Computational Study of Mobilities and Diffusivities in bcc Ti-Zr and bcc Ti-Mo Alloys
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ection I: Basic and Applied Research
Computational Study of Mobilities and Diffusivities in bcc Ti-Zr and bcc Ti-Mo Alloys Yajun Liu, Lijun Zhang, and Di Yu
(Submitted December 14, 2008; in revised form April 5, 2009) Based on the abundant experimental diffusion data and the thermodynamic parameters in the literature, the atomic mobilities of bcc Ti-Zr and bcc Ti-Mo alloys are critically assessed by means of the CALPHAD technique in this work. Comprehensive comparisons between the calculated and experimentally measured diffusion coefficients are made, where the presently obtained mobility parameters can satisfactorily reproduce most of the experimental data. Moreover, the atomic mobilities derived in the present work are successfully applied to reproduce some measured concentration profiles from diffusion couples in both binary systems and the displacements of Kirkendall makers in the Ti-Mo binary system. It is believed that the proposed atomic mobility parameters contribute to the establishment of a general Ti mobility database, which is useful in designing novel high-temperature Ti alloys.
Keywords
bcc, CALPHAD, diffusion, mobility, Ti-Zr, Ti-Mo
1. Introduction Because of the attractive combination of high specific strength, high modulus, low thermal expansion, good fatigue strength, and good corrosion resistance, Ti alloys can provide ideal engineering properties over a wide temperature range up to about 823 K for lighter and stronger structures in aerospace industries.[1] In addition, Ti, Zr, and Mo have been identified as non-toxic elements by tissue-reaction studies, as they show no adverse effect in human body.[2-4] As such, Ti alloys containing Zr and Mo are desirable biomedical alloys in that their biological and physical properties play an important role in the longevity of prostheses and implants. Knowledge of both thermodynamic and kinetic characteristics of Ti alloys is of critical importance in understanding how temperature, time, and compositions affect alloy microstructures in heat treatment.[5] Such information is not only useful to determine alloy stability under long-term service conditions but also valuable in dealing with processing designs. Inspired by the CALPHAD (CALcula˚ gren[6] tion of PHAse Diagram) method, Andersson and A suggested a similar procedure to represent the atomic mobilities of individual species in a multicomponent
Yajun Liu, Western Transportation Institute, Montana State University, Bozeman, MT 59715, USA; Lijun Zhang, State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, People’s Republic of China; Di Yu, American Water Chemicals Inc., Tampa, FL 33619, USA. Contact e-mail: pcbook@ hotmail.com and [email protected].
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solution phase. The activation enthalpy term and the logarithm frequency term are expanded by the RedlickKister polynomials, which provide a convenience means of advancing into higher order systems from the mobility parameters assessed for unary and binary systems. Such a treatment can provide an in-depth evaluation
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