多晶Ni-Mn-Ga-Gd高温记忆合金的双程形状记忆效应

Ni-Mn-Ga high-temperature shape memory alloy (HTSMA) exhibits two-way shape memory effect (TWSME), and a large two-way reversible strain can be obtained along the certain direction of some single crystal Ni-Mn-Ga, which makes it become a potential HTSMA. However, the TWSME of polycrystalline Ni-Mn-Ga HTSMA has not been widely researched. The applicant's previous studies have found that the hard second phase introduction is advantageous to TWSME of polycrystalline Ni-Mn-Ga. Thus, hard second phase is formed by rare earth Gd doping, and polycrystalline Ni-Mn-Ga with dual-phase is obtained in the present project. The influence and mechanism of second phase on TWSME and thermo-stability is studied to establish the best criteria of composition. The influence and physical nature of thermomechanical training method on TWSME is investigated, and the training method is optimized to improve TWSME. The oriented polycrystalline Ni-Mn-Ga is prepared by directional solidification. In order to further enhance the two-way shape memory properties, the effect of grain orientation on TWSME is observed. The relationship among the process of preparation and thermomechanical training, TWSME and thermo-stability is bulit up. The HTSMA with high TWSME and high thermo-stability is developed.

Ni-Mn-Ga高温记忆合金具有双程形状记忆效应，沿其单晶的某些方向可获得极大的双程可逆应变，是一种发展潜力巨大的高温记忆合金。但关于多晶Ni-Mn-Ga高温双程记忆效应的研究目前仍未完全展开。申请者的前期研究发现，硬性第二相有利于提高多晶Ni-Mn-Ga合金的双程形状记忆效应，因此本项目拟采用Gd掺杂方法引入硬性第二相，制备多晶双相Ni-Mn-Ga-Gd高温记忆合金。研究第二相对双程形状记忆效应及其热稳定性的影响规律和微观机制，确立最佳合金成分设计准则；阐明热机械训练方法对双程形状记忆效应的影响规律和物理本质，优化训练工艺，提高双程形状记忆效应；采用定向凝固方法制备取向多晶，研究晶粒取向对双程记忆效应的影响，进一步提升双程形状记忆性能；建立合金制备与热机械训练工艺、双程形状记忆效应及热循环稳定性的内在联系，研制出具有高热稳定性、大双程形状记忆效应的高温记忆合金。

双程形状记忆效应是记忆合金一种重要的应用特性，目前对双程记忆效应研究较成熟的记忆合金主要是Ti-Ni合金，但Ti-Ni合金无法在高温条件下使用，所以发展高温记忆合金的双程形状记忆效应十分必要。Ni-Mn-Ga合金是一种发展潜力巨大的高温记忆合金，关于其多晶高温双程记忆效应的研究目前仍未完全展开。本项目通过引入第二相及定向凝固的方法提高多晶Ni-Mn-Ga合金的双程形状记忆效应。研究结果表明，当选择稀土Gd为添加元素时，Gd含量应该在0.25-0.4 at.%之间，此时合金基体中具有分布均匀、数量适中的硬性第二相，双程形状记忆效应最大。经适当的热机械训练，合金的双程形状记忆效应可以得到进一步提高，其中应力诱发马氏体循环训练效果强于形状记忆效应循环训练，得到的最大双程形状记忆效应可达3.3%；此外，当合金中加入Co元素时，产生的软性第二相可以提升Ni-Mn-Ga合金的双程形状记忆效应的稳定性。晶粒取向对Ni-Mn-Ga-Gd合金的双程形状记忆效应也具有明显影响，沿（2 2 2）方向生长的多晶合金具有更好的双程形状记忆效应，经过形状记忆效应循环和应力诱发马氏体循环训练后，分别可得到3.0%和3.6%的稳定双程形状记忆效应。本项目为研制出性能优异的高温记忆合金驱动材料提供理论基础，具有较好的应用前景。