Investigation on the origin of magnetization in plastically deformed Ni51Ti49 alloy

The article deals with the investigation of structure and magnetic properties of plastically deformed Ni51Ti49 alloy. The magnetic hysteresis loops confirm the presence of ferromagnetic properties in the alloy. The transmission electron microscopy (TEM) detects the appearance of lenticular crystals with bending contours which indicate the large distortion of the crystal lattice. The crystal lattice curvature occurs due to the large atom displacement. As a result, icosahedral clusters with the Frank-Kasper (FK) structure can be formed. The spin-polarized density of electron states and the magnetic moments for both non-deformed (near-spherical structure) and deformed (elongated by 5% along the Z-axis) Ni7Ti5 (FK-12), Ni8Ti5 (FK-13), and Ni10Ti6 (FK-16) clusters are calculated for the explanation of possibility of magnetization appearance in Ni51Ti49 alloy. The calculations show the increase in the magnetic moments for the deformed clusters. The calculated spectra demonstrate the high density of electron states near the Fermi level which is a characteristic feature of ferromagnetic alloys

Магнитные свойства в пластически деформированном никель-титановом сплаве

Ni–Ti alloy has been intensively studied over the past decades. The unique properties of the alloy have allowed using it as a structural material for the creation of instruments and devices in various fields of science and technology, including mechanical engineering, aerospace, instrumentation. Measuring magnetic hysteresis loop is shown that after the deformation of the alloy having ferromagnetic properties. According to the equilibrium phase diagram, the alloys of Ni–Ti at a Ti content above 10 at. % is non-ferromagnetic. Due to lowering of the crystal phase symmetry with a cubic lattice the magnetization appears. In this work we have investigated the magnetic properties and the structure of deformed Ni51Ti49 samples by electron microscopy and X-ray diffraction methods. In Ni51Ti49 samples after plastic deformation the lenticular crystals containing bending contours with a high concentration of internal stresses were found. Bending contours indicate a large distortion of the crystal lattice. The curvature of the crystal lattice occurs due to the large displacements of the atoms. As a result, it can be formed and icosahedral cluster with the structure of the Frank–Kasper. An icosahedron is a twelve vertex polyhedron, which is denoted by FK-12. Furthermore, the crystal can be formed in other Frank–Kasper structures, e. g., FK-16. FK-16 is a sixteen vertex polyhedron with atom located in the center of the cluster. Indexing paintings electron diffraction and X-ray showed that the alloy phase of the Ni–Ti coexist with the structure Ti2Ni and Ni4Ti3. For explaining the possibility of the appearance of magnetization in Ni–Ti alloy samples spin-polarized electron density of states and magnetic moments Ni10Ti6 clusters (FK-16), Ni7Ti5 (FK-12) alloy Ni51Ti49 for electrons with different spin projections: “up” and “down” was calculated. The calculation by the scattered waves (RF) was performed. The results of calculation can be seen that the total electron density of nickel tends to zero faster than the density of titanium. Also shows that nickel becomes negative spin density in the area of r = 3.25–6.7 a. u. and titanium for r > 4.5 a. u. This may result depending on the value of the interatomic distances and to the effects ferromagnetism and antiferromagnetic in order to establish a magnetic clusters. The spectra show a high density of states near the Fermi level that is a characteristic feature of metals, besides there is an increase in the magnetization of the alloy during deformation. The calculations showed that the investigated clusters, not susceptible to deformation, also have a magnetic moment (the average magnetic moment per atom cluster FK-12, is about 1,0 μB, and for the FK-16 is about 0.3 μB. Overall, however, the average magnetic moment is zero, due to the absence of a preferred direction (the chaotic distribution of clusters) for the alloy. However, if the cluster is subjected to tension, the compensation of the magnetic moments of clusters occurs in the alloy, since there is allocated for all atoms direction due to deformation. At the same time, the average magnetic moments of the atoms in the cluster for the Deformed increase to 1.6 μB and 0.8 μB respectively for the FK-12 and FK-16

Magnetic Properties of Nickel-Titanium Alloy during Martensitic Transformations under Plastic and Elastic Deformation

This paper focuses on the processes of the occurrence of magnetization during structure formation in samples of Ni51Ti49 alloy under deformation conditions. The possibility of the existence of a phase with an FCC (face-centered cubic) lattice in titanium nickelide has been demonstrated by electron microscopy and electron diffraction. It has been discovered that the interplanar distances of BCC110 (body-centered cubic), FCC111, and HCP002 (hexagonal close packed) in the alloy under study have similar values, which indicates the possibility of their mutual polymorphic transformation. Based on the modular self-organization, a scheme of martensitic transformations in titanium nickelide from the B2 structure (BCC lattice) to the B19’ structure (HCP lattice) through an intermediate phase with an FCC lattice is proposed. It is shown that lenticular crystals appear in the Ni51Ti49 alloy under tensile deformation until rupture, which is accompanied by the onset of ferromagnetism. The effect of magnetization in Ni51Ti49 samples when immersed in liquid nitrogen has been also discovered. In this case, the reason for the appearance and disappearance of magnetization can be associated with microdeformation processes caused by direct and reverse martensitic transitions that occur during cooling and heating of the samples