TY - JOUR

T1 - A multiscale study of hot-extruded CoNiGa ferromagnetic shape-memory alloys

AU - Samothrakitis, Stavros

AU - Larsen, Camilla Buhl

AU - Woracek, Robin

AU - Heller, Ludek

AU - Kopecek, Jaromir

AU - Gerstein, Gregory

AU - Maier, Hans Juergen

AU - Rames, Michal

AU - Tovar, Michael

AU - Sittner, Petr

AU - Schmidt, Soren

AU - Strobl, Markus

PY - 2020/11

Y1 - 2020/11

N2 - Ferromagnetic shape-memory CoNiGa alloys have attracted much scientific interest due to their potential alternative use as high-temperature shape-memory alloys, bearing a high prospect for actuation and damping applications at elevated temperatures. Yet, polycrystalline CoNiGa, due to strong orientation dependence of transformation strains, suffers from intergranular fracture. Here, two multi-grain CoNiGa samples were prepared by a novel hot extrusion process that can promote favourable grain-boundary orientation distribution and improve the material's mechanical behaviour. The samples were investigated by multiple methods and their micro structural, magnetic, and mechanical properties are reported. It is found that a post-extrusion solutionising heat treatment leads to the formation of a two-phase oligocrystalline homogeneous microstructure consisting of an austenitic parent B2 phase and gamma-CoNiGa precipitates. Reconstruction of the full 3D grain morphology revealed large, nearly spherical grains with no low-angle grain boundaries throughout the entire sample volume. The presence of gamma precipitation affects the transformation behaviour of the samples, by lowering the martensitic transformation temperature, while, in conjunction with the oligocrystalline microstructure, it improves the ductility. Controlling the composition of the B2 matrix, as well as the phase fraction of the gamma phase, is thus crucial for the optimal behaviour of the alloys. (c) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

AB - Ferromagnetic shape-memory CoNiGa alloys have attracted much scientific interest due to their potential alternative use as high-temperature shape-memory alloys, bearing a high prospect for actuation and damping applications at elevated temperatures. Yet, polycrystalline CoNiGa, due to strong orientation dependence of transformation strains, suffers from intergranular fracture. Here, two multi-grain CoNiGa samples were prepared by a novel hot extrusion process that can promote favourable grain-boundary orientation distribution and improve the material's mechanical behaviour. The samples were investigated by multiple methods and their micro structural, magnetic, and mechanical properties are reported. It is found that a post-extrusion solutionising heat treatment leads to the formation of a two-phase oligocrystalline homogeneous microstructure consisting of an austenitic parent B2 phase and gamma-CoNiGa precipitates. Reconstruction of the full 3D grain morphology revealed large, nearly spherical grains with no low-angle grain boundaries throughout the entire sample volume. The presence of gamma precipitation affects the transformation behaviour of the samples, by lowering the martensitic transformation temperature, while, in conjunction with the oligocrystalline microstructure, it improves the ductility. Controlling the composition of the B2 matrix, as well as the phase fraction of the gamma phase, is thus crucial for the optimal behaviour of the alloys. (c) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

KW - Martensitic transformation

KW - Hot-extrusion

KW - Laue three-dimensional neutron diffraction to-mography

KW - Co-Ni-Ga

KW - Ferromagnetic shape-memory alloy

KW - CO-NI-GA

KW - MARTENSITIC-TRANSFORMATION

KW - COMPRESSION ASYMMETRY

KW - PHASE-EQUILIBRIA

KW - SINGLE-CRYSTALS

KW - MAGNETIC-FIELD

KW - MICROSTRUCTURE

KW - PARTICLES

KW - BEHAVIOR

KW - TENSION

U2 - 10.1016/j.matdes.2020.109118

DO - 10.1016/j.matdes.2020.109118

M3 - Journal article

VL - 196

JO - Materials and Design

JF - Materials and Design

SN - 0264-1275

M1 - 109118

ER -