Mössbauer study of a Fe–Zr–B–Cu–(Ge, Co) nanocrystalline alloy series

Amorphous and nanocrystalline Fe–Zr–B–Cu alloys with partial substitution of Co for Fe and Ge for B have been studied by Mössbauer spectrometry (MS). The compositional and microstructural dependence of the different hyperfine parameters were related to the results obtained by X-ray diffraction (XRD) and saturation magnetization measurements. Combination of MS and XRD leads to estimate an interface region, of thickness ∼0.6 nm. The magnetic moment per transition metal of the crystalline phase is reduced with respect to binary crystalline alloys due to the existence of the interfac

The influence of Co addition on the magnetocaloric effect of Nanoperm-type amorphous alloys

The effect of Co addition on the magnetocaloric effect of amorphous alloys with Nanoperm-type composition has been studied for temperatures above room temperature. Co addition produces an increase in the maximum magnetic entropy change and a shift of its associated temperature to higher temperatures. The maximum refrigerant capacity (RC) value obtained in this study is 82 J kg−1 for a maximum applied field H = 15 kOe. This value is �30% larger than that of a Mo-containing Finemet-type alloy measured under the same experimental conditions. However, the RC of the alloys, when calculated from temperatures corresponding to the half-maximum entropy change value, deteriorates with the presence of Co in the alloy. The field dependence of the magnetic entropy change has also been analyzed, showing a power dependence for all the magnetic regimes of the samples. This field dependence at the Curie temperature deviates from mean field predictions

Mössbauer study of FeCoNbBCu hitperm-type alloys

FeCoNbBCu alloys have been studied by Mössbauer spectrometry. A high contribution of the interfacial region in the nanocrystalline samples to the hyperfine field distribution is detected. The possible presence of B in the nanocrystalline grains is also discussed. Low values of the hyperfine field with respect to the disordered α-FeCo phase suggest the presence of the ordered α′α′-FeCo phase. Recrystallization phenomena at the second crystallization stage are confirmed

Thermal stability of a supersaturated Fe-Ge-Nb solid solution produced by ball milling

Thermal evolution of Fe neighbourhood in a supersaturated bcc Fe(Ge,Nb) solid solution, obtained as the final product of mechanical alloying of Fe75Ge20Nb5, was studied. No changes in Fe neighbourhood were detected after heating up to 473 K, although differential scanning calorimetry shows a clear deviation of the baseline at 400 K. After heating up to 723 K, a similar nanocrystalline microstructure is derived from X-ray diffraction. However, Mössbauer spectra evidence changes in the Fe neighbourhood. A proposed deconvolution of the hyperfine field distribution yields a Ge content of ~10 at. %, in agreement with the maximum solubility of Ge in bcc Fe in thermodynamical equilibrium

Non-Isothermal approach to isokinetic crystallization processes: application to the nanocrystallization of HITPERM alloys

A new approach is proposed for the study of crystallization processes under the hypothesis of isokinetic behavior. From a direct extension of the Avrami theory to non-isothermal regimes, an expression of the local Avrami exponent as a function of the crystalline volume fraction is obtained from a single DSC run performed at a constant heating rate and a raw estimation of the activation energy (a variation of the activation energy value between 3 and 5 eV produces a change on the Avrami exponent smaller than ± 0.1). This approximation was applied to the nanocrystallization process of a FeCoNbB(Cu) alloy series and the results found are in good agreement with previous isothermal analysis on the same alloys. The kinetics is characterized by an initial Avrami exponent close to unity, which decreases down to values below 0.5 as the crystalline volume fraction increase

Analysis of nanocrystallization kinetics and crystal size distribution under limited growth approach

Two different simulation approaches have been used to describe nanocrystallization processes: a limited growth approach, which is an extension from instantaneous growth approximation, and an average soft impingement simulation where spherical crystallites grow to a size for which the corresponding region depleted in Fe (or the element enriched in crystalline phase) is comparable to the average distance between crystallites. Both simulations agree describing a local Avrami exponent which decreases down to ∼1 as crystallization fraction increases. Experimental data for evolution of crystal size and crystal size distribution are reproduce

Ball milling nanocrystallization of arc-melted and melt-spun Fe78Co5Nb3Zr3B5Ge5Cu1 alloy: microstructure and magnetic properties

Versión post-print.The viability of developing a nanocrystalline microstructure for the Fe78Co5Nb3 Zr3B5Ge5Cu1 composition using a planetary ball mill was studied. Two starting samples were chosen: an arc-melted ingot and a partially crystallized melt-spun ribbon. The α-Fe phase was detected in both initial samples, although different intermetallic phases were detected in each of them. Milling at 150 rpm, these intermetallics disappeared after 5 h for the melt-spun alloy (RQ) and did not disappear even after 70 h for the arc-melted sample (AM). However, the α-Fe phase became nanosized in both cases. A good thermal stability was found for the RQ alloy after 10 h of milling, although the nanocrystal size increased from ∼8 to ∼20 nm after heating to 973 K. The AM samples exhibited a lower coercivity and a higher magnetization than RQ sample

Partial substitution of Co and Ge for Fe and B in Fe-Zr-B-Cu alloys: microstructure and soft magnetic applicability at high temperature

The partial substitutions of Co for Fe and Ge for B are studied for a Fe83-xCoxZr6B10-yGeyCu1 alloy series (x = 0, 5 and 20; y = 0 and 5) as a possible way to enhance the high temperature applicability of NANOPERM alloys. The devitrification process, the nanocrystallization kinetics and the nanocrystalline microstructure are similar for all the studied alloys. Good soft magnetic properties are observed even at a high crystalline volume fraction of bcc -Fe nanocrystals, which are stable up to ~1000 K. The partial substitution of Co for Fe is very effective to increase the Curie temperature of the residual amorphous matrix (TCAM). Although the substitution of Ge for B is ineffective to increase TCAM, a clear increase of the saturation magnetization with respect to the Ge-free alloy can be observe

Effect of partial substitution of Ge for B on the high temperature response of soft magnetic nanocrystalline alloys

The effect of partial substitution of Ge for B in Fe78Co5Zr6B10Cu1 alloy on the microstructure, crystallization process and magnetic properties of the amorphous and nanocrystalline alloys was studied. Special attention was focused on the saturation magnetization response at high temperature and its dependence on the microstructure. Although the Curie temperature of the amorphous phase is not affected by the partial substitution of Ge for B, the saturation magnetization increases ∼10% with respect to the alloy without G

The use of amorphous boron powder enhances mechanical alloying in soft magnetic FeNbB alloy: A magnetic study

Saturation magnetization and magnetic anisotropy have been studied during mechanical alloying of Fe75Nb10B15 alloys prepared using crystalline and commercial amorphous boron. The evolution of saturation magnetization indicates a more efficient dissolution of boron into the matrix using amorphous boron, particularly for short milling times. The magnetization of the crystalline phase increases as boron is incorporated into this phase. Two milling time regimes can be used to describe the evolution of magnetic anisotropy: a first regime governed by microstrains and a second one mainly governed by crystal size and amorphous fractio