Analysis of Niobium precipitates effect on the thermo-mechanical behavior of a NiTiNb Shape Memory Alloy

Abstract

Commercial Ni47Ti44Nb9 Shape Memory Alloy (SMA) is generally adopted for connection applications thanks to its wide transformation hysteresis, compared with classical NiTi. Moreover, its sensibility to thermomechanical treatments permits it to be either martensitic or austenitic at room temperature. It has been shown that reverse transformation temperature (As) can increase by 80 ̈C under tensile loading in austenitic state. This phenomenon, about few degrees in classical NiTi [1], is a consequence of martensitic reorientation. This paper deals with the Niobium inclusions effects upon the amplification of this phenomenon for NiTiNb. Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) observations showed the presence of Niobium precipitates in a NiTiNb SMA. This phase has been studied to quantify what kind of inclusion increases reverse transformation temperatures, and what are its properties. It seems that the smallest β-Nb Niobium inclusions have the most important impact on the phenomenon, explained by Niobium low yield stress and important scattering and number of inclusions. In this paper, in the one hand, we describe experimental characterization tests. They are composed by tensile tests on two kinds of samples, wires and rings, by SEM observations, and Differential Scanning Calorimetry (DSC) analysis. We also used nanoindentation techniques to characterize inclusions. In the other hand, we develop a two phases thermo-mechanical model. It describes the global effective behavior of an elastic-plastic inclusion (Niobium precipitates) embedded with SMA Matrix. The constitutive law of the matrix is that developed by Peultier et al. [2] and improved by Duval et al. [3]. The elastic-plastic constitutive law for inclusion is a classical one proposed by Simo and Hughes [5], the Mori-Tanaka scale transition [6] is adopted in order to lead to the effective constitutive law.