Phase Transformations and Formation of Ultra-Fine Microstructure During Hydrogen Sintering and Phase Transformation (HSP
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THE hydrogen sintering and phase transformation (HSPT) process is a new method of producing titanium materials via the powder metallurgy (PM) route.[1–3] Traditional powder metallurgy titanium processes suffer from the trade-off between cost and performance. The blended elemental (BE) approach is, in general, a truly low cost approach;[4] however, the mechanical properties of BE Ti have fallen short of perceived potential. The pre-alloy (PA) approach typically delivers better mechanical properties, compared to BE Ti, with values for ductility and strength comparable to wrought Ti. However, the cost-difference between PA Ti and wrought Ti is often deemed insufficient to warrant the selection of PA Ti over wrought Ti. One of the issues that has plagued PM Ti, especially PM Ti-6Al-4V alloy, is that the microstructure of PM Ti is always too coarse PEI SUN and YANG XIA, Postdoctoral Fellows, ZHIGANG ZAK FANG and K.S. RAVI CHANDRAN, Professors, MARK KOOPMAN, Research Assistant Professor, and JAMES PARAMORE, Graduate Research Assistant, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] YANG REN, Scientist, is with the X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439. JUN LU, Scientist, is with the Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439. Manuscript submitted November 29, 2014. Article published online September 8, 2015 5546—VOLUME 46A, DECEMBER 2015
after high temperature sintering in vacuum. Specifically, the Ti-6Al-4V alloy typically has a coarse lamellar structure that is undesirable from the viewpoint of mechanical properties. Additional thermo-mechanical working can alter the microstructure and improve the properties, but doing so increases the cost of production. The HSPT approach addresses this coarse microstructure issue directly by achieving ultra-fine microstructure in the as-sintered state. Compared to conventional powder metallurgy approaches, which typically sinter Ti alloy powder compacts in high vacuum, HSPT sinters Ti powder compacts in a partial hydrogen atmosphere. Hydrogen partial pressure, and thus the hydrogen content in the Ti alloy, is controlled during the sintering and cooling processes. Hydrogen is used as a temporary alloying element to control the phase transformations during cooling, giving rise to the formation of ultra-fine microstructure in the as-sintered state. In contrast, conventional vacuum sintering of Ti-6Al-4V alloy yields a coarse lamellar structure with the width of a generally greater than 10 microns, depending on the cooling rate, while HSPT yields ultra-fine a lamellae with the width of a in the submicron range. Figure 1 illustrates the dramatic differences between the microstructure of Ti-6Al-4V alloys sintered in vacuum and in hydrogen. In both cases, titanium hydride (TiH2) powder blended with 60Al/40V master alloy powder was used as the starting material.[1]
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