Abstract
Ni-38Cr-3.8Al (mass%) and Ni-38Cr-4.2Al alloys have γ single phase and low hardness as approximately 200 HV in solution treatment condition. On the contrary, cellular precipitation, which consists of α-Cr and γ/γ′ initiates from grain boundary by ageing treatment, and the hardness reaches over 700 HV after fully precipitated in whole grain. It has been reported so far that the cellular microstructure is formed by discontinuous precipitation, which is also observed in Ni-Cr binary alloys, and the addition of Al enhances the growth rate of the cell. However, the growth mechanism has not been cleared sufficiently. In this study, quantitative results of the microstructure applied on an ordinary cellular boundary diffusion model with thermodynamic equilibrium data to discuss the kinetics of the cellular precipitation growth in Ni-38Cr-Al alloys. Consequently, it was revealed the obtained diffusive activation energy was corresponding to that of Ni-Cr binary alloys, so that the cellular boundary diffusion dominated the cell growth in Ni-38Cr-Al alloys as well as Ni-Cr binary alloys. Meanwhile Al increases the activation energy, Al also increases the amount of Cr in the saturated matrix and pre-exponential factor in the diffusive Arrhenius equation extremely. Therefore Al enhances the growth rate of cellular precipitation in two or three order of magnitude more than those of Ni-Cr binary alloys.
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