Phase transformations in an Fe-8.8Al-30.0Mn-6.0Mo-1.1C alloy
T. F. Lui
當此合金在600℃時效兩小時後，在晶界上可以觀察到一個γ→α+B2+M2C+M6C的反應，此B2相在鐵鋁錳鉬碳合金中未被發現過。另外此合金在750℃時效兩小時後，在晶界上可以觀察到 一個γ→α+B2+M6C的反應，在這裡我們可以觀察到 M6C-type 碳化物存在的溫度比M2C-type碳化物還高。依據實驗的結果，我們知道當此合金在500℃到750℃範圍間做時效處理時，隨著溫度的增加在晶界上連續的相變化為γ→γ+κ-carbides→α+B2+M2C+M6C→α+B2+M6C。|
Abstract Phase transformations in an Fe-8.8Al-30.0Mn-6.0Mo-1.1C alloy have been examined by means of optical microscopy (OM) transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometer (EDS). When the alloy was solution heat-treated and then quenched, the microstructure of the alloy was single-phase austenite. When the alloy was aged at 500℃ for 2 hours, the microstructure of the alloy was austenite phase containing extremely fine (Fe,Mn)3AlCX carbides (κ'-carbides), The κ'-carbides having an L'12 –type structure were formed within the austenite matrix during quenching by a spinodal decomposition. With increasing aging time at 500℃, the κ'-carbides existing in the austenite matrix grew preferentially along  directions, but no grain boundary precipitates could by observed. When the aging time was increased to 8 hours, not only the κ'-carbides grew within the austenite matrix but also granular-shaped κ-carbides started to form heterogeneously on the γ/γ grain boundaries, the κ-carbide is also (Fe,Mn)3AlCx carbide. For convenience, the κ and κ'-carbides represent the (Fe,Mn)3AlCx carbide formed within the austenite matrix and on the grain boundaries, respectively. When the alloy was aged at 600℃ for 2 hours, a heterogeneous reaction γ→α+B2+M2C+M6C could be observed on the grain boundaries. It is noted here that the B2 phase has never been observed by other workers in the Fe-Al-Mn-Mo-C alloy. When the alloy was aged at 750℃ for 2 hours, a heterogeneous reaction γ→α+B2+M6C could be observed on the grain boundaries. It is worthy to note that the M6C-type carbide could be reserved up to a higher temperature than the M2C-type carbide. Based on the experimental result, it is seen that when the alloy was aged at temperatures ranging from 500℃ to 750℃, the phase transition sequence on the grain boundaries as the temperature increased, was found to be γ→γ+κ-carbides→α+B2+M2C+M6C→α+B2+M6C.
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