Partially ordered hierarchical substructure of as-cast γ phase in Ni-Mn-Ga alloys
Partially ordered hierarchical substructure of as-cast γ phase in Ni-Mn-Ga alloys
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DOI：
10.1016/j.matdes.2022.110780
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发表时间：
2022-05
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影响因子：
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通讯作者：
Jian-wen Fan;Guangming Zhao;Y. Ai;Sheng Ouyang;Ye Zhu
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作者：
Jian-wen Fan;Guangming Zhao;Y. Ai;Sheng Ouyang;Ye Zhu
铸态Ni-Mn-Ga形状记忆合金具有较高的热稳定性和孪晶应力，但对合金性能影响显着的第二相γ的显微组织却几乎未知。在这里，我们揭示了铸态富镍 Ni57+xMn25Ga18–x(x= 0, 1, …, 4) 和富锰 Ni54Mn32+xGa14–x(x= 0, 3, 7, 9) 合金中 γ 相的显微组织特征。首次发现γ相的亚结构是由具有高密度位错的分级排列的“微片层中的纳米片层”组成。作为 γ 相的组成部分，纳米片层不是孪生相关的，而是与微片层中交替的无序相和部分有序相形成对。部分有序相可能是无序-有序转变的中间阶段。建立了描述微/纳米片晶之间晶体取向关系的几何模型。这种分层子结构消失，并可能在退火后演变成有序相的孪生相。此外，还比较了铸态合金和退火合金的热/机械性能，并讨论了γ相对其性能的影响。
As-cast Ni-Mn-Ga shape memory alloys have high thermal stability and twinning stress, but the second phase γ, which exerts significant effect on the alloys’ performance, remains almost unknown in terms of its microstructure. Here we unravel the microstructural characteristics of the γ phase in as-cast Ni-rich Ni57+xMn25Ga18–x(x= 0, 1, …, 4) and Mn-rich Ni54Mn32+xGa14–x(x= 0, 3, 7, 9) alloys. For the first time, a substructure of the γ phase is discovered to be composed of hierarchically arranged “nano-lamellae in micro-lamellae” with high-density dislocations. As building blocks of the γ phase, the nano-lamellae are not twin-related but form pairs with disordered and partially ordered phases alternating in the micro-lamellae. The partially ordered phase is presumably an intermediate of the disorder–order transition. A geometric model for describing the crystallographic orientation relationships among the micro-/nano-lamellae is established. This hierarchical substructure disappears and may evolve into twins of ordered phases after annealing. In addition, thermal/mechanical properties of the as-cast and annealed alloys are compared, and the effects of γ phase on the properties are discussed.