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Double twinning in Ni–Mn–Ga–Co / K. Rolfs in Acta materialia, Vol. 58 N° 7 (Avril 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 7 (Avril 2010) . - pp. 2646–2651 Titre : Double twinning in Ni–Mn–Ga–Co Type de document : texte imprimé Auteurs : K. Rolfs, Auteur ; M. Chmielus, Auteur ; R.C. Wimpory, Auteur Année de publication : 2011 Article en page(s) : pp. 2646–2651 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : Heusler  phases  Neutron  diffraction  Twinning  Plastic  deformation Résumé : Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5 at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110 °C respectively up to 160 °C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645409009094 [article] Double twinning in Ni–Mn–Ga–Co [texte imprimé] / K. Rolfs, Auteur ; M. Chmielus, Auteur ; R.C. Wimpory, Auteur . - 2011 . - pp. 2646–2651. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 7 (Avril 2010) . - pp. 2646–2651 Mots-clés : Heusler  phases  Neutron  diffraction  Twinning  Plastic  deformation Résumé : Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5 at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110 °C respectively up to 160 °C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645409009094

Effects of surface roughness and training on the twinning stress of Ni–Mn–Ga single crystals / M. Chmielus in Acta materialia, Vol. 58 N° 11 (Juin 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 11 (Juin 2010) . - pp. 3952–3962 Titre : Effects of surface roughness and training on the twinning stress of Ni–Mn–Ga single crystals Type de document : texte imprimé Auteurs : M. Chmielus, Auteur ; K. Rolfs, Auteur ; R. Wimpory, Auteur Année de publication : 2011 Article en page(s) : pp. 3952–3962 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : Magnetic  shape  memory  alloys  Mechanical  properties  Mechanical  training  Neutron  scattering  Surface  treatments Résumé : Spark eroding, which is commonly employed to cut samples out of magnetic shape-memory alloy single crystals, produces a rough surface layer. Directly after cutting, the single crystals exhibit a high twinning stress. After removal of a surface layer by electropolishing, the twinning stress reduces significantly and stress–strain curves become serrated. The reduction of twinning stress has previously been attributed to the removal of the defective surface layer. In this work, it is shown that different surface treatments in combination with repeated mechanical deformation experiments significantly reduce the twinning stress, regardless of whether or not electropolishing is used. The reduction of the twinning stress is due to softening that takes place as a mechanical training effect, which occurs with mechanical testing. In addition, the stress–strain curves of samples subjected to different surface treatments differed in so far as the curves of electropolished samples showed serrated flow, while the curves of unpolished samples and those of mechanically polished samples were smooth. Furthermore, the unpolished samples displayed significant hardening at higher strain. Following subsequent mechanical polishing, this hardening reduced to nearly zero, and the average twinning stress decreased another 30–50% to 1.6 MPa and below. For these samples, the twinning stress stayed at a very low level until twinning was complete. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S135964541000176X [article] Effects of surface roughness and training on the twinning stress of Ni–Mn–Ga single crystals [texte imprimé] / M. Chmielus, Auteur ; K. Rolfs, Auteur ; R. Wimpory, Auteur . - 2011 . - pp. 3952–3962. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 11 (Juin 2010) . - pp. 3952–3962 Mots-clés : Magnetic  shape  memory  alloys  Mechanical  properties  Mechanical  training  Neutron  scattering  Surface  treatments Résumé : Spark eroding, which is commonly employed to cut samples out of magnetic shape-memory alloy single crystals, produces a rough surface layer. Directly after cutting, the single crystals exhibit a high twinning stress. After removal of a surface layer by electropolishing, the twinning stress reduces significantly and stress–strain curves become serrated. The reduction of twinning stress has previously been attributed to the removal of the defective surface layer. In this work, it is shown that different surface treatments in combination with repeated mechanical deformation experiments significantly reduce the twinning stress, regardless of whether or not electropolishing is used. The reduction of the twinning stress is due to softening that takes place as a mechanical training effect, which occurs with mechanical testing. In addition, the stress–strain curves of samples subjected to different surface treatments differed in so far as the curves of electropolished samples showed serrated flow, while the curves of unpolished samples and those of mechanically polished samples were smooth. Furthermore, the unpolished samples displayed significant hardening at higher strain. Following subsequent mechanical polishing, this hardening reduced to nearly zero, and the average twinning stress decreased another 30–50% to 1.6 MPa and below. For these samples, the twinning stress stayed at a very low level until twinning was complete. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S135964541000176X