Variant Selection and Coarsening During Stress Aging and Creep Deformation of HAYNES^® 244^® Alloy

HAYNES^® 244^® alloy is a high-strength, low coefficient of thermal expansion, Ni-base alloy designed for use up to 760°C (1400°F). This alloy leverages a novel Ni₂(Cr, Mo, W) intermetallic phase, labeled the γ′′′ phase, to strengthen the alloy at elevated temperatures. Previous studies have shown that this alloy deforms through deformation twinning during long-term creep deformation at temperatures from 649°C (1200°F) to 760°C (1400°F). This deformation is mediated by the presence of the γ′′′ phase and the six orientation variants that form from aging. At longer creep times, these variants can coarsen along favored low-coherency strain directions to form highly elongated precipitates. However, although the variants can coarsen significantly, there appears to be no significant impact on the mechanical properties, such as strength and ductility, from this overaged coarsened material. This phenomenon is dissimilar to materials that leverage the γ′ intermetallic phase for strength, where rafting can negatively influence creep properties.