EFFECTS OF COOLING RATE AND GAMMA-' MORPHOLOGY ON CREEP AND STRESS-RUPTURE PROPERTIES OF A POWDER-METALLURGY SUPERALLOY

The 650 °C creep and stress-rupture (S/R) behavior of powder metallurgy (PM) RENÉ 95 alloy was investigated under various cooling rates from two subsolvus solution temperatures. The cooling conditions were chosen to alter the morphology of the cooling γ′ (γ′c), which constituted a high volume fraction (≃0.85) of all γ′ precipitates. The dispersion characteristics of γ c ′ changed from very fine at the highest cooling rate to progressively coarser with reduced cooling rate. At small γ c ′ (≃0.05 μm), a marked increase in creep and S/R resistance was observed, whereas at larger γ c ′ (≃0.1 μm), a marked decrease was observed. Also, for a givenγ c ′ size, both properties benefited from a higher solution temperature. The change in properties with the size of γ c ′ was associated with a change in deformation mechanism. The operative mechanism was essentially determined by the mean surface-to-surface spacing (l s ) of the γ c ′ precipitates. At l s > 0.05 μm, the Orowan mechanism of dislocation expansion and looping became favorable, which led to higher primary creep strain and steady-state rate and also reduced S/R life. At l s < 0.05 μm, dislocation motion was significantly restricted, and the low dislocation density and inadequate dislocation sources were responsible for greatly suppressed primary creep strain and steady-state rate, and in some cases, for an incubation period in the creep curve. In addition, the S/R life increased significantly at low l s . At l s ≃ 0.05 μm, a mixed mode of deformation was observed due to inherent distribution of particle spacing in the structure, and this gave rise to intermediate creep and S/R resistance. © 1990 The Metallurgical of Society of AIME.