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

0376005 - ÚFM 2012 RIV US eng J - Journal Article
Olbricht, J. - Yawny, A. - Pelegrina, J.L. - Dlouhý, Antonín
On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys.
Metallurgical and Materials Transactions A. 42A, č. 9 (2011), s. 2556-2574. ISSN 1073-5623. E-ISSN 1543-1940
R&D Projects: GA ČR GA106/09/1913
Institutional research plan: CEZ:AV0Z20410507
Keywords : martensitic transformations * rietveld method * single-crystals
Subject RIV: JG - Metallurgy
Impact factor: 1.545, year: 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. 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.
Permanent Link: http://hdl.handle.net/11104/0208522