On the Stress-Induced Formation of R-Phase in Ultra-Fine-Grained Ni-Rich NiTi Shape Memory Alloys |
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Authors: | J Olbricht A Yawny J L Pelegrina A Dlouhy G Eggeler |
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Affiliation: | 1.BAM Federal Institute for Materials Research and Testing,Berlin,Germany;2.Instituto Balseiro and CONICET, Centro Atómico Bariloche,San Carlos de Bariloche,Argentina;3.Institute of Physics of Materials, Academy of Sciences of the Czech Republic (AS CR),Brno,Czech Republic;4.Institute for Materials, Ruhr University Bochum,Bochum,Germany |
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Abstract: | 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. |
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