Tension/compression asymmetry in yield and creep strengths of ni-based superalloys

The tension/compression asymmetries of yield and creep strengths of three kinds of single-crystal superalloys-PWA1480, CMSX-4, and TMS-75 - and a DS superalloy, Mar-M247LC, were investigated at intermediate and high temperatures. In PWA1480, tensile yield strength was higher than the compressive strength from 20°C to 750°C From the TEM observation, it was found that the asymmetry of yield strengths is primarily due to the microtwin formation associated with a superlattice extrinsic stacking fault (SESF). In CMSX-4 and TMS-75, tensile/compressive yield strengths were comparable at every temperature, and shearing of γ′ precipitates by a/2〈110〉 dislocations pairs was the dominant deformation mechanism in both tensile and compressive tests at 750°C The creep response of these materials were quite different than their yielding response. CMSX-4 and TMS-75 showed distinctive creep tension/compression asymmetry. These two alloys showed large creep strain caused by {111}〈112〉 slip at 750°C under tensile stress, and mechanical twins at 750°C and 900°C under compressive stresses. Tension/compression asymmetry of CMSX-4 and TMS-75 was larger at 900°C than at 750°C because 〈112〉 viscous slip was not observed under tensile stress at 900°C The asymmetric nature of PWA1480 was small at both 750°C and 900°C because the dominant deformation mode in both tension and compression is a combination of both a/2〈110〉 dislocation's bowing on the {111} plane in the matrix and climbing along the γ/γ′ interfaces