Effect of Ti/Al ratio and Cr, Nb, and Hf additions on material factors and mechanical properties in TiAl
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INTRODUCTIOI~
THE compound TiA1 has a superior strength to density ratio t~1 and fairly good oxidation re, istance I2J for rotating high-temperature structural mate dais. However, TiA1 has not been used industrially be~ ause of its brittleness below 1000 K. [3,41 Therefore, t le improvement of ductility is a main problem for deve |opment of new heat resistant materials. There have been ( nly a few studies on the effect of third elements on the mi~ rostructures and structural factors. 15,6~Vujic et al. ts] have s~adied long-range order parameters, S, in rapidly quencl~ ed binary TiA1. The S values increased with increasing a,1 content from 0.46 at 50 at. pct A1 to 0.68 at 55 at. pct ~ .1 in as-quenched materials and from 0.63 at 50 at. pct A1 to 0.80 at 57 at. pct A1 in annealed materials. Whang .'nd Li 17~ studied the effect of annealing treatment and Ti/AI ratio on lattice parameters and axial ratios in rapk ly quenched binary TiA1 alloys. Shong and Kim I81 stu( led the effect of annealing temperatures on lamellar-like microstructures composed of TiA1 (gamma, L10 strut :ure) and TiaAI T. KAWABATA, formerly with Sumitomo Li1 ht Metal Industries, Ltd., Technical Research Laboratories, Nagoya 155, Japan, is with Atom Material Research Laboratory, Send i 982-02, Japan. T. TAMURA, formerly with NKK Corporation, i ,td., Kawasaki 210, Japan, is with Sony Corporation, Ltd., Sendal ?echnology Center, Tagajo 985, Japan. O. IZUMI, formerly Pro: essor, Institute for Materials Research, Tohoku University, is Emeria ~ Professor, Tohoku University, Sendal 980, Japan. Manuscript submitted April 22, 1991. METALLURGICAL TRANSACTIONS A
(alpha 2, D019structure) plates in a Ti-43 at. pct AI alloy. Vujic et al. 151have studied lattice parameters and the axial ratio as a function of annealing treatments and alloying elements of V, Cr, Mn, Fe, Co, Ni, and Ge ( = M ) in rapidly solidified (Ti-55A1)-xM (in at. pct, x = 0 - 10) alloys. The rapid quenching suppressed long-range ordering and reduced the axial ratio, c / a , of TiAI. Lipsitt e t a l . r41 have shown that the ductility of Ti-40 wt pct A1 (Ti-54.2 at. pct A1) was about 0.1 pct in fracture strain at room temperature. Whang and Hahn studied the effect of V [9] and Nb u~ addition on plastic deformation, revealing that the V addition increased compressive ductility due to active deformation twins, but Nb did not change the compressive ductility. The Mn addition improved ductility for Ti-rich TiA1, changing the microstructure with highly dense twins owing to a decrease of stacking fault energy and Ti3A1 thin plates on interfaces of the twins. [H,12] The additions of W stabilized the beta phase in TiAI and caused ductility improvement, u3J The Ag addition resulted in an increase of ductility but a decrease of strength at high temperatures because of the presence of Ag solid solution as a second phase. [~41The effect of third elements on the axial ratio was studied by Hashimoto et al., 161 Vujic et al., 151 and Huang and Hall. tJS~ TiA1 has the L10 structure with an axial ratio o
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