Explosive martensitic transformation of supercooled austenite in CuZr-based thin-film shape memory alloys
Explosive martensitic transformation of supercooled austenite in CuZr-based thin-film shape memory alloys
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DOI：
10.1016/j.actamat.2020.08.081
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发表时间：
2020-11
期刊：
影响因子：
9.4
通讯作者：
Y. Miao;J. Vlassak
中科院分区：
文献类型：
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作者：
Y. Miao;J. Vlassak
CuZr 基合金被认为是用于高温应用的潜在形状记忆合金。我们使用多晶薄膜样品研究了几种合金元素对这些合金形状记忆性能的影响。在这里，我们报告了过冷 CuZr、CuZrNi 和 CuZrCo 样品中马氏体的爆炸形成。这种爆炸转变行为的特征在于以下观察结果： 1) 高温奥氏体相可以过冷至低于马氏体完成温度Mf。在低于 Mf 的临界温度下，整个样品的奥氏体在不到一微秒的时间内转变为马氏体。 2）临界温度分布较窄，随冷却速率的增加略有降低。 3) 过冷和爆炸转变行为的观察取决于奥氏体完成温度 Af 以上的温度历史。如果样品在Af以上加热后立即淬火，则在Ms以下冷却时逐渐形成马氏体；如果样品在 Af 上方停留几秒钟，马氏体就会爆炸形成。我们认为，逐步转变是通过在连续转变周期中积累的缺陷上马氏体生长来进行的。然而，如果允许样品停留在高于 Af 的温度，这些缺陷就会消失，并且转变将受到成核限制。马氏体的形核需要显着的过冷。缺陷消灭过程对温度高度敏感，表观活化能为 326 kJ/mol，这对于简单的扩散限制过程来说太大了。 CuZrCo 样品的透射电子显微镜表明这些缺陷可能与残余马氏体的存在有关。
CuZr-based alloys are being considered as potential shape memory alloys for use in high-temperature applications. We have conducted a study on the effects of several alloying elements on the shape memory properties of these alloys using polycrystalline thin-film samples. Here we report on the explosive formation of martensite in supercooled CuZr, CuZrNi and CuZrCo samples. This explosive transformation behavior is characterized by the following observations: 1) The high-temperature austenitic phase can be supercooled below the martensite finish temperatureMf. At a critical temperature belowMf, austenite transforms to martensite across the entire sample in less than a microsecond. 2) The critical temperature has a narrow distribution and decreases slightly with higher cooling rate. 3) Observation of supercooling and explosive transformation behavior depends on the temperature history above the austenite finish temperatureAf. If a sample is quenched immediately after heating aboveAf, martensite forms gradually on cooling belowMs; if a sample is allowed to dwell a few seconds aboveAf, the martensite forms explosively. We suggest that the gradual transformation proceeds by martensite growth on defects that accumulate during successive transformation cycles. If the sample is allowed to dwell at a temperature aboveAf, however, these defects are annihilated and the transformation is nucleation-limited. Nucleation of martensite then requires significant supercooling. The defect annihilation process is highly sensitive to temperature and has an apparent activation energy of 326 kJ/mol, which is too large for a simple diffusion-limited process. Transmission electron microscopy of CuZrCo samples suggests that the defects may be related to the presence of residual martensite.