Thermodynamic consideration of formation mechanism of α₁ plate in βCu-base alloys

Abstract

For the possible ordering structures of the parent phase β′—i.e., B2 in Cu-Zn alloys, DO₃ in Cu-Al alloys, and B2 and L2, in Cu-Zn-Al alloys—the driving forces for the a, plate formation have been calculated for the three βCu-base alloys. The driving force, ΔG, is larger than zero for the β′ →α₁ shear mechanism and less than zero for the β′ → β′₁ + α₁ diffusional mechanism. Furthermore, the equilibrium temperature, T_o, between the parent β′ phase and the α in, plate has been evaluated to be much less than the experimental start temperature, B_s, of the α₁ plate formation. Therefore, the a! plate formation cannot be initiated by a diffusionless β′ → α₁ shear reaction without other causative factors such as stress field, defects, etc. However, the α₁ plate can be formed through a diffusional β′ → β′₁ +α₁ reaction. Results in Cu-Al alloys also show that the composition of the α₁ plate produced in the diffusional reaction has to deviate from that of the parent phase to a certain extent. For some compositions of Cu-Zn-Al alloys, ΔG^{α1(dis.)→α1(ord.)} < 0 within a temperature range. Thus, the fresh α₁, plate produced after the diffusional reaction may further transform into the ordered α₁ plate within this temperature range. Calculated driving forces for the disordered parent phase reveal that the ordering of the parent phase resists the α₁ formation in Cu-Zn and Cu-Zn-Al alloys and enhances it in Cu-Al alloys.