钽、铁、钨三种体心立方金属裂纹的多尺度模拟及韧脆性分析

采用多尺度准连续介质法计算模拟了钽、铁、钨三种体心立方(body-centered-cubic,BCC)金属的I型裂纹断裂过程.观察了加载过程中裂纹尖端区域原子的位错、孪晶等塑性变形现象,以及裂纹的脆性开裂和扩展现象.模拟结果表明,不同BCC金属材料的裂纹在相同的加载下有不同韧脆性表现.在一定变形范围内,钽裂纹主要表现出的是裂纹尖端附近区域原子的位错和形变孪晶等塑性变形现象;铁裂纹在变形过程中先后表现出了塑性变形和脆性扩展现象,与实验结果吻合;钨裂纹在变形过程中则主要变现出脆性扩展现象.计算了三种金属材料的广义层错能曲线,得到其不稳定层错能;并分别用两种不同的韧脆性准则,对三种材料断裂模型的韧脆性行为进行分析,计算分析结果与模拟结果一致,从而验证了模拟结果的正确性.     

晶体相场方法模拟纳米孪晶结构

应用双模晶体相场模型,模拟共格纳米孪晶结构.结果表明:球状晶粒生长成的共格孪晶片层,在共格面上的原子排列有变形,容易出现位错;条状晶粒凝固生长成的共格孪晶界,比用球状晶粒长大生成的共格孪晶界的原子排列整齐.应用晶体相场模型,可以精确计算纳米孪晶带的厚度,设计和控制带内的原子层数,实现人工操纵纳米共格孪晶片层结构,指导实验研究纳米孪晶结构及其与性能的关系.     

晶体相场法研究应力状态及晶体取向对微裂纹尖端扩展行为的影响

采用晶体相场模拟研究了单向拉伸作用下初始应力状态、晶体取向角度对单晶材料内部微裂纹尖端扩展行为的影响, 以(111)晶面上的预制中心裂纹为研究对象探讨了微裂纹尖端扩展行为的纳观机理, 结果表明: 微裂纹的扩展行为主要发生在<011>(111)滑移系上, 扩展行为与扩展方向与材料所处的初始应力状态及晶体取向紧密相关. 预拉伸应力状态将首先诱发微裂纹尖端生成滑移位错, 进而导致晶面解理而实现微裂纹尖端沿[011]晶向扩展, 扩展到一定程度后由于位错塞积, 应力集中, 使裂纹扩展方向沿另一滑移方向[101], 并形成锯齿形边缘; 预剪切应力状态下, 微裂纹尖端首先在[101]晶向解理扩展, 并诱发位错产生, 形成空洞聚集型长大的二次裂纹, 形成了明显的剪切带; 预偏变形状态下微裂纹尖端则直接以晶面解理形式[101]在上进行扩展, 直至断裂失效; 微裂纹尖端扩展行为随晶体取向不同而不同, 较小的取向角度会在裂纹尖端形成滑移位错, 诱发空位而形成二次裂纹, 而较大的取向角下的裂纹尖端则以直接解理扩展为主, 扩展方向与拉伸方向几近垂直.     

α-Fe纯I型裂纹扩展中位错形成过程的三维分子动力学模拟

通过分子动力学方法模拟了三维 α-Fe I型裂纹的单向拉伸实验中的裂纹扩展过程。研究了在不同温度下裂纹扩展时位错的形成过程和断裂机理。计算结果表明,裂纹扩展过程是位错不断发射的过程。 裂纹尖端附近先形成无位错区和层错,当裂纹处应力增加到KI=0.566 MPam1/2时,裂纹尖端附近的某一层原子会逐渐分叉形成两层原子,分层后的原子层继续分离形成位错;当应力KI 达到0.669MPam1/2时第一个位错发射。随着温度的升高,临界应力强度因子逐渐降低,同时位错发射也相应地加快。     

α-Fe纯I型裂纹扩展中位错形成过程的三维分子动力学模拟

通过分子动力学方法模拟了三维 α-Fe I型裂纹的单向拉伸实验中的裂纹扩展过程。研究了在不同温度下裂纹扩展时位错的形成过程和断裂机理。计算结果表明,裂纹扩展过程是位错不断发射的过程。 裂纹尖端附近先形成无位错区和层错,当裂纹处应力增加到KI=0.566 MPam1/2时,裂纹尖端附近的某一层原子会逐渐分叉形成两层原子,分层后的原子层继续分离形成位错;当应力KI 达到0.669MPam1/2时第一个位错发射。随着温度的升高,临界应力强度因子逐渐降低,同时位错发射也相应地加快。     

堆垛层错和温度对纳米多晶镁变形机理的影响

本文采用分子动力学模拟方法研究了在拉伸载荷下,堆垛层错和温度对纳米多晶镁力学性能的影响,在模拟中,采用嵌入原子势描述镁原子之间的相互作用．计算结果表明：在纳米晶粒中引入堆垛层错能明显增强纳米多晶镁的屈服应力,但堆垛层错对纳米多晶镁杨氏模量的影响很小;温度为300．0K时,孪晶在晶粒交界附近形成,孪晶随着拉伸应变的增加而逐渐生长．当拉伸应变达到0．087时,一种基面与X—Y面成大约35°角且内部包含堆垛层错的新晶粒成核并快速增长．也就是说,孪晶和新晶粒的形成和繁殖是含堆垛层错的纳米多晶镁在300．0K温度下的主要变形机理．模拟结果也显示,当温度为10．0K时,位错的成核和滑移是含堆垛层错的纳米多晶镁拉伸变形的主要形式．     

晶界对纳米多晶铝中冲击波阵面结构影响的分子动力学研究

用分子动力学方法研究了纳米多晶铝在冲击加载下的冲击波阵面结构及塑性变形机理.模拟研究结果表明:在弹性先驱波之后,是晶界间滑移和变形主导了前期的塑性变形机理;然后是不全位错在界面上成核和向晶粒内传播,然后在晶粒内形成堆垛层错、孪晶和全位错的过程主导了后期的塑性变形机理.冲击波阵面扫过之后留下的结构特征是堆垛层错和孪晶留在晶粒内,大部分全位错则湮灭于对面晶界.这个由两阶段塑性变形过程导致的时序性塑性波阵面结构是过去未见报道过的. 关键词： 晶界 塑性变形 冲击波阵面 分子动力学     

α-Fe裂纹的分子动力学研究

通过分子动力学方法，模拟了α-Fe裂纹的单轴拉伸实验中的形变过程.研究了不同晶体取向裂纹的形变特点和断裂机理，观察到各种形变现象，如位错形核和发射，位错运动，堆垛层错或孪晶的形成，纳米空洞的形成与连接等.计算结果表明，裂纹扩展是塑性过程和弹性过程相结合的过程，其中塑性过程表现为由裂尖发射的位错导致的原子切变行为，而弹性过程的发生则是由无位错区中的原子断键所导致.同时还研究了α-Fe裂纹的形变特点和断裂机理与温度场和应力场的依赖关系.     

晶粒尺寸对纳米多晶铁变形机制影响的模拟研究

利用分子动力学模拟方法研究了拉伸荷载作用下晶粒尺寸对纳米多晶铁变形机制的影响.研究结果表明杨氏模量随着晶粒尺寸的减小而减小.当晶粒尺寸小于15.50 nm时,纳米多晶铁的峰值应力和晶粒尺寸之间遵循反常的Hall-Petch关系,此时晶粒旋转和晶界迁移是其塑性变形的主要变形机制;随着晶粒尺寸的增大,变形孪晶和位错滑移在其塑性变形过程中逐渐占据主导地位.裂纹的形成是导致大晶粒尺寸模型力学性能降低的主要因素.纳米多晶铁在塑性变形中会出现孪晶界的迁移和退孪晶现象.此外还研究了温度对纳米多晶铁变形机制的影响.     

体心立方Ta的广义面错能及在Ⅱ型裂纹尖端初始塑性研究中的应用

基于嵌入原子势考察体心立方(bcc)金属Ta的广义层错能和广义孪晶能并获得广义层错能和广义孪晶能曲线. 研究表明,bcc Ta的广义层错能曲线与面心立方金属的广义层错能曲线有明显差异,Ta的广义层错能曲线不存在明显的能量极小值,位错主要以全位错的形式发射. 不同原子厚度的广义孪晶能曲线表明4个原子层的孪晶能曲线开始出现亚稳定的能量极小值,5个原子层的孪晶能曲线出现稳定的能量极小值. 为进一步验证广义层错能和广义孪晶能曲线揭示的塑性变形机理,采用准连续介质力学多尺度方法研究Ⅱ型裂纹尖端的初始塑性变形过程. 关键词： 广义层错能 广义孪晶能 体心立方金属钽 Ⅱ型裂纹     

Crack-stimulated generation of deformation twins in nanocrystalline metals and ceramics

A theoretical model is proposed that describes the generation of deformation twins near brittle cracks of mixed I and II modes in nanocrystalline metals and ceramics. In the framework of the model, a deformation twin nucleates through stress-driven emission of twinning dislocations from a grain boundary distant from the crack tip. The emission is driven by both the external stress concentrated by the pre-existent crack and the stress field of a neighbouring extrinsic grain boundary dislocation. The ranges of the key parameters, the external shear stress, τ, and the crack length, L, are calculated within which the deformation-twin formation near pre-existent cracks is energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model account for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation is treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.     

Generation of glide split-dislocation half-loops by grain boundaries in nanocrystalline Al

A three-dimensional model for the generation of split dislocations by grain boundaries in nanocrystalline A1 is proposed. In terms of this model, rectangular glide split-dislocation half-loops nucleate at glide lattice dislocation loops pressed to grain boundaries by an applied stress. The level of the applied stress and the grain size at which the emission of such dislocation half-loops becomes energetically favorable are determined. The dependences of the stacking-fault width on the grain size and the applied stress are found. The anomalously wide stacking faults experimentally detected in nanocrystalline A1 are shown to be caused by high internal stresses forming in the stages of preparation, treatment, or local loading of nanocrystalline samples.     

Nanovoid generation due to intergrain sliding in nanocrystalline materials

A theoretical model is suggested which describes the generation of nanoscale voids (nanovoids) at grain boundaries (GBs) in deformed nanocrystalline and nanocomposite materials. In the framework of the model, nanovoids are generated in the stress fields of the dislocations characterized by large Burgers vectors and formed at GB steps and triple junctions due to intense intergrain sliding. The model accounts for experimental observations of nanovoids at GBs in deformed nanomaterials, reported in the literature.     

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

Effect of stress state on deformation and fracture of nanocrystalline copper:Molecular dynamics simulation

Deformation in a microcomponent is often constrained by surrounding joined material making the component under mixed loading and multiple stress states. In this study, molecular dynamics(MD) simulation are conducted to probe the effect of stress states on the deformation and fracture of nanocrystalline Cu. Tensile strain is applied on a Cu single crystal,bicrystal and polycrystal respectively, under two different tension boundary conditions. Simulations are first conducted on the bicrystal and polycrystal models without lattice imperfection. The results reveal that, compared with the performance of simulation models under free boundary condition, the transverse stress caused by the constrained boundary condition leads to a much higher tensile stress and can severely limit the plastic deformation, which in return promotes cleavage fracture in the model. Simulations are then performed on Cu single crystal and polycrystal with an initial crack. Under constrained boundary condition, the crack tip propagates rapidly in the single crystal in a cleavage manner while the crack becomes blunting and extends along the grain boundaries in the polycrystal. Under free boundary condition, massive dislocation activities dominate the deformation mechanisms and the crack plays a little role in both single crystals and polycrystals.     

Atomistic mechanisms of fatigue in nanocrystalline metals

We investigate the mechanisms of fatigue behavior in nanocrystalline metals at the atomic scale using empirical force laws and molecular level simulations. A combination of molecular statics and molecular dynamics was used to deal with the time scale limitations of molecular dynamics. We show that the main atomistic mechanism of fatigue crack propagation in these materials is the formation of nanovoids ahead of the main crack. The results obtained for crack advance as a function of stress intensity amplitude are consistent with experimental studies and a Paris law exponent of about 2.     

Mechanism of deformation-twin formation in nanocrystalline metals

A theoretical model is proposed to describe the nucleation of deformation twins at grain boundaries in nanocrystalline materials under the action of an applied stress and the stress field of a dipole of junction or grain-boundary wedge disclinations. The model is used to consider pure nanocrystalline aluminum and copper with an average grain size of about 30 nm. The conditions of barrier-free twinning-dislocation nucleation are studied. These conditions are shown to be realistic for the metals under study. As the twin-plate thickness increases, one observes two stages of local hardening and an intermediate stage of local flow of a nanocrystalline metal on the scale of one nanograin. In all stages, the critical stress increases with decreasing disclination-dipole strength. The equilibrium thickness and shape of the twin plate are analyzed and found to agree well with the well-known results of experimental observations.     

Deformation induced microtwins and stacking faults in aluminum single crystal

Microtwins and stacking faults in plastically deformed aluminum single crystal were successfully observed by high-resolution transmission electron microscope. The occurrence of these microtwins and stacking faults is directly related to the specially designed crystallographic orientation, because they were not observed in pure aluminum single crystal or polycrystal before. Based on the new finding above, we propose a universal dislocation-based model to judge the preference or not for the nucleation of deformation twins and stacking faults in various face-centered-cubic metals in terms of the critical stress for dislocation glide or twinning by considering the intrinsic factors, such as stacking fault energy, crystallographic orientation, and grain size. The new finding of deformation induced microtwins and stacking faults in aluminum single crystal and the proposed model should be of interest to a broad community.