Influence of Storage Conditions on Powder Surface State and Hot Deformation Behavior of a Powder Metallurgy Nickel-Based Superalloy

Powder metallurgy (PM) FGH96 superalloy powders are stored under vacuum, argon, ambient air, and oxygen atmospheres for as long as 500 days. The surface analysis results demonstrate that the chemical state of Ni, Ti, Cr, Co, O, and C remain unchanged after 90 days storage. However, the oxygen content and NiO/Ni(OH)(2) layer thickness increase from initial values (similar to 120 ppm and similar to 3.8 nm) to stabilized values (similar to 200 ppm and similar to 10 nm) after a short time storage (7-15 days), and remain basically unchanged with the extension of storage time. Powders stored in oxygen atmosphere possess the highest oxygen content (maximum 213 ppm) while the lowest in vacuum, the gap can reach to 25 ppm. The hot isostatic-pressed (HIPed) parts that consolidated from original and stored powders are isothermally compressed at different conditions. The results indicate that HIPed parts with more oxygen will cause higher activation energy and narrower processing window due to a lower degree of dynamic recrystallization (DRX). Discontinuous DRX (DDRX) dominates the DRX nucleation mechanism of HIPed FGH96 superalloys with similar to 120-200 ppm oxygen content, while continuous DRX (CDRX) is the auxiliary mechanism. The increased oxygen content and surface NiO/Ni(OH)(2) layer thickness of superalloy powders generate higher oxygen content of corresponding HIPed parts, thus decreasing the hot workability.