An Analysis of Void Deformation Caused by Power Law Creep and Diffusion Enhanced by Mechanical Stress in HIP Process

A dominant deformation mechanism in an isostatic pressure sintering process of a powder compact is known as power law creep. The deformation of a pore was predicted from the existing stress analysis based on power law creep assuming a hollow sphere and a hollow cylinder. The deformation at high temperature (T> 0.4 Tm) involves both power law creep and diffusion enhanced by mechanical stress. The analysis performed by previous investigators included only the deformation caused by power law creep. The analysis in this paper is considering for both the deformation caused by power law creep and diffusion in the final stage of the HIP process for a hollow sphere and hollow cylinder models. The contribution of the diffusion mechanism to the total densification is investigated. The experimental results published for CoO are compared with the analytical result for power law creep only and for power law creep and diffusion. The results show that the effect of diffusion on the total densification is insignificant for densities of the order of 80 percent of theoretical at low applied pressure, and for a small vacancy diffusion coefficient (D_v). However, the contribution of diffusion is increased in the high densification region (ρ>0.95), with high applied stress and high D_v. It is concluded that the diffusion mechanism enhances the densification and its rate in the final stage of HIP process.