This study has been undertaken to investigate the site occupancy of carbon atoms in the ordered b.c.t. τ phase of Mn-Al alloys. The phase transformation mechanism induced by the presence of carbon atoms in ε phase lattice leading to τ phase has also been investigated. The phase constitution and the lattice parameter relationships in the rapidly solidified and heat-treated $(Mn_0.53Al_0.47)_{100-x}C_x$ pseudobinary alloys (x=0~5.36) have been investigated by means of X-ray diffraction and transmission electron microscopy. The melt-spun alloy was in single ε phase (c.p.h.) in the 0.63 ~ 4.0 at.% C composition range. Below the lower carbon composition limit traces of $γ_2$ were found while $Al_4C_3$ carbide was found beyond the upper limit. Heat treatment of the melt-spun alloys at 823 K produced single τ phase (ordered b.c.t., Cuau type I, $Ll_0$) in the $0.63~3.6 at.% C range. The lattice parameters (a and c) of ε phase were observed to incease with the carbon content. In contrast, c parameter increased markedly with the carbon content while a decreased slightly in τ phase, thereby a large value of the c/a ratio resulted. The lattice parameter data for both the ε and τ phases clearly indicated an increase of the unit cell volume with the carbon content, which supports a notion that carbon atoms in the τ lattice occupy interstitial site.
In order to investigate the site occupancy of carbon atoms in the ordered body-centered structure fo τ phase in carbon doped Mn-Al alloys, a series of structure analyses by x-ray diffraction has been carried out. Four different carbon atom occupancy models were presented, i.e, a substitutional (aluminum) site (1/2,1/2,1/2,), octahedral interstitial site (1/2,1/2,0), (0,0,1/2) site and intersititial random occupation. The structure factor for each model is formulated by introducing a disorder parameter that is defined by the number of aluminum atoms at the manganese site, (0,0,0), therefore, the semi-empirical integrated intensities are calculated for $(Mn_0.53Al_0.47)_100-x C_x (x=0~3.15) τ phase using an experimentally determined disorder parameter of carbon free alloy and employing an assumption that it would not be significantly influenced by the carbon addition. The semi-empirical intensity ratio $I_{200}/I_{100}$ calculated for each model was compared with the observed intensity ratio as a function of the carbon content, from which it becomes evident that carbon atoms preferentially occupy the octahedral interstitial site (1/2,1/2,0). Further discussions relevant to this conclusion are also presented.
The phase transformation of τ phase and orientation relationship between the τ phase and the matrix in the rapidly solidified $(Mn_0.53Al_0.47)_96.86C_3.14$ alloy have been investigated by the transmission electron microscopy. After heat treatment of the melt-spun alloy at 723K, single ε phase transformed to ordered ε phase and τ phase. Nucleation of τ phase took place at the grain boundaries as well as subgrain boundaries of the initial ε phase. But two types of τ phase nucleation occurred simulataneously at the grain boundaries. In the first case, an orientation relationship between nuclei of τ phase and the adjacent grain is established. In this case, the nuclei of τ phase is in the platelike shape and they grow to the direction of plate. It is seen that nuclei plates are parallel with the $(0002)_ε$ habit plane of the adjacent grain. Nucleation takes place via the diffusion controlled process which may be interpreted as a discontinuous precipitation reaction. In the second case, another orientation relationship between τ phase and matrix is established. In this case, streaks in the SAD pattern of τ phase appears and the tip of τ phase front shows the sharp martensitic character. Thus, it is assumed that this mechanism proceeds via martensitic transformation.
