Effects of heat treatment on microstructures and mechanical properties of a directionally solidified cobalt-base superalloy

DZ40M alloy is a newly developed directionally solidified cobalt-base superalloy. The present work investigated microstructures, room-temperature tensile and stress–rupture properties at 980°C/83 MPa of the alloy in as-cast, solutionized as well as aged states. The microstructure of the DZ40M alloy can be modified by heat treatment. Solution treatment at 1280°C for 4 h dissolved the primary carbides essentially and the alloy became a single-phase supersaturated solid solution. Incorporation of aging treatment at 950°C for 12 h produced a profusion of secondary M23C6 precipitation throughout the matrix. The room-temperature mechanical properties of the alloy are mainly dependent on the microstructures of the matrix. During the stress–rupture tests, the microstructural evolution occurred in the alloy, the primary carbides dissolved sluggishly and the secondary M23C6 precipitated heavily. The precipitation hardening is the most important strengthening mechanism at high temperature for the alloy in all three states. The stress–rupture properties were dominated by both the matrix and the boundaries of grains and interdendrites. The aged alloy has a superior stress–rupture property, which is attributed to its good microstructural combination of the matrix and the boundaries of grains and interdendrites.