Materials Science Forum Vols. 534-536 (2007) pp. 813-816 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland
Influence of High-energy Milling and Sintering Cycle on Obtaining TiAl from Elemental Ti and Al Powders P.G.Estebana, E.Gordob, E.M. Ruiz-Navasc Department of Materials Science and Engineering, Universidad Carlos III de Madrid, Avda. de la Universidad, 30, 28911 Leganés (Spain) a
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Keywords: TiAl, Titanium Aluminides, high-energy milling, reactive sintering, Self-propagating High-temperature Synthesis (SHS).
Abstract. The present work studies the influence of high-energy milling (HEM) and sintering cycle of Ti and Al powders on the obtainment of TiAl. This study shows that HEM modifies the diffusion processes during the sintering stage. The ignition temperature of the exothermic reaction (SHS) that occurs between Ti and Al, was considered as the key parameter of the sintering process, leading to the study of sintering cycles that avoid uncontrolled processes caused by the SHS reaction. The samples were obtained by cold uniaxial and isostatic pressing, pre-sintered at different temperatures, and then heated up to the sintering temperature. This study also shows the effect of powder additions processed by HEM on the sintering behavior of blended elemental Ti and Al powder compacts. Introduction TiAl research have drawn great interest due to its low density and applicability at high temperatures [1]. TiAl can be processed by various methods such as casting, forging, extrusion, etc. However, new forming processes continue to be developed, allowing materials to be manufactured in a more economical and competitive manner and also guaranteeing its properties [2,3,4]. From the powder metallurgy point of view, cold pressing of TiAl powders are difficult due to the intermetallic nature of the material. Hot isostatic pressing (HIP) is also another possible method but it is very costly [5]. Therefore, manufacturing TiAl through reactive sintering of a powder mixture was proposed [6]. This process does not guarantee that a material that is free of porosity will be produced, and although it is more economic, it is not comparable to the HIP process in terms of the quality of the final material. In that respect, there is great demand for a more cost effective way to manufacture TiAl, especially in the automobile industry. The main problem surrounding reactive sintering is the exothermic reaction (also known as Self-Propagating High Temperature Synthesis, SHS) that occurs between Ti and Al at certain temperature [7], leading to very porous material and which does not have any control over the dimensions of the product to be obtained. This study investigates the reactive sintering of Ti and Al for obtaining TiAl. Different sintering cycles were employed to study the influence of the SHS reaction on the final material. After studying previous works [8,9,10] showing the benefits of mechanical alloying on TiAl formation, Ti and Al powders were high-energy milled (HEM) to compare its behavior during sintering versus the behavior of blended elemental powders. Mechanical alloying is able to produce powders with properties far from the thermodynamic equilibrium [11,12], lowering the activation temperatures of some diffusion processes. Experimental Procedure Elemental Ti (
