Effect of the linear velocity during the melt spinning process on shape memory transformation of Ni-Ti ribbons

Abstract

The usual process to produce NiTi shape memory alloys is vacuum induction melting (VIM). Currently a new alternative process to produce NiTi shape memory alloys by rapid solidification structures called Melt Spinning has been studied. In this work, results of ribbons with a chemical composition Ti-55.2 Ni (wt %) alloy prepared by this method are presented. The ribbons are prepared at two different linear velocities: 30 m/s and 50 m/s. After that, samples are heat treated at 350 °C during 1 hour. The alloys are characterized by Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction. According to the cycled DSC test, transformation peaks are associated with the B2→R→B19´ transformation during cooling and B19´→R→B2 during heating, showing transformation in multi-peaks. The martensite B19´ start (M s ) is varying from 35 to 39°C and the martensite finish (M f ) from 15 to 21°C, 42-47°C for austenite B2 start (A s ) and 65-69°C for austenite finish (A f ) approximately. All analyzed ribbons show very similar values of transformation hysteresis temperatures at 50% of transformation of around 28°C. In order to change solidification rate, linear velocity is varied during the melt spinning process. Results indicate that linear velocity affects directly the temperature of transformation. When the linear velocity is increasing, crystallographic Ti-rich precipitates are developed, but dendritic growth disappears, changing the microstructure and decreasing these transformation temperatures. Then changes in linear velocity can dramatically affect shape memory properties, and in this case a velocity of 50 m/s produces a more homogenous alloy.

Effect of Spun Velocities and Composition on the R‒phase and Thermomechanical Behavior in Ti‒Ni Ribbons Electrically Heated

“…It has been reported that the appearance of R-phase on Ti-Ni is due to the increase in Ni content, thermal cycling and aging. Besides, the microstructural defects produced during the melt-spinning at high speed enhance the formation of R-phase 28 , which corroborates the results obtained in figure 2. The appearance of martensitic R-phase is characterized by the high increase in ER (Electrical Resistance) and then a decrease when B19´martensitic phase appears 29 .…”

A Smart Polymer Composite Based on a NiTi Ribbon and a Magnetic Hybrid Material for Actuators with Multiphysic Transduction

“…During cooling, a small imperfection is observed on the peak (see the red circle on Figure 5). We assume that this imperfection is caused by the presence of the R-phase [15]. Thus, the transformation seems to occur in three steps, from austenite to R-phase and finally to martensite.…”

“…Figure 1a shows a schema of the melt-spinning process and Figure 1b shows a sample of the NiTi ribbon. More information about the preparation of the SMA ribbons is described by Nuñez-Mendoza et al [15].…”