On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys

Abstract

DOI: 10.1007/s11661-011-0679-y First Published On-line 09 April 2011 Phase transformations in binary ultra fine grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress strain response of the R-phase can be isolated from the overall stress strain data. R-phase always forms prior to B19´ when good pseudoelastic properties are observed. It is proven that the stress induced B2 to R-phase transition does proceed in an homogeneous manner, contrary to the localized character of the B2/R to B19´ transformations which proceed by the propagation of Lüders bands. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19´) and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes.