Abstract
Precipitate shearing mechanisms during quasi-static elevated temperature tensile deformation and high temperature creep of Immm Ni2(Cr, Mo, W) strengthened HAYNES® 244® alloy have been investigated. Interrupted tensile tests from room temperature to 760 °C (1400 °F) and creep tests at 138 to 693 MPa (20 to 100 ksi) and 649 °C to 760 °C (1200 °F to 1400 °F) were conducted to experimentally determine the active deformation mechanisms under relevant operating conditions. Stacking faults were observed to form in precipitates at the start of deformation by transmission electron microscopy diffraction contrast analysis. During subsequent deformation in these conditions, these stacking faults thickened into deformation microtwins at higher strains to accommodate plastic strain, which is contrary to many other Ni-based alloy systems where superlattice intrinsic and extrinsic stacking faults or coupled antiphase boundary shearing take place. This microtwinning deformation offers unique advantages to ductility and prevents debits in yield strength due to over aging. Microtwinning was observed across all strain rates, stresses, and temperatures investigated. The relative fault energies for the formation of these planar defects are discussed and compared to Density Functional Theory calculations.
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