Effect of Stabilizing Treatment on Precipitation Behavior of Alloy 706

Abstract

Ni-Fe-base superalloy 706 has recently been used for high temperature services. A stabilizing treatment between solution-annealing and agehardening treatments has been proposed for this alloy to improve its creep rupture life. However, the relationship between stabilizing treatment and precipitation behavior has not been well understood. The precipitation behavior of Alloy 706 was investigated and related with creep rupture properties. Samples taken from a gas turbine disk forging were solution-treated at 980°C for 3h and stabilizing-treated in a range of 780 to 900°C for 1.5 h, followed by the double-aging at 720°C for 8h and at 620°C for 8h. Precipitation behavior of these samples was examined by TEM, and creep rupture tests were conducted at three conditions with varying temperature and applied stress.Fine Y'-~" co-precipitates having the core of y ' being overlayed with y " on its top and/or bottom and large y ' -y ' co-precipitate having the core of y ' being completely covered with y " were identified in the grain matrix together with large y ' and fine y It phases that have been found to precipitate in Alloy 706. 7 was identified at the gram boundary and found to be accompanied with a serrated grain boundary and denuded zone. Such precipitation behavior was significantly affected by stabilizing temperature, especially below 840°C. and so was creep rupture property accordingly. It is concluded that the best creep rupture properties are obtained when an optimum combination is established between intragranular fine y '/ y " co-precipitates, intra-granular large y '/ y " co-precipitates, and the inter-granular 7 phase.

Effect of Cooling Rate from Solution Treatment on Precipitation Behavior and Mechanical Properties of Alloy 706

“…The selected area diffraction pattern indicates that the 77 phase has a specific orientation relationship with the y matrix, as [Oil J, // [211011 and (111)9 // (OOOl)y. This is consistent with our previous work (11,13). The 7 phase appears parallel to each other in order to meet this orientation relationship.…”

“…These facts on p phase are thought to be due to Figure 7 : SEM micrographs at the gram boundary of aged Alloy 706 after cooled from the solution treatment temperature at rates of (a) 85"C/min, (b) 3"C/min and (c) O.S"C/min. relatively low cooling rates in Group B, and are consistent with the prediction from the "on cooling ITP" and the phenomena of Alloy 706 stabilized and aged (11).…”

“…In both cases, the coherency between y , y ' and y " is maintained. These two types of co-precipitates were also found in Alloy 706 stabilized and aged (11).…”

“…That TIP diagram indicates that four types of precipitates, that is y ', Y '0 overlaid y '-y " co-precipitate (so called, "non-compact morphology") and 7 , appear in Alloy 706. However, double cuboidal y '-y " co-precipitate (so called, "compact morphology"), which is known to be formed in Alloy 706 (11) under some specific heat treatment conditions, dose not appear.…”

The Role of Niobium in Wrought Precipitation-Hardened Nickel-Base Alloys

“…Like alloy 706, alloy 718 precipitates γ′′ as fine coherent platelets in the γ matrix. It is possible for the γ′′ to surround cubic γ′ on all its six faces under certain thermal conditions and specific ratios of (Al + Ti)/Nb (42). This morphology has been proven to retard coarsening.…”