Hierarchical Microstructure and Strengthening Mechanism of Haynes 188 Alloy Manufactured by Laser Powder Bed Fusion

Abstract

Co and Ni superalloys have been widely used in aerospace. Abundant studies have been focused on additive manufacturing of Ni superalloys. However, the equally important Co superalloy has less been fabricated by additive manufacturing. In this work, Haynes 188 alloy, a kind of Co-based superalloy, is fabricated by using laser powder bed fusion. The microstructure and mechanical properties before and after a stress relief treatment of the additive manufactured Haynes 188 alloys are investigated in detail. For as-built alloys without stress relief treatment, the thermodynamic modeling predicts that a phase of M 6 C exists in the specimen, which is confirmed by a TEM EDS mapping result. Arrayed stacking faults, dislocations, and M 6 C precipitations are widely found inside grains of the specimen without stress relief treatment. The yield strength and elongation of the specimen without stress relief treatment are 645 MPa and 57.3%, respectively. For the specimen with stress relief treatment, the dislocation density is reduced, the carbide precipitations are dissolved into the matrix, and stacking faults are arrayed near grain boundaries. As a result, the yield strength slightly decreases to 485 MPa but the elongation increases to 68.1%. The mechanical properties of fabricated Haynes 188 alloys in this work are superior to that of the published Ni or Co superalloys. This work provides an effective method to improve the mechanical properties of Co superalloys by additive manufacturing, which is important for the development of high-performance superalloys and their manufacturing methods.

Keywords: Co superalloy, Laser powder bed fusion, Dislocation, Stacking fault, Precipitation