Phase Transitions and Mechanical Properties of the Fe-Al-Mn-C Alloys
|關鍵字:||鐵鋁錳碳合金;相變化;機械性質;(Fe,Mn)3AlC碳化物;穿透式電子顯微鏡;Fe-Al-Mn-C alloys;Phase Transitions;Mechanical Properties;(Fe,Mn)3AlC carbides;TEM|
(三)、在淬火狀態下Fe-9wt.%Al-30wt.%Mn-1.0wt.%C合金的顯微結構為單一沃斯田鐵相，在625℃短時間時效處理後，細微的κ'碳化物會出現在沃斯田鐵相基地內；在625℃延長時效時間做時效處理後，細微的κ'碳化物會在基地內成長，且可以在晶界上觀察到γ + κ'碳化物→ γ + 粗大κ碳化物的反應，此（γ + κ）的混合相具有層狀的結構。由拉伸測試的結果知道雖然此合金在經過96小時的長時間時效後會有（γ + κ）的層狀結構出現在晶界上，此合金仍然具有很好的28%伸長率。這是因為（γ + κ）的層狀結構所佔的比率很小，尚不至於對延性影響太大。|
Abstract Phase transitions in an Fe-9Al-30Mn-2.0C alloy, and the mechanical properties of the Fe-9Al-30Mn-1.0C and Fe-9Al-30Mn-2.0C alloys have been investigated. On the basis of the experimental examinations, some results can be summarized as follows: . The as-quenched microstructure of the Fe-9wt.%Al-30wt.%Mn-2.0wt.%C alloy was austenite phase containing fine (Fe,Mn)3AlC carbides. The fine (Fe,Mn)3AlC carbides having an L'12 structure were formed by spinodal decomposition during quenching. When the as-quenched alloy was aged at 550-900oC for moderate times, the fine (Fe,Mn)3AlC carbides grew within the austenite matrix and coarse (Fe,Mn)3AlC carbides started to occur on the austenite grain boundaries. When the alloy was aged at 900-1100oC and then quenched, both of large and extremely fine (Fe,Mn)3AlC carbides could be observed simultaneously within the austenite matrix. This feature has never been observed by other workers in the Fe-Al-Mn-C alloy systems before. Furthermore, the Al and Mn concentrations in the coarse (Fe,Mn)3AlC carbides formed on the grain boundaries were found to vary drastically with the aging temperature. . The mechanical properties of the Fe-9wt.%Al-30wt.%Mn-2.0wt.%C alloy, prepared by conventional casting process, were examined. Tensile tests revealed that the optimal combination of mechanical strength and ductility of the alloy was the as-quenched specimen which had good ultimate tensile strength (UTS) of 1060 MPa with an excellent 57% elongation. When the as-quenched alloy was aged at 750 oC for 3-96 h, both the tensile strength and ductility were significantly decreased. Interestingly, both of the mechanical strength and ductility of the as-quenched specimen were much better than those of the aged specimens. It is worthwhile to note that the mechanical properties of the austenitic Fe-Al-Mn-C alloys with C > 1.3 wt.% in the as-quenched condition have never been investigated by other workers before. In addition, the γ/κ lamellar structure of the aged specimens could not improve the tensile ductility because sub-cracks initiated at coarsened κ carbides and linked up to trigger cleavage. . The as-quenched microstructure of the Fe-9wt.%Al-30wt.%Mn-1.0wt.%C alloy was a single austenite (γ) phase. When the alloy was aged at 625oC for short times, fine (Fe,Mn)3AlC carbides (κ' carbides) were observed to precipitate within the γ matrix. After prolonged aging at 625oC, the fine (Fe,Mn)3AlC carbides grew within the γ matrix and a γ + κ' → γ + coarse (Fe,Mn)3AlC carbide (κ carbide) reaction occurred on the grain boundaries. The mixture of (γ + κ carbides) had a lamellar structure. Tensile tests revealed that although the γ/κ lamellar structure occurred on the γ/γ grain boundaries after aged at 625oC for 96 h, the present alloy still exhibited good 28% elongation. Because the area fraction of the γ/κ lamellar structure exhibited in the aged alloy was still very small, its influence on the ductility wasn’t pronounced.
|Appears in Collections:||Thesis|
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