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MECHANIZM TWORZENIA SIĘ PASM UGIĘCIA W MONOKRYSZTAŁACH Ni2MnGa


  W niniejszym artykule przedstawiono mechanizm tworzenia się pasm ugięcia w ściskanych monokryształach Ni2MnGa o orientacji środka trójkąta stereograficznego. Przeprowadzone testy mechaniczne wykazały, że obszar sprężysty charakterystyki naprężenie‐odkształcenie tego materiału jest zakłócony gwałtownym pojawieniem się niestabilności mechanicznej wywołaną lokalizacją odkształcenia w formie pasma ugięcia. Niestabilność ta prowadzi do zniszczenia próbki, której mogą towarzyszyć pasma ugięcia drugiego rzędu. W oparciu o wyniki pomiarów pojedynczych orientacji techniką dyfrakcji elektronów wstecznie rozproszonych (EBSD) oraz wykorzystując metodę macierzy korespondencji zaproponowano krystalograficzno‐ mechaniczny model tworzenia się pasm ugięcia pierwszego i drugiego rodzaju. Słowa kluczowe: pasma ugięcia, monokryształy, odkształcenie plastyczne THE MECHANISM OF KINK BANDS FORMATION IN Ni2MnGa SINGLE CRYSTALS In the present paper a mechanism of the kink bands formation in compressed single glide oriented Ni2MnGa single crystals is shown. The compression tests showed that the elastic part of the material stress‐strain curve is interrupted by the entry of a heavy mechanical instability resulting from strain localization in a form of kink band. This instability leads to the sample failure which may be accompanied by the second order kink bands. On the basis of the measurements of local crystal orientation performed by means of the SEM‐EBSD technique and by using the correspondence matrix method a mechanical crystallography model of the first and second order kink bands formation is proposed. Keywords: kink bands, single crystals, plastic deformation Wprowadzenie Tworzenie się pasm ugięcia jest często spotykanym mechanizmem odkształcenia plastycznego wielu materiałów krystalicznych, szczególnie podczas prób ściskania minerałów oraz kryształów metali i ich stopów. Pionierskie prace doświadczalne dotyczące mechanizmów twor[...]

A note on the kink bands in compressed Ni2MnGa single crystals

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In the last decade Ni-Mn-Ga single crystals had been a subject of intense structural and magnetic studies, because of their enormous magnetic shape memory effect (MSM), which is observed when the alloy possesses a tetragonal or orthorhombic crystal structure [1÷3]. Large interest has been paid to the Curie temperature and to the critical temperature of martensitic transformation, where upon cooling the high temperature cubic phase is replaced by the tetragonal (orthorhombic) structure [4, 5]. Recently, there has been focused a greater interest on the mechanical properties of the Ni2MnGa type alloys of cubic phase, because of their capability to adopt large plastic strains at high temperatures [6, 7] and hence the effect of the plastic deformation on the degree of deprivation of the MSM is also studied [8]. In this context, studies of the brittle-ductile transition which is observed in the cubic Ni2MnGa single crystals at elevated temperatures are of particular significance. However, as it was very recently reported in [9], Ni-Mn-Ga single crystals of both of the cubic and tetragonal structure, subjected to the room temperature compression are still capable to accommodate locally small plastic strains, before they fracture at the strength level of about 1 GPa. It seems important to make a proposal that the brittle-ductile transition in these alloys may be strongly correlated with temperature stability of the mechanism responsible for the accommodation of these small plastic strains. In this work, deformation geometry of the discussed accommodation mechanism will be shown in detail. Basing on the experimental observations of the Ni2MnGa single crystals compressed at 475 K it will be shown that the accommodation mechanism consists of a successive nucleation and further propagation of the kink bands and it is strongly correlated with the appearance of the mechanical instabilities on the stress-strain curve. The analysis presented i[...]

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