The constitution of alloys of iron and manganese with transition elements of the first long period

Abstract

Using metals of the highest purity, the constitutions of iron-rich and manganese-rich binary alloys with the sequence of elements Ti-V-Cr-(Mn, Fe)-Co-Ni-Cu have been investigated by specially accurate thermal analysis, supplemented by microscopical and X-ray work. The liquidus and solidus curves have been determined accurately, and also the forms of the A₄(Y/O) transformations in the iron alloys, and of the (Y/O) and (Y/B) transformations in the manganese alloys. The complete liquidus and solidus were determined for the system Fe—Mn, and the equilibrium diagrams of the systems Fe-Ti and Mn-Ti were studied as far as 52 at. % Ti. O-Mn (b.c.c.) and y-Mn (f.c.c.) have the same crystal structures as O-Fe and y-Fe respectively, and the corresponding parts of the equilibrium diagrams are thus structurally comparable. In general, there is a remarkable correspondence between the effects of the different elements in stabilizing the b.c.c. or f.c.c. phases. The closed y-loops in the systems Fe-Ti, Fe—V and Fe-Cr are paralleled by closed y-fields in the Mn alloys, whilst Mn (in Fe), Fe (in Mn) and Ni produce open or expanded y-fields; Cu behaves abnormally in both series of alloys. In the Mn alloys, the stability and range of the B-Mn phase reach a maximum in the system Mn-Co, where the B-Mn phase is so stable that it comes into equilibrium with the liquid, and prevents the formation of a continuous series of solid solutions between Y-Mn and B-Co (f.c.c.). For the iron alloys the size factors are favourable for all the solutes examined, except Ti, which lies on the borderline of the favourable zone. With O-Mn and Fe-Mn as solvents, the size factors are all favourable, although that for Ti is still appreciable ( ca. 10%). The effect of size factor is shown by steep depression of the O-liquidus and O-solidus curves in the systems Fe-Ti and Mn—Ti. For the iron alloys the relative depressions of liquidus and solidus at equal atomic percentages of solute are in the order Ti > V > Cr and Co < Ni < Cu, and these sequences are the same as those for the lattice distortions produced by equiatomic percentages of the same solutes; the effect of Mn lies out of sequence in both cases. These and other relations are discussed. When compared with previous diagrams, the general effect of the present work is to indicate a much narrower two-phase (liquid and solid) range for both O-Fe and Y-Fe solid solutions. In marked contrast to previous diagrams, the present liquidus and solidus curves can all be reconciled with the existence of monatomic solutions in both liquid and solid phases. For Fe-Mn alloys the equations for dilute solutions are in good agreement with thermal data for pure iron up to 30 at. % Mn.