A New Set of Cr eq and Ni eq Equations for Predicting Solidification Modes of Cast Austenitic Fe-Mn-Si-Cr-Ni Shape Memor
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emory alloys (SMAs) have attracted attention owing to their shape memory effect (SME) and superelasticity. Fe-Mn-Si-based SMAs exhibit lower cost, better workability, and weldability as compared with Ni-Ti-based and Cu-based SMAs. Thus, there have been many studies on Fe-Mn-Si-based SMAs since Sato and his co-workers discovered the SME behavior in FeMn-Si single crystals.[1–14] Unfortunately, the recovery strain is only 2 to 3 pct in polycrystalline Fe-Mn-Sibased SMAs prepared by conventional processing techniques.[3–14] To improve the recovery strain, the processed Fe-Mn-Si-based SMAs have to be subjected to training,[4–6] thermo-mechanical treatment,[9–11] or ausforming.[8] However, the training is not only too complicated to be performed for components with complicated shapes, but also increases the production cost. The thermo-mechanical treatment and ausforming are essentially a training step. As a result, the processed Fe-Mn-Si-based SMAs have not been used commercially so far. Current research focuses mainly on developing training-free Fe-Mn-Si-based SMAs. Very recently, Wen et al.[12] developed a novel trainingfree cast Fe-18Mn-5.5Si-9.5Cr-4Ni alloy which solidifies primarily as delta ferrite. Its recovery strain reaches 6.4 pct,
HUABEI PENG, Lecturer, YUHUA WEN, Professor, YANGYANG DU, JIE CHEN, and QIN YANG, Postgraduates, are with the College of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, People s Republic of China. Contact e-mail: [email protected] Manuscript submitted July 15, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
which is higher than that of the trained Fe-14Mn-5Si-8Cr4Ni alloy.[13] Manufacture of parts with this alloy system is simple and less costly compared to Fe-Mn-Si-based alloy systems which require the training. Moreover, it is easy to cast components with complicated shapes. Therefore, this alloy system provides a novel way for producing a trainingfree Fe-Mn-Si-based alloy with the high recovery strain. In addition, it can be expected that an even higher recovery strain can be developed in cast Fe-Mn-Si-Cr-Ni SMAs through optimization of alloy compositions, casting parameters, and heat treatment techniques. Fe-Mn-Si-Cr-Ni SMAs are a special kind of austenitic stainless steels. There exist four types of solidification mode in austenitic stainless steels, i.e., austenitic (A), austenitic-ferritic (AF), ferritic-austenitic (FA), and ferritic (F) modes. The four solidification modes are determined according to solidification sequence, as follows:[15,16] A mod e : L ðLiquidÞ ! L þ c ! c; AF mod e : L ! L þ c ! L þ c þ d ! c þ d; FA mod e : L ! L þ d ! L þ d þ c ! d þ c; F mod e : L ! L þ d ! d ! d þ c: Different solidification modes produce distinctive ascast microstructures, inevitably leading to different SME behaviors. Obviously, it is of importance to investigate the effect of solidification modes on solidification microstructures and their resulting SME. However, our previous work showed that four sets of Creq and Nieq equations (Delong et al.[17] Hull,[18] Hammar
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