Shape memory properties of Ni-Ti based melt-spun ribbons

Abstract

Shape-memory properties of equiatomic NiTi, Ni 45 Ti 50 Cu 5 , and Ni 25 Ti 50 Cu 25 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 ϳ100 MPa loads for the NiTi and Ni 45 Ti 50 Cu 5 ribbons, transforming to B19Ј martensite. The B19 martensite is formed in the Ni 25 Ti 50 Cu 25 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 ϳ150 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 Ni 45 Ti 50 Cu 5 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.

Annealing effects on the crystallization and shape memory effect of Ti50Ni25Cu25 melt-spun ribbons

“…The high h100i texture of the crystallized grains perpendicular to the ribbon surface is due to the heat flow associated with ribbon processing which is normal to the spin wheel during the rapid solidification process. This result is similar to that of the orientation distribution function (ODF) obtained in the Ti 50 Ni 25 Cu 25 melt-spun ribbon [28,29]. Through prolonging the annealing time, however, the intensity of the (110) B2 peak which is widely obtained in most TieNieCu bulk alloys [5] drastically increases and becomes the dominant peak.…”

“…6, the only XRD peak obtained in the as-crystallized ribbon annealed at 500 C for 90 s is (200) B2 and the lattice parameter of B2 parent phase is 0.305 nm. This characteristic indicates that the as-crystallized grains have a fiber texture h100i along the normal direction of the ribbon [28]. The high h100i texture of the crystallized grains perpendicular to the ribbon surface is due to the heat flow associated with ribbon processing which is normal to the spin wheel during the rapid solidification process.…”

Microstructure of a Partially Crystallised Ti<SUB>50</SUB>Ni<SUB>25</SUB>Cu<SUB>25</SUB> Melt-Spun Ribbon

“…17,18) The aim of the present work is to investigate the morphology as well as crystallography of the microstructure of the crystals formed during the first steps of the crystallisation by Transmission Electron Microscopy (TEM).…”

Magnetoelastic Anomalies Exhibited by Ni&ndash;Fe(Co)&ndash;Ga Polycrystalline Ferromagnetic Shape Memory Alloy

“…The incomplete recovery in this case is attributed to a residual strain related to some amount of martensite remaining after the thermal cycling. 26,27) As already mentioned, for the first configuration it is notably that at MT start there appears an initial decrease of the strain that recovers when transformation is in progress. On the other hand, the deformation parallel to the microstructure presents a maximum.…”