the fractal dimension due to creep deformation was slightly larger under the higher stress (118 MPa) than under the lower stress (98 MPa), while the increase of the fractal dimension with strain was a little larger in the specimens tensile-strained at room temperature (293 K) than in the crept specimens. These results were explained by the grain-boundary sliding and the diffusional recovery near grain boundaries, which lowered the increase of the fractal dimension with the creep strain. The fractal dimension of the grain boundaries in the parallel direction was slightly larger than that in the transverse direction in both creep at 973 K and tensile deformation at room temperature, especially at the large strains. This could be correlated with the shape change of the grains by creep or plastic deformation. Grain-boundary cracks were principally initiated at grain-boundary triple junctions in creep, but ledges, steps and carbide precipitates on serrated grain boundaries were not preferential nucleation sites for the cracks.     

Production of a superconducting Bi-Pb-Sr-Ca-Cu oxide wire using a hot-extrusion process

A Bi_1.84Pb_0.34Sr₂Ca₂Cu₃O_y sample, sheathed with Ag, was hot-extruded into a wire with a high reduction ratio of 91 % in area. The c axis of the extruded oxide was normal to the extrusion direction. Subsequent swaging and rolling at room temperature and annealing for 200 h at 1113 K in air resulted in a high-T_c superconducting composite tape exhibiting a J_c value of 5900 A/cm² at 77 K in zero applied field. The hot-extrusion process is useful as the first deformation process to produce the superconducting oxide tape with a favorable orientation structure.     

Flow behaviour and microstructural evolution in cold worked 70∶30 α-brass

Abstract

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70∶30 α-brass over the test temperature range of 298–973 K and strain rate range of 10^?5–5×10^?3 s^?1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553–673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol^?1 below 623 K and 196·4 kJ mol^?1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol^?1, with the time exponent of 0·37.     

One-step and iterative thermo-mechanical treatments to enhance Σ3n boundaries in a Ti-modified austenitic stainless steel

The article discusses the results of a study on low-strain thermo-mechanical (one-step and iterative) processing to enhance Σ3 ⁿ boundaries in a Ti-modified austenitic stainless steel (alloy D9). Solution annealed (SA) specimens were subjected to 10% thickness reduction by rolling followed by annealing at 1173, 1223, and 1273 K for 0.5, 1, and 2 h. Anomalous grain growth with moderate increase in Σ3 ⁿ boundaries was observed after annealing at 1,173 K for 0.5 to 2 h. Prolific multiple twinning with minimum deviation of Σ3 and Σ9 boundaries from ideal orientation was achieved after annealing at 1,273 K for 0.5 to 2 h. A significant disruption in random boundary connectivity was obtained in these conditions due to the presence of large number of Σ3-Σ3-Σ9/Σ3-Σ9-Σ27 triple junctions. Iterative processing (up to 4 cycles) employing 10% thickness reduction followed by annealing at 1,273 K for 0.5 h revealed fluctuations in the evolution of Σ3 boundaries. The Σ3 fraction increased after 2nd and 4th iteration and there is a drop after 3rd iteration. This was attributed to the increased driving force for grain boundary migration due to dislocation pile-up at twin boundaries during earlier iterations. A two step iterative processing comprising of 10% deformation followed by annealing at 1,273 K for 0.5 h is the recommended thermo-mechanical processing to achieve enhanced fraction of Σ3 ⁿ boundaries (~73%) in alloy D9.     

Theory and application of microindentation in studies of glide and cracking in single crystals of elemental and compound semiconductors

The microhardness of Si (MP 1688 K), GaP (1623 K), GaAs (1510 K) and InP (1327 K) single crystals was determined by indentation (Vicker's hardness, VHN) of low-index facets at loads of 5–100g at 296–673 K, complementing earlier work on Ge and InSb. In the brittle range, extending up to about 0.35 T _melt (K), cracking occurred preferentially along the diagonals of the indentations, and was observed at all loads, with the possible exception of the lowest (5 g) in the case of InP at 289 K. At higher temperatures the relative orientations of crack and slip traces on the crystal surface, as observed by SEM, suggested that cracks nucleated preferentially at the slip-band intersection, as was also noted by Hirsch et al. (Phil. Mag. 3 (1985) 759) in GaAs above 600 K. As earlier in Ge, the VHN was found to depend on the load, L, as L ^p , and on the indentation diameter, d, as dⁿ, with p = 1/2 and n = 2, as required by the model of indentation plasticity of Banerjee and Feltham [4, 5], but higher p and n values were found if chipping at the indentation edges was evident. The effect was related to the resulting decrease in indentation diameter due to the work lost, through chipping, by the indenter. Above about 0.35 T _melt (K), relaxation of the dislocation structures entails a decrease of p and n; both parameters tend to zero as T T _melt. Shear and tensile stresses seem to co-operate in the process of plastic deformation, the role of normal stresses, acting across slip planes, predominating in the brittle range.     

Applicability of improved Dyson–McLean approach to creep deformation behaviour of tempered martensitic P9 steel

Modelling creep deformation of tempered martensitic P9 steel in quenched and tempered, and simulated post weld heat treatment conditions has been performed in the framework of improved Dyson–McLean approach for wide range of stresses at 873 K. In this approach, kinetic creep law coupled with the set of first-order differential equations representing the evolution of microstructural internal-state-variables with strain/time has been employed to describe creep deformation behaviour of tempered martensitic steel. The optimised material constants associated with the model such as dislocation storage parameter (K_d) and rate constants associated with the precipitate coarsening (K_p) and solute depletion (K_s) reflect the influence of two different heat treatments on creep characteristics examined in the present investigation. At all test conditions, good agreement between the predicted and experimental creep strain/strain rate-time data at 873 K has been observed. Further, good correlations have been obtained between the experimental and predicted steady-state creep rates and time to reach the specified strain levels for both the heat treatment conditions.     

Electrical properties of underformed and plastically deformed bismuth-thallium and bismuth-lead systems in the temperature range 4.2 to 300 K

Measurements of electrical conductivity and Hall coefficient have been made on undeformed and plastically deformed states of Bi-Tl (Bi + 3.92 at % Tl) and Bi-Pb (Bi + 4.00 at % Pb) single-crystal specimens, from 4.2 to 300 K. The carrier concentrationn and the Hall mobility obtained from these measurements show strong dependence on temperature. From the variation ofn with temperature, there is evidence for bend gaps of 40 meV obtained from observations between 100 and 300 K for Bi-Tl, and 18 meV between 70 and 300 K for Bi-Pb, in the undeformed states of the specimens. Theme band gaps increase due to plastic deformation. In the low-temperature region, the increase and the subsequent decrease inn have been explained on the basis of a thermal activation process and a phonon-induced electron-hole recombination process. The activation energies thus observed in the undeformed state of the specimens have been greatly reduced due to plastic deformation. These results show that the band structure of bismuth is greatly affected by doping, end that of the doped specimens of bismuth is further affected by plastic deformation. There is a correspondence between the increase inn and the decrease in and vice versa, over the entire range of temperature. The dependence of onT has been utilized to identify the scattering mechanisms.