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Influence of Ni on martensitic phase transformations in NiTi shape memory alloys / J. Frenzel in Acta materialia, Vol. 58 N° 9 (Mai 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 9 (Mai 2010) . - pp. 3444–3458 Titre : Influence of Ni on martensitic phase transformations in NiTi shape memory alloys Type de document : texte imprimé Auteurs : J. Frenzel, Auteur ; E.P. George, Auteur ; A. Dlouhy, Auteur Année de publication : 2011 Article en page(s) : pp. 3444–3458 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : NiTi  shape  memory  alloys  Phase  transformation  temperatures  Thermodynamics  Vacuum  arc  melting  Hysteresis  width Résumé : High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on MS, MF, AS, AF and T0, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 [19] regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalized on the basis of the crystallographic data of Prokoshkin et al. 2004 [68] and the theory of Ball and James [25]. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645410001059 [article] Influence of Ni on martensitic phase transformations in NiTi shape memory alloys [texte imprimé] / J. Frenzel, Auteur ; E.P. George, Auteur ; A. Dlouhy, Auteur . - 2011 . - pp. 3444–3458. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 9 (Mai 2010) . - pp. 3444–3458 Mots-clés : NiTi  shape  memory  alloys  Phase  transformation  temperatures  Thermodynamics  Vacuum  arc  melting  Hysteresis  width Résumé : High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on MS, MF, AS, AF and T0, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 [19] regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalized on the basis of the crystallographic data of Prokoshkin et al. 2004 [68] and the theory of Ball and James [25]. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645410001059

Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading / M.L. Young in Acta materialia, Vol. 58 N° 7 (Avril 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 7 (Avril 2010) . - pp. 2344–2354 Titre : Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading Type de document : texte imprimé Auteurs : M.L. Young, Auteur ; M.F.-X. Wagner, Auteur ; J. Frenzel, Auteur Année de publication : 2011 Article en page(s) : pp. 2344–2354 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : NiTi  shape-memory  alloys  Tensile  testing  Stress-induced  martensite  Synchrotron  radiation  Localized  deformation Résumé : An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9 at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1 GPa) and unloading. Both macroscopic stress–strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19′ phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645409008635 [article] Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading [texte imprimé] / M.L. Young, Auteur ; M.F.-X. Wagner, Auteur ; J. Frenzel, Auteur . - 2011 . - pp. 2344–2354. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 7 (Avril 2010) . - pp. 2344–2354 Mots-clés : NiTi  shape-memory  alloys  Tensile  testing  Stress-induced  martensite  Synchrotron  radiation  Localized  deformation Résumé : An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9 at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1 GPa) and unloading. Both macroscopic stress–strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19′ phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645409008635