Isothermal Resistivity Measurements Research Articles

Properties of rapidly solidified Al–12.5 Si–1Ni alloy

Rapidly solidified Al–12.5 Si–1 Ni ribbons were prepared by melt spinning. The Si solid solubility was extended to high values as deduced from X-ray diffraction and TEM. This high solubility of Si was found to have significant effects on the various properties of this alloy. High values of electric resistivity was observed which was about four times the fully annealed values. Furthermore rapid solidification improved the tensile strength and the toughness of this alloy as compared with its classically solidified counter part. The relaxation kinetics was followed by isothermal resistivity measurements. The relaxation follows an equation which has the Johnson-Mehl-Avrami (JMA) form. No metastable phase was observed.

Oxygen diffusion in silver-free and silver-doped single crystals

Oxygen diffusion in undoped and silver-doped (YBCO) single crystals has been investigated by isothermal resistivity measurements in the temperature interval . The diffusion coefficients for silver-doped crystals were found to be an order of magnitude lower compared with undoped ones. Further, the activation energy for oxygen diffusion was found to be considerably higher for Ag-doped crystals. Analysis of data on the equilibrium value of resistivity as a function of temperature and magnetization measurements made on samples quenched from an annealing temperature of , indicates that silver doping stabilizes the YBCO phase with higher oxygen content.

Effect of metalloid substitution on the reversible structural relaxation of Fe–Ni–B(-P, Si) metallic glasses

The reversible structural relaxation of Fe 37.5Ni 37.5B 25− x P x ( x=0, 3, 6, 16, 25) and Fe 37.5Ni 37.5B 25− y Si y ( y=0, 5, 10, 15) metallic glasses was investigated using the activation energy spectra (AES) model by differential scanning calorimetry (DSC) and resistivity measurements. From the deduced activation enthalpy spectra for these amorphous alloys, it is concluded that possibilities for atomic movements with characteristic activation energies are created by the substitution of B with P or Si. It is shown that at least one Gaussian contribution to the AES appears on increasing P or Si content, superimposed on an exponential part previously found for Fe–Ni–B glasses. The amplitude of the Gaussian function increases with increasing P or Si content while its position decreases with increasing P and remains constant with increasing Si content. According to isothermal resistivity measurements, for P substitution the reversible atomic rearrangements connected to the Gaussian contribution increase the resistivity while those associated with the exponential part decrease it. For Si substitution both processes decrease the resistivity.

Oxygen diffusion and defect mechanism in c-axis textured thin films of YBa2Cu3O7−x by resistivity measurements

Isothermal resistivity measurements have been carried out to study the dynamics of oxygen out- and in-diffusion in thin films of YBa2Cu3O7−x in the temperature range 648–773 K. The activation energies for the out- and in-diffusion were determined to be 1·36 and 0·7 eV respectively. We have modelled the resistance-time plots for the oxygen in-diffusion using an equation for one dimensional diffusion into a plane. The microstructural defects such as low angle grain boundaries associated with the c-axis oriented grains are believed to provide the required diffusion paths in thin films of YBCO.

Electrical Resistance and Stress of Bilayer Co(P)/Cu and Ni(P)/Cu Thin Films

The electrical resistance of the metallic bilayer was measured in situ using a resistance furnace. In the isothermal resistance measurements the sheet resistance was measured as a function of time, up to 20 h, at the constant temperature of 400°C. Results show that the sheet resistance of Co(P)/Cu system stays constant for 19 h, while that for the Ni(P)/Cu system continuously changes with time. In the dynamic resistance measurements the sheet resistance was measured as a function of temperature, from 25 to 500°C at the heating rate of 10°C/min. Results show that the dependence of the sheet resistance on temperature for the bilayer Ni(P)/Cu and Co(P)/Cu thin films is at first linear and then deviates from linearity as the temperature increases from room temperature to 500°C. This dependence exhibited irreversibility. The stress of the bimetallic bilayers was measured in situ as a function of temperature in the range from 50 to 500°C using a modified Flexus Z2400S apparatus. Results show that the initial stress at room temperature is compressive. During the first heating cycle the stress state remains compressive and the stress level at the maximum temperature of 450°C is slightly higher than the stress in the as‐deposited film. During the cooling cycle the compressive stress changes into the tensile and continues to accumulate with the decreasing temperature, reaching a maximum value at room temperature.

Nb-driven nanocrystallization of amorphous Fe 75.5Cu 1Nb 3Si 12.5B 8

The process of crystallization of particles with dimensions of the order of 10 nm in amorphous Fe 75.5Cu 1Nb 3Si 12.5B 8 (nanocrystallization) is investigated by three experimental techniques: isothermal resistance measurements, X-ray diffraction, and electron microscopy image analysis. The results show that the crystallization rate decreases slightly with increasing fraction of crystalline material. This decrease is mainly caused by the small growth rate of the larger crystals. Possibly rate-limiting Nb-diffusion and an enrichment in Nb of the amorphous phase are the decisive processes.

The Description of the Isothermal Decomposition Processes in AlSc Alloys by means of Electric Resistivity Measurements and TEM Investigations

AbstractThe decomposition behaviour of two AlSc alloys (cSc = 0.18; 0.36 at.%) after direct quench to room temperature and subsequent ageing at temperatures in the range 200 °C ≦ Ta ≦ 550 °C was investigated by means of isothermal resistivity measurements and TEM. In the temperature range 325 °C ≦ Ta ≦ 400 °C both alloys show independent of the Sc content an “inversion” of the decomposition kinetics, obviously caused by the transition of the coherent into the incoherent Al3Sc phase.

On the description of isothermal resistivity measurements using a generalized effective medium approach

Avec seulement deux parametres principaux, cette approche permet de decrire correctement le comportement de resistivite durant le vieillissement thermique. Cette theorie combine l'electrodynamique classique, la physique statistique et la mecanique quantique

Some Results of the Analytical Evaluation of Isothermal Resistivity Measurements with Al – Zn – Mg Specimens

AbstractIsothermal measuring curves of a series of Al‐4.5 at.% Zn‐X at.% Mg (X = 0.2, 0.5, 1.0, 1.5, 2.25, 3.0) were evaluated analytically by means of the Johnson‐Mehl‐Avrami equation. Itturned out to be possible to fit the curves completely only by assumption of two process stages. The course of the parameters obtained their dependence upon the Mg content is interpreted by means of suitable model assumptions.

Determination of the Effective Vacancy Concentrations and the Vacancy Losses after the Quench of Al–Zn–Mg Alloys

AbstractOn the basis of the experimental observation of dislocation loops by means of TEM, the conclusion from this that the concentration of vacancies in the dislocation loops exists at the cost of the vacancy concentration primarily present at Tq, and the inversion of the decomposition kinetics of Al–Zn–Mg alloys found out by means of resistivity and microhardness measurements, the effective vacancy concentration cv.eff being at disposal for the GP‐zone formation was found out by means of the relation tR,max (tR,max being the time required to reach the maximum of resistivity) and the Arrhenius plot In tR,max = f(1/Tq) of the isothermal resistivity measurements. From the comparison of cv.eff with the theoretical V concentration at Tq it is possible to derive statements about the loss of vacancies during and after the quench through loop formation, building‐in of vacancies in GP‐zones and annealing of vacancies at lattice defects.From this comparison follows: The ideal ZnV concentration at Tq (calculated by means of the Lomer equation) is sufficiently great to account for the effective vacancy concentration. The part of cMgV in the loss concentration is rising with increasing cMg. Both these statements support the hypothesis that the initial process of the decomposition in Al–Zn–Mg alloys is caused by the ZnV diffusion.

The influence of the direct quench and indirect quench to Ta > Th on the decomposition behaviour of an Al‐2 at.% Zn‐1 at.% Mg alloy

AbstractThe decomposition behaviour of an Al‐2.0 at.% Zn‐1.0 at.% Mg alloy after direct quench (DQ) and indirect quench (IQ:T = 23 °C, t) to the ageing temperature T = 160 °C (above the temperature of the rapid homogeneous nucleation, Th) was investigated by means of TEM and isothermal resistivity measurements. The precipitation density of the η‐particles yields a maximum value at pre‐ageing times about 10 min at T = 23 °C. To clear up the reason for that phenomenon a loop‐formation and clustering model was introduced.

Crystallization kinetics of the amorphous magnetic material 2605 CO (Fe 67Co 18B 14Si 1)

The reaction kinetics of Fe 67Co 18B 14Si 1 have been studied with continuous heating differential scanning calorimetry (both peaks) and isothermal resistivity measurements (first peak only). The first and second peaks can be characterised by constant values of empirical activation energies (2.5 and 4.02 eV, respectively) and pre-exponential factors (In K 0 = 40.65 and 57.35 min -1, respectively). The first transformation involving the formation of α-iron dendrites does not appear to conform to a simple Johnson-Mehl-Avrami relation insofar as it shows a non-linear behaviour in the ln ln(1- x 2) -1 against ln t plot. Two quite distinct slopes, hence two values of Avrami exponent are observed (average values of n 1 = 1 n 2 = 5). The behaviour is confirmed by both measurement techniques. Various possibilities, e.g. surface crystallization or incubation time are not considered responsible for the observed behaviour. Explanations in terms of the morphology of the growing dendrites are briefly discussed.

Resistometric study of short range ordering in metallic glasses having different free volume content

Isothermal resistivity measurements have been performed between room temperature and 570 K on amorphous Fe3ONi36Crl4P14B6 and Fe86B14 prepared by melt spinning at different quenching rates. The variation of the resistivity with annealing time is explained in terms of processes of compositional and topological short range ordering. A coherent shift of the observed resistivity changes towards higher temperatures is observed in alloys prepared with lower quenching rates. Compositional and topological short range ordering processes are equally affected by the variation of the degree of amorphicity associated with different rates of quenching from the melt.

Information on the formation rate of GP zones attained by isothermal resistivity measurements of an Al‐6at.% Zn alloy

Information on the formation rate of GP zones attained by isothermal resistivity measurements of an Al‐6at.% Zn alloy

Crystallization of (Fe1−xNix)80B20 amorphous alloys

The crystallization behavior of a Fe60Ni20B20 amorphous ribbon alloy was investigated by isothermal resistivity measurements and by x-ray diffraction transmission pin-hole techniques. The resistivity decreased appreciably (∼25%) upon crystallization. The time dependence of the resistivity at various isothermal crystallization temperatures displayed a prominent kink indicative of a two stage crystallization process where about 80% of the resistance loss is associated with the first crystallization stage. An activation energy of 85 kcal/mole was found to characterize the kinetics of the first crystallization stage. The x-ray results failed to reveal unambiguously the structural changes suspected to accompany the advent of the second crystallization stage. A comparison of the crystallization results of this study where nickel replaces iron with the results of a previous study where phosphorus replaces boron in the Fe80B20 stoichiometry revealed numerous systematic differences in behavior. An analysis of crystallization in a Fe20Ni60B20 amorphous alloy was hampered by a small apparent resistivity change which prevented an accurate evaluation of the kinetic behavior.

The mechanism of clustering in supersaturated solid solutions of A1-Mg 2Si alloys

The early stages of the precipitation in the Al-Mg 2Si system were investigated by isothermal resistivity measurements. The process of clustering was studied over a wide range of temperature (between − 50 and 200°C) after two different quenching methods. The continuous observation of the resistivity led to the investigation of the kinetics of the process near room temperature. The effect of preageing on the subsequent ageing was also studied by electrical resistivity and microhardness measurements. On the basis of the experimental results a new model is suggested for the process of clustering. The most important feature of the process seems to be the decisive role of Si.