Abstract
An examination has been carried out of the microstructures of hot isostatically processed IN-738 turbine blades. Areas of void closure were found to be atypical of the bulk microstructure and showed recrystallization, twinning, residual gas micropores, and denudation of primary γ′. The effect of heat treatment following hot isostatic processing and solution treatment at 1200°C was investigated. Heat treatments involving controlled cooling at 140°C/hr from 1200°C to the primary aging temperature, followed by secondary aging at 845°C gave rise to the longest rupture lives in tests at 760°C and 586 MPa. Such heat treatments gave rise to finely serrated grain boundaries and a mixture of cuboidal primary γ′ and spheroidal secondary γ′. Appreciably faster or slower cooling rates gave rise to shorter rupture lives, and either planar grain boundaries, or serrated grain boundaries with overaged γ′.
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