鐵-9wt.%鋁-28wt.%錳-1.8wt.%碳合金顯微結構、機械性質及抗腐蝕性質研究

Abstract

本論文利用穿透式電子顯微鏡，掃瞄式電子顯微鏡，X-光能量散佈分析儀，LECO 2000影像分析儀與Instron拉伸試驗機研究觀察Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金顯微結構與機械性質之關係。並研究離子氮化對表面硬度和抗腐蝕性質影響。依據實驗的結果，本論文所得到的具體研究結果如下: (一)、在淬火狀態下， Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金的顯微結構是沃斯田鐵相(Austenite, γ), 且在沃斯田鐵基地內具有高密度、細微奈米級的κ'-碳化物，此奈米級κ'-碳化物是在淬火過程中藉由史賓諾多分解(spinodal decomposition)反應所產生的。此κ'-碳化物的形成機構與過去學者在時效處理後的鐵-(7-10)wt.%鋁-(28-32)wt.%錳-(0.54-1.3)wt.%碳 (C≦1.3wt.%)合金中發現的細微κ'-碳化物截然不同。在C≦1.3wt.%的Fe-Al-Mn-C合金中，淬火狀態下為單一沃斯田鐵相，κ'-碳化物只能在時效後才能產生。由於Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金在淬火狀態下，沃斯田鐵基地內就具有高密度、細微奈米級的κ'-碳化物存在，所以其最大拉伸強度(ultimate tensile strength (UTS))、降伏強度(yield strength (YS))與延伸率(El)分別可達到1080 MPa、868 MPa和55.5%。因此Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金在淬火狀態下之機械性質明顯的優於其它學者所研究的C≦1.3wt.%的鐵錳鋁碳合金在淬火狀態時之機械性質。因為Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金在淬火狀態下即具有κ'-碳化物，所以在達到最佳的強度與延性組合的機械性質所需的溫度與時間均可大幅減少。將淬火合金在450C，時效12小時，可得到最佳強度和延性組合，其最大拉伸強度、降伏強度與延伸率分別可達到1552 MPa、1423 MPa和25.8%。

(二)、Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金的淬火結構為沃斯田鐵相 (austenite，)，且在γ基地內有十分緻密奈米級(nano-sized)的(Fe,Mn)3AlC碳化物(κ'-碳化物)。緻密奈米級的κ'-碳化物是合金在固溶化淬火過程中藉由史賓諾多分解(spinodal decomposition)相變化在γ基地內形成。將在淬火狀態下的Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金，經450C、12小時、壓力範圍133-798 Pa離子氮化處理後，在γ基地內的κ'-碳化物會成長且量變多，而使氮化後合金能具有優異的強度和延性組合，且隨著氣體壓力的增加在合金表面分別可得到6~15 m厚的氮化層，此氮化層結構由X-繞射可知，主要組成為具有面心立方(Face-Centered Cubic, FCC)型之B1-AlN及少量的FCC -Fe4N；藉由離子氮化處理後的Fe-9wt.%Al-28wt.%Mn-1.8wt.%C合金表面硬度可高達1710 Hv，基材硬度為540 Hv，且在3.5%鹽水中具有極佳抗腐蝕性質，這些特性都遠優於一般經最佳氮化處理後的高強度合金鋼，工具鋼，麻田散鐵不銹鋼及析出硬化型不銹鋼。

The relationships between microstructures and tensile properties of an Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy have been examined by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, LECO 2000 image analyzer and Instron tensile testing machine, respectively. Microstructures, mechanical properties and corrosion behaviors of the plasma-nitrided Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy have also been investigated. On the basis of the experimental examinations, the results can be summarized as follows：

[1] The as-quenched microstructure of the Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy is austenite phase containing a high density of extremely fine nano-sized κ'-carbides. The κ'-carbides were formed during quenching by spinodal decomposition. The unique κ'-carbides formation mechanism is quite different from that observed in the Fe-(7~10)wt.%Al-(28~32)wt.% Mn-(0.54~1.3)wt.%C (C≦1.3wt.%) alloys, in which the fine nano-sized κ'-carbides could only be observed in the aged alloys. Owing to the presence of the high density of the extremely fine κ'-carbides within the austenite matrix, the ultimate tensile strength (UTS), yield strength (YS) and elongation (El) of the as-quenched Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy was 1080 MPa, 868 MPa and 55.5%, respectively. Evidently, the mechanical property of the as-quenched alloy is superior to that of the as-quenched FeMnAlC (C≦1.3wt.%) alloy examined by previous workers. Since the '-carbides already exist in the as-quenched alloys, both the aging time and temperature for obtaining the optimal combination of strength and ductility can be significantly reduced. When the alloy was aged at 450℃ for 12 hours, the ultimate tensile strength (UTS), yield strength (YS) and elongation (El) of the Fe-9wt.%Al-28wt.%Mn-1.8wt.% C alloy was 1552 MPa, 1423 MPa and 25.8%, respectively. [2] The as-quenched microstructre of the Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy was austenite (γ) phase containing an extremely high density of nano-sized (Fe,Mn)3AlC carbides (κ'-carbide). These κ'-carbides were formed within the austenite matrix by spinodal decomposition during quenching. The size and the amount of the κ'-carbides increased dramatically when the as-quenched Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloy was plasma-nitrided at 450C for 12h under nitriding pressures ranging from 133-798 Pa. Consequently, the nitrided alloys could obtain an excellent combination of strength and ductility after being plasma-nitrided. The thickness of the nitrided layers obtained are 6, 9 and 15 m-thick under pressure of 133, 399 and 798 Pa, respectively. The nitride layer is composed predominantly of FCC B1-AlN with a small amount of -Fe4N. Due to the surface nitrogen concentration reached up to 18.4wt.% (44.3at.%), the surface hardness (1710 Hv), substrate hardness (540 Hv), and corrosion resistance in 3.5% NaCl solution of the plasma-nitrided Fe-9wt.%Al-28wt.%Mn-1.8wt.%C alloys are far superior to those obtained previously in optimally nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels.