Effect of Refractory Elements on the Coherency Strain and Flow Stress on Nickel-Base Superalloys.

Various refractory elements from groups IVB, VB and VIB are used in nickel-base superalloys to increase their mechanical strength and service life at high temperatures. Group IVB and VB metals partition primarily to gamma, dilate the gamma structure and increase its antiphase boundary energy. Group VIB metals partition primarily to the gamma matrix and dilate the gamma lattice and decrease its stacking fault energy. The alteration of the lattice parameters of gamma and gamma by these elements changes the effective coherency strain. Molybdenum and tungsten also reduce the solubility of aluminum in the matrix, whereas titanium reduces the solubility of molybdenum in the gamma precipitate. These interactions affect the resulting volume fraction of gamma and the gamma - gamma mismatch. Deformation in high coherency alloys proceeds by dislocation particle bypass through Orowan bowing, whereas in low coherency alloys, deformation requires dislocation-particle shear APB shear, SISF-SESF shear. The flow stress in Ni-15Cr-Ti-Al-Mo alloys is shown to be mainly a function of coherency strain at all temperatures.