R. Márquez

Point defects and high-temperature creep of non-stoichiometric NaCl-type oxide single crystals I. NiO

Abstract Creep of NiO single crystals has been investigated by compression along 〈100〉 in the temperature range 0·6T m to 0·8T m, for stresses from 25 to 120 MPa and oxygen partial pressures pO2 from 10−5 to 0·2 atm. The data are analysed using the equation for recovery-controlled creep. The stress exponent decreases from a value of 12 to low temperature to 7 at high temperature; the activation energy is in the range 5·4 to 8·5 eV and the pO2 exponent varies from 0 to -0·17. The rate-controlling mechanism exhibits a change at, about 0·7T m which can be interpreted in two different ways. Oxygen diffusion plays a dominant role in the rate-controlling mechanism of creep, and occurs via a vacancy mechanism.

Diffusional and dislocation creep of NiO polycrystals

Abstract Polycrystals of NiO have been deformed in compression under nominal stresses between 5 and 90 MPa, at temperatures between 1100 and 1450°C. The oxygen activity was changed by the flowing air and an oxygen-argon mixture. At low stresses, the deformation occurs by grain switching, the creep rate being controlled by oxygen self-diffusion along grain boundaries. At high stresses, dislocation recovery creep is the dominant mechanism of deformation; the creep rate is related to oxygen bulk diffusion. The same point defect is responsible for diffusion in the bulk and the grain boundaries.

Non-isothermal crystallization and isothermal transformation kinetics of the Ni68.5Cr14.5P17 metallic glass

Non-isothermal crystallization of the Ni68.5Cr14.5P17 alloy is characterized by differential scanning calorimetry (DSC) and X-ray techniques. Transformation occurs in two stages at peak temperatures 622±1 and 692±1 K (at 20 K min−1), with ΔH1=1.50+0.1 kJ mol−1 and ΔH2=3.0±0.1 kJ mol−1. Precipitation of a nickel phase occurs in the first stage and a (NiCr)3 P phase is formed during the second stage. An approach to the isothermal kinetics of the two crystallization events is derived within the frame work of the Johnson-Mehl-Avrami theory.

On the crystallization of Fe77B16Si5Cr2 alloy

The crystallization of the Fe77B16Si5Cr2 alloy is characterized by calorimetric (DSC) and X-ray diffraction experiments. Two crystallization stages are resolved in DSC records with ΔH1 = 3.6 ± 0.1 and ΔH2 = 3.1 ± 0.1 kJ/mol and kinetics of the two exotherms is derived from isothermal annealing experiments in the Johnson-Mehl-Avrami approach. The precipitation of α-Fe and σ-FeCr phases in the first stage is followed by polymorphic crystallization of α-Fe and Fe2B phases in the second stage in which the σ-FeCr phase is also transformed into α-Fe phase.

Crystallization kinetics of Fe40Ni38Mo4B18 and Fe80B20 metallic glasses

Experimental results for magnetic susceptibility, calorimetry and electrical resistivity for the metallic glasses 2826 MB (Fe40Ni38Mo4Bi18) and 2605 (Fe80B20) are reported. The crystallization kinetics of these alloys is investigated and activation energies are estimated. Time variation of crystallized fraction derived from isothermal electrical runs is interpreted to give the preponderant mechanisms involved in the crystallization process.

Lattice Dynamics and Thermal Crystallographic Parameters in Phenothiazine

A computer program has been developed to study the lattice dynamics of molecular crystals in the harmonic approximation with the external Born-von Karman formalism and an atom-atom potential function. Dispersion curves are obtained for monoclinic phenothiazine together with frequency distribution functions and external mode contribution to thermodynamic functions. Lattice dynamical T, L and S rigid-body tensors are obtained and individual thermal tensors are compared with experiment. The disagreement with respect to experimental results is of the same order as the disagreement with a Schomaker-Trueblood fit of experimental data.

Calorimetric and X-ray characterization of the non-isothermal crystallization of the metallic glass Ni89P11C (wt.%)

Abstract The crystallization of the Ni81P19C (at.%) quenched alloy is studied by calorimetric (DSC) and X-ray diffraction techniques. The transformation occurs in one stage (at 641 K) with ΔH=3.7±0.1 kJ/mol. The crystallization phases formed are Ni(fcc) and Ni3P(bct) and the phase proportions in the crystalline mixture, derived from the X-ray data, agree with those corresponding to the nominal composition of the amorphous alloy.

Crystallization kinetics of an Fe-Co based metallic glass

Crystallization kinetics of the Fe67Co18Si1B14 metallic glass has been studied from electrical conductivity measurements. Results are interpreted on the basis of a modification of the Germainet al. model (J. Non-Cryst. Solids23 (1977) 93). The characteristics of kinetics for different annealing temperatures are analysed and the activation energy is evaluated

Thermal evolution of co-evaporated amorphous thin Ni-Ag films

Abstract Amorphous thin Ni100−xAgx films with thicknesses in the range 500–1500 A were obtained by vacuum co-evaporation on cooled substrates (170 K). The onset of crystallization below room temperature is revealed by an abrupt decrease in the electrical resistivity and is confirmed by electron diffraction. The thermal evolution of the structure is characterized by a mean evolution coefficient which is determined from electrical resistivity data.

Crystallization kinetics of Fe79B13Si8 metallic glass

Abstract Crystallization of 2605S-2 metglass occurs in two stages as revealed by exothermic peaks at 825 and 840 K. Crystallization kinetics was derived from isothermal DSC runs and the activation energies were determined from an Arrhenius fit. Values found are 460 and 390 kJ/mol.