PHASE AND LATTICE-PARAMETER RELATIONSHIPS IN RAPIDLY SOLIDIFIED AND HEAT-TREATED (MN0.53AL0.47)100-XCX PSEUDOBINARY ALLOYS

The phase constitution and the lattice parameter relationships in the rapidly solidified and heat-treated (Mn0.53Al0.47)100-xCx pseudo-binary alloys (x = 0-6) have been investigated by means of x-ray diffraction and transmission electron microscopy. The melt-spun alloys contained a single epsilon-phase (cph) with 0.63-4.0 at. % C, and below and beyond this carbon composition range small traces Of gamma-2-MnAl and Al4C3 compounds were formed, respectively. The heat treatment of the melt-spun alloys at 823 K produced a single tau-phase (ordered bct, CuAu type I, L1(0) with 0.63-3.6 at. % C. The c lattice parameter of the epsilon-unit cell was observed to increase pronouncedly with the carbon content whereas that of the a-axis revealed no apparent change; the corresponding increase of the unit cell volume was taken to indicate an interstitial dissolution of the carbon atoms in the epsilon-lattice. On the other hand, for the tau-phase, the c lattice parameter increased markedly with the carbon content while the a parameter decreased slightly, so that a large increase of c/a ratio was produced. The lattice parameter data for the tau-phase thus indicated an increase of the unit cell volume with the carbon content, providing new evidence that the carbon atoms dissolve interstitially in the bct lattice. In addition, it was deduced that the higher c/a ratio with increasing carbon content may arise from a preferential site occupation of the carbon atoms at a specific type of octahedral interstitial site lying in the manganese atom layers.