A new nickel-base superalloy designated as Allvac ® 718Plus ™ has been developed by ATI Allvac to be used at 700°C. For phase characterization, heat treatment selection and structure stability evaluation in high temperature range, the determination of TTT diagram for alloy 718Plus is not only a basic research issue but also an important matter for the alloy application at high temperatures. The conventional alloy 718 has been also studied for comparison. Both alloys were solution treated at 1040°C for full solution of precipitated phases except stable carbide MC. After solution treatment both alloy samples were heat treated at 649 to 954°C for the times from 3-6 minutes till 100 hrs. Phase identification was mainly by means of SEM, XRD and partially TEM and SAD. Hardness test was also conducted on the samples as the indication of γ" and γ' precipitation hardening. The experimentally determined T-T-T diagrams and discussion on the phase formation in alloys 718 and 718Plus are described in this paper.

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.

Structure stability study on Alloy 71 8 disks with four different forginglheat treatment process combinations, i.e. Standard7 18, Super I 7 18,Super I1 7 1 8 and DA7 1 8, has been conducted with long time thermal exposure at 593 "C (1 1 00°F),649 "C (1 200°F) and 677 "C (1 250°F) till 2000hrs. A b.c.c chromiunl enriched phase a-Cr has been identified by XRD,SEM,TEM and SAD methods. The existence of a-Cr in Alloy 718 is also confirmed by Thermo-Calc method. The weight fraction o f a-Cr has been determined by special designed electrolytic isolation and micro-chemical analyses. a-Cr precipitates are often in small globular particles (-0.3 P m) and form at the vicinity of 6-Ni,Nb phase. More a-Cr particles will precipitate in Alloy 71 8 at higher exposure temperatures and longer times. Retained stress can promote a-Cr precipitation. The fractions of a-Cr in 4 different processed Alloy 7 18 disks are not very high. only less than l%wt. The highest fraction of a-Cr in DA718 after 677"C/2000hrs is about 0.62%wt and the a-Cr fraction in Standard 718 after 677"C/2000hrs is only 0.19%wt. Small amount of a-Cr in Alloy 718 does not has serious deleterious effect on 650°C creep crack propagation rate. However, the impact energy does decrease with the thermal exposure times at 649 "C and 677 "C. Siipcrallo! s 71 8. 625. 706 and Various Derivatives f3Iitt.d b>. E.:\. Loria ThlS (The Minerals. Metals & Materials Society). 2001

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