Structural origin of reversible martensitic transformation and reversible twinning in NiTi shape memory alloy
Structural origin of reversible martensitic transformation and reversible twinning in NiTi shape memory alloy
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DOI：
10.1016/j.actamat.2020.08.039
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发表时间：
2020-10-15
期刊：
影响因子：
9.4
通讯作者：
An, Qi
中科院分区：
文献类型：
--
作者：
Bin Li;Shen, Yidi;An, Qi
在过去的几十年里，人们对NiTi形状记忆合金中有序体心立方（B2）奥氏体向单斜马氏体（B19 '）转变的机制以及马氏体中的孪晶机制的研究取得了很大进展。然而，到目前为止，还没有在原子尺度上进行晶格对应分析，因此，这些机制还没有完全理解的可逆相变和可逆孪晶在马氏体的途径。在这项工作中，原子模拟研究如何B2奥氏体转变为B19'马氏体和如何形成孪晶相变过程中。特别是对结构演化过程中的晶格对应进行了细致的分析，更清晰地揭示了马氏体相变机制和孪晶机制。模拟结果表明，当马氏体晶粒在奥氏体中形核时，它们已经形成孪晶关系，这是晶体结构的自然结果。这样，自适应就自然而然地实现了。随后发生合并，一些马氏体晶粒生长在其邻居的费用，表明界面是弹性移动的，他们的迁移是可逆的。最终形成了孪晶马氏体瓦里。结果表明，在B2 -> B19'变换中，B19'单斜晶体结构可以看作是一种扭曲的六方密堆积结构。结果表明，马氏体相变与体心立方HCP相变基本相似，只涉及原子重排，是可逆的;马氏体中的孪晶与HCP金属中的{10（1）over bar 2}孪晶基本相似，只涉及原子重排，也是可逆的。另一个重要发现是B19'马氏体具有非常特殊的双晶格结构：单斜晶胞的相对弱的Ti-Ti键允许晶格单元容易地重新取向。因此，两种不同取向的晶格单元共存于同一马氏体晶粒中，这失去了长程周期性，为适应外部和内部应变提供了额外的自由度。这些结果很好地解释了为什么B19'马氏体非常适应应变以及为什么变形是可逆的。(C)2020 Acta Materialia Inc.由爱思唯尔有限公司出版。保留所有权利。
Much progress has been made over the past few decades in resolving the mechanism for the transformation from ordered body-centered-cubic (B2) austenite to monoclinic martensite (B19') in NiTi shape memory alloy, as well as the twinning mechanisms in martensite. However, so far no lattice correspondence analyses on the atomic scale have been conducted, as a result, these mechanisms have not been completely understood in terms of the pathways for reversible phase transition and reversible twinning in martensite. In this work, atomistic simulations were performed to investigate how B2 austenite transforms to B19' martensite and how twins are formed during phase transformation. In particular, lattice correspondence in the structural evolutions was carefully analyzed to reveal the martensitic transformation mechanism and twinning mechanism with much better clarity. The simulation results show that, when martensite grains were nucleated in austenite, they already formed a twin relationship as a natural result of the crystal structure. Thus, self-accommodation is achieved naturally. Coalescence occurred subsequently and some martensite grains grew at the expense of their neighbors, indicating that the interfaces are elastically mobile and their migration is reversible. Eventually twinned martensite vari- ants were created. It is shown that, in the B2 -> B19' transformation, the B19' monoclinic structure can be treated as a distorted hexagonal close-packed (HCP) structure. With this treatment, it is demonstrated that the martensitic transformation is essentially similar to BCC HCP transformation which only involves atomic shuffles and is reversible, and twinning in martensite is essentially similar to {10 (1) over bar2} twinning in HCP metals which only involves atomic shuffles and is reversible as well. Another significant discovery is that the B19' martensite exhibits a very special dual-lattice structure: the relatively weak Ti-Ti bonding of the monoclinic unit cell permits easy reorientation of lattice units. As a result, two differently oriented lattice units co-exist in the same martensite grain which loses long range periodicity, providing an extra degree of freedom for accommodating external and internal strains. These results explain well why B19' martensite is extremely adaptive to straining and why deformation is reversible. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.