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Vol. 70 (2006) No. 8 P 666-669



  Advanced Ni-base single crystal superalloys for turbine blade applications are precipitation strengthened by coherent γ′ phase; the particle size and the evolution of its morphology during creep can depict deformation behaviours. A primary ageing is often applied to the alloy after the solution heat treatment to control the γ′ size and refine the precipitates into regular/cuboidal shape for optimal mechanical properties. During turbine blade manufacturing, the primary ageing is combined with coating cycles. The present article examines the effects of primary ageing on a 4th and a 5th generation superalloys, TMS138A and TMS-196, respectively. Experimental observations indicated that alloys treated with 1100°C/4 hours condition could retain the coherency between the precipitates and the matrix, while 1150°C or above would result semi-coherent microstructures. During creep at 1000°C/245 MPa, pre-crept coherent microstructures led to formations of fine/continuous raft for best creep resistance; by contrast, pre-crept partially-coherent microstructures evolved into coarse/discontinuous raft. The effect of pre-crept coherency appeared to be less significant for both alloys during creep at 900°C/392 MPa, when the driving force of microstructural evolution would be lower.

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