Phase Transformations, Mechanical Properties and Corrosion Behaviors in New Austenitic FeMnAlC and FeMnAlCrC Alloys with Spinodal Decomposition
|關鍵字:||鐵錳鋁碳合金;節點分解相變態;K‘-碳化物;離子氮化;耐腐蝕性;氮化鋁;FeAlMnC alloy;Spinodal decomposition;K‘-carbide;Ionitriding;Corrosion Behavior;AlN|
|摘要:||前人研究γ-FeMnAlC合金均集中在0.7≦C≦1.3wt%，固溶狀態(SHT)為單一γ相，強度小。眾多學者發現，經550℃ 15~16小時時效可得最佳強度和延性組合(Y.S.:1094MPa, El.:26%),強化因細微(Fe,Mn)3AlC(κ')碳化物時效時在γ內析出。近年來，我們研究1.8~2.0wt%FeMnAlC，發現大量奈米級κ'(~11nm)在淬火過程中藉史賓諾多相分解（Spinodal Decomposition;S.D）在γ內產生。此合金我們命名為＂Femac＂。Femac在S.H.T.下已有κ'，經較低溫450℃ 12小時時效，其 Y.S:1423MPa和El:25.8%。在相同El下，強度比C≦1.3wt.%提高達30%。Femac技轉生產Power BILT 品牌，獲日本2006-2007年最佳鐵桿金質獎，2009年製作國防用「高性能防護薄鋼板」，通過最新抗彈FJ(SS109)規範。在淬火中即有S.D產生，是我們首度發現，至今尚未有S.D曲線。計劃第一年將分別建立1.4≤C≤2.0wt.%及2.5≤Al≤15wt.%之S.D曲線，並探討淬火和350~500℃時效後相變化與機械性質。
Previously, the studies of γ-FeMnAlC alloys have been mostly focused on the composition range of 0.7≤C≤1.3wt%, which under solution heat treatment (SHT) condition is single γ-phase with small strength. Subsequently, many researchers found that optimal combination of yield strength and ductility (YS: 1904 MPa and El: 26%) can be obtained by carrying out aging treatment at 550 °C for 16 hours. The enhancement of yield strength was attributed primarily to the precipitation of fine (Fe,Mn)3AlC (κ'-carbide) within γ-matrix during aging. Recently, our researches on 1.8-2.0wt% FeMnAlC (termed as “Femac”) have led to the unprecedented discovery of massive amount of nano-scale κ'-carbides dispersed within the γ-matrix via spinodal decomposition (SD) during quenching process. Since the κ'-carbides formed under SHT condition, we were able to obtain a combination of YS: 1423 MPa and El: 25.8% for this alloy by merely aging it at 450 °C for 12 hours. Comparing to the C≤1.3wt% alloys, Femac has demonstrated a 30% enhancement in YS while keeping almost the same ductility. The technology transfer of Fermac to Power BILT had won the 2006-2007 best iron club golden quality award in Japan and the “High Performance Protective Thin Steel Plate” manufactured with Femac had passed the latest FJ bullet-proof regulations in 2009. Since the phenomenon of SD during quench was first discovered by us and the SD curve is yet to be established, thus the first year of the present research proposal will be devoted to establish the SD curve of the alloys containing 1.4≤C≤2.0wt% and 5≤Al≤ 15wt%, respectively. The phase transformations and associated mechanical properties of the as-quenched alloys and those subjected to 350-500 °C aging treatment will be systematically investigated. It is known that, under SHT condition, the corrosion resistance of the C≤1.3 wt% γ-FeMnAlCrC alloys in 3.5%-NaCl solution outperforms that of the martensitic stainless steels (MSS). However, the YS of the γ-FeMnAlCrC alloys is very small under SHT condition and needs aging strengthening. Unfortunately, the 550 °C aging commonly practiced often incorporates precipitation of Cr-carbides and degrades the corrosion resistance significantly. In the second year, we will try to add Cr into the SD-type Femac alloys via delicate alloy designing and then investigate the phase transformations, mechanical properties, as well as the 3.5%-NaCl corrosion resistance for the as-quenched and low-temperature (300-450 °C) aged Cr-added Femac alloys. On the other hand, it has been demonstrated that surface nitrogenized MSS exhibit much more superior corrosion resistance than AISI 304 and 316 stainless steels in various acidic solutions. Since the Femac contains large amount of Al, we expect it will be even easier to be nitrogenized. Consequently, in the third year, we will combine the excellent mechanical properties of the Femac alloys with simultaneous nitrogenization during SHT and subsequent 350-500 °C aging processes. It is expected that a series of novel alloys with breakthrough properties of high strength, high toughness, and superior corrosion resistance will be obtained through the efforts of the present three-year research proposal.