富Mn NiMnGa合金γ相的微观结构与热力学性能研究

Microstructure， Thermal and Mechanical Properties of γ Phase in Mn-rich NiMnGa Alloy

摘要:

传统Ni-Mn-Ga合金脆性大，虽然富Ni的Ni-Mn-Ga合金显著提升了合金的韧性，形状记忆效应却大幅度下降，而富Mn的Ni-Mn-Ga合金具有较高的马氏体相变温度、良好的热稳定性和适中的形状记忆性能。研究了 Ni₅₄Mn_28+xGa_18-x（x=0，4，7，9，13）γ相的微观结构及合金的热力学性能。随着Mn含量的增加，γ相开始出现，晶粒内存在“微米片层包含纳米片层”微观结构。微米片层由两变体组成，变体包含一对呈｛011｝孪晶关系的纳米片层，该片层是面心四方结构。随着Mn含量的增加，压缩应力从914 MPa增加到2175 MPa，压缩应变从14%增加到26%，但其形状记忆效应逐渐降低，马氏体相变温度从352 ℃提高至585 ℃。富Mn Ni-Mn-Ga合金层片γ相的引入对合金韧性增强虽不如富Ni合金，但合金马氏体相变温度更高。

Abstract:

Brittleness of traditional Ni-Mn-Ga alloy is a marjor obstacle for its practical applications, as actuators and sensors. The Ni-rich Ni-Mn-Ga alloy can significantly improve the ductility. However, the shape memory strain is significantly reduced. Higher martensitic transformation temperature, good thermal stability and moderate shape memory property are shown in Mn-rich Ni-Mn-Ga. In the present work, microstructural feature, mechanical properties and thermal property of Ni₅₄Mn_28+xGa_18-x(x=0, 4, 7, 9, 13) were investigated. As the Mn content increases, the γ phase appears, with is a face centered tetragonal (fct) structure, and a γ grain contains a hierarchical "nano-lamellae forming within micro-lamellae" microstructure. A micro-lamella consists of two variants, each variant has a pair of nano-lamellae, and they are {011} twin related. Owing to the introduction of lamellar γ, the ductility is improved. With the increase in Mn content, the compressive stress increases from 914 MPa to 2175 MPa, and the compressive strain increases from 14% to 26%. The martensitic transformation temperature of such series of alloys increases from 352 ℃ to 585 ℃. For Mn-rich Ni-Mn-Ga alloy, the ductility improvement is inferior to that of Ni-rich alloy, but the martensitic transformation temperature is higher.