Discontinuous Cellular Precipitation in a High-Refractory Nickel-Base Superalloy

摘要

A discontinuous precipitation reaction has beeninvestigated in a high-refractory content nickel-base alloys .Thereaction transforms the two-phase γ-γ' parentmicrostructure into a three-phase cellular structurewith a γ' matrix containing Re-rich P-phase andagglomerated γ lamellae .The reaction has beenstudied in polycrystalline material and in bicrystalswith varying degrees of boundary misorientation attemperatures in the range of T/Tm=0.78 to 0.85 .Theearly stages of the reaction are characterized byheterogeneous nucleation of P-phase precipitates andmigration of the grain boundary .At low-angle ,near-tilt boundaries misoriented by less than10 deg ,nucleation of P-phase particles wasobserved ,but the cellular reaction did not occur ,dueto limited boundary mobility and diffusivity .The highdegree of supersaturation of Re and W in the initialγ-γ' alloy appears to be the primary driving forcefor the reaction .Small amounts of creep deformationdid not significantly influence the extent of thetransformation .The diffusivity of Re associated withthe moving boundary was calculated to be 5×10-8cm2s-1at 1093℃ ,which is approximately four orders ofmagnitude greater than the bulk lattice diffusivity of tungsten.