Molecular dynamics simulation on plasticity deformation mechanism and failure near void for magnesium alloy

The plastic deformation processes of magnesium alloys near a void at atomic scale level were examined through molecular dynamics(MD)simulation.The modified embedded atom method(MEAM)potentials were employed to characterize the interaction between atoms of the magnesium alloy specimen with only a void.The void growth and crystal failure processes for hexagonal close-packed(hcp)structure were observed.The calculating results reveal that the deformation mechanism near a void in magnesium alloy is a complex process.The passivation around the void,dislocation emission,and coalescence of the void and micro-cavities lead to rapid void growth.

Molecular Dynamics Simulations of the Orientation Effect on the Initial Plastic Deformation of Magnesium Single Crystals

Molecular dynamics simulation is employed to study the tension and compression deformation behaviors of magnesium single crystals with different orientations.The angle between the loading axis and the basal direction ranges from 0° to 90°.The simulation results show that the initial defects usually nucleate at free surfaces,but the initial plastic deformation and the subsequent microstructural evolutions are various due to different loading directions.The tension simulations exhibit the deformation mechanisms of twinning,slip,crystallographic reorientation and basal/prismatic transformation.The twinning,crystallographic reorientation and basal/prismatic transformation can only appear in the crystal model loaded along or near the a-axis or c-axis.For the compression simulations,the basal,prismatic and pyramidal slips are responsible for the initial plasticity,and no twinning is observed.Moreover,the plastic deformation models affect the yield strengths for the samples with different orientations.The maximum yield stresses for the samples loaded along the c-axis or a-axis are much higher than those loaded in other directions.

Molecular Dynamics Simulation of Grain Refinement in a Polycrystalline Material under Severe Compressive Deformation

Grain refinement in a polycrystalline material resulting from severe compressive deformation was simulated using molecular dynamics. A simplified model with four square grains surrounded by periodic boundaries was prepared, and compressive deformation was imposed by shortening the length in the y direction. The model first deformed elastically, and the compressive stress increased monotonically. Inelastic deformation was then initiated, and the stress decreased drastically. At that moment, dislocation or slip was initiated at the grain boundaries or triple junction and then spread within the grains. New grain boundaries were then generated in some of the grains, and sub-grains appeared. Finally, a microstructure with refined grains was obtained. This process was simulated using two types of grain arrangements and three different combinations of crystal orientations. Grain refinement generally proceeded in a similar fashion in each scenario, whereas the detailed inelastic deformation and grain refinement behavior depended on the initial microstructure.

Temperature effect on nanotwinned Ni under nanoindentation using molecular dynamic simulation

Temperature effect on atomic deformation of nanotwinned Ni (nt-Ni) under localized nanoindentation is investigated in comparison with nanocrystalline Ni (nc-Ni) through molecular simulation.The nt-Ni exhibits enhanced critical load and hardness compared to nc-Ni,where perfect,stair-rod and Shockley dislocations are activated at (111),(111) and (111) slip planes in nt-Ni compared to only SSockley dislocation nucleation at (111) and (111) slip planes of nc-Ni.The nt-Ni exhibits a less significant indentation size effect in comparison with nc-Ni due to the dislocation slips hindrance of the twin boundary.The atomic deformation associated with the indentation size effect is investigated during dislocation transmission.Different from the decreasing partial slips parallel to the indenter surface in nc-Ni with increasing temperature,the temperaturedependent atomic deformation of nt-Ni is closely related to the twin boundary:from the partial slips parallel to the twin boundary (~10 K),to increased confined layer slips and decreased twin migration(300 K–600 K),to decreased confined layer slips and increased dislocation interaction of dislocation pinning and dissociation (900 K–1200 K).Dislocation density and atomic structure types through quantitative analysis are implemented to further reveal the above-mentioned dislocation motion and atomic structure alteration.Our study is helpful for understanding the temperature-dependent plasticity of twin boundary in nanotwinned materials.

Anelasticity to plasticity transition in a model two-dimensional amorphous solid

Anelasticity, as an intrinsic property of amorphous solids, plays a significant role in understanding their relaxation and deformation mechanism. However, due to the lack of long-range order in amorphous solids, the structural origin of anelasticity and its distinction from plasticity remain elusive. In this work, using frozen matrix method, we study the transition from anelasticity to plasticity in a two-dimensional model glass. Three distinct mechanical behaviors, namely,elasticity, anelasticity, and plasticity, are identified with control parameters in the amorphous solid. Through the study of finite size effects on these mechanical behaviors, it is revealed that anelasticity can be distinguished from plasticity.Anelasticity serves as an intrinsic bridge connecting the elasticity and plasticity of amorphous solids. Additionally, it is observed that anelastic events are localized, while plastic events are subextensive. The transition from anelasticity to plasticity is found to resemble the entanglement of long-range interactions between element excitations. This study sheds light on the fundamental nature of anelasticity as a key property of element excitations in amorphous solids.

Anisotropic plasticity of nanocrystalline Ti:A molecular dynamics simulation

Using molecular dynamics simulations,the plastic deformation behavior of nanocrytalline Ti has been investigated under tension and compression normal to the{0001},{1010},and{1210}planes.The results indicate that the plastic deformation strongly depends on crystal orientation and loading directions.Under tension normal to basal plane,the deformation mechanism is mainly the grain reorientation and the subsequent deformation twinning.Under compression,the transformation of hexagonal-close packed(HCP)-Ti to face-centered cubic(FCC)-Ti dominates the deformation.When loading is normal to the prismatic planes(both{1010}and{1210}),the deformation mechanism is primarily the phase transformation among HCP,body-centered cubic(BCC),and FCC structures,regardless of loading mode.The orientation relations(OR)of{0001}HCP||{111}FCC and<1210>HCP||<110>FCC,and{1010}HCP||{110}FCC and<0001>HCP||<010>FCC between the HCP and FCC phases have been observed in the present work.For the transformation of HCP→BCC→HCP,the OR is{0001}α1||{110}β||{1010}α2(HCP phase before the critical strain is defined as α1-Ti,BCC phase is defined as β-Ti,and the HCP phase after the critical strain is defined as α2-Ti).Energy evolution during the various loading processes further shows the plastic anisotropy of nanocrystalline Ti is determined by the stacking order of the atoms.The results in the present work will promote the in-depth study of the plastic deformation mechanism of HCP materials.

Indenter Size Effect on Stress Relaxation Behaviors of Surface-modified Silicon:A Molecular Dynamics Study

Long-lasting constant loading commonly exists in silicon-based microelectronic contact and can lead to the appearance of plastic deformation.Stress relaxation behaviors of monocrystalline silicon coated with amorphous SiO_(2)film during nanoindentation are probed using molecular dynamics simulation by varying the indenter’s size.The results show that the indentation force(stress)declines sharply at the initial and decreases almost linearly toward the end of holding for tested samples.The amount of stress relaxation of SiO_(2)/Si samples indented with different indenters during holding increases with growing indenter size,and the corresponding plastic deformation characteristics are carefully analyzed.The deformation mechanism for confined amorphous SiO_(2)film is depicted based on the amorphous plasticity theories,revealing that the more activated shear transformation zones(STZs)and free volume within indented SiO_(2)film promote stress relaxation.The phase transformation takes place to monocrystalline silicon,the generated atoms of Si-II and bct-5 phases within monocrystalline silicon substrate during holding are much higher than those for smaller indenter.

孪晶取向对Au纳米线变形机制影响的模拟

本文采用分子动力学方法,阐明了Au纳米线各力学特性参数随孪晶取向角度的变化规律,以及不同变形机制发生的孪晶取向角度范围。结果表明:在较小孪晶取向角度下(0°<θ<90°),纳米线具有较高的强度,不全位错与孪晶面相互作用引起的应变局域化主导了其塑性变形;在中等孪晶取向角度下(18°≤θ≤75°),退孪生主导了Au纳米线塑性变形,尤其是30°≤θ≤60°时,完全退孪生引发的应变硬化,使得纳米线具有较高塑性;在较大孪晶取向角度下(75°<θ≤90°),纳米线具有较高的强度,但塑性较差,不全位错连续穿越孪晶界引起的位错链主导了Au纳米线塑性变形。

温度对金纳米线拉伸塑性变形影响的分子动力学模拟研究

小尺寸金(Au)纳米线因具有优异的力学性能而受到广泛关注。而之前的相关研究大多是在常温下进行,不同低温下的研究较少。研究金纳米线在低温下塑性变形行为,能够为其低温下的应用提供理论依据。本文采用分子动力学模拟的方法,研究了不同温度下菱形Au纳米线的力学行为,发现Au纳米线的强度和塑性变形能力会随温度降低而增大。另外,Au纳米线的塑性变形机制受温度影响会发生转变。在175~350 K时,其塑性变形机制主要为Shockley偏位错发射导致纳米线形成大量层错,之后出现切变和过早颈缩。在50~140 K时则转变为以偏位错发射为主,首先形成平滑的孪晶界并发生迁移,之后形成交叉的孪晶界并继续迁移,过程中生成大量孪晶,最终产生40.16%的均匀变形。

Investigation on the Velocity-Dependent Adhesion Hysteresis via Molecular Dynamics Simulation

Adhesion plays an important role in miniaturized devices and technologies,which depends not only on indentation depth but also on the history of contact making and breaking,giving rise to adhesion hysteresis.In the present work,adhesion hysteresis has been investigated via molecular dynamics simulations on approaching and retracting a rigid tip to and from a substrate.The results show that hysteresis in the force-displacement curve that depends on approaching and retraction velocities arises under both elastic and plastic deformation.The underlying mechanisms have been analyzed.The implications of the results in friction have been discussed briefly.

Orientation and strain rate dependent tensile behavior of single crystal titanium nanowires by molecular dynamics simulations

Molecular dynamics simulation was employed to study the tensile behavior of single crystal titanium nanowires（NWs）with[112^-0],[1^-100] and[0001]orientations at different strain rates from 10^8s^-1 to 10^11s^-1.When strain rates are above 10^10s^-1,the state transformation from HCP structure to amorphous state leads to super plasticity of Ti NWs,which is similar to FCC NWs.When strain rates are below 10^10s^-1,deformation mechanisms of Ti NWs show strong dependence on orientation.For [112^-0] orientated NW.{101^-1} compression twins（CTs）and the frequently activated transformation between CTs and deformation faults lead to higher plasticity than the other two orientated NWs.Besides,tensile deformation process along [112^-0] orientation is insensitive to strain rate.For [1^-100] orientated NW,prismaticslip is the main deformation mode at 10^8s^-1.As the strain rate increases,more types of dislocations are activated during plastic deformation process.For[0001]orientated NW,{101^-2} extension twinning is the main deformation mechanism,inducing the yield stress of [0001] orientated NW,which has the highest strain rate sensitivity.The number of initial nucleated twins increases while the saturation twin volume fraction decreases nonlinearly with increasing strain rate.

Deformation Twinning in Nanocrystalline Ni during Cryogenic Rolling

Deformation twinning is evidenced by transmission electron microscopy examinations in electrodeposited nanocrystalline （nc） Ni with mean grain size 25nm upon cryogenic rolling. Two twinning mechanisms are confirmed to operate in nc grains, i.e. heterogeneous formation via emission of partial dislocations from the grain boundary and homogeneous nucleation through dynamic overlapping of stacking faults, with the former being determined as the most proficient. Deformation twinning in nc Ni may be well interpreted in terms of molecular dynamics simulation based on generalized planar fault energy curves.

Atomistic Simulation of the Orientation-dependent Plastic Deformation Mechanisms of Iron Nanopillars

Tensile tests were performed on iron nanopillars oriented along [001] and [110] directions at a constant temperature of 300 K through molecular dynamics simulations with an embedded-atom interatomic potential for iron. The nanopillars were stretched until yielding to investigate the onset of their plastic deformation behaviors. Yielding was found to occur through two different mechanisms for [001] and [110] tensions. In the former case, plastic deformation is initiated by dislocation nucleation at the edges of the nanopillar, whereas in the latter case by phase transformation inside the nanopillar. The details during the onset of plastic deformation under the two different orientations were analyzed. The varying mechanisms during plastic deformation initiation are bound to influence the mechanical behavior of such nanoscale materials, especially those strongly textured.

Mg-6.3Zn-0.7Zr-0.9Y-0.3Nd镁合金的高温塑性变形行为的热压缩模拟

采用Gleeble-1500热压缩模拟试验机进行压缩实验,研究ZK60(0.9Y+0.3Nd)镁合金在变形温度623～773K、应变速率0.001～1s-1的范围内的变形行为,计算应力指数和变形激活能,并采用Zener-Hollomon参数法构建合金高温塑性变形的本构关系。结果表明:在实验变形条件下,合金的真应力—真应变曲线为动态再结晶型;在实验温度范围内,应力指数随着变形温度的升高而增大,变形激活能随着变形温度和应变速率的增加而增大。对比ZK60合金,ZK60(0.9Y+0.3Nd)合金的变形激活能提高38%。

ZK60及ZK60(0.9Y)镁合金高温变形行为的热模拟研究

采用Gleeble-1500热模拟试验机进行压缩试验,研究ZK60和ZK60(0.9Y)镁合金在变形温度为473～723K、应变速率为0.001～1s-1范围内的变形行为,计算了应力指数和变形激活能,并采用Zener-Hollomon参数法构建了合金高温塑性变形的本构关系。结果表明:在试验变形条件范围内,合金的真应力-真应变曲线为动态再结晶型;在573～723K范围内,应力指数随着变形温度的升高而增加,变形激活能随着变形温度和应变速率的改变而变化。对比ZK60合金,ZK60(0.9Y)合金的变形激活能降低了30%,且材料常数n和A值均降低。

Mg-5.3Zn-0.8Zr镁合金高温变形行为的热模拟研究

采用Gleeble-1500热模拟试验机进行压缩实验,研究Mg-5.3Zn-0.8Zr镁合金在变形温度为473～723K、应变速率为0.01～1.00s-1的变形行为。分析合金流变应力与应变速率、变形温度之间的关系,计算高温(573～723K)下合金变形时的应力指数和变形激活能,并采用Zener-Hollomon参数法构建该合金高温塑性变形的本构关系。研究结果表明:在实验变形条件范围内,合金的真应力-真应变曲线为动态再结晶型;在573～723K,应力指数随着变形温度升高而增加,而且增加的幅度逐渐增大,变形激活能随着变形温度和应变速率的改变而发生变化。

Mg-5.6Zn-0.7Zr-0.8Nd合金高温塑性变形的热/力模拟研究

采用Gleebe-1500热/力模拟机研究了 Mg-5.6Zn-0.7Zr-0.8Nd合金在应变速率为0.1,0.01和0.002s^(-1)、变形温度为373—673K、最大变形程度60％条件下的高温塑性变形行为。分析了合金流变应力与应变速率、变形温度之间的关系,计算了高温变形时变形激活能和应力指数,并观察了合金变形过程中显微组织变化情况。结果表明:Mg-5.6Zn—0.7Zr-0.8Nd合金在热变形过程中不同温度下流变应力呈现不同形式,分析可知加工硬化、动态回复和动态再结晶在不同温度和不同应变速率下各自起到了重要的作用,合金变形激活能随应变速率增加而升高.在473K温度以上变形,合金发生明显动态再结晶且动态再结晶晶粒非常细小,晶粒尺寸为 5—10μm。

镁单晶沿不同取向压缩的变形机制分子动力学研究

基于分子动力学理论,建立沿C轴以及<1010 >(垂直C轴)方向进行单轴压缩的模型,结合两种模型的应力-应变曲线,分析镁单晶沿不同取向压缩的微观变形机制。结果表明,沿C轴压缩时模型的压缩弹性模量较大,说明该取向难变形。且该模型先发生基面不全位错滑移(柏氏矢量b1^→=1/3<1010 >)以及锥面位错滑移(柏氏矢量b2=1/6<0223 >),其次在位错畸变区形核产生{1011}孪晶。此外,在晶体内部观察到两种不同类型的{1011}孪晶变体。沿垂直C轴方向压缩过程中,首先会形成大量的紊乱点,为位错以及孪晶的产生提供形核点。进一步加载时,会出现{1012}孪生过程,且{1012}孪晶迅速吞噬基体,模型变为沿C轴方向压缩变形,最后在位错堆积的畸变区形核生成{1011}二次孪晶。

刚性粒子流法模拟自由锻造工艺的探索

基于网格的数值计算方法在处理自由锻造、冲压成形、高速碰撞、裂纹动态扩展等大塑性变形问题时,往往会因网格畸变和网格重构困难使得计算精度丢失甚至无法计算。针对该问题,基于连续介质力学理论和分子动力学基本原理,采用一种求解金属大塑性变形的数值模拟方法——刚性粒子流法,建立了刚性粒子流法的数学模型并探讨了初始粒子配置、粒子搜索算法、数值积分和边界条件等关键数值技术。将该方法应用到核电饼类锻件高温镦粗和大型核电封头终锻成形两个数值算例中,计算结果表明:模拟大塑性变形问题时,刚性粒子流法与实验结果能够较好地吻合,有效地解释了大型饼类锻件夹层裂纹和封头锻件层状撕裂的形成机制。研究结果为进一步采用该方法模拟变形更加复杂的自由锻造工艺提供了参考。

孪晶结构Cu纳米线塑性变形机制的分子动力学模拟研究

孪晶结构的金属纳米线因具有优异的力学性能而受到广泛关注,然而之前的研究对象都为孪晶界垂直于纵轴的孪晶结构纳米线.本文采用分子动力学模拟的方法,研究了孪晶界平行于纵轴方向的Cu纳米线的力学行为.结果发现纳米线的屈服应力随孪晶厚度的减小而不断增大,表明孪晶厚度减小对孪晶结构的Cu纳米线的强度具有显著强化效应.此外,孪晶结构Cu纳米线的塑性变形机制受孪晶厚度的影响.当孪晶厚度>3个原子层时,它们的塑性变形由Shockley偏位错与孪晶界相交主导;当孪晶厚度减小到3个原子层时,Cu纳米线的塑性变形通过晶格畸变和原子重排导致新的晶粒形成来实现.