TY - JOUR A1 - Schaefer, A. A1 - Wagner, M.F.-X. A1 - Pelegrina, J.L. A1 - Olbricht, Jürgen A1 - Eggeler, G. T1 - Localization events and microstructural evolution in ultra-fine grained NiTi shape memory alloys during thermo-mechanical loading N2 - Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures). PY - 2010 U6 - https://doi.org/10.1002/adem.201000063 SN - 1438-1656 VL - 12 IS - 6 SP - 453 EP - 459 PB - Wiley-VCH Verl. CY - Weinheim AN - OPUS4-28616 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Olbricht, Jürgen A1 - Yawny, A. A1 - Pelegrina, J.L. A1 - Eggeler, G. A1 - Yardley, V. A. T1 - Characteristics of the stress-induced formation of R-phase in ultrafine-grained NiTi shape memory wire N2 - The transformation between the cubic B2 and monoclinic B19' phases in ultrafine-grained pseudoelastic NiTi can occur as a two-step process involving the intermediate rhombohedral R-phase. Experimental work using differential scanning calorimetry, electrical resistance measurements and transmission electron microscopy has demonstrated the formation of this intermediate phase during thermal cycling and during mechanical loading. In the present paper, complementary mechanical and thermographic results are presented which allow to further assess the character of the stress-induced R-phase formation. The transformation from B2 to R-phase is demonstrated to occur homogeneously within the gauge length rather than via advancing Lüders-type transition regions as it is the case in the localized transformation from B2 or R-phase to B19'. KW - NiTi shape memory alloys KW - Pseudoelasticity KW - Stress-induced phase transformations KW - Localization KW - R-phase PY - 2013 U6 - https://doi.org/10.1016/j.jallcom.2013.06.056 SN - 0925-8388 SN - 1873-4669 VL - 579 SP - 249 EP - 252 PB - Elsevier B.V. AN - OPUS4-28833 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Olbricht, Jürgen A1 - Yawny, A. A1 - Pelegrina, J.L. A1 - Dlouhy, A. A1 - Eggeler, G. T1 - On the stress-induced formation of R-phase in ultra-fine-grained Ni-Rich NiTi shape memory alloys N2 - 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. KW - Phase transformation KW - Shape memory KW - Pseudoelasticity KW - NiTi KW - R-phase PY - 2011 U6 - https://doi.org/10.1007/s11661-011-0679-y SN - 1073-5623 SN - 1543-1940 VL - 42A IS - 9 SP - 2556 EP - 2574 PB - The Minerals, Metals and Materials Society CY - Warrendale AN - OPUS4-24968 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Pelegrina, J.L. A1 - Yawny, A. A1 - Olbricht, Jürgen T1 - Comment on: "Isothermal nature of the B2-B19' martensitic transformation in a Ti-51.2 Ni (at.%) alloy" N2 - Fukuda et al. published a paper (Scripta Mater. 68 (2013) 984–987) in which they claimed to be able to determine the isothermal nature of the martensitic transformation in a Ti–Ni alloy. They analyzed the evolution of electrical resistance during isothermal holding stages and concluded that the variations were a consequence of the isothermal transition between austenite and martensite. In the following we will demonstrate that the discussed data do not allow the interpretation presented and that the isothermal nature is not proved. KW - Thermal activation KW - Titanium–nickel alloy KW - Shape memory alloy KW - Nucleation PY - 2015 U6 - https://doi.org/10.1016/j.scriptamat.2014.10.009 SN - 1359-6462 SN - 1872-8456 VL - 98 SP - 68 EP - 70 PB - Elsevier Ltd. CY - Oxford AN - OPUS4-32401 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Pelegrina, J.L. A1 - Yawny, A. A1 - Olbricht, Jürgen A1 - Eggeler, G. T1 - Transformation activity in ultrafine grained pseudoelastic NiTi wires during small amplitude loading/unloading experiments N2 - Martensitic phase transformations were studied in ultrafine grained Ni-rich pseudoelastic NiTi wires during cyclic deformation under small imposed strain amplitudes. Small strain variation tests were complemented by a thermographic analysis of the emerging temperature distributions in the specimens. The characteristics of the observed thermal profiles result from the specific phase transitions which take place at different stress levels. Homogeneous temperature changes, corresponding to a non-localized transformation activity along the specimen length, were observed throughout the whole range of applied stresses, starting from values as low as 100 MPa. This behavior is in line with previous literature reports for the stress-induced transformation from B2 austenite to R-phase as long as the critical stress for B19' martensite formation is not reached. In the present study, similar type of transformation activity could also be demonstrated at higher strains/stresses, even after the stress induced transformation to B19' was apparently completed. These findings suggest that transformation activity involving the B2 phase is present throughout the whole pseudoelastic stress–strain cycle; i.e., it is not restricted to the initial loading portion. Finally, non-localized transformation to or from B19' was identified during small amplitude strain variations in the plateau-like coexistence ranges of the pseudoelastic cycle. KW - NiTi shape-memory alloys KW - Pseudoelasticity KW - Stress-induced phase transformations KW - R-phase KW - Thermal analysis PY - 2015 U6 - https://doi.org/10.1016/j.jallcom.2015.08.159 SN - 0925-8388 SN - 1873-4669 VL - 651 SP - 655 EP - 665 PB - Elsevier Sequoia CY - Lausanne AN - OPUS4-34012 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -