초급냉 Al-Cr계 합금 의 열적안정성 에 관한 연구

Abstract

In this paper decomposition and thermal stability of the supersaturated solid solution of Al-Cr alloys were investigated. Al-1.2 wt%Cr alloys were rapidly solidified by using a small droplet chill quenching method. The decomposition process of the supersaturated Al-Cr solid solution by annealing at 350℃ to 600℃ and up to 60 hours was examined by means of microhardness and X-ray diffraction analysis. Hardness measurements on supersaturated Al-Cr solid solution annealed isothermally showed no sign of age hardening. The activation energy for decomposition was estimated to be 52.6㎉ and the thermal stability to be 0.76. The microstructural change during isothermal aging was that the fine, globular particles, which were nucleated heterogeneously along the cell boundary, were grown into chunky particles. The decomposition process was considered by establishing a cylindrical cell model and by using Johnson-Mehl equation. At the initial stage of the decomposition process the decomposition was governed by the diffusion controlled growth of a fixed number of particles (that was, the time exponent of Johnson-Mehl equation n＝3/2), and at the middle and final stage the decomposition was governed by the combination of the diffusion controlled growth of cylinders in axial direction only and growth on dislocation (that was, n＝1∼2/3).