Nanocrystallization Mechanism Research Articles

The role of metallic (M) elements in the ageing behavior of amorphous Fe–Si–B–M (M=Nb, Zr, or V) alloys

To clarify the mechanism of nanocrystallization, the ageing-induced behavior of amorphous Fe 84− X Si 6B 10M X (M=Nb, Zr, or V) alloys was examined using differential scanning calorimetry (DSC), transmission electron microscopy, and high-frequency magnetic measurements. In a comparison between the Fe 82Si 6B 10M 2 and Fe 80Si 6B 10M 4 (M=Nb or Zr) alloys, a notable difference in the peak morphology was observed on the DSC curves. Ageing for 3.6 ks in the first exothermic temperature range produced ultrafine α-Fe grains only for the latter alloys, and their effective permeability was markedly enhanced by prolonged ageing below 623 K. However, no appreciable difference in such ageing-induced behavior was determined between the Fe 82Si 6B 10V 2 and Fe 80Si 6B 10V 4 alloys. These results were reasonably explained as being related to a structural change in the amorphous phase resulting from the increase in the Nb or Zr content. It was concluded that the formation of an increased number of α-Fe nuclei originates from the concurrent stabilization of the amorphous structure, because the mobility of Fe and Si atoms is extremely reduced due to the large atomic size of Nb or Zr and their attractive interactions with the Fe atoms.

The mechanism of nanocrystallization driven by the Fe/Si ratio in Fe73.5Cu1Nb3Si22.5−xBx alloys

The kinetics of nanocrystallization were studied in two amorphous Fe–Cu–Nb–Si–B based alloys with different Fe/Si ratios. Differential scanning calorimetry and transmission Mössbauer spectroscopy data were collected in a range of time-temperature space. Nanocrystallization is preceded by a relaxation of the amorphous phase that appears to involve changes in its short range order (SRO) parameters [T. Pradell, N. Clavaguera, J. Zhu, and M. T. Clavaguera-Mora, J. Phys.: Condens. Matter 7, 4129 (1995)] which are consistent with Cu-clustering. This process is followed by formation of Fe(Si) embryos with ∼20 at. % Si, regardless of alloy composition. Nanocrystals with DO3 structure grow, and change composition via a diffusion controlled process that is stopped by soft impingement [D. Crespo, T. Pradell, M. T. Clavaguera-Mora, and N. Clavaguera, Phys. Rev. B 55, 3435 (1997)]. The rate of this process depends on both temperature and alloy composition. Nanocrystallization is enhanced by prior annealing at conditions which induce SRO relaxation, however, prior annealing at an intermediate temperature (where crystal nucleation occurs) will retard nanocrystallization.

The role of Nb in the nanocrystallization of amorphous Fe–Si–B–Nb alloys

The aging behavior of amorphous Fe 84 − x Si 6B 10Nb x alloys was investigated using differential scanning calorimetry (DSC), transmission electron microscopy, and high-frequency magnetic measurements. A marked change in the peak morphology was observed on the DSC curve for the alloys containing more than 3 at.% Nb. The right side of the first exothermic peak extended to the higher temperature side, while the second exothermic peak was drastically reduced in its intensity. After aging for 3.6 ks in the first exothermic temperature ranges, all the alloys with a Nb content up to 6 at.% were composed of a mixed structure of the α-Fe phase and an amorphous matrix; however, the addition of more than 3 at.% Nb caused a significant grain refinement of the crystallization-induced α-Fe particles. Furthermore, the low-temperature aging produced a notable increase in the effective permeability only for the alloys containing more than 3 at.% Nb, even though the amorphous phase remained unchanged without crystallization. Based on these results, the mechanism of nanocrystallization was reasonably explained as being related to a change in the amorphous structure which occurs in the vicinity of 2–3 at.% Nb.

Magnetic phase transitions in Fe-(Nb, Zr)-B-Al amorphous and nanocrystalline alloys

Nanocrystalline Fe±(Nb, Zr)±B alloys, prepared by primary crystallization of melt-spun amorphous ribbons, are new soft magnetic materials with a low coercivity and a high saturation magnetization [1]. The microstructure consists of a large proportion of body-centred cubic (b.c.c.) a-Fe nanograins embedded in a residual amorphous matrix. It was reported that both the reduced magnetocrystalline and magnetoelastic anisotropies provide the basis for the magnetic softness of these materials [2]. The nanocrystallization mechanism was described by the nucleation-and-growth and grain-growth models [3, 4]. In this letter we report the magnetic phase transitions in Fe83:5Nb7B8:5Al1 and Fe85:5Nb4Zr4 B5:5Al1 (compositions given in atomic per cent) amorphous and nanocrystalline alloys. The amorphous alloys were prepared by the single-roller melt-spinning method. The thickness and width of the resulting ribbons were about 30 im and 1.5 mm, respectively. The amorphous ribbons were annealed in a conventional furnace under an argon atmosphere. The magnetic phase transitions were followed by thermogravimetric analysis (TGA) (Perkin±Elmer 7) by heating from room temperature to about 800 8C at a heating rate of 10 8C miny1 under a small applied magnetic ®eld. The microstructure was observed by transmission electron microscopy (TEM) using a JEM-2010 operating at 200 kV. Fig. 1a, b and c shows the TGA curves of Fe83:5Nb7B8:5Al1 alloy as quenched, annealed for 1 h at 460 8C and annealed for 1 h at 655 8C, respectively.

Nanocrystallization mechanisms in Finemet-type alloys from calorimetric studies

Enthalpy changes during crystallization of Fe 73.5Cu 1Nb 3Si 13.5B 9 alloy have been obtained by differential scanning calorimetry (DSC) using both isothermal and constant heating rate runs. Residual enthalpy contents after isotherms were measured with a constant heating rate up to 1000 K to determine the crystallized fraction. X-ray diffraction (XRD) on samples annealed in DSC apparatus were carried out to obtain the time dependence of mean grain size and crystallized fraction along isotherms. Taking into account these results, we propose a model where, below 800 K, the crystallization is mainly governed by nucleation and growth process and, above 800 K, by grain growth mechanism. This model gives good agreement with experimental data obtained both along isotherms, by assuming saturation of the nucleation sites, and, also with scan results, by using an activation energy for the growth rate. This formalism gives additional support to the current ideas on the role of copper and niobium atoms in the nanocrystallization of Finemet.