Modelling elastoplastic and room-temperature creep behaviors in Titanium alloys with hierarchical structures

Titanium (Ti) alloys possess excellent performance on the strength and creep resistance, which can be effectively manipulated through microstructure and crystal orientations. Most works and models have focused on single-order approaches, one microstructural feature, or phenomenological frameworks. There are few deformation-mechanism-driven and theoretical models with integrate hierarchical microstructure composite. In this work, the elastoplastic and room-temperature creep behaviors are measured in these dual-phase Ti alloys, and the hierarchical structures are experimentally observed in the dual-phase Ti alloys: the first order of hierarchical structures consists of the soft phase and the hard phase attributed to differences in crystal orientation at mesoscale, and the second order is about the composite structure composed of the equiaxed α phase and the secondary (α+β) transition phase at the microscopic level. Within the framework of the micromechanics, the elastoplastic constitutive model for the hierarchically structured Ti alloys is developed through considering the microstructural compositions from SEM and the grain-crystallographic orientation from EBSD, as well as the dislocations pile-up along the phase boundaries. Furthermore, the room-temperature creep constitutive model is also addressed through modifying the Theta model and involving the creep steady-state dislocation motion for the dual-phase Ti alloys with hierarchical structures. Theoretical results demonstrate that the simulated elastoplastic curve and creep constitutive curves are agreeable well with experimental measurements, including the yield stress, strain hardening, and the steady-state creep rates under different applied stresses. The proposed constitutive models are further utilized to forecast the mechanical properties and the creep features of dual-phase Ti alloys with various grain size and volume fraction of phases: the larger grain size and the higher volume fraction of soft phase can lead to the lower yield strength and the higher steady-state creep rates, while the increased interfaces ratio results in the higher yield strength and the lower steady-state creep rates. Crucially, the proposed theoretical modeling framework introduces a novel methodology to characterize the mechanical behaviors of hierarchically structured materials. Namely, based on experimental observations and the framework of elastoplastic mechanics, a theoretical model describing the mechanical properties of materials is established through the micromechanical theory and the crystal dislocation theory. These findings and the proposed models could be helpful for optimizing the mechanical properties of dual-phase Ti alloys such as the yield strength, strain hardening rate, creep performance by designing the composites orientations and distributions of microstructures.