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 degrees C. Mar-M247 and modified Mar-M247 alloys consisted of dual precipitation of primary and secondary gamma' particles in the gamma matrix. An increase in Re content led to a reduction in the primary gamma' phase with the gamma' phase becoming finer; however, the addition of Re did not have an obvious influence the secondary gamma' phase. Tensile and creep tests (871 degrees 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 gamma matrix and decreasing the inter-particle spacing. In the later creep stage, the linear fraction of the gamma' raft in stress axial direction increased following the addition of Re, resulting in improved creep resistance. Observation of microstructure evolution indicated that only secondary gamma' phase coarsened directionally into a rafting structure and the primary gamma' phase was unaffected by creep. An interrupted creep tests verified that the gamma' 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/gamma 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. (C) 2012 Elsevier B.V. All rights reserved.