Фазовая диаграмма системы B-Ti

К оглавлению: Другие диаграммы (Others phase diargams)


B-Ti (Boron-Titanium) J.L. Murray, P.K. Liao, and K.E. Spear The equilibrium phases of the Ti-B system are (1) the terminal solid solutions- high-temperature bcc (bTi), low-temperature cph (aTi), and rhombohedral (bB); ( 2) two intermediate compounds, TiB and TiB2, whose structures and melting mechanisms are well established; and (3) Ti3B4, which forms from the melt only in a narrow temperature range and remains to be verified by an independent study. There are several discrepancies in the literature concerning the structures and stability ranges of the equilibrium phases, in part because many of the reported binary phases are derived from studies of ternary systems. The major studies of the binary sytem were conducted by [54Pal], [64Fen], and [66Rud]. The work of [54Pal] was not used to construct the assessed diagram in the high- temperature range because of the evident effect of contamination on their specimens. [64Fen] and [66Rud] exercised considerable caution to avoid contamination; nevertheless, there are also discrepancies between these two studies, primarily concerning the existence of Ti3B4 and the temperatures of the invariant reactions. For the invariant temperatures, the data of [66Rud] are preferred, because their technique for determining the incipient melting temperatures has been successfully tested for a number of high-temperature systems and because results were verified by differential thermal analysis for a large number of samples. [64Fen] and [66Fen] observed Ti3B4, a phase not reported by [66Rud]. It was reported to be isomorphous with that of Ta3B4. The composition of this phase is 58.1 at.% B, as determined by chemical extraction. A second phase, thought to be possibly a high-temperature form of Ti3B4, was found with a completely different structure from any other phase of the system. During heating, Ti3B4 is transformed at about 2010 C, but the transformation is not reversible. High- and low-temperature forms of Ti3B4 are not distinguished in the assessed diagram, because the irreversibility of the transformation strongly suggests that it is a contamination effect. [64Fen] indicated that Ti3B4 forms from the melt by a peritectic reaction at 2020 C, 20 C above the peritectic reaction involving TiB. In the assessed diagram, the peritectic reactions are separated by 20 C, but positioned at 2180 and 2200 C, according to the work of [66Rud]. Reported melting points of TiB2 range between 2790 [52Gla] and 3225 C [66Rud]. [64Fen] noted that large volumes of vapor prevented the accurate determination of the TiB2 melting point. The major difficulty is reaction with the crucible material, which results in the detection of a ternary eutectic temperature lower than the melting point of the binary compound. The highest reported congruent temperature was therefore used to construct the assessed diagram. 52Gla: F.W. Glaser, Trans. AIME, 194, 391-396 (1952). 54Pal: A.E. Palty, H. Margolin, and J.P. Nielsen, Trans. ASM, 46, 312-328 ( 1954). 64Fen: R.G. Fenish, NRM-138, 1-37 (1964). 66Fen: R.G. Fenish, Trans. AIME, 236, 804 (1966). 66Rud: E. Rudy and St. Windisch, Tech. Rep. No. AFML-TR-65-2, Part I, Vol. VII (1966). Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 1 figure, 3 tables, and 53 references. Special Points of the Ti-B System