Thermal, microstructural and elastic modulus behavior of Ti50Ni50−xNbx (x = 0–25%at) shape memory alloys obtained by plasma arc melting

Abstract

Ternary alloys based on the NiTi system with the addition of Nb have attracted attention for several areas of engineering, due to improvements in the shape memory properties. In this work, six TiNiNb compositions were obtained by plasma arc melting under controlled atmosphere, with Nb contents of 5%, 10%, 15%, 20% and 25% Nb in the Ti50Ni50−xNbx system (% at). This work aims to evaluate the influence of Nb content on the microstructure, phase transformation and elastic modulus behavior of the ternary alloy compared to the equiatomic binary NiTi alloy. Differential scanning calorimetry (DSC) show that after a heat treatment of 900 °C for 1 h, all studied compositions present a single-step phase transformation during cooling and heating. Nb addition has a limited effect over phase transformation temperatures, with the onset of martensite transformation remaining practically constant. The microstructure of the alloys is constituted by B2 NiTi austenite matrix in the samples with 0% and 5% Nb, and by B19′ NiTi martensite in the samples with 10–25% Nb, all surrounded by an eutectic phase rich in Nb (β-Nb). Scanning electron microscopy (SEM) revealed microstructures with the presence of dendrites with a large amount of β-Nb that evolved in size with increasing Nb content. The Nb addition and consequent microstructure changes contributed to the decrease in the average hardness, going from 527 HV in NiTi to values between 259 and 396 HV in TiNiNb alloys. The Nb content, however, had a weaker effect over the highest modulus of elasticity, which was highest in the Ti50Ni30Nb20 alloy (66 GPa) and lowest in the Ti50Ni45Nb5 alloy (52 GPa), representing a less than 10% variation.