(A) study on the phenomenological transformation kinetic equation and the martensite structure in Cu-Zn-Al shape memory alloys

Abstract

Isothermal and non-isothermal aging behaviors of five Cu-Zn-Al shape memory alloys A (Cu-28.3Zn-3.1Al), B(Cu-26.4Zn-3.9Al), C(Cu-23.1Zn-5.1Al), D (Cu-18.2Zn-7.2Al), and E(Cu-12.8Zn-9.2Al) [wt%] have been investigated. The electrical resistivity during isothermal aging from 180 to 340℃ increases with aging time at each aging temperature. During the isothermal aging from 180 to 300℃, a bainitic transformation has occurred for the alloys A, B, and C, while the decomposition to the cubic $γ_2$ and FCC α phases has occurred for the alloy E at temperatures from 240 to 340℃. The time to reach a constant fraction transformed during the isothermal aging increases with increasing Al-content or decreasing Zn-content in the Cu-Zn-Al alloys, except the alloy D. The alloy D exhibits a double aging behavior due to the earlier formation of the $α_1$ bainite and the subsequent formation of the cubic $γ_2$ phase from the $β_1$ matrix phase. The apparent activation energy for the alloy D varies from 107 to 115 kJ/mol with the fraction transformed by the double aging behavior. In contrast, the apparent activation energies for the alloys A, B, and C are about 110 kJ/mol, while that for the alloy E is 100 kJ/mol. They are nearly independent of the fraction transformed. Therefore, the alloys A, B, C, and E exhibit isokinetic behavior. Change in the heat capacity for the five alloys have been measured in the temperature range from 100 to 500℃ at the various heating rates (3~20℃/min) by using DSC. All the reactions are exothermic, and the apparent activation energies estimated in the non-isothermal aging are found to be about equal to those obtained in the isothermal aging. The solvus line temperatures ($T_s$) of the alloys A, B, C, and E are 790, 730, 660, and 605℃, respectively. The higher $T_s$ temperatures of the alloys favor the earlier transformation during aging due to increased supersaturation in the matrix. By generalizing the impingement factor, a new comprehensive transfor...