Shape memory behavior of FeNiCoTi single and polycrystals
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I. INTRODUCTION
IT is well known that a number of Ni- and Cu-based materials display shape memory behavior and also pseudoelastic response.[1,2,3] The shape memory behavior is defined as the recoverability of deformation upon heating above a critical temperature. The pseudoelastic response refers to the forward transformation upon loading and reverse transformation upon unloading under isothermal conditions. The shape memory behavior has been observed in other materials such as certain iron alloys but the research in iron-based alloys lagged its Ni- and Cu-based counterparts. Previously, the primary focus has been on FeMnSi alloys,[4] where the reversibility of the transformation has been curtailed by the underlying slip deformation. In the present work, we study FeNiCoTi alloys, which are a new class of materials exhibiting thermoelastic martensite, hence, reversible transformations from austenite to martensite. The transformation can be induced via a change in temperature or the application of stress at temperatures exceeding the martensite start temperature, Ms . Three major groups of researchers have studied these classes of alloys. The group of Kokorin et al.[5] in the Ukraine has considered Co-rich (⬎20 wt pct) compositions to reduce the tetragonality ratio and minimize the volume change upon deformation. They demonstrated recoverable strains of less than 0.5 pct in temperature cycling experiments in early work.[6] In more recent work,[7] they have demonstrated recoverable strains near 0.7 pct. In addition to the length change measurements, they noted the changes in magnetic transition from ferromagnetic to paramagnetic HUSEYIN SEHITOGLU, Grayce Wicall Gauthier Professor, X.Y. ZHANG, Visiting Scholar, T. KOTIL, Visiting Scholar, on sabbatical leave from the Aeronautical and Astronautical Engineering Department, Istanbul Technical University, Istanbul, Turkey, and D. CANADINC, Research Assistant, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801. Contact e-mail: huseyin @uiuc.edu Y. CHUMLYAKOV, Professor, is with the Siberian PhysicalTechnical Institute, Tomsk, 6340502, Russia. H.J. MAIER, Professor, is with Lehrstu¨hl fur Werkstoffkunde, University of Paderborn, 33095 Paderborn, Germany. Manuscript submitted May 15, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
states upon transformation from austenite to martensite. The second group who conducted extensive work on FeNiCoTi alloys was Hornbogen and colleagues[8,9] in Germany. They have considered compositions with lower Co (⬍20 pct) and higher Ni (⬎20 pct) contents compared to Kokorin. Their main emphasis has also been to optimize aging treatments and to strengthen the austenite phase with the introduction of fine precipitates. Finally, Maki[10] in Japan has made the observation that thin martensites are responsible for the reversible transformations. We note that these previous treatments have not considered theoretical calculations of transformation strain levels and also reported transformation
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