Ligas com memória de forma Ti50Ni50-XNbx (x= 0 a 25%at): obtenção por fusão a arco e reprocessamento por centrifugação em molde cerâmico.

Abstract

NiTi shape memory alloys (SMA) with the addition of Nb have attracted attention for several areas of engineering, due to improvements in a series of mechanical and functional properties. Thus, this work has as general objective to evaluate the influence of Nb content on the microstructure, phase transformation and mechanical behavior of the ternary TiNiNb alloys compared to the equiatomic binary Ni50Ti50 (%at) alloy. Six TiNiNb compositions were obtained by plasma arc melting under argon atmosphere, with Nb contents of 5, 10, 15, 20 and 25%at Nb in the Ti50Ni50-xNbx system. Compositions with 0, 10 and 15%at of Nb were selected according to the amount of Nb to verify the possibility of manufacture of screens by a centrifugal investment casting process. For alloys obtained by arc melting the results show, by means of differential scanning calorimetry (DSC) tests, that after a heat treatment of homogenization at 900°C for 1h, all studied compositions present a single-step phase transformation during cooling and heating, with the transformation enthalpies decreasing almost linearly with the increase in the Nb content. On the other hand, Nb addition has a limited effect over phase transformation temperatures, with the martensite start temperature (Ms) remaining practically constant (~38°C). The microstructure of the alloys is constituted by NiTi matrix (Bβ and B19’) and some secondary phases (Ni4Ti3 and Ni3Ti) for the binary alloy and Nb-based phases (ȕNb, NiTi2, Nb2Ni and NbTi) for the ternary alloys. Scanning electron microscopy (SEM) revealed microstructures with the presence of dendrites with a large amount of ȕNb that evolved in size with increasing Nb content. The Nb addition and consequent microstructure changes contributed to the decrease in the average hardness, from 527 HV in NiTi to values between 259 and 396 HV in TiNiNb alloys. The Nb content, however, had a more limited effect over the behavior modulus of elasticity, which was highest in the Ti50Ni30Nb20 alloy (66 GPa) and lowest in the Ti50Ni45Nb5 alloy (52 GPa), with an overall variation of less than 20%. In general, the good castability by investment casting of the TiNiNb SMA was confirmed in the manufacture of thin screens. These screens showed higher levels of hardness and modulus of elasticity compared to the base alloys obtained by arc melting.