Effects of Al 3 Ti and (Al,Ni) 3 Ti Addition on the Sintering of TiB 2
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Effects of Al3Ti and (Al,Ni)3Ti Addition on the Sintering of TiB2 Masashi Yoshida Ube National College of Technology, 2-14-1 Tokiwadai, Ube, Yamaguchi 755-8555, Japan
ABSTRACT Titanium diboride has been sintered using spark plasma sintering (SPS). With the addition of tetragonal Al3Ti or cubic (Al,Ni)3Ti, the Vickers hardness, Hv, of TiB2 increases up to as high as 2000 by the sintering at 1273K, while that of the sample sintered without additives is as low as 20. Such a remarkable improvement is caused by the improvement of direct link between TiB2 grains in addition to the effect that Al3Ti or (Al,Ni)3Ti fills in the space between TiB2 grains and act as a binder. INTRODUCTION Titanium diboride has many desirable properties such as high hardness (Hv=3400), high melting point (3000K), low density, high electrical conductivity and good corrosion resistance [1]. It is a promising candidate for aircraft and automotive application, space vehicle thermal protection and so on [2]. However, a high sintering temperature above 2000K, is required to obtain TiB2 of high quality, which limits its application [2-5]. Titanium trialuminide (Al3Ti) has low density, high melting temperature (1600K), good high-temperature strength and good oxidation resistance [6-8], so it is also a promising candidate for aerospace and automotive applications. Tetragonal D022 structure of Al3Ti can be changed to cubic L10 structure by substituting some (~10%) of the Al by transition metals such as Ni, Cr and Fe.[9] Fu et al have calculated the adhesion work among TiB2, Al and Al3Ti and found a good binding between TiB2 and Al3Ti [10]. In the present study, TiB2-Al3Ti and TiB2-(Al,Ni)3Ti composites are fabricated using spark plasma sintering at 1273K. Structures and mechanical properties of the composites have been investigated. It has been shown that TiB2 and Al3Ti coexist in equilibrium at 1273K. High Vickers hardness, Hv, of around 2000 has been obtained for the composition of Al3Ti or (Al,Ni)3Ti of 30wt%. EXPERIMENT The raw materials were commercially available TiB2 powders of 99.9% purity, Ti powders of 99.5% purity Ni powders of 99.9% purity and Al powders of 99.9% purity (all powders by Furuuchi Chemical Corp.). The diameter of TiB2 powder particles was several Pm and those of Al Ni and Ti powders were around 10 Pm. To obtain fine Al3Ti and (Al,Ni)3Ti powders, Ti powders were hydrogenated by heating at 873K during 1 hour in H2 environment. Then, powders of TiHx, Al and Ni were mixed, pulverized and heated at 973K during 1 hour in Ar environment. By this procedure, fine powders of Al3Ti or (Al,Ni)3Ti of several Pm in diameter could be obtained.
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The weight ratio of Al3Ti in the starting materials were chosen as 0% (pure TiB2), 10%, 20%, 30%, 40%, 50% and 100% (pure Al3Ti). The weight ratio of (Al,Ni)3Ti in the starting materials were chosen as 20% and 30% where the mole ratio of Ni:Al is chosen as 15%. A graphite die, having an internal diameter of 20mm and a wall thickness of 10mm was filled with 5g of the mixed powders, sealed by two g
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