Effect of Manganese on Microstructures and Solidification Modes of Cast Fe-Mn-Si-Cr-Ni Shape Memory Alloys
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Fe-Mn-Si-BASED shape memory alloys (SMAs) have attracted considerable attention owing to their shape memory effect (SME) and cheap cost since Sato and coworkers[1–13] discovered the excellent SME in FeMn-Si single crystals. In the past few decades, polycrystalline Fe-Mn-Si, Fe-Mn-Si-Cr, and Fe-Mn-Si-Cr-Ni SMAs were developed.[3,4,6,9] Among them, austenitic stainless Fe-Mn-Si-Cr-Ni SMAs are the most promising alloys because they show the best SME and good corrosion resistance.[4] Unfortunately, ordinary polycrystalline Fe-Mn-Si-Cr-Ni SMAs free from training suffer from a poor SME with the exception of thin foil specimens.[7,10] However, the training,[5,8] a repeated process of deforming at room temperature and subsequent annealing at about 923 K (650 °C), markedly increases the production cost. In addition, it is too complicated to be performed for components with complicated shapes. Accordingly, current researches mainly focus on developing training-free Fe-Mn-Si-CrNi SMAs. Very recently, we developed a novel trainingfree cast Fe-18Mn-5.5Si-9.5Cr-4Ni alloy.[11,12] Its recovery strain reaches 6.4 pct only after annealing at 973 K (700 °C).[12] The method of casting and subsequent annealing is simpler and cheaper than the above training. Furthermore, this method can manufacture components with complicated shapes. Our finding provides a novel method for developing training-free HUABEI PENG, Lecturer, YUHUA WEN, Professor, and YANGYANG DU, QINXU YU, and QIN YANG, Postgraduates, are with the College of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 22, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
polycrystalline Fe-Mn-Si-based SMAs with a high recovery strain. However, our study is a preliminary work. We only investigated the microstructures of the cast Fe-18Mn5.5Si-9.5Cr-4Ni alloy before and after annealing. The results showed that this alloy solidifies as the F mode.[11,12] Therefore, its microstructure before annealing is characterized by Widmanstatten austenite. After annealing, an excellent SME is obtained, provided that Widmanstatten austenite remained.[11,12] Obviously, a question will arise: Is the microstructure of Widmanstatten austenite the key factor guaranteeing the high recovery strain in the cast Fe-Mn-Si-Cr-Ni alloys? As is well known, the solidification mode decides the as-cast microstructures of austenitic stainless steels.[14] Accordingly, to answer this question, the effect of solidification modes must be first known on the as-cast microstructure and SME of cast Fe-Mn-Si-Cr-Ni SMAs. Unfortunately, there is no report on this issue. In order to clarify the issue, we must first predict the solidification modes of cast Fe-Mn-Si-Cr-Ni SMAs and subsequently attain different solidification modes of cast Fe-Mn-Si-Cr-Ni SMAs. Two methods may be used to predict the solidification modes of cast Fe-Mn-Si-Cr-Ni SMAs so far. The first one is to use chromium and nickel equivalence equations (Creq/Nieq equations). For austen
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