Synthesis of nanocrystalline Supermalloy powders by mechanical alloying: A thermomagnetic analysis

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Abstract

Nanocrystalline Supermalloy powders (79Ni19Fe5Mo, wt%) were obtained by mechanical alloying under argon atmosphere and subsequent annealing. Several milling times ranging from 2 to 32 h have been used. The alloy formation was checked by X-ray diffraction and magnetic measurements. The Curie temperature measurements illustrate the progressive formation of the alloy until 16 h of milling. The evolution of the ferromagnetic–paramagnetic transition and the formation of the alloy in the milled samples are discussed considering the heating and cooling part of the thermomagnetic measurements. A difference between the Curie temperatures measured on the heating and cooling curves was observed. This difference has been interpreted as resulting from a powder contamination with iron for longer milling times.

Introduction

The production of nanocrystalline materials is an active field of research nowadays in materials science. The exceptional properties of nanocrystalline materials are derived from their large number of atoms residing in defect environments (grain boundaries, interfaces, interphases, triple junctions) compared to coarse-grained polycrystalline counterparts [1], [2]. Besides the incipient crystallization of amorphous solids, mechanical alloying (MA) is one of the widely used preparation techniques to obtain nanocrystalline structures starting from solid phase. Mechanical alloying involves the synthesis of materials by high-energy ball milling, in which elemental blends (or pre-alloyed powders, oxides, nitrides, etc.) are milled to achieve alloys or composite materials [2], [3], [4], [5].

According to the random anisotropy model, the crystallite refinement reduces the magnetocrystalline anisotropy and thus, the nanocrystalline materials have simultaneously low coercivity and high permeability [6]. The Ni–Fe alloys (around Permalloy composition and Ni–Fe–Mo alloy, namely Supermalloy) are well known for their high performance as soft magnetic materials, consequently these materials are widely studied for both fundamental properties and applications. There is the possibility to combine the interesting properties of the nanocrystalline materials with the high soft magnetic properties of the Ni–Fe alloys. In the past years, many studies were reported for obtaining by MA the Ni-rich Ni–Fe powders [7], [8], [9], [10] and Supermalloy powders [11], [12], [13], [14]. Following our previous studies [13], [14], this new one is devoted to a thermomagnetic analysis of the nanocrystalline Supermalloy powders synthesis by MA.

Section snippets

Experimental

For the mechanical alloying process 123-carbonil nickel, NC 100.24 iron and molybdenum powder produced by chemical reduction was used. A mixture of elemental powders having the composition 79Ni17Fe5Mo (wt%) was homogenized for 15 min with a Turbula-type apparatus. Then, the mixture was milled in a planetary ball mill under Ar atmosphere for times ranging from 2 to 32 h. To remove internal stresses and to investigate the influence of annealing on the evolution of alloy formation, samples of milled

Results and discussion

The X-ray diffraction patterns show the progressive disappearance of the Bragg peaks of the elemental Fe (after 4 h of milling) and Mo (after 12 h of milling) and the shift of the Ni peaks toward smaller angles, Fig. 1. The new position of the remaining peaks in the milled samples after 16 h of milling is in good agreement with the Bragg peak position of the cast Supermalloy, indicating the progressive alloy formation by milling [13]. A broadening of the peaks is observed, as a consequence of the

Conclusions

The magnetic 79Ni16Fe5Mo (wt%) nanocrystalline alloy has been obtained by mechanical alloying and subsequent annealing. The alloy is already obtained after 16 h of milling. The magnetic alloys synthesis by the mechanical alloying process can be very good followed by thermomagnetic measurements. After 16 h of milling, the Curie temperature of the milled samples is in good agreement with the Curie temperature of the cast alloy, confirming the alloy formation in this time interval. The annealing

Acknowledgements

The authors would like to thank the Romanian Ministry of Education and Research for Grant CNCSIS 1265/2006-2008 and PNCD II 71-015/2007. In addition, F. Popa and I. Chicinaş thank the University J. Fourier and the Région Rhônes Alpes for financial support.

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