Simulation on Grain Boundary Sliding during Superplastic Deformation Using Molecular Dynamics Method

Grain growth and grain boundary sliding are the two main superplastic deformation mechanisms. In the paper, simulation work is focused on the sliding of a S3 (111) symmetric twist coincidence grain boundary, a S13 (110) asymmetric tilt coincidence grain boundary, and a S3 (110) symmetric tilt coincidence grain boundary in Al, and the energies of grain boundary for each of equilibrium configurations are computed. An embedded atom method (EAM) potential was used to simulate the atomic interactions in a bicrystal containing more than 2000 atoms. At 0 K, the relationships between total potential energy and time steps for S3 (111) symmetric twist coincidence grain boundary and S3 (110) symmetric tilt coincidence grain boundary during sliding at 2 m/s represent the periodic characteristic. However, the relationship between total potential energy and time steps for S13 (110) asymmetric tilt coincidence grain boundary represents the damp surge characteristic. It is found that grain boundary sliding for S3 (110) symmetric tilt coincidence grain boundary is coupled with apparent grain boundary migration.

Grain Boundary Sliding in Fatigue of Pure Aluminum

The distributions of plastic strain near grain boundaries induced by fatigue loading were investigated by the fiducial grid method in pure aluminum specimens, and the resulted grain boundary sliding (GBS) was systematically analysed. The results show that the strain field near a grain boundary is nonuniform. GBS is restricted by the junction of grain boundaries and causes discontinuities of both displacement and strain. A peak value of shear strain was created in short-range area across the grain boundary. GBS plays an important role in cyclic softening and secondary hardening. The control fac- tor of GBS is the relative orientation between two grains and the macro orientation of the grain boundary rather than the ∑ value of the boundary.

Mesoscale deformation mechanisms in relation with slip and grain boundary sliding in TA15 titanium alloy during tensile deformation

Revealing the mesoscale deformation mechanisms of titanium alloy with tri-modal microstructure is of great significance to improve its mechanical properties. In this work, the collective behavior and mechanisms of slip activities, slip transfer, and grain boundary sliding of tri-modal microstructure were investigated by the combination of quasi-in-situ tensile test, SEM, EBSD and quantitative slip trace analyses. It is found that the slip behavior presents different characteristics in the equiaxed α(α_(p)) and lamellar α(α_(l))grains. Under a low level of deformation, almost all the slip deformation is governed by single basal and prismatic slips for both of α_(p)and α_(l),despite small amount of < a >-pyramidal slip exists in α_(l)grains. As deformation proceeds, < a >-pyramidal and < c + a >-pyramidal slip systems with high Schmid factors were activated in quantities. Specially, certain coarse prismatic slip bands were produced across both of single and colony α_(l)grains whose major axes tilting about 40 °–70 ° from the tensile axis. Slip transfer occurs at the boundaries of α_(p)/α_(p)and α_(l)/β under the condition that there exists perfect alignment between two slip systems and high Schmid factors of outgoing slip system. The slip transfer across α_(l)/β boundary can be divided into two types: straight slip transfer and deflect slip transfer with a deviation angle of 5 °–12 °, depending on the alignment of slip planes of two slip systems. The grain boundary sliding along boundaries of α_(l)/β and α_(p)/β was captured by covering micro-grid on tensile sample. It is found that the crystallographic orientation and the geometrical orientation related to loading axis play great roles in the occurrence of grain boundary sliding.

Dislocation Creep Accommodated by Grain Boundary Sliding in Dunite

To investigate the role of grain boundary sliding during dislocation creep of dunite, a series of deformation experiments were carried out under anhydrous conditions on fine-grained （-15 μm） samples synthesized from powdered San Carlos olivine and powdered San Carlos olivine＋1.5 vol.% MORB. Triaxial compressive creep tests were conducted at a temperature of 1 473 K and confining pressures of 200 and 400 MPa using a high-resolution, gas-medium deformation apparatus. Each sample was deformed at several levels of differential stress between 100 and 250 MPa to yield strain rates in the range of 10^-6 to 10^-4 s^-1. Under these conditions, the dominant creep mechanism involves the motion of dislocations, largely on the easy slip system （010）[100], accommodated by grain boundary sliding （gbs）. This grain size-sensitive creep regime is characterized by a stress exponent of n=3.4±0.2 and a grain size exponent of p=2.0±0.2. The activation volume for this gbs-accommodated dislocation creep regime is V＊=（26±3）×10^-6 m2·mol^-1. Comparison of our flow law for gbs-accommodated dislocation creep with those for diffusion creep and for dislocation creep reveals that the present flow law is important for the flow of mantle rocks with grain sizes of 〈100μm at differential stresses 〉20 MPa. Hence, gbs-accommodated dislocation creep is likely to be an important deformation mechanism in deep-rooted, highly localized shear zones in the lithospheric upper mantle.

Superplastic behavior of a fine-grained Mg-Gd-Y-Ag alloy processed by equal channel angular pressing

An extruded Mg-6Gd-3Y-1.5Ag(wt%) alloy was processed by 6 passes of equal channel angular pressing(ECAP) at 553 K using route Bc to refine the microstructure. Electron back-scattered diffraction(EBSD) analysis showed a fully recrystallized microstructure for the extruded alloy with a mean grain size of 8.6 μm. The microstructure of the ECAP-processed alloy was uniformly refined through dynamic recrystallization(DRX). This microstructure contained fine grains with an average size of 1.3 μm, a high fraction of high angle grain boundaries(HAGBs), and nano-sized Mg_(5)Gd-type particles at the boundaries of the DRXed grains, detected by transmission electron microscopy(TEM). High-temperature shear punch testing(SPT) was used to evaluate the superplastic behavior of both the extruded and ECAP-processed alloys by measuring the strain rate sensitivity(SRS) index(m-value). While the highest m-value for the extruded alloy was measured to be 0.24 at 673 K, the ECAP-processed alloy exhibited much higher m-values of 0.41 and 0.52 at 598 and 623 K, respectively,delineating the occurrence of superplastic flow. Based on the calculated average activation energy of 118 kJ mol^(-1) and m-values close to 0.5, the deformation mechanism for superplastic flow at the temperatures of 598 and 623 K for the ECAP-processed alloys was recognized to be grain boundary sliding(GBS) assisted by grain boundary diffusion.

NUMERICAL SIMULATION FOR DEFORMATION OF NANO-GRAINED METALS

Electro-deposition technique is capable of producing nano-grained bulk copper specimens that exhibit superplastic extensibility at room temperature. Metals of such small grain sizes deform by grains sliding,with little distortion occur- ring in the grain cores.Accommodation mechanisms such as grain boundary diffusion, sliding and grain rotation control the kinetics of the process.Actual deformation min- imizes the plastic dissipation and stored strain energy for representative steps of grain neighbor switching.Numerical simulations based on these principles are discussed in this paper.

GRAIN SHAPE EFFECT IN SUPERPLASTIC DEFORMATION

The method controlling grain shape in TMT processing and the effect of grain shape on char- acteristic parameters in superplastic deformation were discussed.The accommodation velocity of grahl boundary sliding,which is the dominant mechanism in superplastic deformation,and the contribution of each mechanism to the total strain,as influenced by grain shape,were ana- lyzed.Grain shape has been shown to be an essential structural factor for superplasticity.Then an analysis was made about the effect of grain shape on the region transition strain rate so that a new concept,critical aspect for superplasticity,was worked out.These predictions were compared with the measured results in an Al-Zn-Mg alloy.

High strain rate superplasticity of rolled AZ91 magnesium alloy

The high strain rate superplastic deformation properties and characteristics of a rolled AZ91 magnesium alloy at temperatures ranging from 623 to 698 K（0.67Tm-0.76Tm） and high strain rates ranging from 10^-3 to 1 s^-1 were investigated.The rolled AZ91 magnesium alloy possesses excellent superplasticity with the maximum elongation of 455% at 623 K and a strain rate of 10-3 s-1,and its strain rate sensitivity m is high up to 0.64.The dominant deformation mechanism responsible for the high strain rate superplasticity is still grain boundary sliding（GBS）,and the dislocation creep mechanism is considered as the main accommodation mechanism.

Superplasticity of a friction stir processed overaged WE54 magnesium alloy

The coarse-grained WE54 magnesium alloy was heat treated in order to have minimum hardness minimizing the effects of precipitates and solid solution. Friction stir processing(FSP) was applied in severe conditions to obtain fine, equiaxed and highly misoriented grains, with grain sizes even less than 1 μm. The high severity of processing demonstrated to have a strong impact in the microstructure. Consequently,the processed materials exhibited excellent superplasticity at the high strain rate 10^(-2)s^(-1), and temperatures between 300 and 400 ℃. The maximum tensile superplastic elongation of 756% was achieved at 400 ℃ thanks to the operation of grain boundary sliding mechanism(GBS). Besides the new data obtained through tensile testing, the paper deals with a transcendental question regarding the large differences in strain rate values at a given stress in the superplastic regime at maximum elongation compared to other magnesium-based alloys. With this is mind, 19 magnesium alloys from 22 different investigations were analyzed to give some light to this behavior that never was treated before. It is proposed that this behavior has to be attributed to the accommodation process, necessary for GBS to occur, which is hindered by reinforcing solutes.

Superplastic Deformation Behavior of Hot-rolled AZ31 Magnesium Alloy Sheet at Elevated Temperatures

Uniaxial tensile tests were carride out in the temperature range of 250-450℃ and the strain rate range of 0.7×10^-3^-1.4×10^-1s^-1 to evaluate the superplasticity of AZ31 Mg aloy .The threshold stress which characterizes the difficulty for grain boundary sliding was calculated at various temperature .The surface relieves of superplastically deformed specimens were observde by using a scanning elctronic microscops （SEM）.Results show that ,at the temperature of 400℃ and strain rate of 1.7×10^-3^-1,the strain rate sensitivity exponent ,i e,m value reaches 0.47 and the maximum elongation of 362.5% is achieved .Grain boundary sliding （GBS）is the primary deformation mechanism and characterized by a pronounced improvement in the homogeneity with increasing temperatures.A large number of filaments were formed at the end of deformation and intergranular cavities produced with the necking and fracture of filaments.Finally ,the model for the formation of intergranular cavities was proposed.

Critical review of superplastic magnesium alloys with emphasis on tensile elongation behavior and deformation mechanisms

The tensile elongation behavior and deformation mechanisms of superplastic Mg alloys and Mg composites were examined by extensively reviewing the literature published from the time of the first report on the superplasticity of Mg alloys to the present day.Studies on the superplasticity of Mg alloys have been conducted mainly on Mg-Al-Zn(AZ)series alloys,Mg-Zn-Zr(ZK),Mg-Li and Mg-RE(rare earth)alloys,and in recent years,Mg-RE alloys have attracted the greatest attention.The effect of grain size and the type and amount of secondary phase particles on the superplasticity of Mg alloys was systematically examined and reviewed.The alloys processed by severe plastic deformation(SPD)and powder-metallurgy methods have smaller grain sizes and exhibit superior superplasticity compared to conventionally processed(by extrusion and rolling)Mg alloys.For the AZ alloys,as the volume fraction of the Mg17Al12phase increases,smaller grains are obtained,and the low-temperature superplasticity(LTS)and high-strain-rate superplasticity(HSRS)characteristics become enhanced.The ZK60 alloy with finely dispersed Mg Zn2particles exhibits excellent LTS,while the Mg-RE alloys with a high fraction of thermally stable particles exhibit excellent HSRS.Mg-Li alloys can exhibit LTS even at room temperature due to the presence of a high-volume fraction of the body centered cubic(BCC)phase where atomic diffusivity is high.Grain boundary diffusion-and lattice diffusion-controlled grain boundary sliding are found to operate as the dominant deformation mechanisms below~473 K and above~673 K,respectively,at small grain sizes.Deformation mechanism maps were constructed based on the analysis of the deformation behavior of superplastic Mg alloys,and from the maps,the critical conditions for achieving LTS,HSRS and simultaneous achievement of LTS and HRSR were calculated and proposed,and their importance was discussed.

Superplasticity Enhanced by Two-stage Deformation in a Hot-extruded AZ61 Magnesium Alloy

A two-stage strain rate deformation method is proposed to enhance the superplasticity in a hot extruded AZ61 alloy. In the stage-one of deformation, a relatively high strain rate was applied in order to obtain fine grains through dynamic recrystallization. The optimum strain rate for DRX at 300℃ was identified as -5×10-3s-1. Stage-two is conducted at relatively low strain rate in order to utilize the fine grains refined by DRX during stage-one to make the grain boundary sliding operate more smoothly, which resulting in enhanced superplastic elongation from 350% to 440%.

Unexpected high-temperature brittleness of a Mg-Gd-Y-Ag alloy

Rare earth(RE)can produce excellent precipitation hardening in Mg alloys.However,when forming a solid solution,it also deteriorates formability,a problem that can usually be overcome by raising deformation temperature.Here we report an unexpected observation of high temperature brittleness in a Mg-Gd-Y-Ag alloy.As the temperature reached 500℃,the formability decreased drastically,leading to severe intergranular fracture under only 0.5% strain.This was caused by failure of grain boundaries,which are weakened by segregated interfacial compounds.

High Temperature Ductility Loss of 16MnCr5 Pinion Steels

A wide ductility trough covering from 700 to 1100 is observed in the curve of Reduction of Area (RA) vs.tempetature for 16MnCr5 pinion steel. At 750℃, corresponding to the minimum of RA, it is grain boundary slidingthat controls its hot ductility rather than usual Deforming Induced Ferrite (DIF), which can only appear just below750℃ and slightly improve hot ductility. The volume fraction of ferrite is dependent on the strain and strain rate.Firstly a critical strain must be necessary for formation of DIF then with strain rate increasing, the volume fractionof DIF decreases but RA is elevated. In the γ phase region, hot ductility is seriously deteriorated because of grainboundary sliding promoted by oxidcs and sulfides at the grain boundary and recovered because of dynamic recrystal-lization at higher temperature; when strain rate increases, ductility is improved as there is insufficient time for cracksto propagate along the γ grain boundary as well as dynamically precipitating, and ductility trough becomes narrowerbecause the temperature for onset of dynamic recrystallization decreases. In addition, γ→α phase transformationintroduced by temperature drop before the tensile test encourages precipitation of AlN and impairs ductility.

Property and microstructure analysis of ZK60 alloy superplastic sheet

It was investigated that the superplastic mechanical properties of fine-grained ZK60 magnesium alloy sheets at the temperature range of 200-420 ℃ and strain rate range of 5.56 × 10-4 -5.56 ×10-2 s-1 by tensile tests.And the microstructure evolution during the superplastic deformation of ZK60 magnesium alloy was examined by metallurgical microscope and transmission electronic microscope (TEM).The results showed that fine-grained ZK60 magnesium alloy starts to exhibit superplasticity from 250 ℃ and the maximum elongation is about 1106% at 400 ℃ and 5.56 × 10-4 s-1.The strain rate sensitivity is significantly enhanced with the increase of temperature and with the decrease of strain rate.The predominate superplastic mechanism of ZK60 magnesium alloy is grain boundary slide (GBS) at the temperature range of 300-400 ℃.The grains of ZK60 alloy remain equaxial after superplastic deformation,and dynamic continuous recrystallization (DCRX) is an important softening mechanism and grain stability mechanism during the superplastic deformation of the alloy.The curved grain boundaries and crumpled bands at grain boundaries after deformation prove GBS generates during superplastic deformation of ZK60 magnesium alloy.

MICRO-EXPERIMENTAL STUDY OF LARGE PLASTIC DEFORMATION FOR HIGH-PURITY POLYCRYSTALLINE ALUMINUM

This paper aims at revealing various micro- deformation characteristics, such as crystalline slip and grain boundary slide, which are recorded under scanning electronic microscope for high-purity aluminum tensile specimen at room temperature. These experimental data provide us direct evidence for shear localization near the grain boundary network via multi- directional grain boundary slide. The nonuniform deformation induced in the grain interiors would have decisive effect on the plastic flow and failure of polycrystalline materials.

DAMAGE MECHANISMS OF INCONEL 738LC UNDERCREEP-FATIGUE LOADING CONDITIONS

In order to investigate creep-fatigue life characteristics of InConel 738LC, fatigue and creep-fatigue tests were carried out at 850 degree C. Significant life reduction to 30% to 50% of normal fatigue life, occurred when a strain hold period was introduced at the minimum compressive strain. Test results show that the degree of life reduction does not depend on the scale of hold time. To clarify the creep-fatigue damage mechanisms under creep-fatigue loading, in-situ SEM observations were carried out during the tests. In the tensile strain hold test, multi-site grain boundary cracking was observed from the beginning of the test. On the other hand, only a single main crack was observed in the compressive strain hold test. The crack propagates in an intergranular manner, and it is different from the features observed under cyclic fatigue conditions. Through the detailed observations, it was found that the grain boundary sliding damage occurs at most surface grain boundaries. (Edited author abstract) 5 Refs.

Grain size and temperature effect on the tensile behavior and deformation mechanisms of non-equiatomic Fe(41)Mn(25)Ni(24)Co8Cr2high entropy alloy

The effect of the grain size on the tensile properties and deformation mechanisms of a nonequiatomic Fe(41)Mn(25)Ni(24)Co8 Cr2 high-entropy alloy was studied in the temperature range between 298 and 1173 K by preparing the samples with three different grain sizes through severe plastic deformation and subsequent annealing:ultrafine(sub)grain size(≤0.5μm),8.1μm and 590.2μm.In the temperature between 298 and773 K,the material with the large grain size of 590.2μm exhibited the largest tensile ductility(57%-82%)due to its high strain hardening associated with mechanical twinning,but it exhibited the lowest strength due to its large grain size.The material with the ultrafine(sub)grain size exhibited the lowest tensile ductility(3%-7%)due to a greatly reduced strain hardening ability after severe plastic deformation,but it exhibited the highest strength due to the dislocation strengthening and grain refinement strengthening.At tensile testing at temperatures above 973 K,recrystallization occurred in the material with the ultrafine(sub)grains during the sample heating and holding stage,leading to the formation of fine and equiaxed grains with the sizes of 6.8-13.5μm.The deformation behavior of the Fe(41)Mn(25)Ni(24)Co8 Cr2 with different grain sizes in the high temperature range between 973 and 1173 K,where pseudosteady-state flow was attained in the stress-strain curves,could be explained by considering the simultaneous contribution of grain boundary sliding and dislocation-climb creep to total plastic flow.The activation energies for plastic flow for the materials with different grain sizes were similar as^199 kJ/mol.In predicting the deformation mechanism,it was important to consider the change in grain size by rapid grain growth or recrystallization during the sample heating and holding stage because grain boundary sliding is a grain-size-dependent deformation mechanism.The sample with the ultrafine(sub)grains exhibited the large tensile elongations of 30%-85%due to its high strain rate sensitivity,m(0.1-0.5)at temperatures of973-1173 K.The material with the large grain size of 590.2μm exhibited the very small elongations of0.2%-8%due to its small m values(0.1-0.2)and occurrence of brittle intergranular fracture at the early stage of plastic deformation.

纳米多晶316L奥氏体不锈钢高温蠕变行为的原子尺度研究

通过分子动力学模拟研究了纳米多晶316L奥氏体不锈钢在不同峰值应力下的高温蠕变变形行为和蠕变机制.数值模拟结果表明纳米多晶316L奥氏体不锈钢的高温蠕变变形是由位错、晶粒内部和晶界扩散以及晶界滑移之间的交互作用引起.在初始蠕变和稳态蠕变阶段,纳米多晶316L奥氏体不锈钢高温蠕变的主导性机制是晶粒内部和晶界的扩散以及晶界的滑移.在加速蠕变阶段,一些晶界被摧毁后,位错滑移成为纳米多晶316L奥氏体不锈钢的主要蠕变机制.这项工作旨在揭示纳米多晶316L奥氏体不锈钢的高温蠕变机理,为增强型抗蠕变316L奥氏体不锈钢的设计和制造提供指导.

Tensile and fracture properties of an Mg-RE-Zn alloy at elevated temperatures

Magnesium alloy EZ10 （Mg-RE-Zn） was deformed in tension at temperatures from 20 up to 520℃. A rapid decrease of the yield and tensile strength with temperature was observed at temperatures higher than 300 ℃. On the other hand, ductility of sam-ples rapidly increased in the same temperature range. Light microscopy and scanning electron microscopy was used to reveal the rea-son for these behaviours. Intermetallic particles in grain boundaries are responsible for excellent mechanical properties at lower tem-peratures. Diffusional processes occurring at temperatures higher than 300 ℃ significantly influenced the deformation mechanism as well as the fracture character.