Enhanced densification of Ti-6Al-4V/TiC powder blends by transformation mismatch plasticity
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Marc R. Matsen Boeing Research and Technology, The Boeing Company, Seattle, Washington 98124
David C. Dunanda) Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 27 January 2013; accepted 29 March 2013)
Ti-6Al-4V alloy with attractive properties such as corrosion resistance and high specific strength has a broad impact on daily life in the field of aerospace and medicine. The addition of TiC to Ti-6Al-4V is to further improve abrasion resistance and hardness. To have a low processing cost and precise control of the TiC volume fraction and distribution, the composite is densified with a blend of Ti-6Al-4V and TiC powders through a powder metallurgy route. The densification kinetics of the blend is studied for uniaxial die pressing (i) under isothermal conditions at 1020 °C, where b-Ti-6Al-4V deforms by creep and (ii) upon thermal cycling from 860 to 1020 °C, where the a-b transformation leads to transformation superplasticity. Densification curves for both isothermal and thermal cycling for various applied stresses and TiC fractions are in general agreement with predictions from continuum models and finite element simulation models performed at the powder level.
I. INTRODUCTION
Ti-6Al-4V, the workhorse titanium alloy for aerospace and medical applications, has attractive properties, including excellent corrosion resistance and biocompatibility as well as high specific strength and stiffness.1–3 These mechanical properties, as well as abrasion resistance and hardness, can further be improved by incorporation of stable reinforcing ceramic particles such as TiC with high modulus and hardness.4–6 Titanium metal-matrix composites are usually processed by powder metallurgy, using pressure to densify metal/ceramic blends via hot pressing, hot isostatic pressing, or extrusion.4,7–9 Since residual porosity adversely affects ductility of these and most other MMCs, the densification of titanium/ceramic powder blend has been the subject of various studies. Densification modeling using continuum approaches7,10–13 considers plastic deformation of individual powders in the initial stage (for a relative density of up to 90%), and the collapse of individual pores within a continuous matrix in the final stage (for a relative density above 90%). Usually, densification occurs at constant temperature where the main mechanism for powder a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.95 2520
J. Mater. Res., Vol. 28, No. 17, Sep 14, 2013
http://journals.cambridge.org
Downloaded: 16 Mar 2015
deformation is dislocation (or power-law) creep.7,10–12 Ti-6Al-4V has a reversible transformation between its a and b phases, which spans a wide range of temperatures but occurs principally from 860 to 1000 °C.14 Enhanced densification kinetics of Ti-6Al-4V powders can be obtained during hot pressing while cycling about the phase transformation range of Ti-6Al-4V,15 as it activates transformation-mismatch plasticity which is a deformation mechanism
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