A micromechanical study of heat treatment induced hardening in {\alpha}-brass

TL;DR

This study investigates the heat treatment-induced hardening in alpha-brass, revealing that increased deformation twins at specific temperatures enhance strength by impeding dislocation slip, with microstructural analysis confirming the mechanism.

Contribution

It uncovers the specific role of deformation twins in heat-induced hardening of alpha-brass, disproving other microstructural factors as contributors.

Findings

Deformation twins increase at ~220°C, enhancing hardness.

Heat treatment effects are dominated by twin development, not recrystallization.

In-situ XRD confirms dislocation-twin interactions as the hardening mechanism.

Abstract

The mechanisms that govern a previously unexplained hardening effect of a single phase Cu-30wt%Zn {\alpha}-brass after heating have been investigated. After cold-work, the alloy possesses an increased yield strength and hardening rate only when heat treated to temperatures close to 220{^\circ}C, and is otherwise softer. Crystallographic texture and microstructure, explored using electron backscatter diffraction (EBSD), describe the deformation heterogeneity including twin development, as a function of heat treatment. When heated, an increased area fraction of deformation twins is observed, with dimensions reaching a critical size that maximises the resistance to dislocation slip in the parent grains. The effect is shown to dominate over other alloy characteristics including short range order, giving serrated yielding during tensile testing which is mostly eliminated after heating. In-situ X-ray diffraction during tensile testing corroborates these findings; dislocation-related line broadening and lattice strain development between as worked and heated {\alpha}-brass is directly related to the interaction between the dislocations and the population of deformation twins. The experiments unambiguously disprove that other thermally-induced microstructure features contribute to thermal hardening. Specifically, the presence of recrystallised grains or second phases do not play a role. As these heat treatments match annealing conditions subjected to {\alpha}-brass during deformation-related manufacturing processes, the results here are considered critical to understand, predict and exploit, where appropriate, any beneficial process-induced structural behaviour.