Computational modelling of ti-6al-4v microstructure evolution during thermomechanical processing

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Bhatt, Sagar
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Electronic thesis
Mechanical engineering
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The microstructure that results from thermomechanical processing of metals and alloys is directly responsible for the material’s mechanical, electrical, and thermal properties. Hence, the goal of this work is to model and simulate Ti-6Al-4V microstructure under thermo-mechanical loading in order to understand the mechanisms that guide the evolution of themicrostructure during processing. It is a dual-phase alloy that is characterized by a vanadium stabilized body-centered cubic (BCC) β phase and an aluminum stabilized hexagonal close-packed (HCP) α phase at room temperature. Ti-6Al-4V is typically deformed at elevated temperatures, followed by a prescribed heat treatment schedule designed to generate a suitable microstructure. On heating above 600℃, the α phase starts transforming into β phase. The phase transformation during this process is governed by the Burgers orientation relationship, where the α → β (heating) transformation can result in six possible β orientation variants and the β → α (cooling) transformation can result in twelve possible α variants due to crystal symmetry. Experimental observations show that variant selection, i.e. an apparent preference for certain variants over others, during heating (α → β) is negligible. The final microstructure is largely dominated by the β → α transformation where the initial α grains show features of the final texture, meaning that the mechanisms leading to variant selection occur mainly during the initial stages of cooling. Hence, it is vital to understand this process in order to understand why certain microstructures form under a given processing condition. As the material is cooling down, the β → α phase transformation can lead to significant deformation, well beyond the elastic limit of the surrounding β grain. A vast majority of current models, however, use elastic analysis to compute the effects such deformation may have on the local energy and α growth. The first part of this work looks at the impact of plastic relaxation during transformation-induced deformation on the subsequent growth of α. A model is developed to simulate the deformation caused by phase transformation using a finite element crystal plasticity method. Transformation of a single lath of α is simulated and the phase transformation at each time step is introduced as a deformation gradient. 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 simulations with and without plasticity to determine the impact of plastic relaxation on the driving energy in phase transformation. We show that when the surrounding β crystals are allowed to undergo plastic relaxation, the resulting strain energy is significantly lower when compared to the results from elastic analysis. This shows that existing models need to consider elastic-plastic deformation to accurately compute the driving energy of transformation. It was also found that increasing the growth rate of α increases strain energy density in and around the lath – suggesting that deformation due to transformation may also be an important consideration to understand microstructure morphology as a result of processing conditions. While it is important to study the cooling cycle, it is also necessary to look at the heating cycle and the warm working regime for a better understanding of processes that end at warm deformation and do not heat the material above β-transus. In order to study the microstructure evolution during the warm deformation conditions, following industriallyrelevant processing conditions, both the effects of annealing and deformation need to be studied simultaneously. The crystal plasticity model is calibrated for both phases at 800℃, and the deformation of an equiaxed α + β microstructure is simulated at 800℃ under 15% compressive load at a strain rate of 10−3 s−1. The Monte-Carlo Potts model, used to model grain-growth during annealing, is calibrated for various temperatures using literature data. For temperatures above β-transus, the model is calibrated using grain growth data at 1088℃. For temperatures below β-transus, the model is calibrated using grain growth data at three different temperatures – 650℃, 775℃, and 815℃, in order to capture the growth behavior in the warm working temperature range. To integrate these models, the polycrystal used in deformation simulations is used as the starting microstructure for the Monte Carlo model and the strain energy as a result of the deformation is mapped on to the Monte Carlo grid for the grain growth simulations. Grain growth simulations are conducted with and without strain energy density to compare the impact of prior deformation on grain growth. In presence of strain from deformation simulation, the grain growth gets more localized to the regions where there is significant gradient in energy across grain boundaries whereas without the strain energy, the grains grow irrespective of the phase to minimize the grain boundary energy. The soft β grains having relaxed first, have significantly lower strain energy than the surrounding α resulting the grain growth getting concentrated at the α/β interfaces. This work provides a process of integrating the stored elastic energy from deformation in Monte Carlo framework to study the effect of prior deformation on grain growth, with further work on multi-phase grain and phase evolution this integrated approach can be used to predict microstructure evolution during warm working accurately.
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Rensselaer Polytechnic Institute, Troy, NY
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