P. P. Pal-Val

Low-temperature acoustic properties of nanostructured zirconium obtained by intensive plastic deformation

The temperature dependences of the logarithmic decrement and dynamic Young’s modulus of polycrystalline coarse-grained and nanostructured Zr are studied at temperatures of 2.5–340K. A nanostructured state of samples with grain sizes on the order of 100nm was produced by intensive plastic deformation (IPD). The measurements were made using a two-component vibrator technique at frequencies of 73–350kHz. A relaxation peak in the internal friction near 250K was discovered in the coarse-grained, annealed Zr which is retained after IPD, but its height increases by roughly a factor of 10 and the localization temperature shifts to lower values. In addition, after IPD a new internal friction peak shows up at moderately low temperatures near 80K. The activation parameters for the observed peaks are estimated and it is shown that they arise from different thermally activated dislocation processes: interactions of dislocations with impurities and kink pair formation in dislocations. It was found that IPD is accompani...

Low-temperature relaxation processes in a Cu–Nb nanostructured fiber composite

The elastic and dissipative properties of nanostructured superconducting fiber composites Cu–32 vol.% Nb at frequencies of the order of 70kHz are investigated in the temperature range 2–320K. The composites, prepared by the method of intensive plastic deformation, consist of a fragmented copper matrix uniformly filled with niobium fibers having diameters of 200–500nm. It is found that the acoustic properties of the composite are mainly determined by the properties of the copper matrix. Near 90K a relaxation peak of the internal friction is observed, with activational parameters close to those of the Bordoni peak in copper. Below 12K the temperature dependence of the dynamic Young’s modulus is described by the thermally activated relaxation of two-level systems in a highly disordered medium. The influence of high-temperature annealing on the observed effects is investigated.

Statistical analysis of the low-temperature α peak of the internal friction in iron single crystals

The effect of changes in the ultrasonic frequency or in the dislocation structure of the samples on the parameters of the low-temperature α peak of the internal friction and the Young’s modulus defect corresponding to it is investigated experimentally in single crystals of pure iron. A statistical interpretation of the temperature dependence and structural sensitivity of the characteristics of the dynamic relaxation in the neighborhood of the α peak is proposed, based on the assumption of a random scatter of the values of the activation energy of the elementary relaxators. Empirical estimates are obtained for the attempt period and for the mean value and variance of the activation energy of the relaxation process responsible for the α peak of the internal friction in iron.

Dynamic elastic moduli of niobium at low temperatures: their temperature dependence in the normal state, the influence of the superconducting transition, and dislocation effects

The low-temperature elastic properties of niobium crystals of different purity and orientation are investigated in the normal (N) and superconducting (S) states. The experiments are done by the composite vibrator technique at frequencies ∼90kHz. The temperature dependence of Young’s modulus E(T) and of the shear modulus G(T) are obtained in the temperature range 2–12K in the N and S states. It is found that both the temperature dependence of the individual components of the elastic moduli in the normal state, MN(T), and the temperature dependence of the difference ΔMNS(T)=MN(T)−MS(T) can in some cases have anomalies that cannot be satisfactorily explained in a theoretical description of a direct interaction of the acoustic deformations of the crystal lattice of the metal with the electronic and phonon excitations. In particular, it is found that for fast-cooled samples there exists a temperature region in which an anomalous increase of the elastic moduli at the N–S transition is observed. These anomalies ...

Spontaneous magnetostriction in the Gd–Y system: analysis of phase transformations

The transformations of the states of the magnetic and lattice subsystems of Gd100−xYx (x=0,5.5, 7.5, 10.2) alloys are studied at temperatures 5–370 K in magnetic fields up to 4 kOe. The temperature dependences of elastic moduli, thermal expansion, low-field magnetization, and magnetic anisotropy parameters are obtained. The linear spontaneous striction ΔL(T)/L=ξη2 is analyzed, and the effective order parameters η and magnetostrictive parameters ξ are evaluated for the four magnetic phases (canted ferromagnetic, deformed ferromagnetic helix, ferromagnetic helix, and the ferromagnetic cone, respectively): ξcf=2.2×10−5, ξdh=−5.3×10−5, ξfh=7.1×10−5, ξfc=1.4×10−3.

Nonlinear acoustic effects arising from dislocations in niobium single crystals

The temperature dependence of the logarithmic decrement δ and dynamic Young’s modulus E in niobium single crystals of high purity is investigated in a wide interval of low temperatures 3 K⩽T⩽300 K. Measurements are made by means of a two-component composite vibrator method under excitation of longitudinal vibrations with a frequency of 88 kHz and a relative amplitude of the acoustic strain varying in the range 5×10−9 e0c and their transformation with temperature are investigated in detail. Analysis shows that the nonlinear effects are due to the breakaway of ultrasonically excited dislocations from ...

Kinetics of the low-temperature structural transformation in the In–4.3 at. % Cd solid solution

The reversible structural transformation of the hysteretic type observed previously by the authors in the In–4.3 at. %Cd alloy on low-temperature thermocycling is investigated further. In the hysteresis region (150–290 K) a pronounced instability of the macroscopic characteristics of the alloy is observed, which is manifested as time dependence of the dynamic Young’s modulus and resistivity of the samples at a constant temperature fixed during the cooling or heating run. The kinetic effects are investigated in detail near the boundaries of the hysteresis, where they are most clearly expressed. Less clear signs of the structural transformation are observed on the temperature dependence of the microhardness of the alloy. It is found that the structural transformation is governed by thermally activated processes with several characteristic relaxation times: the analytical form of the kinetic curves and the temperature dependence of the relaxation times are substantially different for the direct (on cooling) ...

Change of Parameters of the Koiwa–Hasiguti Dynamic Dislocation Relaxation in Nanostructured and Polycrystalline Zirconium after Severe Plastic Deformation and Annealing

The temperature dependences of acoustic properties of nanostructured and polycrystalline zirconium are investigated in the temperature range of 100–340 K. The effect of severe plastic deformation and subsequent annealing on key parameters of the Koiwa–Hasiguti acoustic relaxation in zirconium is studied in detail. It is established that, due to intensive plastic deformation, the relaxation strength considerably increases, and the temperature and the width of the corresponding relaxation peak systematically decrease with reduction of the mean grain size in the samples. Annealing leads to a partial recovery of the relaxation strength and the peak temperature back to the initial values in undeformed samples, but the width of the relaxation peak shows an additional decrease. The majority of the effects observed can be explained by changes in dislocation subsystems of the samples during intensive plastic deformation and annealing. An influence of a random scatter of the relaxation time on the main parameters of the Koiwa–Hasiguti peak is established using the statistical analysis based on the lognormal distribution. It is shown that the parameter β of the lognormal distribution determines the width, height, and asymmetry of the peak and also allows estimating the relaxation strength from the peak height. An algorithm for retrieving the parameter β from experimental data is presented.

Low Temperature Elasticity and Magnetic Susceptibility of Gd-Y Alloys

The longitudinal and torsional complex elastic modulus of the polycrystalline alloys Gd 100-x Y x (x = 0, 5.5, 7.5, 10.2, 12.0) have been measured by ultrasonic technique at the frequencies near 91 and 55 kHz correspondingly in the temperature range 5-300 K. The effective a.c. magnetic susceptibility χ eff at the frequency 100 Hz was being measured simultaneously. It was revealed an increase of elasticity between x = 0 and x = 7.5 in the whole temperature range, as well as four anomalies of the temperature dependences of elastic modules for yttrium containing alloys, which were interpreted as: a) conventional features in the region of Curie points T c ; b) first order magnetic phase transition (evidently from ferromagnetic phase to a sort of helicoid) located for all the alloys in the narrow temperature range 245.5 - 248 K); c) two adjacent second order phase transitions from helicoid tofinal angle ferromagnetic phase, with transitions temperatures monotonously decreasing in the interval 83 < T < 146 K when increasing the yttrium contents.

Curious Magnetic Phase Transition in Polycrystalline Gd-Y Alloys. Ultraacoustic Study

43 TUESDAY MORNING, JANUARY 6,1998 Session AS CRITICAL PHENOMENA, SPIN GLASSES AND FRUSTRATION (POSTER SESSION) Norm Koon, Cochair Naval Research Laboratory, Code 6340, Washington, DC 20375 Jian-Qing Wang, Cochair AMRI-Science Building, University of New Orleans, New Orleans, LA 70148