Oxygen-induced amorphization of metallic titanium by ball milling
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Amorphization of metallic titanium by ball milling was presented. With the introduction of continuous pickup of impurities, hexagonally close-packed (hcp) titanium transformed gradually into an amorphous phase without experiencing any intermediate stage of forming a detectable metastable compound phase. The crystallization temperature of the obtained Ti metal glassy phase is about 640 K. The total concentration of the impurities (oxygen, nitrogen, iron, etc.) in the final product of the milled powders that was obtained after 60 h of milling was 10.85 at.%. The amorphization of metallic titanium may account for the combined effects of the pickup oxygen impurity in small amount and the Gibbs–Thompson effect.
I. INTRODUCTION
Mechanical alloying (MA), a highly developed synthesis method of metastable materials, has been widely used to produce nanocrystalline and quasi-crystalline materials, intermetallics, solid solutions, amorphous alloys, etc.1,2 In the process of MA, metal powders would be inevitably more or less contaminated. Contamination of the metal powders can be traced to (i) impurity of the starting powders, (ii) milling equipment (vial and grinding medium), (iii) atmospheric impurities in handling of the powders, and (iv) milling atmosphere. The impurity can lead to formation of nitrides in metallic Ti and Zr,3 amorphization of Ti-Zr, Ni60Nb40, and Nb25Sn powders.4–6 Meanwhile, the impurity, such as O, N, and Fe, can give rise to distinct differences in the behavior and products of crystallization for milled amorphous alloys, lattice parameter of milled crystalline phases, and viscosity of prepared amorphous alloys.5–7 Among all the impurities, the effect of oxygen on structural transformation of starting powders during MA has attracted great attention due to its chemical reactivity. Unfortunately, the effect of oxygen on structural transformation of starting powders has been documented mostly in binary and multicomponents alloy systems.1,2,5–7 It was reported that formation of metal glassy phase did not take place in TiAl24Nb11 alloy with 4.8 wt% O and in Ti6Al alloy with 44.8 at.% O.1 Koch et
al. prepared amorphous Ni60Nb40 alloys with 3.4 wt% O and without O, and Nb25Sn amorphous alloy containing 11.3 to ∼23.3 at.% O by MA.5,6 In such cases, oxygen influences on structural transformation of some base metallic element may be concealed or weakened by those from other metallic elements and/or the other impurities. Recently, Lucks et al. found that amorphization of metallic molybdenum did not occur with the introduction of about 33 at.% O.8 More recently, Manna et al. studied the effect of oxygen on structural transformation of metallic Ti9; they did not detect any metal glassy phase even under 16.5 at.% O. Hence, the exact influence of oxygen impurity on the formation of a metal glassy phase in binary alloy systems is not very clear. From a theoretical viewpoint, it is also of interest to know whether a metal glassy phase can be formed in a single metallic element under the effect of oxygen. Perhaps, proper
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