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

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

Mechanical alloying of Fe100−x−yNbxBy (x = 5, 10; y = 10, 15): from pure powder mixture to amorphous phase

The mechanical alloying process of Fe75Nb10B15 and Fe85Nb5B10 systems has been studied from an initial mixture of elemental powders. The amorphization process is monitored by X-ray diffraction and Mössbauer spectrometry. An amorphous phase is formed after 400 h milling only for Fe75Nb10B15 alloy, whereas a bcc supersatured solid solution is the final product after milling Fe85Nb5B10 alloy. For both cases, a dispersion of ∼10% in the Fe content of the powder particles persists after 400 h milling. Powder particle size, Cr content and lattice parameter of bcc phase are larger for the alloy with the highest Nb conten

Comparison of equivalent ball milling processes on Fe70Zr30 and Fe70Nb30

Dynamical analysis of the movement of a single ball in a planetary ball mill yields a cubic law with the frequency of the mill for the power released during the process. This fact has been explored for two binary compositions (Fe70Zr30 and Fe70Nb30) using two different milling frequencies. The experimental techniques used, in general, support the predictions of the equivalent milling time model. However, some deviations appear which could be ascribed to differences in the temperature inside the vial during millin

Influence of Milling Time on the Homogeneity and Magnetism of a Fe70Zr30 Partially Amorphous Alloy: Distribution of Curie Temperatures

In this work, the mechanically alloyed Fe70Zr30 (at. %) composition has been used to study the influence of milling time on its homogeneity and magnetic properties. The microstructure and Fe environment results show the formation of an almost fully amorphous alloy after 50 h of milling in a mixture of pure 70 at. % Fe and 30 at. % Zr. The soft magnetic behavior of the samples enhances with the increase of the milling time, which is ascribed to the averaging out of the magnetocrystalline anisotropy as the crystal size decreases and the amorphous fraction increases. The formation of a non-perfectly homogenous system leads to a certain compositional heterogeneity, motivating the existence of a distribution of Curie temperatures. The parameters of the distribution (the average Curie temperature, T C ¯ , and the broadening of the distribution, ∆ T C ) have been obtained using a recently reported procedure, based on the analysis of the approach towards the saturation curves and the magnetocaloric effect. The decrease of ∆ T C and the increase of T C ¯ with the milling time are in agreement with the microstructural results. As the remaining α-Fe phase decreases, the amorphous matrix is enriched in Fe atoms, enhancing its magnetic response.AEI/FEDER-UE Project MAT 2016-77265-

On the use of JMAK theory to describe mechanical amorphization: a comparison between experiments, numerical solutions and simulations

The kinetics of amorphization during ball milling is generally analyzed using two different approaches: the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and Delogu and Cocco’s model for which a region deterministically transforms after it reaches a certain number of collisions. The application of JMAK analysis to the latter model predicts Avrami exponents to be higher than the experimental ones (typically close to one). We develop simulations based on the probabilistic character of the nucleation phenomenon and concave growth of the amorphous phase in the core of a nanocrystal. The predictions of our simulations are in good agreement with the low Avrami exponents and with the size evolution of the remaining crystallites found experimentally. From these values, the parameters involved in the simulated model (growth rate and probability of nucleation) can be estimated.AEI/FEDER-UE (Project MAT-2016-77265-R)Junta de Andalucía (Grupo PAI

Milling effects on magnetic properties of melt spun Fe-Nb-B alloy

Fe75Nb10B15 amorphous ribbons were grinded via ball milling to produce powder samples preserving the amorphous microstructure. A continuous increase of the Curie temperature with the milling time is observed as well as an enhancement of spontaneous magnetization, average hyperfine field, and magnetocaloric effect. This enhancement in the magnetic character of the samples as milling progresses is ascribed to an increase of the Fe-Fe distance. However, the peak entropy change reduces after grinding the ribbon sample. This effect could be related to a broader distribution of Curie temperatures in powdered samples

Extracting the composition of nanocrystals of mechanically alloyed systems using Mössbauer spectroscopy

Determining the composition at the nanoscale generally requires the use of experimental techniques such as 3D atom probe or nanoanalysis, which have limited availability, involve high economic cost and, moreover, imply aggressive sample preparations. However, the combination of Mössbauer spectrometry (MS), X-ray diffraction (XRD) and magnetization measurements can supply very detailed information on the average values of composition of tiny elements of the microstructure such as nanocrystals and boundary regions. Unlike nanoscale techniques, those techniques are widely accessible to most of the scientific community and do not require any special sample preparation, especially for powder samples. Two methods are proposed: the first method uses the ratio between the high field contributions to the MS spectra to extract the composition of the nanocrystals and allows us to follow its evolution; the second method uses average values of the hyperfine field and XRD data to study nanocrystalline samples. These procedures have been applied to two FeNb(B) powder samples obtained by mechanical alloying. The proposed procedures can be easily extended to systems containing other isotopes suitable for Mössbauer spectroscopy or to data from nuclear magnetic resonance experiment

Influence of the demagnetizing factor on the magnetocaloric effect: Critical scaling and numerical simulations

In recent years, the magnetocaloric effect is studied not only for the search of potential magnetic refrigerant materials but also for the analysis of critical phenomena. In both cases, the demagnetizing field might have a notable influence on the results. In this work, we carry out a systematic study, based on theoretical simulations, of the influence of the demagnetizing factor on the magnetocaloric properties. On the one hand, we show that demagnetizing factor affects only slightly the magnetic entropy change ( D S M ), reducing its magnitude and shifting the peak to higher temperatures. On the other hand, it dramatically affects the exponent n of field dependence ð D S M / H n Þ at temperatures below the peak. We demonstrate that scaling of the magnetocaloric curves can be used to remove the influence of the demagnetizing field and, to which extent, critical exponent determination can be affected. Results of numerical simulations are compared with experimental data from a ball milled powder alloy

Role of starting phase of boron on the mechanical alloying of FeNbB composition

Mechanical alloyed Fe75Nb10B15 systems, prepared with crystalline or commercial amorphous boron and a similar composition with the same Fe/Nb ratio but no boron, have been studied as a function of milling time in the aim of enhancing the homogenization of boron and determining its role on the mechanical alloying process. Neither boron addition nor boron phase used affect the developed microstructure at very early stages. After 4 h milling at 350 rpm, the formation of an amorphous phase was observed for boron containing compositions while for the boron-free alloy a supersaturated solid solution was found in the final microstructure. The alloy prepared using commercial amorphous boron showed a larger fraction of amorphous phase than that prepared using crystalline boron for the same milling time, suggesting that amorphous boron accelerates the formation of the amorphous phase during mechanical alloyin