The Role of Boron in the Mechanical Milling of Titanium-6 %Aluminium-4% Vanadium Powders
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a grain refining second phase [e.g., 1, 2]. Prior to this study, the clean mechanical alloying of gas-atomised Ti-6%AI-4%V (Ti-6-4) with 0, and 2 at.% boron powder under a purified argon atmosphere has been carried out [3]. These powders were consolidated by hot isostatic pressing (HIPing) at temperatures between 600 and 900 TC. For the 0 % boron alloy, it has been shown that milling results in nanocrystalline oc-Ti-6-4 (an hcp phase), which on consolidation grows rapidly (highlighting the low contamination levels in the milling process). The addition of 2 at.% boron to the alloy produces highly faulted/twinned boride plates and needles on consolidation (e.g., Fig. 1). These precipitates do provide good localized grain refinement, however unfortunately they are distributed inhomogeneously throughout the alloy. Therefore, it is clearly of interest to understand how the boron mixes in the milling process and so powder mixtures of gas-atomised Ti-6-4 with 25 and 50 at.% boron powders have been milled. Boron is known to add intensity to the ball-mill by decreasing the sticking of material to the vial [e.g., 4], but it has low solid solubility in titanium (< 0.2 at.%) [5]. Preliminary thermodynamic modelling of the titanium-boron system can be used to investigate any potential phase formation during milling (Fig. 2). Energetically it may be possible for boron to atomically mix with titanium to form an amorphous alloy. However, a mixture of cc-titanium and TiB with the same overall composition as an equivalent amorphous alloy may have a lower energy than the amorphous alloy. So, given that the MA 265 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
process can provide nanoscale mixing of the boron, there is potential for either of these phases to be formed. The present study will describe the microstructural development of the milled Ti - B powders and investigate the mixing of the boron.
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Figure 1, Energy filtered TEM images of a faulted/twinned boride precipitate in the titanium matrix of the consolidated 2% boron alloy, a) Bright field image of the precipitate, b) boron map (EELS jump ratio) and c) titanium map (EELS jump ratio). The precipitate shows boron enrichment and titanium deficiency relative to the matrix. Selected area electron diffraction patterns of needle and plate shaped precipitates in the 2 % boron alloy can be indexed to TiB in either the B27 or the Bf orthorhombic structures. TiB is expected to form in the B27 structure, however twinning of this can produce intergrowths of the Bf structure [6]. Furthermore vanadium boride forms in the Bf structure and so vanadium may stabilize the Bf precipitates. It is interesting to note that both these structures are similar and are based on a modified fcc titanium framework (afc, - 4.5 A). Figure 2, Gibbs free energy curves, at 300 K, of the Ti-B phases that may be formed during milling. The energies of a-titanium [7], an amorphous Ti-B mixture [7, 8], TiB and TiB 2 are plotted [6]. Mixtures with < 50 % B will have lower energy
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