Review paper
Improvement of Creep strength in a nickel-base single-crystal superalloy by heat treatment

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Abstract

The effect of two heat treatments on the creep behaviour of CMSX-2, a recently developed nickel-base single-crystal turbine blade alloy, was investigated in the temperature range 760–1050 °C. It is shown that the heat treatments had no effect on the γ′ volume fraction, the composition of precipitates and the coherency strains. However, the heat treatment resulting in cuboidal aligned precipitates, 0.45 μm in size, leads to a two-fold improvement in creep lives over the corresponding values obtained with the other heat treatment which produces smaller and more odd-shaped particles. Transmission electron microscopy on specimens crept to various strains at 760 °C showed that, in contrast with the odd-shaped particles, the cuboidal precipitates promoted a very homogeneous deformation which led to the lower creep rates. The precipitate shearing mechanism in the late stages of primary creep is shown to be by {111}〈112〉 slip regardless of the heat treatment. In secondary creep the shearing occurs by (α/2)〈110〉 dislocation pairs but the difference in creep behaviour is thought to be associated with the higher stability of dislocation networks associated with cuboidal precipitates. At 1050 °C the more perfect rafted structure produced by the cuboidal precipitates is responsible for lower creep rates. The γ′ rafts at high stresses were sheared by (α/2)〈110〉 dislocation pairs.

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