Nanocrystalline titanium-magnesium alloys through mechanical alloying
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Wright-Patterson Air Force Base,
F. H. Froes Institute for Materials and Advanced Processes, University of Idaho, Moscow, Idaho 83843-4140 (Received 27 December 1989; accepted 6 June 1990) The solid solubility of magnesium in titanium under equilibrium conditions is reported to be extremely small. Mechanical alloying of a mixture of titanium and magnesium powders resulted in the formation of nanocrystalline (10-15 nm in size) grains of Ti-Mg solid solution. This solid solution has a metastable fee structure with a = 0.426 nm and contains about 3 wt. % (6 at. %) magnesium in it. It is suggested that the fee structure has formed as a result of the heavy mechanical deformation of the hep structure introduced during milling. High temperature annealing of the metastable solid solution led to its decomposition forming the equilibrium phases, viz., elemental titanium and magnesium.
I. INTRODUCTION
There is at present a great deal of interest in developing light-weight structural materials for aerospace applications. Addition of magnesium to titanium can produce very light alloys with a high specific strength. Several attempts have been made in the past to produce titanium-magnesium alloys by conventional methods and these have met with only a limited success. This was mainly because magnesium, with its boiling point well below the melting point of titanium, cannot be retained in the molten metal using conventional melting methods. On the basis of theoretical considerations (HumeRothery criteria) there should be appreciable solid solubility of magnesium in titanium. From Table I it is clear that (i) the atomic size factor is favorable with a difference of only about 10%, (ii) the crystal structures are the same (hep), (iii) the electronegativities are not very different, but (iv) the valency of titanium is 4 while that of magnesium is only 2. It is only the last factor which can prevent the possibility of forming extensive solid solutions in this system. The solid solubility of magnesium in titanium under equilibrium conditions has been reported to be small.1 Most of the experiments have involved alloying in the liquid phase, with high pressures superimposed to suppress magnesium evaporation. However, other non-conventional methods such as rapid solidification from the melt,2 solid-state reaction in the form of sin"'Permanent address: Department of Metallurgical Engineering, Banaras Hindu University, Varanasi-221 005, India.
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http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 9, Sep 1990
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tering,3 sheath rolling,4 and extrusion5 have also been investigated, again with only a marginal success. Recently, mechanical alloying (a completely solid-state powder processing technique) has been shown to be capable of dissolving about 3 wt. % magnesium in titanium in a metastable state.6'7 In the light of recent reports8"12 that nanocrystalline materials can be produced through mechanical alloying, further investigations have been conducted and the purpose of the present paper is to report on
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