The Effects of thermal cycling on the creep performance of a single crystal alloy

International audienceThe superalloy CMSX-4 was subjected to thermal cycling between 900-1050ºC and constant load (200 MPa). Testpieces were given 3 mins exposure at 1050ºC, alternating with 9, 18 or 27 minutes at the lower 900ºC. The resulting creep strains and microstructures are compared with isothermal creep under each of the two conditions. In particular the dislocation configurations were analysed in the TEM using a variety of techniques. Thermal cycling was shown to accelerate creep strain, compared to the incremental addition of the isothermal exposure. Thermal cycling also increased the rate at which the rafts formed. The TEM revealed substantial dislocation activity in the  phase, increasing with the shorter thermal cycles. It is argued that the movement of the / interface, caused by the change in volume fraction with the temperature step, assists the entry into the  of dislocations from the misfit networks, thus increasing the creep rate particularly during the longer low temperature exposures compared to isothermal creep