Hiroshi Kubo1 and Ken-ichi Hirano1
1Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai
Growth rates of bainite plates in Ag-27.6 wt%Zn and Ag-27.1 wt%Zn alloys have been measured and the activation energies for the growth have been determined.
In Ag-27.6 wt%Zn alloy, the activation energy for the growth of bainite plates in the lower temperature range (185 ∼ 225°C) is found to be 17 kcal/mol and different from that in the upper temperature range (above 225°C), 32 kcal/mol. In Ag-27.1 wt%Zn alloy, their growth in the upper temperature range is too fast to be measured, and the activation energy has been determined only in the temperature range of 195 ∼ 225°C to be 16 kcal/mol.
In both alloys, the mechanism of the bainitic transformation in the initial stage seems to be different from that in the later stage. In the initial stage the bainite plates are likely to grow by the shear mechanism accompanied with short range inter-diffusion, while in the later stage the transformation seems to proceed with the diffusion of both the species across the non-coherent interphase boundary.
From the values of the activation energy, it is deduced that the longitudinal growth of the bainite plates in the lower temperature range is controlled by the short range diffusion in the matrix, and in the upper temperature range it depends on the short range diffusion in the bainite plate.
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