块状LaNi_5基纳米合金的正电子湮灭研究

Positron Annihilation Research of Bulk LaNi_5 Based Nanometer Alloys

利用高温高压技术,在不同的压力和温度(～4.5GPa,～800℃)下将LaNi5基快淬合金粉直接压制成了块状纳米晶合金。X射线衍射分析表明,高压使其晶粒内部发生了明显的压致晶粒碎化,其平均晶粒尺寸在4.5GPa下从75.5nm降至24.6nm。利用正电子湮灭技术研究了这种晶粒碎化效应对纳米合金内部缺陷结构的影响。测试结果表明,在高温高压的作用下,由于界面上原子的迁移和弛豫加剧,导致晶界上尺寸较大的微孔隙缺陷逐渐转化为尺寸较小的自由体积缺陷,使得纳米合金的致密度逐渐增强,显微硬度逐渐升高,从而在高温高压下得到致密的块状纳米合金材料。

The LaNi5 based alloy powders were compacted into bulk LaNi5 based nanometer alloys by using the effect of pressureinduced crystallite breaking under high pressure and high temperature up to 4.5 GPa and 800 ℃. The Xray diffraction analysis results show that there is no change in the crystal structure of this nanoalloy under high pressure, but their mean grain size is reduced from 755 to 24.6 nm after compacted under 4.5 GPa at room temperature, and their mean dislocation density also increases for nearly fifteen times. This obvious crystallite breaking effect has a strong influence on their interface defect structure. It can be seen from the analysis of positron annihilation measurements that high pressure and high temperature have a strong influence on the defect structure of these LaNi5 based nanosolids. With the increase of temperature from room temperature to 800 ℃, most of the interface microvoid defects with larger size are changed gradually into the smaller free volume defects by the transport and diffusion of interface atoms, and so the densification of the nanosolids is enhanced gradually.