Stress Evolution in Sputter-deposited Fe-Pd Shape-memory Thin Films
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Fe–Pd films with Pd content varying between 26 and 30 at.% have been deposited by means of magnetron sputtering of elemental Fe and Pd targets. As-deposited films are highly supersaturated solid solutions of Pd in Fe that have a body-centered-cubic crystal structure and a very fine grain size. Substrate curvature measurements indicate that the films undergo an irreversible densification when heated above 100 °C. This densification is attributed to a structural change that is also observed in other supersaturated systems with a substantial atomic size difference between the constituents. It is possible to retain the high-temperature austenite phase at low temperature by annealing the films at 900 °C followed by rapid cooling. Depending on film composition, this metastable austenitic phase transforms to either a body-centered tetragonal (bct) or a face-centered tetragonal (fct) martensite around room temperature. Substrate curvature measurements show that formation of the fct martensite is reversible, while that of bct martensite is not. The fct transformation occurs at lower Pd content and higher temperature than reported for bulk materials. Both the fct and the fcc phase show a strong Invar effect at lower temperature and Pd content than observed in the bulk.
I. INTRODUCTION
Shape-memory alloys can recover large strains due to a thermoelastic martensitic transformation that takes place in these materials. They have been studied extensively over the last 50 years, and Ni–Ti alloys in particular have been developed successfully into actuators and sensors in various engineering applications. If the martensitic or austenitic phases are ferromagnetic, it is sometimes possible to control the martensitic transformation through application of a magnetic field. In some materials, for instance, the transformation can be induced by applying a strong enough magnetic field.1 Alternately, a magnetic field can be used to convert martensitic variants or twins in the martensitic phase. Application of a magnetic field causes variants with their easy axis of magnetization aligned with the externally applied field to grow at the expense of others. The result is a macroscopic shape change. This magnetic shape memory effect is distinctly different from the well-known magnetostriction phenomenon in that the strain obtained in the magnetic shape memory is orders of magnitude larger than for magnetostriction.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0283 J. Mater. Res., Vol. 20, No. 9, Sep 2005
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A number of ferromagnetic materials have been studied in the past including the Fe–Ni, Fe–Pt, Fe–Pd, Co– Ni, Fe–Co–Ni–Ti systems in addition to intermetallics based on the Heusler-phase Ni2MnGa. These studies2–8 have shown that among these alloy systems, Ni2MnGa, Fe–Pd, and ordered Fe3Pt have the potential to be used as ferromagnetic shape-memory alloys. Thus far, only limited research9–13 has been conducted on Fe–Pd thin films and
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