Effect of Heat-treatment on Microstructure and Tensile Properties in Cast Alloy 718

Abstract

The effect of various types of heat-treatment on the mechanical properties of cast Alloy 718 has been investigated. Cast Alloy 718 bars were subjected to ‘standard heat-treatment’_(SHT), ‘HIP (Hot Isostatic Pressing) heat-treatment’_(HHT), and ‘HIP-sim-ulated heat-treatment’_(HS). In the absence of long time high temperature heat-treatment, a small amount of Laves phase remained in the ‘SHT’ specimen, and needle shaped δ precipitated in the vicinity of the Laves phase. Due to the formation of the Laves and δ phases in the ‘SHT’ specimen, it exhibited lower tensile properties than those of the others_specimens. On the other hand, the Laves phase was completely dissolved into the matrix after ‘HHT’ and ‘HS’ treatments. It is known that isostatic pressure reduces the self-diffusion coefficient, because of the lower self-diffusivity under HIP conditions in the interdendritic region, Nb segregation and the high amount of γ ” precipitation that occurs. Due to the higher fraction of coarse γ ” phases, the ‘HHT’ treated Alloy 718 showed excellent tensile