Effect of rapid solidification and alloying addition on lattice distortion and atomic ordering in L1₀ tial alloys and their ternary alloys

Abstract

Binary and ternary L1₀ type titanium alloys were rapidly quenched into thin foils by the hammer-anvil quenching technique. The nominal alloy compositions include Ti_1-xAl_x and (Ti_1-xAl_x)_{100-@#@ y}M_y, wherex = 0.5 to 0.6; y= 1 to 4; and M is V, Cr, Mn, Fe, Co, Ni, and Ge. Lattice parameters of Ll₀ phases in these alloys were studied by X-ray diffractometer using the foils and their powders. Simultaneously, long-range order parameters of these Ll₀ phases in the as-quenched foils and the annealed powders were determined from the superlattice intensity measured by X-ray diffractometer. Rapid quenching suppresses long-range ordering (S) during solidification and, thereby, reduces lattice distortion (c/a) in Ll₀ titanium alloys. A small amount of alloying element has a large effect on thec/a ratio. The atomic size effect and probably substitutional site preference play an important role in such ac/a reduction. From the density measurement, it is found that antisite defects are a predominant mode in Al-rich off-stoichiometric TiAl compound alloys. Determined long-range order value is significantly low in as-quenched binary TiAl due to rapid quenching, and the highest ordering occurs at as-quenched Ti₄₅Al₅₅ alloy. The relationship betweenc/a andS in Ll₀ Ti₄₅Al₅₅ may be written asc/a = 1 + 0.025 ·S.