The effect of nb, ti, al on precipitation and strengthening behavior of 718 type superalloys

Abstract

Inconel 718 characterizes with unique mechanical properties and has wild application in different high temperature industries. However, the ceiling temperature for Alloy 718 is 650 °C because of the instability of main strengthening phase γ-″-Ni3(Nb,Ti,Al). At high temperatures the meta-stable γPrime; changes for stable phase δ-Ni3 with large size and plate-like morphology. In result of that Alloy 718 looses structure stability and strengthening effect. Recently, many researchers have studied the stability of Alloy 718 and try to develop modified 718 Alloys to be used beyond 650 °C. The basic idea of this paper is to raise the stability of γPrime; and γ strengthening phases in the following ways: raising the solution temperature of y" and /, retarding the γ-″ phase change for δ-Ni3 and changing the precipitation behavior of γ-″/γ/ phase. The experimental heats were prepared for this study in variation of Nb, Ti, Al and the atomic ratio among them on the base chemical composition of Alloy 718. The precipitated phase γ-″ and γ behavior was studied in detail by means of SEM and TEM and the weight fraction of γ-″+γ phases was exactly determined by electrolytic isolation and followed by micro-chemical analyses. The mechanical properties including hardness, tensile properties and stress rupture for several heats were also tested. The structure of a new developed 718 type Alloy-Allvac® 718 Plus™ has been also adopted for comparison. A detail discussion has been conducted for 718 type superalloys in structure stability and strengthening effect both.