熱處理對鐵-9鋁-30錳-1碳合金機械性質影響

Abstract

本篇論文的目的是經由光學顯微鏡、掃描式電子顯微鏡及穿透式電子顯微鏡來觀察鐵-9鋁-30錳-1碳合金經不同熱處理後的顯微結構，並探討不同顯微結構對機械性質和破斷機構的影響。合金在經過1100℃固溶熱處理之後可以得到完全的沃斯田鐵相顯微結構，擁有良好的強度和韌性。合金經過550℃、6小時的時效熱處理後，細小的κ´-碳化物在沃斯田鐵晶粒內析出，造成了析出硬化的效果，且此條件下無析出物在晶界上析出，所以材料在沒有明顯損失韌性的情況下明顯的得到了更佳的強度。隨著時效溫度的上升，在經過650℃、6小時的時效熱處理後，不但在沃斯田鐵晶粒內有細小的κ´-碳化物析出物，在晶界上也開始有一些粗大的κ-碳化物異相析出，所以雖然材料因為析出硬化而在強度上達到最佳，但卻因為晶界上析出而造成材料靭性稍微下降。而在經過850℃、6小時的時效熱處理後，在晶粒內的κ´-碳化物變的很少，而在晶界上析出了大量粗大的κ-碳化物，因而造成材料的強度及靭性都大幅地下降。 本篇論文除了比較不同時效熱處理溫度下所得到的機械性質外，也可以比較出鑄造試片及軋延試片的不同。鑄造試片經過各不同時效熱處理後，破斷模式是比較偏向脆性的穿晶、沿晶破斷。而軋延試片在經過各不同時效熱處理後，破斷行為以延性的酒窩狀凹洞為主，隨著時效溫度上升，輔以稍具脆性的穿晶破斷。

The purpose of this study is to examine the microstructural development of the Fe-9Al-30Mn-1C alloy through different heat-treatments by means of optical microscopy, scanning electron microscopy and transmission electron microscopy. Besides, the effects of the microstructures on the mechanical properties and fracture mechanisms were also made. In the as-quenched condition, the microstructure of the alloy was single austenite phase. Therefore, the alloy possesses good strength and toughness. When the alloy was aged at 550℃ for 6 hours, fine κ´-carbides were formed within the austenite matrix and no precipitates could be observed on the grain boundaries. It is thus that the alloy could get better strength without losing much toughness. When the alloy was aged at 650℃ for 6 hours, not only the fine κ´-carbides formed within the austenite matrix but also a few of the coarse κ-carbides started to precipitate heterogeneously on the γ/γ boundaries. This leads the alloy to get the best strength and cause the toughness down slightly. When the alloy was aged at 850℃ for 6 hours, there was deficient κ´-carbides existing within the austenite matrix but much coarse κ-carbides occurred on the grain boundaries. Obviously, the strength and the toughness of the alloy would go down greatly. In the present study, the comparison of the mechanical properties between the cast alloy and the rolled alloy was undertaken. Under the same heat-treatment condition, the cast alloy trended to be more brittle. The main fracture mechanism of the cast alloy was intergranular decohesion with a certain proportion of transgranular cleavage. The main fracture mechanism of the rolled alloy was ductile dimple fracture. With increasing the aging temperature, it would be accompanied with a certain proportion of transgranular cleavage.