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 a 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.     

焊后塑性变形对搅拌摩擦焊AZ31镁合金耐腐蚀的影响

设计了AZ31镁合金的母材(BM)、搅拌摩擦焊接头(FSW)、焊后横向拉伸5%(FSW+T)和焊后拉伸加退火(FSW+T+A)等4组试样。通过对比4组试样在质量分数为3.5%的NaCl溶液中的耐腐蚀性能,探索焊后塑性变形对接头耐腐蚀性能的影响。结果发现,AZ31镁合金的耐腐蚀性能在搅拌摩擦焊接后显著改善,但焊后横向拉伸将显著降低接头的耐腐蚀性能,而拉伸后退火处理使接头耐腐蚀性能增强。4组试样的腐蚀速率由大到小为:BM>FSW+T>FSW≈FSW+T+A。焊后横向拉伸在AZ31镁合金焊接接头中引入拉伸孪晶及位错滑移,两者对接头的耐腐蚀性能产生不利影响,但位错滑移所起的作用更显著。     

纳米压痕诱导单晶铜弹塑性变形分析与破坏机理研究

目的 从微观角度实现对微机电系统中微器件局部接触区域弹塑性演化过程和原子迁移演变规律的探寻.方法 运用经典分子动力学法,基于EAM和Morse混合势函数,对硬质金刚石探头与软质金属铜基底展开纳米压痕接触特性研究.结果 纳米压痕中位错环构型生成与演变有规可循,位错环在受载荷影响时,有着4个演变阶段,即位错环萌芽期→生长期→繁衍期→维持期.当载荷达到一定程度时,压痕中铜基质内密排六方结构的HCP容易与附近类似结构发生关联耦合效应,产生刃型位错和形成螺旋式位错结构,随后以脱落方式构成棱柱形位错结构,并向基底底部发射.另外,整个纳米压痕中,铜基质亚表面损伤最为严重,探头与基底接触区域两侧的位错环迁移处应力较集中.结论 纳米接触中铜基质内位错环出现与演变过程,是衡量局部接触塑性变形强弱程度的重要依据和非接触区域损伤程度的内在表现.此次研究结果对细观尺度接触变形行为有着深层次认识,也对纳尺度设计出优异摩擦学性能的微结构有着重要借鉴作用.     

铱单晶纳米线在不同应力状态下的变形行为

基于分子动力学模拟方法计算了铱单晶纳米线在室温下的拉伸和压缩变形行为。计算结果表明,铱单晶纳米线在拉伸和压缩时的力学性能均存在尺寸效应。铱单晶纳米线的拉伸和压缩变形行为具有非对称性,在拉伸条件下其伸长率为14%,而在压缩条件下表现出较好的塑性,在压缩过程中有堆垛层错产生,压缩应变达到0.499后模型还未发生断裂。     

单晶铜在不同温度下的纳米切削机理研究

目的 虽然纳米切削是21世纪超精密加工技术的重要发展方向之一,但现有的纳米切削机理仍不完善。因此,采用数值模拟方法,从晶体结构、力学和粒子运动等方面对纳米切削机理进行补全。方法 首先,基于分子动力学方法对纳米尺度下的单晶铜进行了拉伸模拟,总结其在不同温度下的韧脆性特征;其次,对纳米尺度下的单晶铜进行了切削模拟,系统性地研究了切削过程中晶体结构、切削力、应力应变分布,以及原子运动特征在不同材料韧脆性下的变化规律。结果 拉伸模拟结果表明,低温下单晶铜脆性特征显著,但仍具有一定的韧性。随着温度的升高,单晶铜脆性减弱,韧性增强。切削模拟结果表明,靠近工件自由面的材料沿主剪切方向发生持续的剪切滑移和周期性的长距离错动,形成多种晶体结构有序分布的块状切屑。靠近刀具的材料在推挤作用下由晶体结构变为非晶结构,之后持续流动形成切屑。随着切削温度的升高,块状切屑中的长距离错动频率提高,通过剪切形成的块状切屑尺寸减小,而通过推挤形成的流动状切屑厚度增加。结论 切屑的形成方式包括剪切和推挤2种类型。低温下,剪切切屑形成过程占据主导地位,切屑呈现明显的块状。随着温度升高,切屑形成机理从剪切向推挤转变。     

单晶铱取向有关的拉伸变形行为的原子尺度模拟

单晶铱与其它面心立方金属相比表现出反常的变形行为,其本征变形断裂机制仍存在争议。本文进行分子动力学模拟研究了单晶铱在1K下沿[100]、[110]和[111]取向的拉伸变形行为。研究结果表明:单晶铱在3个取向应力-应变曲线上的变形行为差异明显。由于变形机制不同,包括弹性模量、屈服强度、抗拉强度以及延伸率在内的力学性能在几个拉伸取向上或多或少存在差异。在拉伸载荷作用下,[100]取向单晶铱变形主要通过位错滑移还有少量空位聚集;[110]取向的塑性变形由堆垛层错引起;而[111]取向单晶铱断裂前产生的塑性变形量很少。     

Hot Deformation Stability of Extruded AZ61 Magnesium Alloy Using Different Instability Criteria

The hot deformation stability of extruded AZ61 magnesium alloy was investigated by means of hot compression tests at the temperature range of 250-400 ℃ and strain rate range of 0.001-1 s~(-1).The 3D instability maps considering the effect of strain were developed to delineate the regions of unstable flow on the basis of Jonas' s,Semiatin's,Prasad's,Murty's,Gegel's and Alexander's criteria.Since non-uniform deformation occurs due to the initial microstructure inhomogeneity,the friction,etc.,finite element simulations were performed to determine the position of the specimens which can mostly represent the preset deformation parameter.Detailed microstructural investigation on such position was carried out to examine the validity of the instability maps,and the results indicate that for extruded AZ61 magnesium alloy:(1) Jonas' s and Semiatin's criteria conservatively predict the instability regions;(2) Gegel's and Alexander's criteria inadequately predict the instability regions;(3) Prasad's and Murty's criteria provide more effective predictions of the instability regions than Jonas' s,Semiatin's,Gegel's and Alexander's criteria.     

AZ80镁合金热变形过程中的动态再结晶动力学和运动学模型

采用热模拟实验方法测试了AZ80镁合金材料的真实应力-应变曲线, 变形温度范围533K - 683K, 应变速率范围0.001 - 10 s-1, 变形程度为50%。动态再结晶的晶粒尺寸随着变形温度的升高和应变速率的降低而增大。确定了AZ80镁合金的热激活能, 确定了AZ80镁合金材料热变形时的本构方程。根据Sellars方程, 确定了AZ80镁合金的动力学模型, 其定义为描述发生动态再结晶体积分数与变形温度和应变速率的函数关系。确定了AZ80镁合金的运动学模型, 其定义为描述动态再结晶晶粒尺寸与Z函数之间数学关系. 动态再结晶晶粒尺寸的模型计算结果与实验结果相吻合,相对误差小于11.8%。确定了临界应变和稳态应变与Z函数之间数学关系。     

Atomistic simulation of the structural evolution in magnesium single crystal under c-axis tension

Molecular dynamics simulation is applied to investigate the microstructure evolution of magnesium single crystals under c-axis extension at different temperatures. At low temperatures, both {10 ˉ 12} and {10 ˉ 11} twins are observed. At elevated temperatures, {10 ˉ 11} twining decreases quickly with increasing temperature, while the amount of {10 ˉ 12} twins increases. The {10 ˉ 12} twin is found to be the main deformation mechanism under the c-axis tension in the magnesium single crystal. Meanwhile, shear ...     

Ti3Al单晶塑性变形行为的晶体塑性有限元模拟

基于晶体塑性理论,建立了滑移变形机制下Ti_3Al单晶晶体塑性的细观本构关系,并利用ABAQUS/UMAT用户子程序接口开发本构关系子程序,将其应用于不同取向Ti_3Al单晶的单向压缩模拟。分析不同取向时晶体滑移系的启动情况,以及滑移系启动与取向的相互关系。结果表明:基面滑移系(0001)1120,柱面滑移系{1010}1120和锥面滑移系{1121}1126均被激活,但由于滑移系之间临界剪切应力和施密特因子的不同,各滑移系的启动情况存在显著差异。基面和锥面滑移系的临界剪切应力较大,仅在施密特因子较大时启动;特别是锥面滑移系,其临界剪切应力最大,因而该滑移系只有在压缩轴接近[0001]方向时才被激活。柱面滑移系的启动较为容易,对塑性变形的贡献也最大。同时与实验结果相比较,模拟结果基本与之吻合。     

AZ31镁合金散热器等温挤压成形金属流动规律研究

根据等温压缩实验所得AZ31变形镁合金应力-应变数据,通过回归法得出材料温成形数学模型,应用刚塑性有限元法模拟AZ31变形镁合金散热器等温挤压成形,着重探讨AZ31变形镁合金等温挤压成形过程中,变形力及金属流动规律.根据模拟得到的应力场、应变场、速度场及加载变化等,也可预测变形时产生的缺陷,为该类零件等温挤压成形工艺提供科学的依据.     

单晶Al3Ti拉伸与剪切变形的分子动力学模拟

采用分子动力学方法研究单晶Al3Ti模型的拉伸和剪切力学性能。模拟Al3Ti在常温、恒定应变速率下的拉伸和剪切变形过程,讨论了温度和应变速率对体系拉伸和剪切性能的影响。结果表明,Al3Ti室温下很脆,弹性变形阶段结束后在短时间内体系产生的孔洞和位错迅速发展导致材料破坏。温度升高会导致Al3Ti的抗拉强度、杨氏模量、剪切强度和切变模量降低;应变速率增大能提高材料的拉伸和剪切强度,但不影响杨氏模量和切变模量大小。     

Analysis on Dissociation of Pyramidal I Dislocation in Magnesium by Generalized-Stacking-Fault Energy

The generalized-stacking-fault energies are calculated to illustrate the dissociation of〈c+a〉dislocation on pyramidal I plane in magnesium.The c surfaces of f10 "11g plane and its adjacent planes f30" 34 g and f30 "32g are presented using Liu embedded-atom-method potential method,and one possible dissociation path of 1=3h11" 23 i dislocation on f10 "11g plane with minimum energy is predicted.Meanwhile,another two reasonable dissociation paths of 1=3h11" 23 i dislocation successively on f30 "34g and f30" 32 g planes are also proposed.Moreover,based on molecular dynamics simulations of magnesium single crystals under c-axis compression,the possible slip path is further examined and discussed.     

Orientation-Dependent Mechanical Responses and Plastic Deformation Mechanisms of FeMnCoCrNi High-entropy Alloy:A Molecular Dynamics Study

Mechanical properties of high-entropy alloys (HEAs) with the face-centered cubic (fcc) structure strongly depend on their initial grain orientations.However,the orientation-dependent mechanical responses and the underlying plastic flow mecha-nisms of such alloys are not yet well understood.Here,deformation of the equiatomic FeMnCoCrNi HEA with various initial orientations under uniaxial tensile testing has been studied by using atomistic simulations,showing the results consistent with the recent experiments on fcc HEAs.The quantitative analysis of the activated deformation modes shows that the ini-tiation of stacking faults is the main plastic deformation mechanism for the crystals initially oriented with[001],[111],and[112],and the total dislocation densities in these crystals are higher than that with the[110]and[123]orientations.Stacking faults,twinning,and hcp-martensitic transformation jointly promote the plastic deformation of the[110]orientation,and twinning in this crystal is more significant than that with other orientations.Deformation in the crystal oriented with[123]is dominated by the hcp-martensite transformation.Comparison of the mechanical behaviors in the FeMnCoCrNi alloy and the conventional materials,i.e.Cu and Fe50Ni50,has shown that dislocation slip tends to be activated more readily in the HEA.This is attributed to the larger lattice distortion in the HEA than the low-entropy materials,leading to the lower criti-cal stress for dislocation nucleation and elastic-plastic transition in the former.In addition,the FeMnCoCrNi HEA with the larger lattice distortion leads to an enhanced capacity of storing dislocations.However,for the[001]-oriented HEA in which dislocation slip and stacking fault are the dominant deformation mechanisms,the limited deformation modes activated are insufficient to improve the work hardening ability of the material.     

YL112压铸铝合金的半固态压缩变形特性

半固态等温热处理作为一种有发展前途的非枝晶组织坯料制备方法,研究其制备出的非枝晶组织坯料的变形特性具有重要的理论意义和实际应用价值.基于半固态等温热处理,以570 ℃×120 min条件下制备出的YL112压铸铝合金非枝晶组织坯料为研究对象,对其在不同变形速率和变形温度下的半固态压缩变形特性进行了研究.研究结果表明:在固液相区,随着变形温度的升高,变形抗力降低;随着变形速率的增加,变形抗力增加;随着变形程度增加,变形抗力先增加然后降低.     

纳米尺度单晶锗各向异性摩擦磨损性能试验研究

利用纳米压痕仪和原子力显微镜,分别对单晶锗Ge(100)、Ge(110)、Ge(111)3种晶向的表面进行纳米尺度下的摩擦磨损试验。在较大载荷的条件下,3种不同晶面取向的单晶锗磨损情况均呈现沟槽形式,沟槽两侧出现明显的碎屑堆积现象。在划痕试验过程中,单晶锗的磨损性能受晶面取向影响较小;单晶锗的摩擦力随着滑动速度的增加而增加。而且,随着滑动速度的增大,晶体表面出现严重的磨损-沟槽损伤,沟槽两侧碎屑堆积的体积也越来越大,沟槽的深度也逐渐增大;单晶锗在较低载荷下,摩擦力基本保持稳定,但随着载荷的增大,单晶锗的摩擦力呈非线性增长,载荷增大一定值时,晶体表面发生明显的由塑性变形向脆性破坏转变的脆塑转变过渡过程,导致单晶锗表面发生脆性剥离,形成沟槽两侧碎屑堆积。     

纳米级铱单晶不同温度拉伸的分子动力学解析

针对面心立方金属铱单晶独特的韧脆变形特征,采用分子动力学方法研究了纳观尺度下的铱单晶在不同温度下的拉伸变形行为。通过分析不同温度拉伸过程中的应力应变关系,势能变化和原子构型图,认为随着温度的上升,纳米级铱单晶沿[100]晶向的弹性模量逐渐下降,抗拉强度也逐渐降低。温度为300 K时拉伸变形过程中在晶体内仅有少量空位和位错产生,600和800 K拉伸变形过程中在晶体内有滑移,位错和空位产生。     

Recent Research on the Deformation Behavior of Particle Reinforced Magnesium Matrix Composite: A Review

Particle reinforced magnesium matrix composite(PMMC) possesses the merits of high specific strength, high specific modulus, better dimensional stability, good wear resistance and lower production cost, which is thought as a promising material in the field of aerospace, automobile, electronic communication, etc. To eliminate the casting defect, the PMMC is usually experienced hot deformation process. The present paper mainly focuses on the deformation behavior of PMMCs. First, the development of PMMCs based on particle size is introduced. Then, the hot deformation technology and deformation mechanism of PMMCs at elevated temperature are given and analyzed, respectively. After reviewing the dynamic recrystallization and texture of PMMCs, its future development is suggested based on the current research progress.     

一种无铼二代镍基单晶高温合金蠕变机制研究

利用层层静电自组装技术将聚乙烯亚胺(PEI-Ag+)、PdCl42-交替沉积在基底上,然后用硼氢化钠还原,构筑了含银/钯复合纳米粒子的PEI-Ag/Pd纳米复合薄膜.通过扫描电子显微镜(FESEM),X射线光电子能谱(XPS)和循环伏安(CV)等手段对复合膜的成分、微结构和性质进行了测试分析.膜上生成了不规则和立方体状的银/钯纳米复合物,导致膜表面有一定的粗糙度.结果表明,双金属{PEI-Ag/Pd}n复合膜比单金属{PEI/Pd}n或{PEI-Ag/PSS}n膜对多巴胺的氧化有更好的电催化活性.     

一种无铼二代镍基单晶高温合金蠕变机制研究

通过对一种镍基单晶合金中温高应力条件下的蠕变曲线的测定和微观组织及断裂后合金中位错组态的衍衬分析,研究中温高应力条件下单晶合金的组织演化及变形特征。结果表明:在中温高应力条件下,该合金的蠕变激活能Q为(462±20)kJ/mol,表观应力指数na=4.34。表明在试验温度和应力范围内,合金具有较好的蠕变抗力。位错组态衍衬分析表明,蠕变期间切入相内的<110>超位错既可在{100}立方体滑移系中运动,也可在{111}八面体滑移系中运动;位错在运动中相遇发生位错反应,形成的超位错可交滑移至{100}立方体滑移面。位于2个不同{100}六面体滑移面的位错在运动中相遇,可发生位错反应,生成的位错可在{111}八面体滑移系中运动。