Comment on the paper 'Experimental determination of phase equilibrium in the Fe-Co-Sb ternary system' by Pongsaton Amornpitoksuk, Hongxiao Li, Jean-Claude Tedenac, Suzana G. Fries, Didier Ravot (Intermetallics 15 (2007) 475-78)

摘要

In their paper "Experimental determination of phase equilibrium in the Fe-Co-Sb ternary system" Amornpitoksuk et al. [1] describe phase equilibria in the ternary Co-Fe-Sb system along three isopleths (see Fig. 1 in Ref. [1]), among them one at a constant Sb content of 30 at. percent in the temperature range between 950 and 1200 deg C. We think that this isopleth, as it is shown in Fig. 4 in Ref. [1], must be wrong. The diagram shows a continuous two-phase field labeled (bcc-A_2 + B8) extending from 0 at. percent Fe (Co_(0.7)Sb_(0.3)) to 70 at. percent Fe (Fe_(0.7)Sb_(0.3)). As it is well known, Co crystallizes above 422 deg C in an fcc (Al) cubic lattice, whereas bcc (A2) Co is metastable over the entire temperature range [2]. In the literature, the binary Co-Sb phase diagram shows at 30 at. percent Sb a two-phase field (fcc-Co + B8) which ends in a eutectic reaction L = fcc-Co + B8 at 1113 deg C (according to Massalski et al. [3]) or 1118 deg C (according to Hanninger et al. [4]), whereas in Fig. 4 of Ref. [1] an invariant reaction is indicated in the Co-Sb binary system that involves the three phases L, bcc-A2, and B8. Looking at the binary Fe-Sb phase diagram, one can see that at 30 at. percent Sb a two-phase field (bcc-Fe + B8) exists up to the eutectic temperature of 996 [5] or 1009 deg C [6], and the fcc Al-structure is limited to a very narrow composition range forming a so-called gamma-loop. This is a clear proof that a continuous two-phase field (bcc-A2 + B8), as it is given in Fig. 4 of the paper, is impossible.