Development of Tough and Strong Cubic Titanium Trialuminides
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Development of Tough and Strong Cubic Titanium Trialuminides Robert A. Varin1, Les Zbroniec3 and Zhi Gang Wang3 1 Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 2 National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan 3 Institute of Aeronautical Materials, Bejing 100095, P.R. China ABSTRACT In this work, the recent breakthroughs in the understanding of the fracture behavior and fracture toughness of L12-ordered titanium trialuminides are described and discussed. First, it is shown that, as opposed to many other intermetallics and specifically those with an L12 crystal structure, the fracture toughness of L12 titanium trialuminides is insensitive to testing in various environments such as air, water, argon, oxygen and vacuum (∼1.3x10-5 Pa). Second, it is reported here that by increasing the concentration of Ti combined with boron (B) doping, the room temperature fracture toughness of a Mn-stabilized titanium trialuminide can be improved by 100% from ~4 MPam1/2 to ~8 MPam1/2 and by 150-250% at 10000C to ~(10-12) MPam1/2 with a simultaneous suppression of intergranular fracture (IGF) to ~(40-50%). Almost three fold increase in yield strength to ~550 MPa is attained at room temperature for high Ti, boron-doped trialuminides. Both Vickers microhardness and strength increase linearly with increasing concentration of (Ti+B) indicating a classical solid solution strengthening response. INTRODUCTION The L12-ordered titanium trialuminides are derived from D022-ordered Al3Ti by alloying with fourth-period transition elements such as Cr, Mn, Fe, Co, Ni, Cu, and Zn [1,2]. They have attracted much attention as high temperature structural materials because of their low density, high melting point, oxidation resistance better than that for TiAl-based alloys, and expected improvement of room temperature tensile ductility and fracture toughness due to their cubic lattice structure. Regrettably, despite numerous efforts in the last ten years their tensile ductility and fracture toughness have not been improved. Most surprisingly, such brittle materials also exhibit quite a low strength (hardness) [3]. The first breakthrough that has been achieved in our laboratory demonstrates that, as opposed to many other intermetallics and specifically those with L12 crystal structure, the L12 titanium trialuminides are immune to the environmental embrittlement [4,5]. This important finding prompted us to focus further research efforts on other factors. Specifically, some years ago we found that the microhardness of cubic titanium trialuminides stabilized with various transition elements Fe, Mn, Cr and Cu depends nearly linearly on the Ti concentration [6]. Regarding boron effects Winnicka and Varin [7,8] found that the boron–doped, Cu–stabilized cubic titanium trialuminide exhibited entirely different fracture surface than its boron-free counterpart. In addition, fine cracks were formed around a 20 kg indentation in a boron-doped trialuminide as opposed
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