As heat treated, Rene' 88 DT was found to contain some 42.5 % y ' , with both the cooling (24.5%) and aging (18.0%) forms exhibiting a very low positive (0.05%) mismatch to the matrix. Small amounts of gram boundary MsB2 TiNblCJ, and traces of MsBs,were also present. The rate of coohng from the solutioning temperature could be related to the resultant cooling y ' diameter by a regression equation, as could the aging conditions. The rate of cooling, as well as the grain size, were found to effect the gram boundary sermtions that were formed on cooling from the supersolvus annealing temperature. Both of these features, and thus the cooling rate, mav effect mechanical oronerties. On nrolonaed hiah tempemture' exposure, the y ' phases 1 grow, and more M&z precioitates above 650°C (1200°F 1. followed bv MT&L above 705 '"C (i300°F), and an inbagranul~ p phase at ?6O"C (1400°F). Simple regression equations were fitted to the kinetics of growth of both the cooling and aging y ' by the use of a Larsen-Miller parameter to normalize the-time and temperature of exposure. This abowed the mediction of the growth of the Y ' Dhases and the possible iden&ication of the average service tempe&ure that a part experienced. The growth of y ' during high temperature exposure, rather then any precipitation of deleterious phases, was found to limit the service range of this structurally stable alloy. Indeed, the formation of M&e and n start only after the aging y' had comnletelv re-solutioned. Re& 8SDT is an extremely stable alloy for prolonged 650°C (1200°F) service. Its sunerior orooerties reflect a hieh Y' content and a y "whose low positive mismatch allows a highly coherent and finer y' precipitation. These advantages are optimized by a super solvus annealing practice that eliminates large, sub-solvus y' phases, which promote crack nucleation in fatigue, and interfere with the formation of serrated grain boundaries.

and a gamma matrix high in Cr, Co and MO.

This study was partially funded by USAF Contract F33657-846-2011.

No abstract

Measurements of gamma prime (y') and gamma double prime (y") size were made on Inconel alloy 718 exposed for up to 50,000 hr's, in the temperature range of 538 to 704 "C (1000 to 1300 "F ). All materials were from either the original Barker, Ross and Radavich study, where the specimens were exposed under stress, or the more recent Korth work that utilized unstressed exposures. The diameter of the y' and y" phases, D' and D" ( in pm ), were measured and could be correlated through a Larsen -Miller parameter ( P = T [ 25+ log t 1 ) to the temperature ( T in OK ) and the time of exposure ( t in hr's ) . The following relationships were established : These relationships were derived using materials of varying chemistry, processing and heat treatment, exposed with and without stress. In addition, where suitable mechanical data were available, it was possible to establish regression equations between the y " diameter and simple mechanical properties. High resolution scanning electron microscopy, that allows accurate precipitate measurement, can thus be used to estimate service temperature and approximate residual properties in parts after prolonged service. This technique is effective regardless of the stress level, or slight, in specification, variations in chemistry, processing or heat treatment.Detailed AEM studies were performed on material exposed 50,000 hr's. at 649 C ( 1200 OF ). These lead to the definite identification of c(-Cr, a high Cr and MO Iaves phase, and small amounts of a sigma phase, as forming during prolonged exposure, in addition to the standard y ' , y I', delta, and MC phases. More extensive SEM studies, backed up by energy dispersive x -ray analysis, were utilized to follow these precipitation reactions over the whole range of time and temperature studied here. It was possible to conclude that the precipitation of a-Cr and the other minor phases in alloy 718 are driven by the almost complete rejection of Cr and MO that results in advance of the growing delta phases.