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

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.