热处理工艺对Ni-Bi合金晶界硬度影响的纳米力学探针分析

晶界脆化问题一直是提高金属材料服役性能所必须关注的重要问题,但目前并没有直接的实验方法来确定晶界的硬度,因而也无法直接判定晶界的脆化状况。以经过不同工艺热处理的Ni-Bi二元合金为研究对象,利用光学金相显微镜和透射电镜观察了合金的微观组织,同时利用纳米力学探针进行了合金晶界和晶粒基体载荷—位移曲线的测定,并计算出了晶界和晶粒基体硬度的变化规律。结果表明,Ni-Bi二元合金的晶界和晶粒基体都不存在沉淀相;晶界硬度要低于晶粒基体的硬度,而且时效时间较长的合金,其晶界硬度更低。由此,确认了纳米力学探针在测量晶界脆化方面的可行性,并分析确定了晶界软化的原因:Bi的晶界偏聚。

Inf l uence of heat treatment on the hardness at grain boundary of Ni-Bi alloy analyzed by nano indenter

Grain boundary embrittlement is an important problem for the service of metallic materials. However, there is no experimental method to detect hardness of grain boundary directly, and therefore embrittlement of grain boundary cannot be evaluated. Ni-Bi alloys after various heat treatments were used to study hardness of grain boundary. The microstructure was observed by optical microscopy and transmission electron microscope. Load-displacement curves for both grain boundary and bulk were obtained by nano indenter. Afterwards, the hardensses of grain boundary and bulk were calculated and the variations in hardness were analyzed. The results show that there is no precipitate in bulk or at grain boundary. The hardness of grain boundary is lower than that of bulk. The longer the aging time of sample, the lower the hardness of grain boundary. On the bases, the feasibility of nano indenter in studying hardness (or strength) of grain boundary is confirmed. Besides, the cause of the lower hardness of grain boundary is determined: grain boundary segregation of Bi.