Structure stability study on a newly developed nickel-base superalloy - Allvac® 718Plus™

Abstract

Recently a new Ni base superalloy to be used at 700°C has been developed by ATI Allvac designated as Allvac® 718Plus™. The critical criterion for this alloy is long time structure stability at 700°C that is closely related to the nature of phases and their stability in this alloy. In this study phase identification of 718Plus at as-heat treated and long-term thermal exposed conditions was conducted by various means. Phase prediction of 718Plus was calculated by Thermo-Calc software and the detailed phase identification was conducted by SEM, TEM, EDS, SAD and XRD. Quantitative determination of different phases were conducted by electrolytic phase isolation and followed by micro-chemical analyses. The results show that alloy 718Plus is predominantly γ' strengthened alloy with very good structure stability at 700°C. The formation of δ phase occurred during heat treatment and thermal exposure. A new phase Ni3Al 0.5Nb0.5 with HCP structure was found. The phase precipitation behavior in combination with good mechanical properties makes 718Plus a very good candidate superalloy to be used at 700°C.