탄소를 함유한 Mn-Al 합금의 구조 및 상변태에 관한 연구

Abstract

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