A Study on the Chemical and Thermal Stability and Mechanical Properties of Nanocrystallization on Zr-4 Alloy Surface Based on Mechanical Lapping

Abstract

The nanocrystallization of the surface of industrial Zr-4 alloy is realized by means of the mechanical lapping technology. XRD diffractometer and transmission electron microscopy are applied to analyze the grain size, the micro-hardness and the micro-structure of Zr-4 alloy at different annealing temperatures. Meanwhile, a X- ray stress gauge is adopted to measure the stress properties of Zr-4 alloy. The results have indicated that mechanical lapping technology can effectively realize the surface nanocrystallization of industrial zirconium, and Zr-4 alloy can form a gradient nanometer crystal structure on its surface after mechanical lapping. After SMAT processing, the diffraction characteristics of Zr-4 alloy have experienced significant changes. The grain orientation has transformed in the process of SMAT treatment and the diffraction peak intensity of the sample changes with the increase of SMAT treatment time. The size of the grain on the alloy surface tends to decrease first and then increase. When the treatment time is 15mins, the grain size reaches a minimum value of 19.6nm. The higher the annealing temperature is, the lower the micro-hardness of the alloy surface (100μm) is. When the annealing temperature is 250°C, the micro-hardness of the material is substantially no difference from that without the annealing process. When the treatment time of SMAT augments, the strain rate and the residual stress of Zr-4 alloy indicate an increasing trend, and the increase process is in rapid growth in the early stage and in slow growth in the later stage. After 60mins of treatment, the residual stress of Zr-4 alloy sample reaches 381.5MPa and the average micro-distortion is 0.42%.