Basic research on the shape memory alloy thin films

在几种用于制造微机械驱动器的性能材料中，Ti-Ni形状记忆合金薄膜具有变形大、恢复力大等优点，而且即使是形状记忆合金固有的缺点，如由于冷却速度的限制，也可以在薄膜中得到很大的改善。因此，发展钛镍形状记忆合金薄膜已成为迫切需要。本项目已成功地在溅射沉积的Ti-Ni薄膜中实现了完美的形状记忆效应。研究发现，热处理、合金成分和溅射条件对相变温度和形状记忆特性有很大影响。用溅射法制备了Ti-Ni-Cu和Ti-Ni-Pd三元形状记忆合金薄膜，其马氏体相变的形状变化率和恢复力分别达到6%和500 Mpa，与块体样品相当。它们分别表现出较小的温度滞后和较高的相变温度，这些特性有效地实现了高响应的致动器特性。Ti-Ni-Pd薄膜在冷却和加热过程中表现出与Ti-Ni二元合金薄膜相似的单相变。相应地，它们在冷却和加热时都表现出单级变形。Ti-Ni-Cu薄膜表现出两阶段相变，即两阶段变形，第一阶段对应于B2与O(正交)相之间的转变，第二阶段对应于O相与M相之间的转变。第一阶段相变伴随着一个小的温度滞后。所有这些薄膜都表现出了完美的形状记忆效应。

Among several types of performance materials proposed for fabricating microactuators to power micromachines, Ti-Ni shape memory alloy thin films have great advantages such as large deformation and recovery force ; besides, even an intrinsic disadvantage of the shape memory alloy such as a slow response due to the limitation of cooling rate can be greatly improved in thin films. Therefore, the development of Ti-Ni shape memory alloy thin films has been keenly demanded. The present project has been successful in developing a perfect shape memory effect in sputter-deposited Ti-Ni thin films. The transformation temperatures and shape memory characteristics were found to be strongly affected by heat-treatment, alloy composition and sputtering condition. The shape change and recovery force associated with the martensitic transformation in Ti-Ni thin films reached up to 6% and 500Mpa, respectively, which were equivalent to those of bulk specimens.Ti-Ni-Cu and Ti-Ni-Pd ternary shape memory alloy thin films were also made by sputtering. They showed a small temperature hysteresis and high transformation temperatures, respectively ; these characteristics are effective to achieve a high response actuator characteristic. The Ti-Ni-Pd thin film showed a single stage transformation upon cooling and heating similarly to a Ti-Ni binary alloy thin films. Corresponding, they showed a single stage deformation both upon cooling and heating. The Ti-Ni-Cu thin film showed a two-stage transformation and hence a two-stage deformation ; the first stage corresponds to the transformation between B2 and O(Orthorhombic)-phase and the second stage between the O- and M-phases. The first stage transformation was accompanied by a small temperature hysteresis. All these thin films showed perfect shape memory effect.