Sub/micro-structural evolution of a Co-Al-W Ta-Ti single crystal superalloy during creep at 900 °C and 420 MPa

In order to clarify the high temperature strengthening mechanism of Co-Al-W-base superalloys, the tensile creep behavior of a Co-Al-W-Ta-Ti single crystal alloy was investigated at 900 degrees C and 420 MPa and sub/microstructural evolutions were analyzed in detail using interrupted creep tests by transmission electron microscopy (TEM). Four distinct creep stages were observed during the creep process. The sharp decrease of strain rate in the deceleration stage should be attributed to the activation of run and stop mechanism of dislocations. Detailed analysis revealed that the planar defects, which were enclosed by 1/6[(11) over bar2] partial dislocations in numerous isolated and contiguous gamma' precipitates, resulted in the increase of strain rate in the first acceleration stage. The planar defects are then activated on multiple {111} planes to form Lomer-Cottrell locks (L-C lock), which were responsible for the increase of creep resistance in the steady-state stage combined with the gamma/gamma' interfacial dislocation networks and the topological inversion of gamma/gamma' microstructures. This indicates both advantages and disadvantages of planar defects as one of the major dislocation configurations in Co-base superalloys.