Review and Analysis of Powder Prior Boundary (PPB) Formation in Powder Metallurgy Processes for Nickel-based Super Alloys

Powder metallurgy of nickel-based super alloys have been developed and used for a wide range of products, owing to their excellent high temperature rupture life, creep strength and fatigue crack growth resistance. Typical processes for high performance PM super alloys include hot isostatic pressing (HIPing), hot extrusion and hot isothermal forging. Hot isostatic pressing is normally conducted at a high temperature by using metal forming standards at low pressure for a long time. However an intrinsic problem, i. e. powder prior boundary (PPB) precipitate networks, can occur in the HIPing process. In this paper, a comprehensive review of PPB formation in the powder metallurgy of super alloys is presented. Research on PPB formation process from room temperature to high temperature and during HIPing, has been reviewed and PPB formation mechanisms are discussed. Methods to reduce PPB effects on the mechanical properties of nickel-based super alloys, including heat treatment, changing the proportions of composition elements, and plastic deformation to break up the PPB networks, are reviewed.