High-Temperature Tensile Properties of a Cobalt-Based Co-20Cr-15W-10Ni Superalloy with a Bimodal Grain Structure

Abstract

Cobalt-based superalloys are common materials for the manufacturing of various components used in aerospace applications. Conventional cobalt-based superalloys with a unimodal grain structure generally exhibit low strength and ductility at high temperatures. A bimodal grain structure of a cobalt-based superalloy, Co–20Cr–15W–10Ni (CCWN), was designed to achieve both high strength and ductility at high temperatures. The deformation behavior and tensile properties of a CCWN alloy with unimodal fine-grain (FG), coarse-grain (CG), and bimodal (FG/CG) structures were investigated at 900 °C. The microstructures and substructures after high-temperature deformation were examined via electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to determine the deformation mechanisms. The microstructural observation showed that the bimodal grain structure consisted of FG and CG domains. During high-temperature deformation at 900 °C, the FG structure was mainly deformed by dynamic recrystallization (DRX), maintaining a similar FG structure. The CG structure was mainly deformed by DRV, resulting in a small amount of DRX grains and a large amount of dynamic recovery (DRV) grains. However, the bimodal grain structures were mainly softened via DRX and transformed into a new bimodal structure, ultrafine grain (UFG) and FG. The FG domains tended to deform via dislocations, and the CG domains via twinning. The high-temperature tensile tests revealed that the bimodal-structured alloy exhibited both higher strength and ductility than those of the alloy samples with unimodal FG or CG structure. This is associated with the newly developed UFG/FG structures in the bimodal grain-structured samples during high-temperature deformation. This work may provide new insight into the development of high-temperature alloys with bimodal grain structures.