Study of Structure and Deformation Pathways in Ti-7Al Using Atomistic Simulations, Experiments, and Characterization
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INTRODUCTION
TITANIUM-BASED alloys are widely used for engineering applications ranging from aircraft turbine engines to biomedical implants.[1,2] Certain industrially important a + b alloys such as Ti-6Al-4V are known to have complex two-phase microstructures with wide ranges in length scale, resulting in complicated deformation mechanisms,[3] leading to difficulties in applying in situ characterization techniques such as high-energy X-ray diffraction microscopy (HEDM)[4] to study their deformation behavior. The composition and deformation behavior of Ti-7Al are similar to the a phase in many near-a and a + b titanium alloys, yet Ti-7Al can be processed to have a more simple microstructure (large equiaxed grains with minimal orientation gradients and without the presence of b),[5,6] which is more suitable for HEDM experiments. Quantifying the elastic
properties and the energy barriers for slip propagation in Ti-7Al would help in understanding the behavior of the a phase in more practically relevant alloys. Furthermore, in HEDM experiments, the values of elastic constants are critical in order to obtain accurate values of grain-averaged stresses from the lattice strains that are obtained directly from experiments. Although there have been recent experimental studies on the deformation behavior of this alloy,[6–15] the structure and the deformation characteristics of the alloy have not, to our knowledge, been studied at the atomic scale. In this work, we have created an atomistic model of Ti-7Al based on TEM imaging of the atomic arrangements, and validated the model by comparing elastic constants calculated from the model to the experimental values. We then utilized the model to explore the deformation behavior of this alloy.
A. Deformation Modes of Ti and Its Alloys AJEY VENKATARAMAN and MICHAEL D. SANGID are with the School of Aeronautics and Astronautics, Purdue University, 701 W. Stadium Avenue, West Lafayette, IN 47906. Contact e-mail: [email protected] PAUL A. SHADE and ADAM L. PILCHAK are with the Materials and Manufacturing Directorate, AFRL/RXCM, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433. R. ADEBISI and S. SATHISH are with the University of Dayton Research Institute, Dayton, OH 45469. G. BABU VISWANATHAN, MATT C. BRANDES, and MICHAEL J. MILLS are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted August 3, 2016. Article published online March 6, 2017 2222—VOLUME 48A, MAY 2017
The structure of Ti-7Al is expected to be composed of nanodomains of ordered a2 phase of Ti3 Al in a solid solution matrix of a phase of Ti, due to the coherency of the nanodomain and the processing route of the material.[16,17] The a phase has a HCP crystal structure and the a2 phase has a D019 crystal structure. The three slip systems for a HCP crystal are shown in Figure 1. The deformation behavior of pure a-Ti has been studied extensively. Titanium, which is the a phase in Ti-7Al, has the lattice parameters of a = 2.95 A˚ and c = 4.68
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