Nanoscale plastic deformation mechanism in single crystal aragonite

Molecular dynamics simulations have been performed to study the dynamic behaviors of single crystal aragonite under indentation, tension, and compression. The elastic modulus and hardness of single crystalline aragonite measured in our simulations are found in good agreement with experimentally measured values. Our simulation results show that the mechanical properties of aragonite crystal, including the elastic modulus, hardness, strength, and toughness, strongly depend on the crystallographic orientations and loading conditions. We have identified that this dependence is resulted from different deformation mechanisms, i.e., phase transformation, amorphous phase formation, dislocation, and twining. This work is an attempt to identify the deformation mechanisms in aragonite and to establish the relationship between the dominant deformation mechanisms and its crystallographic orientations and loading conditions.     

Evolution of the spread of crystal orientation with plastic deformation in a cold-rolled Cu single crystal

Using scanning electron microscopy, we investigated how the microstructure of a Cu single crystal with a {15 12 9}〈9 10 3〉 orientation evolved from cold rolling. The first 50 % rolling caused its crystal orientation to rotate to {211}〈111〉. Although orientation splitting occurred near the surface of the single crystal, band-like regions with near-{211}〈111〉 orientations were still present after the fourth 50 % rolling. We measured the spread of crystal orientation in the near-{211}〈111〉 regions as a function of the total plastic equivalent strain induced by all rolling steps. When 50 % rolling was performed less than twice, the spread of crystal orientation was proportional to the square root of the plastic equivalent strain. Based on our results, we discussed the relationship between the spread of crystal orientation and the plastic equivalent strain generated by rolling.     

Ni2 MnGa合金的价电子结构研究

用“固体与分子经验电子理论”研究了Ni2MnGa合金的价电子结构，研究结果表明，在Ni2MnGa中，Ni的杂化状态为A'种杂化第2阶；Mn的杂化状态为B种杂化第9阶，Ga为A种第9阶杂化。Ni的原子磁矩计算值为3.8666μB十分接近。最强的共价键是Ni-Mn键，其上的共价电子对数为0．577，其次为Ni-Ga键，其价电子对数为0．542，其它键上的共价电子对数小于0．2。     

Temperature dependence of impact deformation behavior of single crystal Ni based superalloy

The impact test was carried out to investigate the intermediate temperature brittleness of single crystal Ni based superalloy. The samples were impacted at the velocity of ～5?m?s^??1. The results showed that the impact toughness also exhibited intermediate temperature brittleness, which is similar to the situation in tensile. The samples showed the highest impact toughness at 600°C but exhibited the lowest impact toughness at 760°C. Results showed that the variety of impact toughness was due to the deformation mechanism. At 600°C, a/2? dislocation slips on the {111} slip system were attributed to the high impact toughness; however, a/2? dislocation slips from octahedral {111} planes to cubic {100} planes resulted in significant work hardening, leading to the decrease of impact toughness.     

一种新型功能晶体Ni2MnGa

Ｎｉ_２ＭｎＧａ晶体同时具有强铁磁性、大磁致伸缩、温控和磁控形状记忆效应,其磁控形状记忆效应的响应频率接近压电陶瓷,输出应变和应力接近温控形状记忆合金,是近年来发现的一类新型功能晶体．Ｎｉ_２ＭｐＧａ晶体７７Ｋ时在［００１］方向,仅２ＭＰａ的预应力即可产生５％的温控可回复应变,已接近 ＴｉＮｉ合金 ６％～８％的可回复应变量;非化学剂量的 Ｎｉ。ＭｎＧａ晶体室温条件下在［００１］方向, ６ ｈＯｅ外磁场已诱发产生 ０．３１％的输出应变,远超过巨磁致伸缩材料Ｔｅｒｆｅｎｏｌ－Ｄ输出应变０．１７％的水平．由于该晶体在磁场控制下表现出的大输出应变和应力,以及响应频率快和可精确控制的综合特性,使其可能在声纳、微位移器、线性马达、微波器件、振动和噪声控制、机器人和智能结构等诸多领域有重要应用,成为未来新一代驱动器和传感器材料．本文综述了Ｎｉ_２ＭｎＧａ单晶的基本性能、磁控形状记忆效应的机理、本质以及影响性能的因素等。     

On the smoothness of the surface deformation profile due to the indentation of an elastic half-space

Although the surface deformation concept has been used in the nanoindentation study, there is a lack of investigation on the smoothness of the surface deformation profile. We find that the surface deformation profile is continuous while its first order derivative is continuous only for incomplete and critical complete contact situations. The presented result may be used to make more realistic assumptions on the surface deformation in the nanoindentation modeling.     

Orientation dependence of fracture toughness measured by indentation methods and its relation to surface energy in single crystal silicon

Fracture toughness of silicon crystals has been investigated using indentation methods, and their surface energies have been calculated by molecular dynamics (MD). In order to determine the most preferential fracture plane at room temperature among the crystallographic planes containing the 〈001〉, 〈110〉 and 〈111〉 directions, a conical indenter was forced into (001), (110) and (111) silicon wafers at room temperature. Dominant {110}, {111} and {110} cracks were introduced from the indents on (001), (011) and (111) wafers, respectively. Fracture occurs most easily along {110}, {111} and {110} planes among the crystallographic planes containing the 〈001〉, 〈011〉 and 〈111〉 directions, respectively. A series of surface energies of those planes were calculated by MD to confirm the orientation dependence of fracture toughness. The surface energy of the {110} plane is the minimum of 1.50 Jm⁻² among planes containing the 〈001〉 and 〈111〉 directions, respectively, and that of the {111} plane is the minimum of 1.19 Jm⁻² among the planes containing the 〈011〉 direction. Fracture toughness of those planes was also derived from the calculated surface energies. It was shown that the K _IC value of the {110} crack plane was the minimum among those for the planes containing the 〈001〉 and 〈111〉 directions, respectively, and that K _IC value of the {111} crack plane was the minimum among those for the planes containing the 〈011〉 direction. These results are in good agreement with that obtained conical indentation.     

Hydrogen‐enhanced plastic deformation during indentation for bulk metallic glass of Zr65Al7.5Ni10Cu17.5

The effect of hydrogen charging on shear bands and plastic zone during Vickers indentation for Zr₆₅Al_7.5Ni₁₀Cu_17.5 bulk metallic glass has been studied. The results showed that hardness increased gradually with charging time and reached saturation. The shear bands and the size of the plastic zone on the surface and subsurface of indentation increased evidently when charging time was less then 40 h at i = 10 mA/cm² or 10 h at i = 100 mA/cm², respectively. After that, the size of the plastic zone began to reduce with charging time because hydrogen blisterings began formation and growth.     

Ni2MnGa合金结构及磁控形状记忆机制的研究进展

Ni2MnGa是一种新型的功能材料,它在磁场下能产生大的应变。本文从结构和磁控形状记忆机制方面阐述近年来国内和国际研究的进展,着重磁控形状记忆机制的3种微观模型。     

Etching-enhanced growth of indentation crack in barium titanate single crystal

The effect of etching on an unloaded indentation crack in a poled BaTiO₃ single crystal whose poling direction is lying in or normal to the indentation plane, has been investigated. The result showed that for the sample whose poling direction was lying in the indentation plane, the indentation cracks and 90° domain switching zones surrounded by two pairs of indentation cracks grew gradually during etching using HCl + HF aqueous solution, and reached saturation after etching for 30 s, however, the effect of etching on the crack length and the size of the domain switching zone was very small for the sample whose poling direction was normal to the indentation plane. Etching-enhanced domain switching and crack propagating is due to adsorption-decreased surface energy.     

Plastic deformation of Al2O3 single crystals by indentation at temperatures up to 750° C

Vickers hardness measurements have been made on three differently oriented sapphire single crystals in the temperature range from room temperature to 750° C. The hardness generally decreased with increasing temperature but varied with specimen orientation. In the regions surrounding the indents, twinning lamellae and slip lines have been studied systematically and could be associated with dislocation systems. Rhombohedral {01¯1 2} and basal (0 0 0 1) twinning as well as prismatic {1 1¯ 2 0} and basal (0001) slip have been observed. Plastic deformation occurred during placement of Vickers microhardness indentations. The hardness was markedly influenced by the number and kind of activated systems. A change in the activated systems caused variation and inversion in hardness ratios. Investigating high-index planes gave much more information on anisotropy effects than would have been obtained had only basally and/or prismatically oriented specimens been used. The latter is commonly the case in the literature. It was shown that in brittle materials, such as ceramics, plastic deformation occurs at temperatures below 0.5T/T _m if a stress field with a large hydrostatic component is applied. It is suggested that this is also the case in abrasion.     

铁磁形状记忆合金Ni2MnGa多晶的磁-力学特性

对外加磁场、应力场共同作用下的铁磁形状记忆合金多晶的磁-力学特性进行了实验测试与研究,分别获得了两种组分Ni52Mn27Ga21和Ni54Mn25Ga21多晶样品在不同磁场倾角下、不同预加应力下的磁化曲线和磁滞回线;以及不同外加磁场及磁场倾角下的应力-应变曲线和磁致应变曲线等磁-力学特性曲线.结果表明铁磁形状记忆合金多晶沿样品轴向所测的饱和磁化强度随磁场倾角的增大而减小,施加预应力并不显著影响样品的磁化曲线和磁滞回线;各种角度时不同磁场对两种多晶样品的应力-应变关系影响均很小.