Abnormal grain growth in the Ni-based wrought superalloy GH4698 bar during heat treatment

Abstract

The microstructure evolution of the wrought superalloy GH4698 bar subjected to annealing was investigated during solution treatment, with managing temperature and duration. The differences in mechanical properties between the core and the edge of annealed rod are attributed to residual stresses introduced by surface grinding in the shallow layers of the bar. Experimental findings of heat treatment revealed that grains in the core of the bar grow normally, with the average size exhibiting exponential growth with respect to the solution temperature and time. Abnormally large grains (ALGs) were observed at the edge of the rod under sup-solvus temperature. The presence of γ′ can restrict grain growth, but the dissolution of γ′ is not the fundamental cause leading to the occurrence of abnormal grain growth (AGG). The initial reason for AGG at the edge position lies in the presence of stored energy (SE). Grains undergoing abnormal growth are not with high SE and the nucleus of ALGs originate from static recrystallization triggered by SE. Differences in grain orientation spread (GOS) between grains serve as the driving force for AGG. The abnormal enlargement of grains coincides with the expansion of twin boundaries (TB). Σ3n (n = 1, 2 and 3) boundaries play an indispensable role in the formation and growth of ALGs.

Keywords

• Wrought superalloy
• Abnormal grain growth
• Stored energy
• Grain orientation spread
• Σ3n boundary