Фазовая диаграмма системы Cr-Mn

К оглавлению: Другие диаграммы (Others phase diargams)

Cr-Mn (Chromium-Manganese) M. Venkatraman and J.P. Neumann The assessed Cr-Mn phase diagram is based primarily on the work of [49Car], [ 52Pea], [64Wac], and [71Lug]. It is characterized by a wide homogeneity range of the (Cr) solid solution and the presence of two intermediate phases, a and s, that occur at about 65 and 75 at.% Mn, respectively. The results of the various investigations are in general agreement concerning the Cr-rich region of the phase diagram, but they vary considerably in the Mn-rich region. It appears that the conflicts are due to the appearance of metastable phase regions in quenched alloys. For this reason, the results of [71Lug] are accepted; they studied the phase relations in the Mn-rich region by means of high-temperature X-ray techniques. The intermediate phase a› forms peritectoidally at 926 C and transforms below 600 C to an ordered a› modification [64Wac]. Its homogeneity range extends from ~60 to ~65 at.% Mn. The s phase has the s-type structure and exhibits three modifications-s (HT), s› (MT), and s› (LT)-corresponding to different states of order. The s phase forms peritectically at 1312 C; it transforms at approximately 1000 C to s›. At about 800 C, the low-temperature modification, s›, is formed. In contrast to the very fast s = a transformation, the s› = s› transformation is sluggish. The metastable solid phases that form when quenching liquid Cr-Mn alloys at cooling rates of 107 to 108 K/s have been investigated by [75Gud]. The composition of the alloys ranged from 72 to 100 at.% Mn; the resultant films had a thickness of approximately 10 mm. From 72 to 82 at.% Mn, a bcc phase was obtained, corresponding to the (dMn) terminal solid solution. From 84 to 100 at.% Mn, the fct phase that occurred can be considered a tetragonally distorted modification of the fcc (gMn) phase. From 82 to 84 at.% Mn, both phases appeared. Apparently the very fast quenching of liquid alloys in the composition region of ~75 at.% Mn prevents the formation of the equilibrium s phase, which requires at least partial ordering, and leads to a metastable extension of the equilibrium (dMn) solid solution toward lower Mn concentrations. In fact, it is possible that (dMn) and (Cr) form a continuous solid solution in the metastable state, because the equilibrium (Cr) field extends to ~71 at.% Mn. The N‚el temperature of pure Cr (~38 C) is increased rapidly by small additions of Mn; it approaches a value of ~500 C at 50 at.% Mn. Alloys with more than ~30 at.% Mn are already in the two-phase region (Cr) + a› and should exhibit a constant N‚el temperature. However, it is very difficult to establish equilibrium at these low temperatures [64Wac], and it is very likely that alloys above 30 at.% Mn correspond to a metastable state of the (Cr) solid solution. 49Car: S.J. Carlisle, J.W. Christian, and W. Hume-Rothery, J. Inst. Met., 76, 169-194 (1949). 52Pea: W.B. Pearson and W. Hume-Rothery, J. Inst. Met., 81, 311-314 (1952). 64Wac: E. Wachtel and C. Bartelt, Z. Metallkd., 55, 29-36 (1964) in German. 71Lug: E. Lugscheider and P. Ettmayer, Monatsh. Chem., 102, 1234-1244 (1971) in German. 75Gud: V.N. Gudzenko and A.F. Polesya, Russ. Metall., (5), 153-156 (1975). Published in Bull. Alloy Phase Diagrams, 7(5), Oct 1986. Complete evaluation contains 3 figures, 3 tables, and 29 references. Special Points of the Cr-Mn System