Adjusting mechanisms for ultrafine-grained microstructures during hot deformation of Ni−38Cr−3.8Al alloy via pre-ageing precipitation
The influence of pre-ageing temperature on dynamic recrystallization (DRX) and microstructure evolution during hot compression of Ni−38Cr−3.8Al alloy was investigated. Five samples with varying pre-precipitation states were fabricated. Pre-ageing treatment precipitates α-Cr phases in lamellar and particle forms. During subsequent hot deformation, pre-precipitated α-Cr lamellae undergo dissolution fragmentation and spheroidization, transforming into finer particles due to elevated temperature and high-density dislocations. At 560 °C, an incomplete discontinuous precipitation (DP) state restrains DRX, producing necklace-like microstructures. Above 640 °C, a complete DP state with fully lamellar structures promotes DRX, yielding ultrafine-grained (UFG) microstructures. Coarse α-Cr particles enhance DRX through particle-stimulated nucleation (PSN) and discontinuous DRX mechanisms, while dissolved α-Cr lamellae promote DRX via continuous DRX. DRX kinetics analysis indicates that increasing pre-ageing temperature accelerates DRX, evidenced by reduced critical strain and peak strain, and increased DRX volume fraction. To achieve UFG microstructures during hot deformation, fully lamellar structures should be precipitated during pre-ageing. These findings provide a processing pathway for tailoring microstructures in high-Cr nickel-based alloys.