Effect of thermal aging on microstructure, hardness, tensile and impact properties of Alloy 617

Influence of thermal aging on microstructural evolution and its effect on hardness, tensile and impact properties have been studied for Alloy 617. Aging was carried out at 1023K up to 20,000h duration on a solution annealed as-received alloy. Detailed microscopic investigation of the alloy revealed the precipitation of γ´ phase (rich in Al and Ti) along with M23C6 carbides during aging treatments. Thermal aging has imparted increased hardness and strength, however with decreased ductility and toughness. Yield stress (YS), though initially increases up to 5000h of aging, and has shown a decreasing trend from 5000 to 10,000h. Further aging up to 20,000h has shown an anomalous increase in YS. Charpy impact test for 20,000h aged sample showed a reduction in fracture energy (~ 85%) w.r.to as-received material along with a shift in fracture mode from transgranular ductile to intergranular brittle. The change in hardness and YS in aged Alloy 617 has been attributed to the aging-induced evolution of γ´ precipitates. The intergranular fracture associated with lower impact energy, as observed after prolonged aging duration, has been attributed to the grain boundary embrittlement originating from precipitation of M23C6 carbides at the grain boundaries.