Alloy design and coarsening phenomenon of L1 2 precipitates in high-temperature Al-2 At. Pct (Ti,V,Zr) systems
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INTRODUCTION
IT seems likely that high-strength Al-base alloys useful up to 698 K can be developed in view of the behavior of nickel-base superalloys, which resist degradation of mechanical properties to 75 pct of their absolute melting temperature. For high-temperature applications, the dispersed hardening phase must not undergo phase transformation to an undesirable phase during long time exposure at the temperature of interest. An additional factor to be considered is the stability of the hardening phase with respect to Ostwald ripening. This coarsening resistance is necessary so that the required strength level can be maintained after the long-time service at high temperatures. Several alloy systems designed for this purpose have appeared, of which the rapidly solidified A1-Fe-V-Si systems have received the most attention, t~.2I While the A1-Fe-V-Si systems appear to be the best, several Al-base systems containing transition metal elements for potential elevated temperature use were suggested by Adam t31 based on low values of diffusivities and solubilities. The equilibrium crystal structures of A13Ti, AI3V, and AI3Zr are tetragonal D022, D022, and D023, respectively. At the high temperatures of interest (around 698 K), vanadium and titanium are mutually substitutable in the form of A13(Ti,V). t41Much of titanium and vanadium can be substituted for zirconium in the D023-type A13Zr compound, creating A13(Ti,Zr) and AI3(V,Zr), respectively. I51 In particular, it has been reported that metastable facecentered cubic (fcc) L12-structured A13M (M = Ti,V,Zr) dispersoids form in the rapidly solidified A1-V-Zr and A1-Ti-Zr systems and that both LI2- and D023-structured A13M phases show slow coarsening kinetics, t6-91 LI2 precipitates may form as spherical or cellular fanshaped morphology, depending on alloying compositions and aging temperatures, t6'91 Spherical precipitates SEUNG ZEON HAN, Graduate Research Assistant, and HYUCK MO LEE, Associate Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, Korea 305-701. SEOG CHUL CHUNG, Researcher, formerly with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, Korea, is with the Goldstar Electron Company, Cheongju, Korea 360-480. Manuscript submitted September 14, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
are coherent and coplanar with the aluminum matrix. The D022-type structure is an ordered one derived from the Llz-type structure by introducing an antiphase boundary (APB) with a displacement vector of the type 1/2 [110] on every (001) plane. When the 1/2 [ll0]-type APB is introduced on every other (001) plane in the L12-type structure, the D023-type structure can be derived. The lattice parameters of tetragonal A13Ti and A13V, a' and c, are very close to ao and 2ao, respectively, where a0 is the lattice parameter of cubic A1 and a' is the half diagonal length of the (001) plane in a unit cell of the D022 structure. A13Zr
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