The effect of rapid solidification and grain size on the transformation temperatures of Ni-44,8wt%Ti melt spun alloy
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The effect of rapid solidification and grain size on the transformation temperatures of Ni-44,8wt%Ti melt spun alloy G. C. S. Anselmo, W. B. de Castro*; C. J. de Araújo UAEM, Universidade Federal de Campina Grande, Aprígio Veloso, 882 – Caixa *Posta 10069 58429-900 – Campina Grande - PB – Brazil [email protected], [email protected]*, [email protected] ABSTRACT Ribbons of the Ni-44.8wt%Ti shape memory alloy are prepared through the melt spinning technique. The study is focused on investigating the effect of the rapid solidification and grain size at characteristic start martensitic (Ms), final martensitic (Mf), start austenite (As) and final austenite (Af) transformation temperatures. Changes on martensitic transformation temperatures in Ti45Ni55 melt spun ribbons are observed as grain size is reduced. Results of optical microscopy and differential scanning calorimetry (DSC) are used to associate grain size with transformation temperatures. Keywords: Shape memory Ni-Ti alloy; Rapid solidification; Martensitic transformation. INTRODUCTION Shape memory alloys (SMAs) represent a unique class of materials that undergo a reversible phase transformation (martensitic transformation) allowing these materials to display dramatic pseudoelastic stress-induced deformations and recoverable temperature-induced shape memory deformations. These materials are used as smart materials in a variety of aerospace, biomechanical, and microelectronics applications. Among the known shape memory alloys, NiTi is the most commonly used because of its excellent mechanical properties, corrosion resistance and biocompatibility [1]. In the recent past, the field of microsystems has been subject to growing attention from both the industry and the research community. Microsystems have been recognized as having the potential to revolutionize the performance of a wide range of products by merging silicon-based microelectronics with micromachining technologies, thus enabling complete systems-on-a-chip to be developed and allowing novel functionalities at reduced costs. In this context, the application of shape memory alloys for actuation of micropneumatic devices might bring a relevant technological breakthrough. SMA materials exhibit the highest energy density amongst current micro-electromechanical systems MEMS compatible materials and, more importantly, as size is reduced towards the micro-scale, they benefit from improved heat transport, which increases their response speed [2]. Many studies have been undertaken to find a method to control the martensitic transformation. According to previous studies, it is significantly affected by the alloy composition, crystallographic defects such as dislocations, precipitation and grain size [3]. Grain boundaries are believed to strengthen parent phases, and therefore Ms decreases with decreasing grain size. In Ti–Ni-based alloys, the critical austenite grain size for martensitic transformation is known to be 50 nm, below which martensitic transformation does not occur. The aim of this work i
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