鐵-8.8鋁-30.0錳-6.0鉻-1.0碳合金相變化

Abstract

摘 要 我們利用光學顯微鏡 (OM)，穿透式電子顯微鏡 (TEM) 以及X-光能量散佈分析儀 (EDS)，研究鐵-8.8鋁-30錳-6鉻-1碳之相變化。 在淬火狀態下，此合金的顯微結構為單一的沃斯田鐵相。當此合金在500℃時效處理2小時後，我們發現其顯微結構為具有L'12結構的細微κ′相碳化物在沃斯田鐵基地內沿<100>方向成長，在晶界上並未觀察到有析出物的存在；在500℃時效處理36小時後，不但在沃斯田鐵基地內κ′相碳化物粗化，而且在沃斯田鐵間的晶界上有粒狀的κ相碳化物析出；在700℃時效處理10小時後，另一種型式的析出物為具面心晶體結構的(Fe, Mn, Cr)23C6鉻化物，以顆粒狀在沃斯田體基地中析出，此時κ′相碳化物在沃斯田鐵基地中並未被發現；在800℃時效處理10小時後，析出物的型態從顆粒狀轉變成板狀並且析出在沃斯田鐵的基地中，此板狀的析出為具六方最密結構的(Fe, Mn, Cr)7C3鉻化物，原有的(Fe, Mn, Cr)23C6鉻化物此時在沃斯田體基地中並無析出。 所以，隨著溫度由500℃至800℃此合金的相變化為γ→γ+κ′相碳化物→γ+(Fe, Mn, Cr)23C6鉻化物→γ+(Fe, Mn, Cr)7C3鉻化物。

ABSTRACT Phase transformations in the Fe-8.8Al-30Mn-6Cr-1C alloy have been examined by means of optical microscopy (OM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectrometer (EDS). In the as-quenched condition, the microstructure of the present alloy was single austenite phase. When the alloy was aged at 500℃ for 2 hours, the microstructure of the alloy was austenite phase containing extremely fine (Fe, Mn)3AlCx carbides (κ′-carbides). The κ′-carbides having an L'12-type structure grew preferentially along <100> directions. No grain boundary precipitates could be observed. However, when the aging time was increased to 36 hours, not only the κ′-carbides grew within the austenite matrix but also the granular-shaped κ-carbides started to appear on the γ/γ grain boundaries. When the alloy was aged at 700℃ for 10 hours, another type of particle-like (Fe, Mn, Cr)23C6 chromium carbides having a face-centered cubic structure started to appear within the austenite matrix and no (Fe, Mn)3AlCx carbides could be detected. When the alloy was aged at 800℃ for 10 hours, the morphology of precipitates changed from particle-like to plate-like within the austenite matrix and no (Fe, Mn)3AlCx carbides could be detected. The plate-like chromium carbides was (Fe, Mn, Cr)7C3, which has a hexagonal closed-packed structure. Therefore, when the alloy was aged at temperatures ranging from 500℃ to 800℃, the phase transformation sequence as the temperature increased was found to be γ→ γ+κ′-carbide→γ+ (Fe, Mn, Cr)23C6 →γ+ (Fe, Mn, Cr)7C3.