鐵-9鋁-30錳-1碳合金衝擊性質與變形微結構之研究

Abstract

本篇論文利用光學顯微鏡、掃描式電子顯微鏡與穿透式電子顯微鏡觀察鐵- 9鋁-30錳-1碳合金經固溶及650℃不同時間時效處理後的顯微結構、顯微結構變化對衝擊性質的影響、在不同溫度下經過衝擊試驗後的破裂行為與變形後的顯微結構變化。合金經過固溶處理後其顯微結構為單一沃斯田鐵相，經650℃、6至48小時的時效處理後，(Fe,Mn)3AlC碳化物分別在沃斯田鐵相基地內與晶界上析出與成長。在室溫衝擊試驗時，固溶處理的試片其破斷面為有許多酒渦狀凹洞的延性破斷面，衝擊值也較經650℃時效處理的試片為高。時效處理後的試片衝擊值隨著時效時間及晶界上碳化物析出量增加而大幅下降。在室溫至-196℃的範圍內，固溶處理後合金的衝擊值隨衝擊溫度下降而降低，並可觀察到韌-脆轉換行為的發生，測試溫度由-70℃降至-140℃時，衝擊值由255 J/cm2急降至136.2 J/cm2，時效處理之合金亦有似之結果。另外我們發現合金經650℃、24小時以上時效處理後，在衝擊試驗後可觀察到形變雙晶的發生，這點以前從未被學者在碳含量高於0.77wt%的沃斯田鐵系鐵鋁錳碳合金中發現過。

The purpose of this study is to examine the microstructure and its influence on the impact property of the Fe-9Al-30Mn-1C alloy after solution-heat-treated (SHT) and aged at 650℃ for various times. The impact tests were carried out at temperatures ranging from 25℃ to -196℃. Besides, the fracture behavior and the deformed microstructures of the fractured specimens were also observed. The microstructures were examined by using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure of the Fe-9Al-30Mn-1C alloy after SHT was a single austenite (γ) phase. When the SHT alloy was aged at 650℃ for 6 to 48 hours, the fine (Fe,Mn)3AlC carbides grew within the austenite matrix and coarse (Fe,Mn)3AlC carbides occurred on the grain boundaries. After the impact tests, the ductile dimple fracture surface was observed on the SHT specimen, which had a higher impact value than those of the aged specimens. The impact value of the aged specimens decreased gradually with the increasing aging time and the amount of the precipitates observed on the grain boundaries. When the impact tests were carried out at temperatures ranging from 25℃ to -196℃, the impact value of the SHT specimens decreased with decreasing temperature. When the test temperature fells from -70℃ to -140℃, the impact energy was dramatically reduced from 255 J/cm2 to 136.2 J/cm2. The aged specimens had a similar results. Moreover, after aged at 650℃ for more than 24 hours, deformation twinning occurred in the Fe-9Al-30Mn-1C alloy during impact tests, which has never been found by other workers in the austenitic FeAlMnC alloy with C content higher than 0.77wt% before.