The Microstructures, Mechanical Properties and Deformation Mechanisms of Fe-Mn-Al-C Alloys with Spinodal Decomposition
|關鍵字:||史賓諾多分解反應;奈米結構材料;機械性質;顯微結構;穿透式電子顯微鏡;掃瞄式電子顯微鏡;Spinodal decomposition;Nanostructured materials;Mechanical properties;Microstructures;Transmission electron microscopy;Scanning electron microscopy|
|摘要:||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影像分析儀與拉伸試驗機，研究觀察鐵-30wt.%錳-8.5wt.%鋁-2.0wt.%碳(合金 A)顯微結構與機械性質之關係和鐵-30wt.%錳-9.5wt.%鋁-2.0wt.%碳(合金 B)拉伸試驗後之變形機構。依據實驗結果，本論文所得的具體研究成果如下：
(一)、在淬火狀態下，合金A和合金B的顯微結構均是具有高密度、細微的κ'-碳化物在沃斯田鐵基地中，且此κ'-碳化物是由淬火過程中藉由史賓諾多分解反應所產生的。此κ'-碳化物的形成機構與過去學者在時效處理後的鐵錳鋁碳合金(C≦1.3wt.%)中發現的細微κ'-碳化物截然不同。由於合金A和合金B在淬火狀態下，沃斯田鐵基地中就具有高密度、細微的κ'-碳化物存在，所以其最大拉伸強度、降伏強度與延伸率分別可達到1105 MPa、883 MPa、54.5%與1150 MPa、 950 MPa、53%。因此合金A與合金B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。
(二)、因為合金A與合金B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。例如：合金A在550℃ 3小時與合金B在450℃ 9小時時效處理後，其降伏強度分別可達到1262 MPa和1406 MPa，延伸率仍保持32.1 和32.5%。與C≦1.3wt.% 經最佳時效處後的鐵錳鋁碳合金相比下，在相同的延伸率時，我們所得到的機械強度高了30%。更重要的是，我們所得到的強度-延性組合已明顯大幅度的超越由美國能源部在2012年提出所要發展之目標和由美國鋼鐵協會所要發展預計2017~2025年欲達到的第三代高強度鋼材(3GAHSS)之機械性質。
(三)、合金B經過450℃，9小時時效處理後，基地內κ'-碳化物以<100>方向性成長，此時κ'-碳化物的體積分率快速增加達68%，因此γ基地受到κ'-碳化物的分割而形成isolated γ phase nano-channels。所以，在拉伸破斷後，並無發現到任何的長滑移線僅在isolated γ phase nano-channels內發現了高密度且均勻分佈的差排。這表示塑性變形是由在isolated γ phase nano-channels的“bursting dislocation nucleation”所產生的。此外，利用穿透式電子顯微鏡在γ/κ'界面上觀察到大量的正負交替dislocations-pairs之現象，此現象為Taylor lattice之特徵。此部份值得一提的是(a)在多晶金屬中，這種「Bursting Dislocation Nucleation」新穎塑性變形機制為我們首度發現。(b)利用高解析電子顯微鏡首度率先觀察到Taylor lattice的原子排列影像。|
The relationship between microstructures and tensile properties of an Fe-30wt.%Mn-8.5wt.%Al-2.0wt.%C alloy (Alloy A), and the deformation mechanisms in an Fe-30wt.%Mn-9.5wt.%Al-2.0wt.%C alloy (Alloy B) after tensile test 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. On the basis of the experimental examinations, the results can be summarized as follows：  The as-quenched microstructure of both the alloys A and B is the austenite phase containing a high density of extremely fine κ'-carbides were formed during quenching by spinodal decomposition. The unique κ'-carbides formation mechanism is quite different that observed in the FeMnAlC (C≦1.3wt.%) alloys, in which the fine 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 both A and B alloys can reach 1105 MPa, 883 MPa and 54.5%, and 1150 MPa, 950 MPa, 53%, respectively. Evidently, the as-quenched mechanical properties of both A and B alloys are superior to those of the as-quenched FeMnAlC (C≦1.3wt.%) alloys 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. For instance, when the alloy A was aged at 550℃ for 3 hours and the alloy B was aged at 450℃ for 9 hours, the obtained YS can be reach 1262 MPa, and 1406 MPa and the El maintains to be 32.1 and 32.5%, respectively. Under the same elongation, our results showed over 30% enhancement in strength as compared with the optimally aged C≦1.3wt.% FeMnAlC alloys. More importantly, the obtained strength-ductility combination has evidently exceeded by a sizeable margin over the targeted specifications put forth by the US DOE in 2012 and by American Steel Association for the third generation advanced high-strength steels (3GAHSS) to be on market in 20172025.  When the alloy B was aged at 450℃ for 9 hours, the nano-size κ'-carbides grew along the <100> directions, and the volume fraction of the grown κ'-carbides increased dramatically to about 68%. Therefore, the austenite matrix was well-divided to isolated γ-phase nano-channels. Consequently, after tensile test fractured, no long slip lines could be observed and a very high density of dislocations homogeneously distributed within the γ-phase nano-channels. Moreover, these dislocation were all oriented normal to the γ/κ'-carbides interfaced. It means that the plastic deformation was dominated by “bursting dislocation nucleation” within the isolated austenite nano-channels. Furthermore, transmission electron microscopy observation revealed that the dislocations near the γ/κ' interface were formed in pair-wise manner, which is a unique feature of Taylor lattice. It is worth while to note here that (a) we discovered, for the first time, a novel deformation mechanism in polycrystalline metals, which is governed by “bursting dislocation nucleation”, and (b) we are the first to directly observe the dislocation Taylor lattice in atomic resolution images.