Issue 18

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 18 (2011) 45-53 ; DOI: 10.3221/IGF-ESIS.18.05 45 Structural transitions in a NiTi alloy: a multistage loading-unload cycle V. Di Cocco University of Cassino, DiMSAT, via G. Di Biasio 43, 03043, Cassino (FR), Italy. v.dicocco@unicas.it C. Maletta University of Calabria, Dip. of Mechanical Engineering, 87036 Rende (CS), Italy. carmine.maletta@unical.it S. Natali University of Rome “Sapienza”, D.I.C.M.A., via Eudossiana 18, 00185 Roma, Italy. stefano.natali@uniroma1.it A BSTRACT . NiTi shape memory alloys (SMAs) are increasingly used in many engineering and medical applications, because they combine special functional properties, such as shape memory effect and pseudo- elasticity, with good mechanical strength and biocompatibility. However, the microstructural changes associated with these functional properties are not yet completely known. In this work a NiTi pseudo-elastic alloy was investigated by means of X-ray diffraction in order to assess micro-structural transformations under mechanical uniaxial deformation. The structure after complete shape recovery have been compared with initial state. S OMMARIO . Le leghe a memoria di forma costituite da nickel e da titanio sono sempre più utilizzate in molti settori dell’ingegneria e della medicina, in quanto combinano speciali proprietà funzionali, quali la memoria di forma e la pseudo-elasticità, con buone caratteristiche meccaniche e di biocompatibilità. Tuttavia, le trasformazioni mico-strutturali che conferiscono tali proprietà funzionali non sono ancora completamente note. In questo lavoro è stata analizzata una lega NiTi pseudo-elastica, mediante indagini diffrattometriche ai raggi X, al fine di mettere in evidenza le modificazioni strutturali che intervengono sotto l’effetto di deformazioni imposte. La microstruttura ottenuta dal completo recupero della forma è stata confrontata con quella iniziale. K EYWORDS . Shape Memory alloys; Stress-induced martensitic transformation; X-Ray analyses. I NTRODUCTION hape memory alloys (SMAs) are an important class of metallic alloy which exhibit unique features with respect to common engineering alloys, such as the shape memory effect (SME) and pseudo-elastic effect (PE). In particular, due to these properties SMAs are able to recover their original shape after high values of mechanical deformations, by heating up to a characteristic temperature (SME) or simply by removing the mechanical load (PE). Different kinds of shape memory alloys have been exploited in the last decades, such as the copper-zinc-aluminum (ZnCuAl), copper- aluminum-nickel (CuAlNi), nickel-manganese-gallium (NiMnGa), nickel-titanium (NiTi), and other SMAs created by S