鐵鋁錳合金之相變化及機械性質

Abstract

本論文研究Fe-9Al-30Mn-2.0C合金之相變化及Fe-9Al-30Mn-1.0C與Fe-9Al-30Mn-2.0C合金之顯微結構與機械性質。依據實驗的結果，本論文所得到的具體研究結果如下:

(一)、在淬火狀態下，Fe-9wt.%Al-30wt.%Mn-2.0wt.%C合金的顯微結構為沃斯田鐵相中包含細微的(Fe,Mn)3AlC碳化物（κ'碳化物）。其中具備L'12結構的細微κ'碳化物是在淬火過程中藉由史賓諾多相分解反應產生，當此合金在550至900℃時效處理後，細微的κ'碳化物會在沃斯田鐵相基地內成長，而粗大的(Fe,Mn)3AlC碳化物（κ碳化物）開始在晶界上出現。當此合金在900至1100℃時效處理後，粗大的κ碳化物及細微的κ'碳化物會同時在沃斯田鐵相基地內出現，這個結果以前未曾被其他學者在鐵鋁錳系合金中觀察到過。另外，在晶界上析出的粗大κ碳化物中，鋁及錳的含量會隨著時效溫度而變化。

(二)、我們研究了以傳統鑄造方式製備的Fe-9wt.%Al-30wt.%Mn-2.0wt.%C合金的機械性質，由拉伸測試的結果我們發現在淬火狀態下此合金具有最佳的抗拉強度及延性組合，此時此合金具有很好的最大抗拉強度（UTS）1060 MPa及極佳的57%伸長率。當此合金在750℃時效處理後，我們發現合金的強度及延性都隨著時間增加而明顯下降，此合金在淬火狀態下的強度及延性都要比時效處理過的合金優異許多。值得注意的是以往未曾有學者研究過高碳含量(大於1.3%)的沃斯田鐵系鐵鋁錳合金的機械性質。另外我們也發現此合金在時效處理後所形成的γ/κ層狀結構並不會改善合金的延性，這是因為細微的裂縫會在κ碳化物中起始並連結造成劈裂。 (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:

[1]. 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.

[2]. 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.

[3]. 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.