Microstructural and texture evolution, deformation mechanism, and dynamic compressive response of electron beam melted Ti-6Al-4V under elevated and high strain rate deformations

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University of New Brunswick
This study presents a comprehensive investigation on the effect of high strain rate compression deformation on the microstructural and texture evolution, mechanical properties improvement, and deformation mechanisms of electron beam melted Ti-6Al-4V samples. Cylindrical rods of Ti-6Al-4V were printed in horizontal and vertical directions. Initial microstructure characterization revealed the microstructure consists of α and β phases in the form of basketweave and lamellar structures and grain boundary α along prior β-grain boundaries. Also, finer microstructure and lower interlamellar spacing were achieved in the vertical sample compared to horizontal ones. Elemental mapping analysis revealed the aluminum-rich α phase and vanadium- and iron-rich β phase. High strain rate compression tests were conducted using Split-Hopkinson pressure bar apparatus at various strain rates from 150s-1 to 2700s-1 and 150s-1 to 1900s-1 in horizontal and vertical samples, respectively. Increasing strain rate led to increments in yield strength, ultimate compressive strength, and total strain. Deformed vertical samples revealed a higher density of dislocations as the strain rate increased than deformed horizontal samples. At elevated strain rates, dislocations were configured randomly; however, dislocations tend to form dislocation cells at high strain rates. Deformation mechanism investigations demonstrated that the basal slip plane is the least favorable in the horizontal sample at each strain rate. 1st-order pyramidal slip planes were the dominant planes in the sample deformed at the strain rate of 2100s-1; however, only <c+a> type was active in the sample deformed at 700s-1. The basal slip plane had the least activity in the deformed vertically printed samples, and the 1st-order pyramidal slip system was the most favorable one. In-depth microstructural characterization of an adiabatic shear band showed α→β phase dynamic transformation, which showed 65-70% volume fraction of β phase. Moreover, ultrafine grains inside the detected adiabatic shear band were observed, which could result from substructures and sub-grains formation due to the movement, annihilation, and rearrangement of dislocations. Constitutive modeling based on the Chang-Asaro and GaoZhang-Yan equations showed that the Chang-Asaro model could perfectly predict the behavior of the mentioned alloy; however, the Gao-Zhang-Yan model is only applicable for the elevated strain rate deformation condition.