Effect of Vanadium on Plastic Deformation in Ll o TiAl Compound Alloys
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EFFECT OF VANADIUM ON PLASTIC DEFORMATION IN Llo TiAl COMPOUND ALLOYS SUNG H. WHANG AND YOO-DONG HAHN Department of Metallurgy & Materials Science Polytechnic University 333 Jay Street, Brooklyn NY 11201 ABSTRACT Ternary Ti-Al-V (Llo) alloys containing vanadium up to 10 at.% and aluminum in the range of 50-55 at.% were prepared. Miniature specimens machined from these alloys were deformed in uniaxial compression at room temperature. The yield stress, and fracture stress and strain were determined with respect to vanadium and aluminum concentration. The deformed alloys were electropolished into thin foils and studied by TEM. In particular, the types of dislocations, stacking faults and twins in the Ti-Al-V alloys were investigated by TEM. INTRODUCTION Excellent high temperature properties such as high specific strength, creep resistance and oxidation resistance of TiAl (Llo) was first acknowledged byMcAndrew and Kessler [1]. The crystal structure of TiAl (Lbo) is based on face-centered tetragonal structure with ordered layers (002) of aluminum and titanium along [001] direction and its axial ratio, c/a, is slightly larger than unity [2], i.e., 1.02, differing from other Llo compounds where the c/a is smaller than unit [3]. Furthermore, the [110] slip does not disturb the ordered structure, which makes it the most favorable slip system in this compound whereas [011] and [101] slips, in principle, destroy the ordered structure and should create a series of complex fault structures. As a result, these latter slip systems may be feasible only if dislocations move in pairs not to disturb the ordered structure, i.e., superdislocations. An earlier theory on dislocation configurations in Llo compounds [4] predicted'that a/2[I1O] dislocation V would split into two partials: a/6[211] and a/6[121.] whi.le a/2[011] type dislocation would dissociate into two partials: a/6[112] and a/6[121] which leaves the antiphase boundary behind them. Therefore, a pair of a/2[011] type dislocations, i.e., superdislocations a[011] is required to restore . the original structure .[4]. Dislocation structures in the deforma ed TiAl (Llo) were investigated by a • number of authors [5-7] showing that first, a/2[110] dislocations have been observed, but without any evidence that the a/2[110] dislocations dissociate into two partial dislocations a/6[211] and a/6[121]; and Al 20 40 60 so Ti second, observed superdislocations a[011] have not been split into four Ti content,at % partial dislocations [2]. The reason for this is not clear as to whether Fig. 1: Equilibrium phase diagram or not the split is too small to be of Ti-Al-V at 10000C [8] observed or it is simply not possible Mat. Res. Soc. Symp. Proc. Vol. 133. '1989 Materials Research Society
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from the viewpoint of their stabi l i ty. Vanadium addition to L] o TiAl increases the phase stability of Llo structures at 1000oC as shown in Fig. 1 [8]. The fact that the phase extension occurs in the direction of constant aluminum content indicates that the excess vanadium goes into the titanium su
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