M. Millán

The magnetocaloric effect in soft magnetic amorphous alloys

The influence of different compositional modifications on the magnetic entropy change and refrigerant capacity of Finemet, Nanoperm, HiTperm, and bulk amorphous alloys is presented. For all the studied alloys, the field dependence of the magnetic entropy change exhibits a quadratic dependence in the paramagnetic regime, a linear dependence in the ferromagnetic temperature range, and a potential law with a field exponent ∼0.75 at the Curie temperature. This exponent can be explained using the critical exponents of the Curie transition. It is shown that for alloys of similar compositional series, the magnetic entropy change follows a master curve behavior.

Thermomagnetic study of devitrification in nanocrystalline Fe(Cr)SiB-CuNb alloys

Abstract Structural changes taking place in the course of the amorphous to crystalline transformation in nanocrystalline Fe 73.5 - x Cr x Cu 1 Nb 3 Si 13.5 B 9 (0≤ x ≤5) alloys were investigated by DSC and thermomagnetic measurements. Heating up to temperatures of about 800 K leads to the formation of a nanocrystalline Fe(Si) phase in the amorphous matrix. The grain size is found to be in the range of 15–20 nm, showing no dependence on the chromium content. The composition of the Fe(Si) phase is estimated by the Curie temperature and a significant decrease of the silicon content in that phase, after heating at higher temperatures, is evidenced. After full crystallization, two main phases are detected, an Fe(Si) phase and another unknown boride bcc phase. The effects of the chromium addition are a small increase of the thermal stability of the amorphous alloy and a significant lowering of the Curie temperature of both the amorphous as-cast alloy and the final boride phase, whereas only a slight variation in T C with composition is observed for the final Fe(Si) phase.

Microstructural evolution of FINEMET type alloys with chromium: An electron microscopy study

Microstructural changes in Fe73.5−xCrxCu1Nb3Si13.5B9 (0⩽x⩽5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe(Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is enhanced with the Cr content of the alloy. In a second stage, around 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type unknown crystal phase with a lattice parameter of a=1.52 nm, and recrystallization of the previous Fe(Si) nanophase also occurs. No qualitative differences were found between dynamic and isothermal crystallization. The size effect for thin samples is limited to a lowering of crystallization temperatures. For isothermal nanocrystallization in the temperature range 775–900 K, the mean grain size of the nanocrystals increases for short annealing times to stabilize at a constant value of about 10–15 nm for long annealing times. The stabilized grain size increases with increasing annealing temperature and slightly decreases with the Cr content of the alloy.

Nanocrystallization in Fe73.5Si13.5B9Cu1Nb1X2 (X = Nb, Mo and V) alloys studied by X-ray synchrotron radiation

Abstract The influence of partial substitution of Nb by refractory elements (Mo and V) on the nanocrystallization process of a FeSiB-CuNb alloy was studied by in situ X-ray diffraction, using a synchrotron radiation source. The crystalline volume fraction, the mean grain size of the crystallites and the nanophase Si content were estimated to describe the microstructural evolution and the kinetics of the process. For the studied alloys, the volume fraction of the Fe,Si nanophase after annealing at the same condition increases in the order Nb

On the isothermal kinetics analysis of transformations in metastable systems: combined use of isothermal and non-isothermal calorimetry

A procedure to optimize the isothermal calorimetric data of very slow transformation processes of metastable systems is proposed. The method uses an experimental baseline to identify the transitory effects due to the equipment. Moreover, the combined use of isothermal and non-isothermal results is shown to be effective in overcoming the intrinsic problems of low signal and signal drift for such processes. The procedure has been applied to the analysis of the nanocrystallization kinetics of the Fe60Co18Nb6B16 alloy at different devitrification stages. Based on microstructural observations, an instantaneous growth approach was assumed and a phenomenological expression of the dependence of the nucleation frequency with both the transformed fraction and the temperature was obtained.

Magnetocaloric effect in Mn-containing Hitperm-type alloys

The magnetocaloric effect of Fe60−xMnxCo18Nb6B16 (x=0,2,4) is studied. Mn addition decreases the Curie temperature of the alloys but also reduces the peak entropy change and the refrigerant capacity of the material. The estimated adiabatic temperature change, for a maximum applied field of 15 kOe, is 1.3 K. Obtained values are comparable to those of some Nanoperm-type alloys. The magnetic entropy change, ΔSM, of the studied samples follows a master curve, which is the same for all of them. The exponent controlling the field dependence of ΔSM scales with reduced temperature in the same way as the master curve does.

An in situ synchrotron study of nanocrystallization in (Fe,Cr)-Si-B(-Cu-Nb) alloys

The nanocrystallization process in (Fe73.5-x Crx)Si13.5B9Cu1Nb3 (x = 0,3 and 5) amorphous alloys has been studied by in-situ X-ray diffraction, using a synchrotron radiation source. The nanocrystalline volume fraction and the mean grain size of the (Fe, Si) crystallites were estimated from an intensity profile analysis of the main diffraction maximum. Cr alloying lowers the crystalline fraction for the same annealing conditions. The mean size of the crystallites initially increases with increasing annealing time but the growth rate slows down and a saturation value of about 12 nm is reached. The Si content of the (Fe, Si) phase initially correlates with the concentration in the as-cast glassy alloy but a final value of about 21 at.% Si is reached. The transformation kinetics are described by an Avrami exponent below unity, as usually found for Finemet alloys.

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 behaviour of FeSiB–XNb (X=Pd, Pt) alloys

Abstract The devitrification of FeSiB–XNb (X=Pd,Pt) amorphous alloys has been investigated by the combined use of calorimetric, thermomagnetic, X-ray diffraction and electron microscopy techniques. Substitution of Cu by Pd or Pt modifies the crystallization properties. In the first stage, beginning around 840 K, α-Fe(Si) and t-Fe 3 B phases are formed. Precipitation of the bcc Fe(Si) phase does not show the kinetics found in Finemet-type alloys. The Fe 2 B phase is formed during a second stage but Fe 3 B subsists over a range of annealing temperatures.

Electron microscopy study of the crystallization of Fe75Co4B18Si3 glass

Abstract The crystallization of Fe75Co4B18Si3 glass was characterized by differential scanning calorimetry as a three-stage transformation. The crystallization onset (at 10 K/min) is at 773 K. Electron microscopy investigation was performed on samples thinned after crystallization and also on samples crystallized in situ in the microscope chamber. α-Fe and Fe2B phases were the final products but Fe3B cubic and tetragonal phases were detected as intermediate products.