Effect of plastic deformation by torsion on the heat capacity of the Ni50.5Ti49.5 alloy

Abstract

883Studies of titanium nickelide based shape memory alloys (see, e.g., [1] and references therein) show that severe plastic deformation by torsion under high pres sure (HPT) in Bridgman anvils causes the transition of the microcrystalline structure of initial samples to the amorphous-nanocrystalline state. In this case, the electronic and lattice properties of the alloys are changed substantially. As a result of HPT, the electrical resistivity increases several times, and the dependence ρ(T) has a negative slope below room temperature. The studies of the magnetic susceptibility and optical properties of the alloys show that HPT decreases the density of states at the Fermi level n(E F ). However, the changes in the thermopower S and the Hall coefficient R 0 observed in the Mott two band model do not agree with the results of the magnetic and optical studies. The behavior of the physical properties of the alloys subjected to plastic deformation testifies that the plas tic deformation suppresses the martensitic transfor mation with which the shape memory effect is com monly associated.The aim of this work is to study the change in the heat capacity C P (T) of the shape memory alloy Ni 50.5 Ti 49.5 as a result of its atomic and structural disor dering by torsion under a pressure of 6 GPa through five revolutions of Bridgman anvils at room tempera ture. The heat capacity C P (T) was measured during cooling the samples from 310 to 2 K on a Quantum Design PPMS 9 device (USA) at the Department of Magnetic Measurements of the Institute of Metal Physics, Ural Branch of the Russian Academy of Sci ences (Yekaterinburg). The initial samples were pre pared from the components 99.99% pure by the elec tric arc melting in a helium atmosphere. According to the data of chemical analysis, the elemental composi tion of impurities in the alloy was as follows (wt %): C = 0.0372, S = 0.0001, O 2 = 0.0167, N = 0.0003. The technique of preparing the alloys was described in more detail in [1,2]. The effect of plastic deformation on the thermal expansion and the kinetic, magnetic, and optical properties of the alloy were studied in [1].The results of measurements of C p (T) of the initial (microcrystalline, with the mean grain size of 40 μm) and plastically deformed (amorphous-nanocrystal line) Ni 50.5 Ti 49.5 alloys are shown in Figs. 1 and 2. It is seen from the data presented in Fig. 1a, that, in the limit of the lowest temperatures (2 K ≤ T ≤ 30 K), the heat capacity of the initial alloy is described by the standard relationship (1) Here, the coefficient γ = = 2 mJ/g atom K 2 characterizes the electronic component, and β = = 6.58 × 10 -5 mJ/g atom K 4 character izes the lattice (Debye) component; k B is the Boltz mann constant; and N A is the Avogadro number. These values of the coefficients γ and β agree satisfactorily with the values obtained in [3]. Hence, we can calcu late the density of electron states at the Fermi level n(E F ) = 0.85 state/eV atom and the Debye tempera ture θ D = 309 K. The electron energy band calcula tions perf...