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 TM . 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 Ni 3 Al 0.5 Nb 0.5 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.

TTT Diagram of a Newly Developed Nickel-Base Superalloy - Allvac 718Plus

“…While the phases found in alloy 718Plus are γ', γ", δ, and Ni 3 Al 0.5 Nb 0.5 phases [12]. The difference between alloys 718 and 718Plus is that the main strengthening phase in alloy 718 is γ", but the main strengthening phase in alloy 718Plus is γ'.…”

“…It confirms that the grain boundary precipitates are δ-Ni 3 Nb. Moreover, selected TEM analyses were conducted to confirm the presence of plate-like Ni 3 Al 0.5 Nb 0.5 phase [12] in addition to plate-like δ phase in alloy 718Plus after aging at 900°C and 954°C for some times.…”

The Effect of Nb, Ti, Al on Precipitation and Strengthening Behavior of 718 Type Superalloys

“…When the ratio of (Al+Ti)/Nb becomes higher in modified 718 alloys with high sum of Al+Ti+Nb(7-7.5%at) such as R=1.1-1.3, the other type of Ni 3 M can be found. In the case of high content of Al (such as Allvac 718PlusA and 718PlusB) the Ni 3 Al 0.5 Nb 0.5 (HCP) phase has been identified [19,20] (see Fig.6b,6c), which is different to δ-Ni 3 Nb not only in chemical composition but also in crystallographic structure, however its morphology is very similar to δ-Ni 3 Nb. Fig.5 The relationship of the γ″/γ′ fraction on the sum of (Al+Ti+Nb)%at contents in modified 718 alloys, the γ″ and γ′ precipitation behavior changes.…”

“…If the Al content is raised to a higher level (such as 1.5%wt in Allvac 718 PlusA),the main strengthening phase is changed from γ″ in Alloy 718 to γ′ in this modified 718 Alloy. Long time structure stability study at 760 result [19] shows that the average size of γ′ after 760 , 500h aging is still less than 100nm. In result of that this alloy characterizes with unique stress rupture properties at 700 .…”

Hold-Time Fatigue Crack Growth of Allvac 718Plus®

“…The observed effects from of long-term exposure at 760°C are precipitate coarsening and transformation of hardening precipitates to δ-phase [15,16]. The observed effects of 350 h exposure at 760°C in [15,16] was an increase in the γ' size, from 20 to 50 nm with a roughly constant volume fraction, and an increased fraction of δ-phase, from 1 to 3%.…”