X-ray diffraction study on the structure of rapidly solidified Fe-Al-C and Fe(Mn,Ni)-Al-C alloys
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phase. Instead, we obtained evidence showing the occurrence of phase decomposition with accompanying sideband phenomenon in a simple supersaturated austenitic phase formed directly from the melt during the rapid solidification process. Inoue et al's suggestion about the new nonequilibrium phase of the LI2 type structure found in their Fe-A1-C and Fe(Mn,Ni)-AI-C alloys leads to the formulation of FesA1 or (Fe,Mn)sA1 and (Fe,Ni)3A1. This is similar to the L'12 (CaTiO3 or filled-Cu3Au) type perovskite carbide phase (K-phase) such as Fe3AICx and (Fe,Mn)3A1Cx which forms in high manganese and/or high aluminum steels. 4-7 Thus, the reported new phase differs from the K-phase only in the degree of order of carbon atom arrangement at the interstitial octahedral sites of each ordered fcc unit cell. If we now assume that the excess iron or manganese atoms occupy the aluminum sites in the L12 type structure, then the relative intensity of the superlattice reflection can be calculated. Such an assumption may be reasonable since the aluminum content of the reported new phase was always less than that of the hypothetical stoichiometric composition. For the intensity calculation, we have used an approximate intensity equation 8 which excludes the absorption factor, the temperature factor, and any correction for the atomic scattering factor due to anomalous dispersion, and the resuits are given in Table I. The calculated intensity ratios of 11oo/12oo are about 0.05 and 0.08 for the hypothetical L12 type structures of Fe-7 pct Al-l.8 pct C and Fe-31 pct Mn8.8 pct AI-1.6 pct C alloys, respectively, and for Fe-31 pct Mn-8.6 pct A1-4.3 pct C alloy, which is a single K-phase after the rapid solidification process, it is about 0.15. Therefore, the measured intensity ratios of the former two alloys should be about one third or half of that of the latter alloy of the K-phase if the reported new phase structure is LI2 type as suggested by Inoue et al. However, contrary to the above expectation, we could not find any superlattice reflections in Debye-Scherrer camera patterns of rapidly solidified Fe-7 pct AI-1.8 pct C and Fe31 pct Mn-8.8 pct AI-I.6 pct C alloys even by use of a microdensitometer (Table I). Instead, rather simple austenite X-ray diffraction patterns were obtained for them. On
Table I. Comparison of Calculated and Measured Relative Superlattice Intensity 11oo/12oo. l)ebye-Scherrer Photographs Were Taken by 57.3 mm Diameter Camera Using Mn-Filtered Fe-K,, Radiation. I1oo/12oo
Alloy Composition Fe-7 pct A1- 1.8 pct C Fe-31 pct Mn-8.8 pct A1- 1.6 pct C Fe-31 pct Mn- 8.6 pct A1- 4.3 pct C *Calculatedfromthe approximateequation, I =
IF,ool
IF ool
6.2 7.3 10.6
61.4 59.2 63.0
(L. P. ) 100 (L. P. ) 200 Calculated* 26.0 5.1 0.05* 25.9 5.1 0.08* 28.2 5.5 0.15'*
Measured** not measurable not measurable 0.13
IFI~P (Le.),
whereF = structurefactor, P = multiplicityfactor,and (L.P.) = Lorentz-polarizationfactor
+_coe:_fl = sin20.cosO ] **Measuredby microdensitometer. *Valuesfor the hypotheticalLI2 typestructure. ttValuefor the L'
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