Influence of microstructure and its evolution on the mechanical behavior of modified MAR-M247 fine-grain superalloys at 871°C

Abstract

Abstract This study added Re to Mar-M247 fine-grain superalloy and investigated the influence of microstructure and its evolution on the mechanical behavior at 871 °C. Mar-M247 and modified Mar-M247 alloys consisted of dual precipitation of primary and secondary γ ′ particles in the γ matrix. An increase in Re content led to a reduction in the primary γ ′ phase with the γ ′ phase becoming finer; however, the addition of Re did not have an obvious influence the secondary γ ′ phase. Tensile and creep tests (871 °C/379 MPa) showed that both tensile and creep strength increased with an increase in Re content up to a maximum of 3 wt%. In the early creep stage, the addition of Re improved creep resistance by increasing the strength of the γ matrix and decreasing the inter-particle spacing. In the later creep stage, the linear fraction of the γ ′ raft in stress axial direction increased following the addition of Re, resulting in improved creep resistance. Observation of microstructure evolution indicated that only secondary γ ′ phase coarsened directionally into a rafting structure and the primary γ ′ phase was unaffected by creep. An interrupted creep tests verified that the γ ′ rafting structure initiated in the primary creep stage and completed in the secondary creep stage. The addition of excessive quantities of Re, such as 5 wt%, resulted in the formation of needle-like P phase, which damaged the tensile and creep properties. During tensile and creep tests, cracks initiated and propagated along grain boundary (GB) in alloys containing 0–3 wt% Re; and propagated along both the GB and P/γ interface in an alloy containing 5 wt% Re. In conclusion, optimal results were obtained for fine-grain Mar-M247 following the addition of 3 wt% Re.