鐵-9鋁-30錳-2碳合金之顯微結構與機械性質

Abstract

摘要 本篇論文目的是利用光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)與穿透式電子顯微鏡(TEM)來觀察鐵-9鋁-30錳-2碳合金在淬火狀態和經過不同時效熱處理溫度及時間後，其顯微結構的變化，並探討顯微結構對機械性質的影響。 合金經過1200℃固溶處理後，可得到顯微結構為沃斯田鐵母相中包含微細的(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，且晶界上沒有任何析出物產生，此時合金擁有良好的最大抗拉強度(UTS)160Ksi及極佳的55％延伸率組合。此時合金的破裂模式也為具有許多酒渦狀凹洞的延性破壞。合金經過低溫450℃的時效熱處理後，細微的κ′碳化物會在沃斯田鐵相基地內成長，隨著時效時間的增加，當時效時間達到18小時，此時合金析出硬化效果達到最佳，其最大抗拉強度(UTS)240Ksi，且具有適當的伸長率(10％)。此時合金的破斷面呈現小的酒渦狀凹洞與劈裂的河流紋，故其破裂模式由延性破壞轉變為兼具延性破壞與脆性破壞。值得注意的是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碳合金經低溫(450℃)時效的機械性質與破壞行為。而隨著時效溫度的增加，合金可在較短時間達到最大強度峰值，但是其強度值較450℃時效差。如在550℃、6小時的時效熱處理，其強度值(196Ksi)達最大。由於藉由高溫時效熱處理時，經顯微結構的觀察，發現在沃斯田鐵晶界上有κ碳化物的形成，且隨時效時間的增加，κ碳化物會繼續成長，因此導致合金的強度及延性大幅下降。此時合金的破斷面呈現大量的劈裂河流紋，故合金的破壞模式為脆性破斷。

Abstract The purpose of this study is to examine the microstructural development of the Fe-9Al-30Mn-2C alloy after solution-heat-treated (SHT) and aged at various temperatures and different timeperiods by means of optical microscopy (OM) , scanning electron microscopy (SEM) and transmission electron microscopy (TEM) . Besides , the effect of the microstructural development on the mechanical properties has also been investigated . The as-guenched microstructure of the Fe-9Al-30Mn-2C alloy was austenite phase containing fine (Fe,Mn)3 AlC carbides . The fine (Fe,Mn)3 AlC carbides having an L′12 structure were formed by spinodal decomposition during quenching . No precipitate could be observed on the grain boundaries . The as-guenched alloy had good combination of ultimate tensile strength (UTS) of 160 ksi with an excellent 55％ elongation . The fracture mechanism of the as-guenched alloy was typically ductile dimple fracture . After aged at low temperatures (450℃) , the fine (Fe,Mn)3 AlC carbides grew within the austenite matrix . With increasing the aging time to 18 hours , optimal precipitation hardness effect could be achieved . The UTS was 240ksi , with suitable 10％ elongation . The fracture surface was a mixture of small dimples and cleavage river patterns . Therefore , the fracture mechanism was transferred from typically ductile fracture to a mixture of ductile and cleavage fracture . It is worthwhile to note that the mechanism properties and fracture behavior of the austenitic Fe-Al-Mn-C alloy with C＞1.3wt.％ aged at low temperatures (450℃) have never been investigated by other workers before . With raising the aging temperatures , the UTS peak value could be achieved , but the UTS peak value was poorer than that aged at 450℃ . For instance , when the alloy was aged at 550℃ for 6 hours , the UTS peak value was 196 ksi . Microstructural observation reveals that after age at higher temperature (550℃) , (Fe,Mn)3AlC carbides started to occur on the austenite grain boundaries . With increasing aging times , the κ carbides grew continuously . Therefore , the strength and the elongation of the alloy droped drastically . The aged specimen exhibited a cleavage fracture surface with obvious cleavage river patterns .