| Summary: | 碩士 === 國立交通大學 === 材料科學與工程系 === 87 === Phase transformations in the Fe-9.5Al-30.0Mn-5.0Cr-1.5C alloy has been examined by means of optical microscopy(OM), transmission electron microscopy(TEM) and energy-dispersive X-ray spectrometer(EDS).
In the as-cast condition, the microstructure in the present alloy is the mixture of theγ+κ''+κ+D03-phases. When the alloy was solution heat-treated and then quenched, the microstructure of the alloy was austenite phase containing extremely fine (Fe,Mn)3AlCx carbides (κ''-carbides). Theκ''-carbides having an L''12-type structure were formed within the austenite matrix during quenching by a spinodal decomposition.
When the alloy was aged at 550℃, theκ''-carbides existing in the as-quenched austenite matrix grew preferentially along ＜100＞ directions, but no grain boundary precipitates could be observed. When the alloy was aged at 650℃, not only theκ''-carbides grew within the austenite matrix but also some plate-likeβ-Mn and granular-shaped κ-carbides precipitated on the grain boundaries. When the alloy was aged at 750℃, the microstructure within the matrix was the mixture of κ''-carbide and D03 phase. The D03 phase was formed by a α→B2→D03 continuous ordering transition during quenching. Moreover, the β-Mn precipitates and κ-carbides were formed on the grain boundaries. When the alloy was aged at 850℃, the coarseκ''-carbides could be observed within the austenite matrix, in addition, the precipitation of (α+κ) phases also occurred on the grain boundaries. When the alloy was aged at 950℃, only extremely fineκ''-carbides could be detected within the austenite matrix and no grain boundary precipitates could be examined, which was similar to that found in the as-quenched alloy. Therefore, when the alloy aged at temperatures ranging from 550℃ to 850℃, the phase transition sequence as the temperature increase was found to beγ→κ-phase +β-Mn→α-phase +κ-phase →γ.
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