Influence of continuum precipitates on intergranular fatigue crack growth of a P/M nickel-based superalloy

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This paper examines the role of microstructure, in particular, the size and volume fraction of secondary (γ' s) and tertiary (γ' t) γ' precipitates, on intergranular crack growth in P/M IN100. A series of different heat treatments were carried out on the as-received material to vary the γ'statistics through control of the different features of the solutioning, stabilization, and aging parts of the heat treatment cycles. Dwell fatigue crack growth experiments have been performed on specimens having as received as well as heat treated microstructures at 650°C and 700°C in air with a 0.5Hz loading frequency with and without a dwell time. Results of the as received material show that the dwell crack growth rate, while dependent on the temperature level, is independent of the hold time period. For the modified microstructures tested at 650°C, the dwell crack growth rate is shown to be sensitive to γ' variations in the surrounding continuum. This sensitivity was illustrated by correlating the crack growth rate and the corresponding continuum yield which shows the dwell crack growth rate decreases with the increase in the yield strength. The influence of the continuum yield on dwell crack growth was first examined numerically by modelling an intergranular crack path surrounded by a continuum region represented by a coarse crystal plasticity model while the far field continuum is represented by an internal state variable model. The grain boundary path considers grain boundary sliding and a critical sliding length which is implemented as a fracture criterion. Results of this model show that the grain boundary sliding is influenced by the viscous strain developing in the surrounding continuum. An increase in the continuum yield strength accompanied with a lower viscous strain results in higher constraint on the grain boundary sliding and thus a decrease in the crack growth rate. This observation was further examined by calculating the individual contributions of the solid solution, γ' s, and γ' t to the total yield strength of the continuum. © 2012 Elsevier B.V.

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Materials Science and Engineering A