Superelasticity in bcc nanowires by a reversible twinning mechanism
Superelasticity in bcc nanowires by a reversible twinning mechanism
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通过可逆孪晶机制实现 BCC 纳米线的超弹性
DOI：
10.1103/physrevb.82.205435
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发表时间：
2010-11-29
影响因子：
3.7
通讯作者：
Saxena, Avadh
中科院分区：
文献类型：
--
作者：
Li, Suzhi;Ding, Xiangdong;Saxena, Avadh
大块材料的超弹性（SE）是由结构变化的马氏体转变引起的，马氏体转变为形状恢复提供了体积热力学驱动力。另一方面，结构不变的变形过程，如孪晶和位错滑移，导致塑性变形，不能为形状恢复提供动力。我们使用分子动力学模拟表明，与上述传统观点相反，一些bcc金属纳米线通过“可逆”孪晶机制表现出SE。我们发现这种可逆孪晶是由孪晶态和非孪晶态之间的表面能变化驱动的。鉴于最近在fcc纳米线中类似的发现，我们认为SE是立方纳米线中的普遍现象，并且形状恢复的驱动力来自于最小化表面能。此外，我们发现bcc纳米线的SE在以下几个方面是独特的：首先，由部分位错产生的< 111 > /{112}层错总是优于< 111 > /{110}和< 111 > /{123}全位错滑移。bcc纳米线中< 111 > /{112}孪晶滑移或完全位错滑移的发生取决于相邻{112}面的后续部分位错发射与同一平面的部分位错发射之间的竞争。其次，与fcc相比，bcc纳米线具有更高的孪晶成核能垒，但孪晶迁移能垒较低。这导致了bcc纳米线中SE具有低能量耗散和低应变硬化等独特特性。第三，某些难熔bcc纳米线，如W和Mo，可以在非常高的温度下显示SE，这比几乎所有报道的高温形状记忆合金都要高。我们的工作为纳米线的超弹性提供了更深入的理解，耐火bcc纳米线是在宽温度范围内工作的纳米机电系统中应用的潜在候选者。
Superelasticity (SE) in bulk materials is known to originate from the structure-changing martensitic transition which provides a volumetric thermodynamic driving force for shape recovery. On the other hand, structure-invariant deformation processes, such as twinning and dislocation slip, which result in plastic deformation, cannot provide the driving force for shape recovery. We use molecular-dynamics simulations to show that some bcc metal nanowires exhibit SE by a "reversible" twinning mechanism, in contrast to the above conventional point of view. We show that this reversible twinning is driven by the surface energy change between the twinned and detwinned state. In view of similar recent findings in fcc nanowires, we suggest that SE is a general phenomenon in cubic nanowires and that the driving force for the shape recovery arises from minimizing the surface energy. Furthermore, we find that SE in bcc nanowires is unique in several respects: first, the < 111 > / {112} stacking fault generated by partial dislocation is always preferred over < 111 > / {110} and < 111 > / {123} full dislocation slip. The occurrence of < 111 > / {112} twin or full dislocation slip in bcc nanowires depends on the competition between the emission of subsequent partial dislocations in adjacent {112} planes and the emission of partial dislocations in the same plane. Second, compared to their fcc counterparts, bcc nanowires have a higher energy barrier for the nucleation of twins, but a lower energy barrier for twin migration. This results in certain unique characteristics of SE in bcc nanowires, such as low energy dissipation and low strain hardening. Third, certain refractory bcc nanowires, such as W and Mo, can show SE at very high temperatures, which are higher than almost all of the reported high-temperature shape memory alloys. Our work provides a deeper understanding of superelasticity in nanowires and refractory bcc nanowires are potential candidates for applications in nanoelectromechanical systems operating over a wide temperature range.