H. Miranda

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.

Time-scaling and crystallization kinetics of three Fe-B-based metallic glasses

Time-scaling properties of the isothermal transformation kinetics have been tested for three Fe-B-based metallic glasses exhibiting two crystallization stages. The time scale was defined as the time at which crystallization has reached half completion and is derived from calorimetric (DSC) data. The temperature dependence of the time-scaling parameter shows a lowtemperature freezing behaviour and can be described by empirical functions based on free-volume considerations.

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.

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.

Annealing effects on the Curie temperature of a Fe77Cr2B16Si5 glass

Abstract Changes in the Curie temperature of a Cr-containing FeBSi glass were studied using DSC measurements. The Curie temperature increases monotonically with annealing temperature and time and the equilibrium value is inversely proportional to the annealing temperature. A uniformly linear log (time) kinetics, as in other Fe glasses containing two metalloids, is found. Results can be explained in terms of the activation energy spectrum model.

High-temperature deformation of cordierite glass-ceramic/SiC platelet composites

Composites based on barium-containing cordierite glass-ceramic matrices reinforced with 20 and 30 vol% SiC platelets were fabricated by hot-pressing. The ceramed specimens were tested in compression at 1000, 1200 and 1300 °C in air, parallel and perpendicular to the hot-pressing direction. Compressive fracture stresses up to 460 MPa were recorded at 1000 °C. Lower stresses were observed at 1200 and 1300 °C. The compressive stress decreased with increase in the SiC platelet content and with the compressive direction being parallel to the hot-pressing axis. The mechanical behaviour at high temperatures was related to the presence of a residual glassy phase within the cordierite matrix, as well as to the mode of the crack nucleation within the composites.

Room-temperature Perminyar effect and wide domain walls in nearly-zero magnetostrictive amorphous ribbons

Abstract The low-field ac complex susceptibility χ ∗ of as-quenched melt-spun amorphous ribbons with nearly-zero magnetostriction λ s has been measured as a function of the field amplitude H m and frequency ƒ. It has been found that there is a jump on the χ ∗ versus H m curve at H m ≈2 A/m. Before the jump, χ ∗ is constant. This is a room-temperature Perminvar phenomenon, due to domain wall pinning by local induced anisotropy. The constant susceptibility is dominated by domain magnetization rotations, while the jump is ascribed to domain wall depinning. Using a new eddy-current losses model together with relaxation analysis, it is further deduced that only very few wide (around 4 μm) walls exist in such ribbons.

Crystallization behaviour of Ni63Co17B13Si7 alloy

Abstract Dynamic crystallization of the Ni 63 Co 17 B 13 S 17 alloy is characterized by differential scanning calorimetry (DSC) and X-ray diffraction. Crystallization occurs in two stages at peak temperatures of 640 and 780 K (at 20 K/min), with enthalpies ΔH 1 = 2.0 and ΔH 2 = 7.2 kJ/mol. A slight precipitation of a nickel phase occurs in the first stage and a Ni 3 Si-type phase is formed during the second stage of crystallization. Isothermal kinetics of the second exotherm is analysed.