The effect of applied stress on the high-temperature creep behaviour and microstructure of NiMoCr Hastelloy-N® alloy

Abstract

The high-temperature creep behaviour and microstructural evolution of Hastelloy-N® was investigated using miniaturised creep samples tested under vacuum at 973 K (700 °C) and stresses of 100 MPa and 165 MPa. The higher applied stress reduced the creep life of the alloy sevenfold, and the creep mechanism at 165 MPa was predominately dislocation-creep while the creep mechanism at 100 MPa was a combination of dislocation creep, diffusion creep and grain boundary sliding. The post-creep microstructure examination using Electron Back-Scatter Diffraction (EBSD) technique showed significantly larger number of Low-Angle Grain Boundaries (LAGBs) and Geometrically-Necessary Dislocations (GNDs) formed during creep at 165 MPa than at 100 MPa. On the other hand, the microstructure of the sample tested at 100 MPa revealed more pronounced precipitation of secondary carbides along High-Angle Grain Boundaries (HAGBs) due to the longer exposure to high temperature. The precipitation of secondary carbides along grain boundaries resulted in grain boundary embrittlement and the promotion of intergranular cracking, which then resulted in low strain-to-failure in the low-stress creep test sample. In addition, it is shown that the prolonged exposure to the elevated temperature lead to Cr depletion from the matrix, reducing solid solution strengthening during creep.