The Structure of Rene' 88DT

Abstract

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 super solvus 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.