Effects of Grain Boundary Precipitation on Creep Rupture Properties of Alloys 706 and 718 Turbine Disk Forgings

Abstract

Creep rupture strength and ductility of Alloys 706 and 718 turbine disk forgings were investigated in relation to the effect of grain boundary precipitation introduced by an intentional stabilizing treatment in the temperature range of 780 to 900 “C. After the two step age hardening treatment at 720 and 620 “C, creep strain curves to rupture were continuously measured at test temperatures of 600°C. Significant improvements of the rupture life and ductility were found in the wavy grain boundary microstructures of stabilized alloy 706 with needle shaped and cellular precipitations of 7 phase. These precipitates appear to originate at a grain boundary and to grow in specific intragrain directions, which result in wavy shapes of grain boundary. In contrast with this, straight grain boundaries with globular and plate-like precipitate were observed in the stabilized alloy 718, which always caused the decreasing of creep rate and rupture life. It may be concluded that the beneficial effect of stabilizing treatment for alloy 706 is attributed to the wavy grain boundary formation and the dispersions of creep strain at these wavy boundaries. Introduction Ni Fe base superalloys 706 and 718, which are age-hardened by combined precipitation of fine y ’ and y ” in austenitic structure, have been widely used for higher temperature services such as gas turbine disks ‘JU5). For Alloy 706, a stabilizing heat treatment is proposed to improve the creep rupture life, which is performed to produced grain boundary precipitation between solution annealing and age-hardening heat treatments @+I. In order to understand the effect of the stabilizing heat treatment on tensile and creep rupture properties of Alloy 706, metallurgical and mechanical examinations were carried out in relation to the effect of grain boundary precipitation. In this study, morphology of the precipitates and shape of the grain boundary of Alloys 706 and 718 were intentionally varied by changing the stabilizing temperature between 780 and 900°C. And, results obtained in short-term tensile and creep tests for Alloys 706 and 7 18 were compared based on metallurgical points of view. Significant improvements of rupture life and ductility were Superalloys 718,625,706 and Various Dcrivatlves Edited by E.A. Loria The Minerals, Metals & Materials Society, 1994