Phase relations and crystal structure of \u3c46-Ti2(Ti0.16Ni0.43Al0.41)3

Ti2(Ti0.16Ni0.43Al0.41) 3 is a novel compound (labeled as \u3c46) in the Ti-rich region of the Ti-Ni-Al system in a limited temperature range 870 < T < 980 \ub0C. The structure of \u3c46-Ti2(Ti,Ni,Al)3 was solved from a combined analysis of X-ray single crystal and neutron powder diffracton data (space group C2/m, a = 1.85383(7) nm, b = 0.49970(2) nm, c = 0.81511(3) nm, and \u3b2 = 99.597(3)\ub0). \u3c46-Ti 2(Ti,Ni,Al)3 as a variant of the V2(Co 0.57Si0.43)3-type is a combination of slabs of the MgZn2-Laves type and slabs of the Zr4Al 3-type forming a tetrahedrally close-packed Frank-Kasper structure with pentagon-triangle main layers. Titanium atoms occupy the vanadium sites, but Ti/Ni/Al atoms randomly share the (Co/Si) sites of V2(Co 0.57Si0.43)3. Although \u3c46 shows a random replacement on 6 of the 11 atom sites, it has no significant homogeneity range ( 3c1 at. %). The composition of \u3c46 changes slightly with temperature. DSC/DTA runs (1 K/min) were not sufficient to define proper reaction temperatures due to slow reaction kinetics. Therefore, phase equilibria related to \u3c46 were derived from X-ray powder diffraction in combination with EPMA on alloys, which were annealed at carefully set temperatures and quenched. \u3c46 forms from a peritectoid reaction \u3b7-(Ti,Al)2Ni + \u3c43 + \u3b12 \u2194 \u3c46 at 980 \ub0C and decomposes in a eutectoid reaction \u3c46 \u2194 \u3b7 + \u3c44 + \u3b12 at 870 \ub0C. Both reactions involve the \u3b7-(Ti,Al)2Ni phase, for which the atom distribution was derived from X-ray single crystal intensity data, revealing Ti/Al randomly sharing the 48f- and 16c-positions in space group Fd3\u304m (Ti2Ni-type, a = 1.12543(3) nm). There was no residual electron density at the octahedral centers of the crystal structure ruling out impurity stabilization. Phase equilibria involving the \u3c46 phase have been established for various temperatures (T = 865, 900, 925, 950, 975 \ub0C, and subsolidus). The reaction isotherms concerning the \u3c46 phase have been established and are summarized in a Schultz-Scheil diagram