Phase diagrams of iron alloys with zirconium and hafnium carbides

Experimental reinvestigation of the Co–Nb phase diagram

“…[2] is in good agreement with the present one; older literature values are 1440 C [8] and 1420 C [12]. Similar to the case of the C36 Laves phase, the eutectoid decomposition reaction is so slow that large amounts of the hexagonal Laves phases are maintained at room temperature after cooling which only dissolve after long-term heat treatments below the eutectoid temperature.…”

“…Literature values are in the same range of compositions (49-54 at.% Nb [8], 48-54 at.% Nb [10], 50.5-52.7 at.% Nb [11], 49-56 at.% Nb [12], and 47.0-51.7 at.% Nb [17]), although a smaller homogeneity range of 50.3-53.3 at.% Nb is given in the assessed diagram in Refs. [1,2].…”

“…The temperature and composition of the congruent melting point of the m phase have not been determined in the present study. Literature values vary strongly between 1475 C at 52 at.% Nb [8], 1430 C at 50 at.% Nb [12], and 1403 C at 51.5 at.% Nb [11]. The assessed values of Hari Kumar et al [2] are 1424 C and 51.8 at.% Nb, which are adopted for the present phase diagram.…”

“…Similar to the case of the C36 Laves phase, the eutectoid decomposition reaction is so slow that large amounts of the hexagonal Laves phases are maintained at room temperature after cooling which only dissolve after long-term heat treatments below the eutectoid temperature. The diffusion couple studies show that the C14 phase does not occur up to 1230 C but is present at 1280 C. The only information obtainable from the literature is from the work of Shurin and Dmitrieva [8], who observed the C14 phase after annealing alloys at 1250 C but not at 1200 C. For the present phase diagram, a value of 1250 AE 20 C was chosen for the eutectoid reaction temperature.…”

“…[3]. Although the phase diagram of the Co-Nb system has already been studied by several authors [4][5][6][7][8][9][10][11][12][13][14][15][16][17], significant uncertainties do exist not only with respect to the existence of intermediate phases but also regarding their homogeneity ranges and the invariant reactions. Even in the most recent version of the phase diagram ( Fig.…”