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

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2011-09, Vol.42 (9), p.2556-2574
• Main Authors: Olbricht, J., Yawny, A., Pelegrina, J. L., Dlouhy, A., Eggeler, G.
• Format: Article
• Language: English
• Subjects: Alloys; Applied sciences; Characterization and Evaluation of Materials; Chemistry and Materials Science; Exact sciences and technology; Intermetallic compounds; Intermetallics; Materials Science; Metallic Materials; Metals. Metallurgy; Nanotechnology; Nickel base alloys; Nickel compounds; Nickel titanides; Phase transitions; Shape memory alloys; Stress measurement; Stress-strain relationships; Structural Materials; Surfaces and Interfaces; Thin Films; Titanium compounds; Transformations
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-011-0679-y

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. The R-phase always forms prior to B19′ when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19′ transformations. 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.