Phase equilibria and transformations in a Ti-Zr-Si system
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I.
INTRODUCTION
Silicon, often in conjunction with zirconium, is employed as an alloying addition in a range of commercial titanium-based alloys. Silicide precipitates can be formed during solidification and also during subsequent low- and high-temperature heat treatments. Silicides also are important for their potential as high-temperature materials in their own right. However, there is little systematic information concerning the nature of, and phase equilibria between, silicides and the metal matrix in the ternary Ti-Zr-Si system. The purpose of the present work is to redress this deficiency.
II.
LITERATURE SURVEY
Ti~Si3 was first observed by Pietrokowsky and Duwez IS1 in 1951. It was found to have a hexagonal D88 structure with lattice parameters a = 0.7465 and c = 0.5162 nm. TisSi3 phase was originally considered by Hansen to be the most titanium-rich silicide. 171It should be noted that while experimental data obtained to date is consistent with Murray's assessment, Goldschmidt I6~ has noted that interstitial elements including oxygen and nitrogen may be responsible for stabilizing Zr (and probably Ti) silicides of the MnsSi3 type in the D88 structure and that in ultrapure form, they exhibit tetragonal crystal structure. These silicides and other silicides in the system (TisSi4, TiSi, and TiSi2), which do not come into equilibrium with the metal, are reviewed in Murray's assessment of the Ti-Si phase diagram. I~1
A . Ti-Si B i n a r y Silicides
B . Z r - S i B i n a r y Silicides
There are two equilibrium solid phases which come into equilibrium with titanium in the binary Ti-Si phase diagram. According to Murray's assessed phase diagram, tq titanium-rich silicide Ti3Si is stable below 1170 ~ It first was reported by Schubert et al. t21 and Schubert and Von Rossteutscher t3j in 1964 and (by comparison with Zr3Si) was assigned the tetragonal Ti3P-type crystal structure. The lattice parameters are a = 1.0196 and b = 0.5097 rim. They proposed a peritectoid reaction for its formation, which was extremely sluggish and only partially replaced TisSi3 (formed at higher temperatures) by Ti3Si after 72 hours at 1000 ~ + 20 hours at 750 ~ They concluded that the peritectoid reaction temperature could be between 1000 ~ and 1100 ~ Later, Svenchnikov et al. 141 observed Ti3Si phase formation at 1000 ~ (1220 ~ h + 1000 ~ h), with lattice parameters a = 1.0206 and c = 0.5069 nm, but not at 800 ~ (1220 ~ h + 800 ~ h). They also noted that even at 1000 ~ the peritectoid reaction forming Ti3Si by reaction between /3Ti and TisSi3 was incomplete.
The early Zr-Si phase diagram proposed by Hansen, jTl which was mainly based on the experimental work of Lundin et al., t81 has been subject to some uncertainty concerning the stoichiometry of certain silicides and their temperature range of stability. The most zirconiumrich silicide was considered to be "Zr4Si." This first was observed in 1952, and since then many authors have reported it. Iv1 However, Schubert et al. 12] and Von Rossteutscher and Schubert 13j identified the si
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