Chemical sensitive positron-annihilation study of amorphization and nanocrystallization processes by repeated roll bonding

摘要

A positron annihilation study of free volumes was performed on Cu-Zr and Al-Sm alloys in the course of repeated cold rolling (RCR) in order to contribute to the microscopic understanding of the complex processes of solid-state amorphization and nanocrystallization. In addition to positron lifetime spectroscopy, which yields information on the size of free volumes, two-dimensional Doppler broadening technique was applied in order to study the local chemical environment of free volumes on an atomistic scale. Both in Cu_(60)Zr_(40) and Al_(92)Sm_8 a characteristic variation of the chemical environment of free volumes with the number of folding and rolling (F&R) cycles could be observed. In the first (Al-Sm) and intermediate state (Cu-Zr) of F&R, free volumes with an enhanced amount of Sm or Zr content in the local environment occur, indicating interfacial segregation or the formation of solute-vacancy complexes. Upon further F&R cycling, a complete (Cu-Zr) or partially amorphous structure (Al-Sm) is obtained with the free volumes exhibiting a chemical environment characteristic of the average chemical composition. In contrast to melt-spun or ball-milled amorphous alloys, free volumes of the size of a few missing atoms are found in amorphous alloys prepared by RCR presumably due to strong athermal conditions of F&R cycling.