Modeling and Simulation of Deformation Caused by Phase Transformation from β to α in Ti-6Al-4V during Processing
Date:
Understanding the relationship between process conditions and microstructure evolution is important to materials manufacturers so they can control the processing conditions to produce a desired microstructure and the resulting macroscale properties. In this work, we are studying Ti-6Al-4V – a dual-phase alloy that is characterized by a vanadium stabilized body-centered cubic β phase and an aluminum stabilized hexagonal close-packed α phase. Due to the α+β nature of this alloy, a wide variety of microstructures are possible as a result of the thermomechanical processing. During the cooling cycle, the β to α transformation itself causes significant deformation that can influence the resulting α growth. For an unconstrained single β crystal, this transformation leads to ~10% contraction along <010>β direction and ~1.5% and ~10% expansion along the two perpendicular <101>β directions. This level of strain imposed on the surrounding β grain is expected to be well beyond the elastic limit. The vast majority of current models use elastic analysis to compute the effects such deformation may have on the local energy and α growth. For a more accurate model, we expect that plastic deformation must be considered. Understanding the material response and energy state as a result of the transformation is key to understanding the mechanics of transformation and will give us insight into why certain microstructures form under a given thermomechanical processing condition.
Here, we present a model to simulate the deformation caused by β to α phase transformation using a crystal plasticity finite element method. Transformation of a single lath of α is simulated and the α growth rate from [1] is used to define the time over which the transformation occurs. The phase transformation at each time step is introduced as a deformation gradient and the elastic and plastic deformations that can accommodate such a deformation are computed. The resulting strain energy, which contributes to the local free energy of the system and thus affects the nucleation and growth of α, is compared for simulation with and without plasticity to determine the impact of plastic relaxation on the driving energy in phase transformation.
This material is based on work supported by NSF under Award No. CMMI-1729336, DMREF: Adaptive control of Microstructure from the Microscale to the Macroscale.
References
[1] G.A. Kane, D. Andersen, M.D. Frey, and R. Hull, “The effect of cooling conditions on Ti6Al4V microstructure observed using high-temperature in-situ scanning electron microscopy,” J. Mater. Res., vol.36, no.3, pp.717–728, 2021