Kinetic Pathways of Phase Transformations in Two-Phase Ti Alloys
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INTRODUCTION
TITANIUM (Ti) alloys have been extensively utilized in aerospace applications, biomedical devices, and chemical processing equipment owing to their excellent strength to weight ratio and corrosion resistance.[1] Pure Ti has two allotropic forms, hexagonal-close packed (hcp) a and body-centered cubic (bcc) b, and it undergoes b to a allotropic transformation upon cooling. Most commercial Ti alloys in structural applications display (a + b) two-phase microstructures for high strength.[2] Incorporating 3d transition metals such as V, Mn, Fe, and Mo as alloying components which stabilize the b phase[3] makes it possible for both phases to coexist. The mechanical properties of Ti alloys are very sensitive to the spatial configurations of the two phases in the microstructure. Therefore, the prediction of microstructural evolution of the phases plays a key role in the optimization of the mechanical properties of Ti alloys. Different thermo-mechanical processing routes produce a wide spectrum of complex (a + b) two-phase microstructures such as fully lamellar structure (or basket-weave and Widmansta¨tten structures) and bimodal (duplex) structure containing lamellae with primary a phases displaying globular morphology.[2,4] There have been a number of experimental efforts to understand the phase transformations and microstructural evolution of binary[3,5–12] or multicomponent[13–16] Ti alloys. The phase transformations in Ti alloys are TAE WOOK HEO, formerly Postdoctoral Scholar with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, is now Postdoctoral Research Staff Member with the Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, CA 94550. Contact e-mail: [email protected], [email protected] DONALD S. SHIH, Technical Fellow, is with the Boeing Research & Technology, St. Louis, MO 63166. LONG-QING CHEN, Distinguished Professor, is with the Department of Materials Science and Engineering, The Pennsylvania State University. Manuscript submitted February 3, 2014. Article published online April 2, 2014 3438—VOLUME 45A, JULY 2014
associated with complicated competitions between nucleation-and-growth and spinodal decomposition, or between continuous and discontinuous displacive transformations. It is a significant challenge to distinguish the different mechanisms experimentally. For example, the spinodal decomposition process of an intermediate a¢ or a¢¢ phase with high solute content or b phase during the phase transformation is not easily detectable in experiments, since the decomposed solute-rich and solutepoor phases exhibit a very small difference in lattice parameters. Systematic theoretical analyses on phase transformation mechanisms have previously been employed to understand coupled kinetic processes. For example, Khachaturyan, Lindsey, and Morris theoretically investigated the equilibrium between a disordered solid solution and an L12 ordered phase as well as the phase transformation paths from a quenched disord
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