Age-hardenable alloy parts having melting points in excess of 1000.degree. C., in particular high temperature superalloys, characterized by the presence of such structural defects as cast micropores, and/or grain boundary voids or internal microcracks resulting from high temperature service, are improved in mechanical properties by subjecting said parts to hot isostatic pressure in an autoclave at selected elevated solution temperatures in excess of 50% of the absolute melting point of the alloy and superatmospheric pressures sufficient to remove substantially said defects followed by rapidly cooling the parts in situ from the selected temperature while maintaining the parts under superatmospheric pressure in the autoclave.