Creep of precipitation-hardened nickel-base alloy single crystals at high temperatures

Abstract

Single crystals of a γ′ precipitation-hardened nickel-base super alloy, Mar-M200, were tested in constant load creep at 1575°F. It was found that shear of the γ′ precipitate by pairs of α/2 (110) dislocations controlled deformation in both primary and steady-state creep. This contrasts with 1400°F creep behavior where shear of γ′ is dominated by α/3 (112) dislocations in primary creep, but by pairs of α/2 (110) dislocations in steadystate creep. The orientation dependence of the steady-state creep rate at 1575°F is explained by the nature of dislocation junction reactions for the different orientations. Crystals along the [001]-[1•11] boundary have the greatest creep resistance because of the formation of stable dislocation networks at the matrix-particle (γ⊃ interfaces, whereas the lower creep resistance of crystals oriented along the [001]-[0•11] boundary is a consequence of the low probability for the formation of stable junction reactions. Finally, evidence, in the form of resolvable α/2 (110) dislocation pairs within the γ′ precipitate, is presented for a reduction in the local antiphase boundary energy of γ′ at high temperatures.