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10/30/03

12:32 PM

Page 2793

Dehydrogenation of Nanocrystalline TiH2 and Consequent Consolidation to Form Dense Ti V. BHOSLE, E.G. BABURAJ, M. MIRANOVA, and K. SALAMA Dehydrogenation of nanocrystalline TiH2, produced by pulverization of commercially available powder, has been examined in detail by a combination of thermal analysis, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The dehydrogenation to form Ti occurs as a two-step process involving the formation of an intermediate phase, TiH. In-situ experiments on dehydrogenation inside a transmission electron microscope reveal the possibility of a powder-metallurgy process for consolidation of Ti components by vacuum annealing of nanocrystalline TiH2 at
0.5Tm, where Tm is the melting point of Ti. Near-full densification of Ti has been achieved by sintering nanocrystalline TiH2 under vacuum at
0.5Tm.

I. INTRODUCTION

ONE of the processes for producing Ti powder is the hydride-dehydride process. In this process, Ti sponge is hydrided to make the brittle TiH2, the hydride is pulverized to make fine powder, and this is then dehydrogenated under vacuum at about 700 °C to 800 °C to make the Ti powder.[1] A number of hydrides of Ti are identified in the literature, and the well-documented phases are TiH2, TiH1.924, TiH, and TiH0.71.[2,3,4] Even though dehydrogenation of the highesthydrogen-containing phase, TiH2, has been studied by many researchers, the sequence of transformation leading to the formation of Ti is still not understood. A basic understanding of the dehydrogenation process is essential for improving the known applications of Ti-H compounds and solid solutions and to develop new applications. Zhang and Kisi[5] and Padurets and Shilov[6] recently examined the decomposition of TiH2 by thermal-analysis techniques. Zhang and Kisi compared the dehydrogenation of nanocrystalline TiH2 produced by reaction milling with commercially available powder, using thermogravimetric analysis (TGA). Their study showed a difference in dehydrogenation temperatures between nanocrystalline hydride produced by reaction milling and that obtained from commercial sources. The high rate of decomposition of ultrafine TiH2 as compared to micronsized powders has been attributed to the large surface energy associated with a nanoscale crystal size. The formation of solid solutions or stoichiometric compounds in the Ti-H system and the subsequent removal of hydrogen from the binary system by heat treatments have been utilized in different materials-processing applications. Dehydrogenation of
-Ti-H solid solution has been utilized to manipulate the microstructure of Ti, by using hydrogen as a temporary alloying element.[7] The hydride-dehydride process also finds applications in storage of hydrogen[8] and synthesis of fine-grained Ti-based alloys.[9–15] In fine powder form, TiH2 finds applications in bonding. In the mid-1950s, Hall demonstrated the use of TiH2 for bonding diamond V. BHOSLE, Graduate Student, E.G. BABURAJ, Research Associate Professor, M. MIRANOVA,