In situ transmission electron microscopy and nanoindentation studies of phase transformation and pseudoelasticity of sha
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Transmission electron microscopy (TEM) and nanoindentation, both with in situ heating capability, and electrical resistivity measurements were used to investigate phase transformation phenomena and thermomechanical behavior of shape-memory titanium-nickel (TiNi) films. The mechanisms responsible for phase transformation in the nearly equiatomic TiNi films were revealed by heating and cooling the samples inside the TEM vacuum chamber. Insight into the deformation behavior of the TiNi films was obtained from the nanoindentation response at different temperatures. A transition from elastic-plastic to pseudoelastic deformation of the martensitic TiNi films was encountered during indentation and heating. In contrast to the traditional belief, the martensitic TiNi films exhibited a pseudoelastic behavior during nanoindentation within a specific temperature range. This unexpected behavior is interpreted in terms of the evolution of martensitic variants and changes in the mobility of the twinned structures in the martensitic TiNi films, observed with the TEM during in situ heating.
I. INTRODUCTION
Shape-memory alloys such as titanium-nickel (TiNi) have spurred interest due to their unique thermomechanical properties, characterized by shape-memory effect and remarkable pseudoelastic (also referred to as superelastic) behavior. As a consequence, TiNi alloys have been increasingly used in various biomedical and technology applications.1–3 Significant effort has been devoted to study the shape-memory effect and pseudoelastic response of TiNi alloys in their bulk form. Recently, the increasing demand for thin-film microdevices and functional materials where TiNi can be used as the structural material, such as microelectromechanical systems (MEMS) and bioimplantable components (e.g., stents), has provided impetus for basic research dealing with the deformation behavior and phase changes in TiNi films. However, an in-depth knowledge of the underlying deformation mechanisms and associated phase changes that are responsible for the unique nanoscale behavior of TiNi films has not been obtained yet.4 This presents a major barrier to the effective use of thin TiNi films in various leading-edge technologies, such as MEMS and biotechnology.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0226 1808
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 27 Nov 2014
Recent studies have demonstrated a remarkable pseudoelastic behavior of austenitic TiNi films subjected to quasi-static and dynamic nanoindentation.5,6 An anomalous increase of the indentation hardness of bulk TiNi alloy has been observed with the increase of the temperature.7 This behavior has been attributed to stress-induced phase transformation. Thin films demonstrating large reversible deformation and significant damping under dynamic contact loading without surface damage are of great importance in MEMS applications. However, understanding of the thermomechanical deformation of TiNi fi
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