Modulated structure and magnetic properties of age-hardenable Fe-Mn-Al-C alloys
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Microstructural changes in the age-hardenable Fe-(30 to 34) wt pct Mn-(8 to 11) wt pct Al-(0.9 to 1.0) wt pct C alloys during aging in the temperature range between 773 and 823 K have been investigated by means of transmission electron microscopy (TEM) and X-ray diffraction. The wavelength of the modulated structure was found to be nearly constant for short aging times and then to increase on further aging, whereas the compositional modulation amplitude was found to increase rapidly from the beginning of aging. The growth of a spinodally modulated structure along the orthogonal (100) directions results in a periodic arrangement of the K-carbide precipitates, (Fe, Mn)3A1Cx, in the austenite matrix. The increases in hardness and residual and saturation inductions in the early stage of aging were in accord with the increase in the amplitude.
I.
INTRODUCTION
THE austenitic Fe-Mn-A1-C alloys, whose compositions are about 30 pct Mn, 8 to 11 pct A1, and about 1 pct C, show age hardening when aged at 723 to 923 K. H'2,31 The structure of the alloys after aging re-
vealed a precipitation of coherent metastable particles of the K-carbide, (Fe, Mn)3A1Cx, in the austenite matrix. I21 Recently, Han et al. t41 observed a modulated structure along the orthogonal (100) directions with superlattice reflections by TEM for an Fe-30Mn-8AI-1.3C* alloy aged *All in weight percent unless specified otherwise.
at 823 K for 300 minutes. They indicated that the superlattice reflections result from the L' 12 ordering which occurred in the carbon-rich zones. Oshima and Wayman I51 also observed the same type of carbide, Fe3A1Cx, which is finely precipitated in the retained austenite of a quenched Fe-7A1-2C alloy. In both cases, it was suggested that the precipitation of the K-carbide results from spinodal decomposition. More recently, we have shown that a coherency strain accompanied by spinodal decomposition can be responsible for the age hardening of an Fe-30Mn9A1-0.9C alloy, t61 However, the evidence which shows spinodal decomposition of the Fe-Mn-AI-C system seems rather limited. Thus, we have made a detailed examination of changes in microstructures during aging of Fe-Mn-AI-C alloys. Further, changes in hardness and magnetic properties have also been investigated as functions of the structural variations: modulation amplitude and wavelength.
KAZUNORI SATO, Research Associate, and YASUNOBU INOUE, Professor, are with the Analysis Center, Nagaoka University of Technology, Nagaoka, Niigata 940-21, Japan. KAZUHIRO TAGAWA, formerly Graduate Student of Mechanical Engineering, Nagaoka University of Technology, is with Tochigi Research Laboratory, Kao Corporation, Tochigi 321-34, Japan. Manuscript submitted July 6, 1988. METALLURGICALTRANSACTIONSA
II.
EXPERIMENTAL PROCEDURES
A. Specimen Preparation and Heat Treatments
Fe-Mn-A1-C ingots were initially prepared by air induction melting. High-purity iron, electrolytic manganese, high-purity aluminum, and electrode-grade graphite were cast into approximately 10-kg ingots. They were homogenize
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