Phase transformations in an Fe-8.0Al-8.0Ni-1.3C alloy
Lin Chin Shih
當合金在550℃到650℃的溫度區間進行不同時間時效後，我們觀察到微細的(Fe,Ni)3AlC碳化物會在沃斯田鐵相(γ)中成長，同時，我們亦觀察到肥粒鐵相(α)會在沃斯田鐵相(γ)中形成，並且在肥粒鐵相(α)中發現細微的B2相。當時效時間增加時，沃斯田鐵相(γ)會完全轉變為(Fe,Ni)3AlC碳化物，並且細微的B2相會在肥粒鐵相(α)中成長成較大顆粒。因此，在550℃到650℃的溫度間時效，其穩定結構為(Fe,Ni)3AlC碳化物、肥粒鐵相及B2相的混合。當合金在850℃溫度時效淬火後，在沃斯田鐵相中可以發現粗大的(Fe,Ni)3AlC碳化物；另外，粗大的B2相和細微的B2*相同時在肥粒鐵相(α)中被觀察到，其中粗大的B2相和細微的B2*顆粒是分別經由時效過程和淬火過程形成的。當時效溫度增加至1050℃~1150℃時，B2相會在沃斯田鐵基地內形成。此合金在1200℃或更高溫度的顯微結構為單一沃斯田鐵相。因此本合金在550℃至1200℃的相變化為(κ-carbide+α+B2) à (γ+κ-carbides+α+B2*+B2) à (γ+B2) à γ。這種相變化的過程之前不曾被其他研究者在鐵-鋁-鎳-碳合金中發現過。|
ABSTRACT Phase transformations in an Fe-8Al-8Ni-1.3C alloy have been examined by means of optical microscopy (OM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectrometer (EDS). When the Fe-8Al-8Ni-1.3C alloy was solution heat-treated and then quenched, the microstructure of the alloy was austenite phase (γ) containing fine (Fe,Ni)3AlC-carbides. The fine (Fe,Ni)3AlC-carbide has an L´12-type structure, which was formed during quenching by a spinodal decomposition. When the as-quenched alloy was aged at the temperatures ranging from 550℃ to 650℃ for short times, (Fe,Ni)3AlC-carbides existing in the austenite phase grew and (fine B2+α) phases were formed within the austenite phase. After prolonged aging at 550℃, the austenite phase was fully transformed into (Fe,Ni)3AlC-carbides and B2 particles grew within the ferrite phase. Consequently, the stable microstructure of the alloy at 550℃~ 650℃ was (α+B2+κ-carbide). When the alloy was aged at 850℃ and then quenched, coarse (Fe,Ni)3AlC-carbides were formed within the austenite phase. In addition, the coexistence of large B2 particles and extremely fine B2* particles could be observed within the ferrite phase. The coarse B2 particles and the fine B2* particles were formed during the aging and quenching from the aging temperature, respectively. This feature has never been found by other workers in various Fe-Al-Ni-C alloys before. When the alloy was aged at 1050℃~1150℃, the B2 particles were formed within the austenite matrix. When the alloy was aged at 1200℃ or above, the microstructure was single-phase austenite. As a result, when the alloy was aged at temperatures ranging from 550℃ to 1200℃, the phase transition sequence was found to be (κ-carbide+α+B2) à (γ+κ-carbides+α+B2*+B2) à (γ+B2) à γ. It is noted here that the phase transformation sequence has never been reported by other workers in the Fe-Al-Ni-C alloys before.
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