Shape memory properties of Ni-Ti based melt-spun ribbons
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2/7/04
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Shape Memory Properties of Ni-Ti Based Melt-Spun Ribbons R. SANTAMARTA, E. CESARI, J. PONS, and T. GORYCZKA Shape-memory properties of equiatomic NiTi, Ni45Ti50Cu5, and Ni25Ti50Cu25 ribbons made by melt spinning have been studied by temperature inducing the martensitic transformation under constant tensile loads. Recoverable strains above 4 pct can be obtained under 100 MPa loads for the NiTi and Ni45Ti50Cu5 ribbons, transforming to B19 martensite. The B19 martensite is formed in the Ni25Ti50Cu25 ribbon after crystallization, and according to the lowering in transformation strain as Cu content increases, the recoverable strain is close to 2.5 pct for 150 MPa load. The transformation temperatures exhibit a linear dependence on the applied stress, which can be quantitatively described by means of a Clausius–Clapeyron type equation. The NiTi and Ni45Ti50Cu5 ribbons exhibited some degree of two-way shape-memory effect (TWSME) after thermomechanical cycling. Texture analyses performed on the different ribbons allow us to better understand the transformation strains obtained in each ribbon. The amounts of shapememory effect (SME) and nonrecoverable strain shown by the studied ribbons are of the same order as those already observed in bulk materials, which makes melt spinning an ideal substitute to complicated manufacturing processes if really thin samples are needed. However, applicable stresses in melt-spun ribbons are limited by a relatively “premature” brittle fracture caused by irregularities in ribbon thickness.
I. INTRODUCTION
NI-TI shape-memory alloys (SMAs), and especially nearequitomic Ni-Ti, show very good mechanical and functional properties (large recovery strains and stresses, high corrosion resistance, good fatigue properties, and biocompatibility), being the most widely used SMA system for applications with transformation temperatures close to room temperature. However, some features of these alloys are not suitable for all the present or potential new applications. This has led to the study of additions of a third or more elements to the binary Ni-Ti or to the use of nonconventional production techniques, in order to modify the martensitic transformation temperatures, hysteresis, and the thermal stability of the alloys, to improve some mechanical properties, etc.[1–4] Substituting Cu for Ni in the binary Ni-Ti lowers the transformation hysteresis, which can be beneficial for some applications, and decreases the effect of cycling processes in the transformation[5,6] without modifying significantly the transformation temperatures. The addition of copper also reduces the sensitivity of the transformation temperatures to compositional changes and prevents Ti3Ni4 precipitation,[5] but it may produce a change in the transformation sequence. For Cu amounts not much higher than 5 at. pct, the B2 parent phase transforms to monoclinic B19 martensite (such as in the near equiatomic Ni-Ti). In the range from 5 to 15 at. pct, the transformation sequence is often split into B2 : orthorh
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