Introduction

As an important advanced functional material, the memory effect of nitinol shape memory alloy (SMA) is the focus of research. According to the current research, the memory function of the alloy decreases after welding, and there is no sufficient explanation for the phenomenon.

Objectives

For the problem, this research is to explore the underlying causes of the decrease of shape memory function after welding by analyzing the microstructure and micro defects.

Methods

The vacuum electron beam welding tests of 1 mm thick Ni₅₀Ti₅₀ alloy plate was carried out to determine the appropriate welding process parameter. And the shape memory function of the welded joint was compared with that of the base metal to analyze the change of memory function.

Results

It was found that the shape memory function of the welded joint decreased significantly under different strain variables. And the phase transition temperature also changed.

Conclusions

This was due to the micro stress field produced by non-equilibrium solidification in molten pool promoted the formation and propagation of dislocations, increasing the dislocation density in the martensite. Dislocations entangled with each other in the martensite, showing a grid-like distribution, which destroyed the integrity of martensite substructure. At the same time, the twin substructure of martensite was often accompanied by vacancies, dislocations, stacking faults, and a consequently large stress field formed between twin planes due to lattice distortion. Secondary twin was identified inside martensite under micro shear stress, where the martensite showed the bending state. The habitual relationship between martensite phase and parent phase was destroyed, resulting in the decrease of shape memory function.

中文翻译：

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电子束非平衡凝固过程中镍钛诺合金形状记忆功能退化机理的理解

介绍

作为一种重要的先进功能材料，镍钛诺形状记忆合金（SMA）的记忆效应是研究的重点。根据目前的研究，焊接后合金的记忆功能下降，对此现象尚无充分的解释。

目标

针对该问题，本研究通过对显微组织和微观缺陷的分析，探讨焊后形状记忆功能下降的根本原因。

方法

_{对1 mm厚的Ni 50} Ti ₅₀合金板进行真空电子束焊接试验以确定合适的焊接工艺参数。并将焊接接头的形状记忆功能与母材的形状记忆功能进行比较，分析记忆功能的变化。

结果

发现不同应变变量下焊接接头的形状记忆功能显着降低。并且相变温度也发生了变化。

结论

这是由于熔池中非平衡凝固产生的微应力场促进了位错的形成和扩展，增加了马氏体中的位错密度。位错在马氏体中相互缠绕，呈网格状分布，破坏了马氏体亚结构的完整性。同时，马氏体的孪晶亚结构常伴有空位、位错、层错，从而因晶格畸变在孪晶之间形成较大的应力场。在微剪应力作用下，在马氏体内部发现了二次孪晶，马氏体呈现弯曲状态。马氏体相与母相的惯常关系被破坏，导致形状记忆功能下降。