The role of 〈112〉{111} slip in the asymmetric nature of creep of single crystal superalloy CMSX-4

A number of slip systems are known to operate in commercial Ni-base single crystal superalloys, one of these being the {111}〈112〉 system which is particularly influential in the primary creep regime. The asymmetric nature of this dislocation activity with regard to the direction of slip under creep conditions is highlighted in this paper. Testing has been undertaken on CMSX-4 at temperatures of 1223 and 1023 K. The [001] and [256] orientations have been investigated under tensile and compressive loading conditions. It has been demonstrated that in addition to stacking fault generation, twinning of the complete two-phase structure is a major source of creep strain accumulation under certain conditions. These are dependent upon activation of suitable a/3〈112〉{111} partials which are a function of the sign of the resolved shear stresses. TEM analysis and EBSD measurements have shown that twin nucleation is dependent upon the activation of suitable 〈110〉{111} type slip within the matrix channels, but once it has taken place deformation can proceed rapidly, only being hindered if a suitable network of dislocations within the matrix channels can be created to halt the progress of the twinning partials.