Highly mobile twinned interface in 10 M modulated Ni–Mn–Ga martensite / L. Straka in Acta materialia, Vol. 59 N° 20 (Décembre 2011)  

Public ISBD [article]  inActa materialia > Vol. 59 N° 20 (Décembre 2011) . - pp. 7450–7463 Titre : Highly mobile twinned interface in 10 M modulated Ni–Mn–Ga martensite : Analysis beyond the tetragonal approximation of lattice Type de document : texte imprimé Auteurs : L. Straka, Auteur ; O. Heczko, Auteur ; H. Seiner, Auteur Année de publication : 2012 Article en page(s) : pp. 7450–7463 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : Microstructure Twinning X-ray diffraction Heusler phases Magnetic shape memory Résumé : The huge strains that Ni–Mn–Ga magnetic shape memory alloys can achieve are usually described in a tetragonal unit cell approximation of a five-layered modulated (10 M) crystal structure. Here we analyze the impact of a slight orthorhombic and monoclinic distortion of the 10 M structure in Ni50.2Mn28.3Ga21.5at.% single crystal. Combining dedicated experiments to probe the microstructure, structure and mechanical properties with calculation using elastic continuum theory, we prove the existence of fine a/b-laminates within modulation macrotwins of the order of 100 micrometers in size. This complex twin microstructure containing a Type II macrotwin interface is associated with an extraordinarily low twinning stress of between 0.05 and 0.3 MPa, while Type I twins exhibit twinning stress of about 1 MPa. The findings provide important guidelines for designing the martensitic microstructure for more efficient actuators. ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645411006574 [article] Highly mobile twinned interface in 10 M modulated Ni–Mn–Ga martensite : Analysis beyond the tetragonal approximation of lattice [texte imprimé] / L. Straka, Auteur ; O. Heczko, Auteur ; H. Seiner, Auteur . - 2012 . - pp. 7450–7463. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 59 N° 20 (Décembre 2011) . - pp. 7450–7463 Mots-clés : Microstructure Twinning X-ray diffraction Heusler phases Magnetic shape memory Résumé : The huge strains that Ni–Mn–Ga magnetic shape memory alloys can achieve are usually described in a tetragonal unit cell approximation of a five-layered modulated (10 M) crystal structure. Here we analyze the impact of a slight orthorhombic and monoclinic distortion of the 10 M structure in Ni50.2Mn28.3Ga21.5at.% single crystal. Combining dedicated experiments to probe the microstructure, structure and mechanical properties with calculation using elastic continuum theory, we prove the existence of fine a/b-laminates within modulation macrotwins of the order of 100 micrometers in size. This complex twin microstructure containing a Type II macrotwin interface is associated with an extraordinarily low twinning stress of between 0.05 and 0.3 MPa, while Type I twins exhibit twinning stress of about 1 MPa. The findings provide important guidelines for designing the martensitic microstructure for more efficient actuators. ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645411006574

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Inapplicability of the Maxwell relation for the quantification of caloric effects in anisotropic ferroic materials / R. Niemann in International journal of refrigeration, Vol. 37 N° 1 (Janvier 2014)  

Public ISBD [article]  inInternational journal of refrigeration > Vol. 37 N° 1 (Janvier 2014) . - pp. 281–288 Titre : Inapplicability of the Maxwell relation for the quantification of caloric effects in anisotropic ferroic materials Titre original : Inapplicabilité de la relation de Maxwell pour la quantification des effets calorifiques dans des matériaux ferroïques anisotropiques Type de document : texte imprimé Auteurs : R. Niemann, Auteur ; O. Heczko, Auteur ; L. Schultz, Auteur Année de publication : 2014 Article en page(s) : pp. 281–288 Note générale : Refrigeration Langues : Anglais (eng) Mots-clés : Magnetocaloric  shape memory alloys  multiferroic  Ni-Mn-Ga  Maxwell relation Résumé : Giant caloric effects were reported in elasto-, electro- and magnetocaloric materials near phase transformations. Commonly, their entropy change is indirectly evaluated by a Maxwell relation. Additionally to the well-known spurious “colossal” results near first-order phase transitions and limitations in the presence of magnetic domains, we report the fundamental failure of this approach for materials that show structural reorientations in external fields. We analyze exemplarily the magnetic shape memory alloy Ni-Mn-Ga. Application of a magnetic field causes magnetically induced reorientation of the crystal structure in the martensite phase. This results in a spurious inverse magnetocaloric effect that disappears when the measurement procedure is repeated. This failure is universal as the vector character of the applied field is not considered in the common scalar evaluation of a Maxwell relation. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713002314 [article] Inapplicability of the Maxwell relation for the quantification of caloric effects in anisotropic ferroic materials = Inapplicabilité de la relation de Maxwell pour la quantification des effets calorifiques dans des matériaux ferroïques anisotropiques [texte imprimé] / R. Niemann, Auteur ; O. Heczko, Auteur ; L. Schultz, Auteur . - 2014 . - pp. 281–288. Refrigeration Langues : Anglais (eng) in International journal of refrigeration > Vol. 37 N° 1 (Janvier 2014) . - pp. 281–288 Mots-clés : Magnetocaloric  shape memory alloys  multiferroic  Ni-Mn-Ga  Maxwell relation Résumé : Giant caloric effects were reported in elasto-, electro- and magnetocaloric materials near phase transformations. Commonly, their entropy change is indirectly evaluated by a Maxwell relation. Additionally to the well-known spurious “colossal” results near first-order phase transitions and limitations in the presence of magnetic domains, we report the fundamental failure of this approach for materials that show structural reorientations in external fields. We analyze exemplarily the magnetic shape memory alloy Ni-Mn-Ga. Application of a magnetic field causes magnetically induced reorientation of the crystal structure in the martensite phase. This results in a spurious inverse magnetocaloric effect that disappears when the measurement procedure is repeated. This failure is universal as the vector character of the applied field is not considered in the common scalar evaluation of a Maxwell relation. En ligne : http://www.sciencedirect.com/science/article/pii/S0140700713002314

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