Continuous Solid Solution of -K-phase in the Third Period Transition Metals-Aluminum-Carbon Pseudo-Ternary Systems
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These features come from the nature of the L12 crystal structure. However, there exist a relatively small number of stable and binary L12 aluminides and silicides: Ni3AI is the only one L12 aluminide in the third-period transition-metals and aluminum binary systems. Many trials have been done to stabilize L12: it is well known that D0 22 Al 3Ti can be transformed to L 12 by macroalloying with substitutional elements such as Cu, Fe and Ni [1]. On the other hand, a few reports exist that L12 is made by interstitial additions: L1 2 V 3Au and Nb 3Si can be stabilized by adding oxygen [2, 3]. Since dislocation glide is generally dependent on the type of crystal structure, some mechanical properties are similar between compounds having similar crystal structures. For example, some E2 1 (called L1 2', perovskite or K-phase) Co 3A1C-base alloys show a positive temperature dependence of strength around 1000K [4]. These E2 1 compounds and L12 compounds having interstitial elements (metalloid atoms) are classified into perovskite phase crystal structures, which include metalloid-free (unfilled) compositions [5]. The perovskite-type compounds such as L12, E21 and L' 1 are closely related to each other, and it is known that they often dissolve each other [5]. Hereafter, these perovskite phases are called "K-phases" even if the K-phase is unfilled with interstitial elements (that is L1 2 structure). It is also known that, in the M-AI-C (or N) ternary systems, the body center sites of E21 M3AIC (M3A1N) are not perfectly filled with interstitial elements [5-7], and L12 Ni 3A1 can dissolve relatively large amount of carbon with keeping L12 [6,7] in which C atoms preferentially occupy the body center sites. Therefore, there is possibility to make continuous solid solutions between these K-phase compounds. For example, Pr3A1 forms a continuous solid solution to Pr3A1N [8-103. In this study, the phase stability of Kc-phases between Ni 3AI, Co 3AIC, Fe3AIC and Mn 3AIC was investigated in the third period transition metals (M: Mn-Cu) -aluminum (Al) -carbon (C) pseudo-ternary systems.
KK8.31.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
EXPERIMENTAL PROCEDURE The concentrations of transition metals were systematically changed from 60%Mn to 20%Ni+40%Cu by 20% along the periodic table under the nominal compositions of CMl+CM2= 6 0 % (M1 and M2 stand for Mn, Fe, Co, Ni and Cu, and M1 and M 2 are neighbors in the periodic table), CA1=20% and Cc=20%, where Cx (X = MI, M 2, Al and C) is the concentration of element X. These compositions correspond to the "stoichiometric" E2, if transition metals are not discriminated. The chemical compositions (mol%) used in this study are listed in Table 1. It should be emphasized that the atomic ratio of (M1+M 2):AI used is 3:1. These compositions correspond to the "stoichiometric" L12 if all carbon atoms are evacuated. It should be noted that K-phase formed at the composition of 60%Ni-20%A1-20%C is Ni3A1 containing 6.8%C and graphite [6,7]: no other ternary compound is recognize
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