Microstructure and transformation behavior of Ni24.7Ti50.3Pd25 high temperature shape-memory alloy with Sc micro-addition

NiTiPd shape-memory alloys (SMAs) are potential functional materials for use as solid-state actuators in the temperature range 100-250 °C. The present study investigates the effect of 1.0 at.% Sc micro-addition to Ni24.7Ti50.3Pd25 alloy, Sc replacing either Ti or Ni. Results show that all the three alloys studied have stable transformation behavior on stress-free thermal cycling and hence, are suitable for cyclic actuation applications. However, the addition of Sc to NiTiPd alloy leads to decrease of transformation temperatures, the magnitude of decrease being greater for the alloy with Sc replacing Ni. The martensite finish (Mf) temperature of 181 °C for the NiTiPd alloy decreased to 139 °C for Sc replacing Ti and 83 °C for Sc replacing Ni. Also, the indentation modulus of NiTiPdSc (Sc replacing Ni) alloy is found to be significantly low compared to the other alloys. Analysis indicates that the observed differences in the alloy properties are related to the solubility of Sc in the NiTiPd matrix. While the quaternary NiTiPdSc alloy, Sc replacing Ti, has a single phase microstructure, the alloy with Sc replacing Ni shows the presence of Sc-rich and TiPd-type second phases in the microstructure. TEM examination revealed that the TiPd-type phase has a distinct rod-like morphology (30-50 nm) arranged in a grid-like structure. The transformation and indentation behavior of the alloys is elucidated using thermodynamic calculations of frictional energy and an electronic structure based analysis.