Formation mechanism of nano-sized η and ω structures in β phase in ECP treated Cu-40Zn alloy

Abstract

Two nano-scaled hexagonal distortions in high temperature BCC phase during cooling is commonly observed in many alloys systems. Although efforts have been made on studying the lattice softening of the BCC structure during cooling, the distortion mechanisms are less addressed. Thus, the formation of two hexagonal structures in the β precipitates in a Cu-40Zn alloy treated by ECP was thoroughly investigated. Results show that the β precipitates contain two kinds of nano-sized and diffuse atomic clusters one with a η structure obeying the Burgers OR and the other a ω structure obeying the Blackburn OR. They were each formed through a two-stepped atomic displacement. For the η structure, the first step is the atomic shuffle of each second 110β plane in the <11¯0>β direction and the second is a structure change mainly by a {1 1¯ 2} < 1¯11>β shear. For the ω structure, the first is an atomic shuffle on each second and third {112¯}β plane in the ± [111]β directions and then normal strains in three mutually perpendicular directions. The concomitant formation of the two structures minimizes the lattice distortion of single formation. The present results provide new information on the formation mechanism of the two hexagonal structures in a BCC matrix.