大可逆应变Ni-Mn-In多晶的形变及回复机理研究

Brittleness and small reversible strain are the factors which restrict the engineering application of Ni-Mn-In magnetic shape memory alloy. Excellent mechanical property, physical property and driving behavior can be provided by the nanotwin in metals. This proposal plans to obtain Ni-Mn-In ribbons with martensitic twin variants in nanometer dimension by means of Co doping and melt-spinning technology, and prepare bulk polycrystals by spark plasma sintering the ordered piled melt-spun ribbons. The increase of shape recovery due to the nanotwins is needed to be found out. On the improvement of mechanical properties, the stress-strain behavior as well as the movement of nanotwin with different dimension during each stage of the deformation and recovery process are managed to be detected. The relationship between the dimension of nanotwin and recovery strain induced by heat or magnetic field will be built. The interaction of twin boundaries and dislocations are also needed to be investigated. Thus, based on the results obtained above, the mechanisms of improvement on shape recovery induced by.heat and the influence of nanotwin on magnetic field induced strain will be clarified. The aim of this proposal is to provide theoretical and practical guide for the application of magnetic shape memory alloy with large mangnetic field induced strain and high ductility.

脆性大、可逆应变量小是目前制约Ni-Mn-In磁性形状记忆合金实际工程应用的重要因素。而纳米孪晶独特的微结构使得金属材料具有优异的综合力学性能、物理性能和使役行为。本项目拟通过掺杂Co元素和改变急冷甩带工艺参数获得具有纳米尺寸马氏体孪晶变体的Ni-Mn-In合金薄带，并将薄带有序堆叠采用放电等离子烧结的方法制备纳米孪晶Ni-Mn-In多晶块材，探究纳米孪晶提高形状记忆可回复应变的原因。在力学性能显著提高的基础上，考察纳米孪晶马氏体的应力应变行为特征，查明不同尺寸纳米孪晶在形变各阶段和卸载后回复过程中的迁动方式与特点，建立纳米孪晶尺寸与热致形状记忆可回复应变及磁致应变量的关系，观察纳米孪晶界面与位错等缺陷的相互作用，阐明纳米孪晶提高Ni-Mn-In多晶热致可回复应变的内在机理及对磁致应变的影响规律，为获得大可逆应变、高韧性的Ni-Mn-In磁性形状记忆合金提供理论及实验指导。

本项目主要采用第四组元掺杂和放电等离子烧结等方法，通过细晶强化和降低孪晶片层厚度，改善了Ni-Mn-In合金的力学性能，提高了可回复应变量和可回复应变率。同时，由于外场驱动马氏体相变进行得更完全，相变过程中的绝热温度变化显著增加，循环稳定性也有明显改善。. 选择掺杂的第四组元主要包括Co、Zr、Ag、Fe和Cu，其中Ag、Fe和Cu对合金力学性能及弾热效应的提高效果最佳，且均可在一定程度上细化晶粒。随掺杂元素增加至一定量，合金基体析出第二相粒子，第二相体积分数较少时有利于力学性能，第二相体积增大数量增多时，力学性能反而下降。力学性能的增加使得合金在受到外加载荷作用时延迟开裂，相比较未掺杂Ni-Mn-In合金，外场诱发马氏体相变及逆相变进行得更加完全，其中的热效应能更加完全的释放，在一定程度上可以提高其绝热温度变化。. 放电等离子烧结具有在加压过程中烧结的特点，脉冲电流产生的等离子体及烧结过程中的加压有利于降低粉末的烧结温度，并使粉末快速烧结致密。采用放电等离子烧结的方法可以将气雾化法制备的Ni-Mn-In合金粉末、急冷甩带法制备的Ni-Mn-In合金薄带，烧结制成致密块材。利用这种方法，可以进一步改善Ni-Mn-In合金的力学性能，提高其断裂应力和断裂应变。经此方法制备的合金块材，晶粒尺寸小于15μm，孪晶片层厚度约为100nm～300nm，有效改善了其力学性能，提高了合金循环加载卸载过程中的稳定性。结果表明，放电等离子烧结Ni50Mn34In15.5Ag0.5合金在环境温度为45 ℃、应变量为6.5 %时的循环加载卸载300次循环后样品依旧保持完整，绝热温度变化也无明显降低，表明该合金具有优异的循环稳定性。