基于磁场诱发奥氏体相变的可回复大磁致应变效应研究

Ferromagnetic shape memory alloys (FSMAs) with large reversible magnetostrain are promising in the applications in high resolution micro-displacement controlling, robots and precision machining. The large magnetostrain realized via magnetic-field-induced twin variants reorientation can be up to 6%, but the output stress is less than 2 MPa. The output stress of the magnetic-field-induced phase transition is over 100 MPa and the magnetostrain is up to 3%, but the large mangetostrain is irreversible. The largest reversible magnetostrain of the magnetic-field-induced phase transition as reported over the world is only 0.4%. Studies on the magnetic shape memory alloys with large reversible magnetostrain is of crucial importance..Recently the applicant has designed Ni(CuCo)MnGa alloys by composition designing and phase transition tailoring, realized the magnetic-field-induced transition from paramagnetic martensite to ferromagnetic austenite, and obtained reversible magnetostrain of 2% by preliminary thermo-mechanical training..In this project the effect of the residual stress induced by thermo-mechanical training and the applied magnetic field on the phase transition temperatures of the Ni(CuCo)MnGa will be revealed, the magnetocrystalline features of the reversible magnetostrain of the martensitic variants with different orientations will be determined, and mechanism responsible for the reversibility of the preferentially orientated martensitic variants realized by thermo-mechanical training will be clarified, and large reversible magnetostrain will be developed.

铁磁形状记忆合金在航空航天高精度微位移控制、机器人、精密加工等领域应用前景广阔。该类合金中磁场驱动孪晶再取向的磁致应变大(>6%)，但输出力小(<2MPa)且不可回复；磁场诱发奥氏体相变的输出力大(>100MPa)，但已报道的3%大磁致应变不可回复，而可回复的磁致应变报道最大值为0.5%。研究兼有大输出力和可回复特性的大磁致应变效应有重要意义。.近期，申请者通过相变和磁性调控，设计了Ni(CuCo)MnGa合金，实现了磁场诱发奥氏体相变，初步热机械训练，获得了2%可回复磁致应变。本项目拟优化热机械训练方法，揭示热机械训练对相变温度和马氏体变体择优取向的影响规律，实现可逆磁场诱发奥氏体相变，获得高度择优取向的马氏体变体组织，建立热机械训练、磁场诱发奥氏体相变、择优取向变体组织与可回复磁致应变之间的关系，阐明热机械训练实现马氏体变体择优取向的内在机理，获得可回复大磁致应变效应。

铁磁形状记忆合金在航空航天高精度微位移控制、机器人、精密加工等领域应用前景广阔。该类合金中磁场驱动孪晶再取向的磁致应变大(>6%)，但输出力小(<2MPa)且不可回复；磁场诱发奥氏体相变的输出力大(>100MPa)，但已报道的3%大磁致应变不可回复，而可回复的磁致应变报道最大值为0.5%。研究兼有大输出力和可回复特性的大磁致应变效应有重要意义。本项目通过相变、磁性和组织调控，设计了Ni-(CuCo)-Mn-Ga合金，实现了磁场诱发相变可逆和马氏体变体高度择优取向，获得了高达5%的可回复大磁致应变。.针对NiMnGa合金奥氏体相磁性弱、磁场驱动奥氏体相变困难的问题，设计了Ni-Co-Mn-Ga四元合金，揭示了合金的相组成、相变温度、磁性能与成分的关系，明确了Co元素添加显著提高磁性，但过量添加Co产生析出相，甚至马氏体相变消失。Co取代Ni使马氏体相变温度剧烈降低，奥氏体居里温度显著升高，并显著增强奥氏体相的磁性。Cu含量小于20at%时合金为单相组织，Cu含量含量20at%时，开始析出γ相。但是当Cu含量超过25at%时，马氏体相变消失。在Ni-Co-Mn-Ga双相合金中获得了“弱铁磁马氏体-强铁磁奥氏体”热磁耦合马氏体相变，相变温度高达500 K，是目前国际报道最高值。γ相的出现，还提高合金的塑性，但合金的相变滞后过大。.进一步设计了Ni-(CuCo)-Mn-Ga合金五元合金，获得了单相组织，实现了热磁耦合马氏体相变对外磁场的高度灵敏相应，获得了可逆的磁场诱发相变。建立了该合金体系反映相变温度与成分之间关系的相图，在宽成分窗口和宽温度窗口内，均获得了“弱磁马氏体-铁磁奥氏体”热磁耦合马氏体相变。通过提高Co含量，耦合相变的磁化强度差增大、相变熵变减小，耦合相变对外磁场的响应灵敏度升高，耦合相变的饱和强化强度差最高可达96 Am2kg-1的热磁耦合相变，1T磁场使相变温度变化3.8K，从而，获得了可逆的磁场诱发相变。.阐明了热机械训练对Ni-(CuCo)-Mn-Ga合金磁相变行为、马氏体变体组织和磁致应变性能的影响规律，实现了磁场诱发相变过程中马氏体变体一直高度择优取向，从而成功获得了高达5％的磁致应变效应，是目前磁性形状记忆合金中报道的自由状态下可回复磁致应变的最大值。