Influence of milling container internal geometry on the mechanical alloying process of the Fe75Si15B10 system

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Abstract

57Fe Mössbauer spectroscopy and X-ray diffraction were used to study the influence of container internal geometry (flat-endings or round-endings) on the mechanical alloying process of the Fe75Si15B10 system. The Mössbauer spectrum for the sample processed for 19 h in a round-endings container indicates the formation of an amorphous Fe–B–Si phase, with 96% of the Fe total volume. Oppositely, the alloy processed in a flat-endings container for 12 h has a spectrum with 82% of Fe atoms in nanocrystalline Fe,Si and Fe2B phases and the remaining 18% are dispersed at the disordered grain boundary region (amorphous state). This result suggests that a single phase alloy is hard to be obtained in this container. It is also shown that the samples processed in the flat-endings container can be de-mixed to α-Fe phase by milling at times above 18 h. The sample preparation procedures have been repeated twice in order to prove the results reproducibility.

Introduction

The mechanical alloying method was firstly employed by Benjamin [1] and by Koch [2] as a new technique for material processing. Nowadays, it is well known that it could be used to prepare a broad variety of equilibrium (intermetallic compounds) and non-equilibrium phases (amorphous, nanocrystalline and quasicrystalline, and supersaturated solid solutions [2], [3], [4]), starting from chemical elemental powders or pre-alloyed powder.

Mechanical alloying (MA) is a complex method due to the large number of variables involved in the milling process. In order to get desired phases and/or microstructures, consequently avoiding misleading products and guaranteeing the reproducibility of the results, it is necessary to have a quite precise control of the milling process variables involved. Among the milling variables that have an important role in the final microstructure of the milled powders alloy, one should consider the mill quality as well as its internal shape (geometry). The former has an influence on the final composition of the milled material, since during the impact of the grinding tools (balls) on the container inner walls, some material would be dislodged and incorporated into the initial powder, contaminating and changing the desired alloy nominal composition. The later seems to play an important role in the case where an amorphous single phase is expected, as will be discussed in this paper.

Harringa et al. [5] have investigated the effect of the container internal geometry on the Si–Ge system using both flat-endings (FE) and round-endings (RE) SPEX mill containers. They have found that the solid-state reaction among the Si and Ge powders occurs at significantly higher rates in the FE container than in the RE one.

FeSiB-based alloys have been extensively studied due to their soft magnetic properties displayed when the alloys are in the amorphous state; the latter often produced as melt-spun ribbons. Some efforts have already been made to synthesize amorphous FeSiB alloy from elemental powders using ball-milling method [6], [7], [8], [9]. However, the reported results suggest that the milled materials are purely nanocrystalline solid solution [6] or a mixture of nanocrystalline and amorphous phases [7], [8]. In this paper, mechanosynthesis method has been applied in order to investigate the influence of the milling container internal geometry on the alloying process of the Fe75Si15B10 system, using both RE and FE hardened steel milling containers.

Section snippets

Experimental procedure

Fe75Si15B10 powder alloys were prepared by mechanical alloying (MA) from a mixture of elemental powders Fe (99.9+%, below 10 μm), Si (99.999+%, below 10 μm) and B (99.99+%, below 30 μm) using a vibratory ball mill SPEX 8000 unit. The elemental powders, at the desired composition, were sealed in both RE or FE SPEX mill containers together with stainless steel balls under high purity argon (HPA) atmosphere. The sample manipulation and the containers sealing procedures were performed under HPA

Results and discussion

Transmission Mössbauer spectra (TMS) for the powders processed in RE and FE milling containers are shown in Fig. 1(a) and (b), respectively, with specified milling time (mt). It should be stressed that the milling process is different in both cases if one compares the TMS of the samples milled in the FE and RE milling containers for similar milling times (mt = 0.5 h or 5 h).

The Mössbauer results for the alloys processed in FE milling containers (Fig. 1(b)) shows a fast decrease of the α-Fe phase,

Conclusions

The results discussed in this paper suggest that the Fe75Si15B10 products obtained by MA are strongly depended on the container internal geometry. It is shown that 96% of Fe atoms are in an amorphous FeSiB phase and only about 4% of Fe atoms are in a n-(Fe,Si) solid solution in case of the sample processed for 19 h in RE milling container. In case of the sample processed for 12 h in FE milling container, the 76% of Fe atoms are distributed in the n-(Fe,Si) solid solution and n-Fe2B phase and 18%

Acknowledgement

The authors express their thanks to the Latin American Center of Physics for partial financial support of this research.

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