鐵-9鋁-30錳-6鉻-1.8碳合金之顯微結構與抗腐蝕性質

Abstract

本研究論文主要目的是利用光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)以及穿透式電子顯微鏡(TEM)來觀察Fe-9 wt.%Al-30 wt.%Mn-1.8 wt.%C合金(合金A)及Fe-9 wt.%Al-30 wt.%Mn-6 wt.%Cr-1.8 wt.%C合金(合金B)在淬火狀態下及經過400℃不同時間時效熱處理後，其顯微結構的改變。此外，利用恆電位儀及歐傑電子能譜分析儀(AES)來測量合金在淬火狀態及時效熱處理後在3.5% NaCl水溶液中的動態極化曲線，進一步探討合金顯微結構變化對其抗腐蝕性質的影響。 本研究發現，合金A(0 Cr)和合金B(6 Cr)在淬火狀態下，晶粒內為沃斯田鐵相，並且可以在基地相中觀察到細微的κ′-碳化物析出，而在晶界上並無任何析出物。此外，κ′-碳化物是具有L′12結構的(Fe,Mn)3AlC碳化物。κ′-碳化物是在淬火時以史賓諾多相分解(spinodal decomposition)的方式在基地相內析出。與未添加鉻的Fe-9 wt.%Al-30 wt.%Mn-1.8 wt.%C合金(合金A)相比，添加鉻的合金B(6 Cr)具有更加良好的抗腐蝕性質。 當合金在400℃時效熱處理3小時之後，基地相中的κ′-碳化物會略為成長，此時晶粒內仍由γ＋κ′所組成，並且在晶界上並無析出物，所以此時的抗腐蝕性質和淬火狀態類似。當時效時間延長至24及36小時，晶界上會有析出物形成，此析出物為具有L′12結構的(Fe,Mn)3AlC碳化物，我們稱之為κ-碳化物。同時，可以觀察到晶界析出物附近形成明顯的無析出區，我們由TEM分析得知該區域為γ0，表示晶界上有γ＋κ′ →γ0＋κ反應發生。而晶界上的析出物會導致孔蝕較容易發生，所以此時抗腐蝕性質會略為下降。當時效時間拉長至96小時以上，我們可以在晶界上發現鉻的碳化物析出，此碳化物經TEM分析後證實為具有六方最密堆積結構(Hexagonal Close-Packed, HCP)的富鉻之M7C3碳化物。由於此碳化物在晶界上異質析出，鄰近的鉻含量會大幅降低，形成鉻的缺乏區，在合金表面無法形成鉻氧化物的保護層，導致抗腐蝕性質大幅下降。

The purpose of this study is to examine the microstructural developments of Fe-9 wt.%Al-30 wt.%Mn-1.8 wt.%C alloy (Alloy A(0 Cr)) and Fe-9 wt.%Al-30 wt.%Mn-6 wt.%Cr-1.8 wt.%C (Alloy B(6 Cr)) after being solution heat-treated (SHT) and then aged at 400℃ for various times by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, the corrosion resistant properties of the as-quenched and aged alloys in 3.5% NaCl solution were also examined by electrochemical measurements and auger electron spectroscopy (AES). In the as-quenched condition, the microstructure of both alloys A(0 Cr) and B(6 Cr) was single austenite (γ) phase containing fine κ′ carbides, and no precipitates could be observed on grain boundaries. The fine κ′ carbides are (Fe,Mn)3AlC carbides with an L′12 structure, which were formed within the γ matrix by spinodal decomposition during quenching. In the as-quenched condition, the corrosion resistance of the alloy B(6 Cr) was much better than that of the alloy A(0 Cr). When the as-quenched alloy B(6 Cr) was aged at 400℃ for 3 h, the fine κ′ carbides grew significantly and no precipitates were formed on grain boundaries. Therefore, the corrosion resistant property of the alloy B(6 Cr) aged at 400℃ for 3h is similar to that of the alloy B(6 Cr) in the as-quenched condition. However, when the aging time was increased to 24h, some coarse (Fe,Mn)3AlC carbides (κ carbides) having an L′12 structure started to appear on grain boundaries. With increasing the aging time at 400℃, the coarse κ carbides grew into adjacent austenite grains through γ＋κ′ →γ0 (carbon-deficient austenite)＋κ carbide reaction. The precipitation of κ carbides on the grain boundaries would deteriorate the pitting corrosion noticeably. After further prolonged the aging time, Cr-rich M7C3 carbides were also found to appear on grain boundaries. The M7C3 carbide has a hexagonal close-packed (HCP) structure. Owing to the heterogeneous precipitation of Cr-rich M7C3 on grain boundaries, the corrosion resistance of the alloy B(6 Cr) in 3.5% NaCl solution dropped drastically.