The Expansion of the Cu-Zn Binary Mixed Powder Compacts during Heating Process

Abstract

In order to investigate the mechanism of thermal expansion during heating the Cu-Zn binary mixed powder compacts containing up to 90 wt% Zn, their dimensional changes were measured continuously by the differential dilatometer. The phase changes and the diffusion behabior during heating were determined by the differential thermal analysis and by the line analysis of the X-ray microanalyser, and the results obtained were summarized as follows.
All the specimens expanded gradually from 200°C to 419°C on the heating process, corresponding to the amount of the γ (Cu₅Zn₈)-phase formed through the zinc evaporation. However, just above the Zn-melting point of 419°C the amount of the γ-phase increased remarkably, and then the β-phase was produced by the interdiffusion between the γ- and the α-phase, where the spaces of the molten zinc which diffused into the γ- and the β-phase were abruptly replaced by the exuvial cavities, corresponding to the amount of the abnormal expansion above 419°C. The amount of the abnormal expansion increased with increasing zinc content up to 60 wt% Zn in the compact, but over this content, decreased with increasing zinc content, where the content of 60 wt% Zn coincides with the chemical composition of the γ-phase.
Consequently, it can be confirmed that the γ-formation induces the swelling of Cu-Zn compacts during heating process.