A coupled, physics-based matrix-grain boundary model for creep of carbide strengthened nickel-based superalloys - II. Experimental results and model application

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Materials which precipitate secondary phases in intergranular regions suppress grain boundary sliding directly as the carbides act as pinning sites resulting in dislocation accumulation and the generation of back stress. Materials which precipitate in intragranular regions suppresses grain boundary sliding indirectly by limiting the rate at which dislocations arrive to the boundary, Reference [1]. It is considered that grain boundary sliding necessitates a supply of extrinsic dislocations that are released from the matrix during creep. As carbides act as obstacles to dislocation motion, their presence in the matrix suppresses the rate at which dislocations can arrive to the boundary to facilitate grain boundary sliding. Aging and stress relaxation experiments have been conducted on the nickel-based superalloy, Inconel 617, to investigate the role of intragranular and intergranular carbides on grain boundary sliding. All specimens were solutioned for 2 h at 1200 °C and water quenched prior to testing to dissolve remaining secondary phases. Following solutioning, aging at 1000 °C for increasing durations produced unique microstructural parameters of carbide size, spacing, and volume fraction within the grain and along the grain boundary. Carbide size, spacing, and volume fraction and grain size were all observed to increase with increasing exposure; for up to 30 h of aging. Stress relaxation tests at 780 °C with an initial stress of 200 MPa were then conducted on the aged specimens to investigate and analytically model the rate controlling properties of matrix and grain boundary carbides as they pertain to grain boundary sliding according to the model formulation presented in Reference [1]. Relaxation rate of Inconel 617 drastically decreased after aging at 1000 °C for 3 h with little variation in relaxation rate up to 30 h of aging prior to testing. Using the model presented in Reference [1] combined with the experimental results presented below, a reload test has been simulated, considering the combined role of both the matrix and grain boundary on the overall sliding rate.

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Materials Science and Engineering A