Strain-Induced Phase Transformation and Anisotropic Lattice-Strain Development in a Cobalt-Based Superalloy

This dissertation addresses two issues concerning the deformation behavior of a cobalt-based superalloy: 1) the strain-induced face-centered-cubic to hexagonal-closepacked phase transformation and 2) the anisotropic lattice-strain development. Cobaltbased alloys are known to exhibit a metastable face-centered-cubic crystal structure at room temperature. This structure can transform to the thermodynamically-stable hexagonal-close-packed phase upon application of a load. While this phenomenon has been studied and documented before, this work offers a more in-depth study on this issue, as it addresses the phase transformation under several loading modes: monotonic tension, monotonic compression, stress-controlled high-cycle fatigue, and strain-controlled lowcycle fatigue. The experimental technique utilized in this research is neutron diffraction. This nondestructive method is particularly well suited to the problem because it can be used to study the transformation with the mechanical load applied in-situ.

Anisotropic lattice strain development is a natural problem to study, as neutron diffraction is also well suited for this measurement. Individual crystallites in a polycrystalline matrix by nature have directionally-dependent responses to the applied stress. When neighboring anisotropic grains interact, inhomogeneities known as intergranular strains develop. This dissertation assumes that the total lattice strain is the sum of two contributions: the linear-elastic contribution and the plasticity-induced intergranular contribution. Within the context of this analysis, the effect of the phase transformation on the deformation behavior is also inferred.