Mechanism-based Constitutive Modeling of L12 Single-crystal Plasticity

Ni 3Al, an L12 structure intermetallic crystal, is the basic composition of the y' precipitates in nickel-based superalloys and is a major strengthening mechanism contributing to the superalloys' outstanding high-temperature mechanical properties. Many L12-structure crystals present unusual macroscopic mechanical properties, including the anomalous temperature-dependence of yield strength and strain hardening rate. To date, extensive research has been carried out to reveal the underlying mechanisms. However, none of the resulting models has satisfactorily quantified the macroscopic behavior based on microscopic phenomena. Mechanism-based constitutive modeling and simulation provide an effective method in this respect, assisting in the understanding and development of current existing models, and potentially providing a convenient path for engineering applications. In light of recent theoretical developments and experimental evidence, a single-crystal continuum plasticity model for the L12-structure compound Ni 3A1 is developed. Both the superkink-bypassing mechanism and the self-unlocking mechanism have been modified and combined to describe the unlocking of sessile screw dislocations and the deformation-induced evolution of dislocation "states" in the yield anomaly region. The proposed model has been implemented within a finite-element framework to investigate the mechanical properties observed in constant strain-rate uni-axial tension/compression tests. Results of numerical simulations successfully capture major features of the mechanical behavior of Ni 3Al-based single crystals, including the anomalous temperature-dependence and the strong orientation-dependence of yield strength and hardening rate, the yielding properties of tension-compression asymmetry and the strain-rate insensitivity of yield strength and strain hardening rate. Diffusional processes corresponding to the uniaxial deformation of [001]-orientation at very high temperatures, and the property of yield strength reversibility presented by the Cottrell-Stokes experiments are also discussed. Thesis Supervisor: David M. Parks Title: Professor of Mechanical Engineering 2