Phase Formation in Isothermally Annealed (Co 0.95 Fe 0.05 ) 89 Zr 7 B 4 Nanocrystalline Alloys
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INTRODUCTION
NANOCRYSTALLINE soft magnetic alloys are of technological interest due to their impressive loss characteristics at high frequencies. Although many compositions have been investigated for this class of alloys, common processing and metallurgical approaches have been used in their development.[1] The most successful process for creating the necessary amorphous precursor has been the single wheel melt spinning technique. The typical alloy produced by melt spinning (referred to as ‘‘as-spun’’) is compositionally homogeneous and substantially amorphous with a long, thin, ribbonlike morphology. These ribbons are subsequently annealed isothermally to promote primary crystallization. Binary phase diagrams of the ferromagnetic transition metals (i.e., Fe, Co, and Ni) with typical alloying elements from the early transition metals (Zr, Nb, Hf, etc.) and some metalloids (B, P, etc.) indicate limited solubility of each alloying element in the primary crystalline phases formed during annealing.[2] For this reason, during crystallization, the early transition metal elements are primarily expelled to the grain boundaries. The resulting enrichment of early transition MATTHEW A. WILLARD, Research Metallurgist, and TODD M. HEIL, NRC Postdoctoral Associate, are with the Naval Research Laboratory, Washington, DC 20375, USA. Contact: e-mail: [email protected] RAMASIS GOSWAMI, Research Scientist is with the SAIC, Washington, DC 20003, USA. This article is based on a presentation made in the symposium ‘‘Phase Transformations in Magnetic Materials’’ which occurred during the TMS Spring meeting, March 12–16, 2006, in San Antonio, TX under the auspices of the Joint TMS-MPMD and ASMI-MSCTS Phase Transformations Committee. Article published online April 5, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
metals in the remaining amorphous phase inhibits the diffusion of the ferromagnetic transition metals, maintaining the desired nanocrystalline microstructure. Therefore, during this process, the crystallites become rich in ferromagnetic transition metals and the remaining amorphous phase is depleted of these elements. As a result, an understanding of the nonequilibrium thermodynamics of these alloys is quite important to tailoring their processing. Nanocrystalline soft magnetic alloys containing the ferromagnetic elements Co and Fe have shown good soft magnetic characteristics and promise for application as inductors for elevated temperature use. The first of these alloys made up of ordered B2 crystallites (i.e., HITPERM) maintains the highest magnetization of all the nanocrystalline soft magnetic alloys and an exceptionally high Curie temperature for the amorphous phase.[3] However, the magnetostrictive coefficients are quite large in these alloys.[4] More recently, cobalt-rich nanocrystalline soft magnetic alloys were found to possess amorphous phase Curie temperatures as high as HITPERM but with a third of the magnetostrictive coefficient size (~13 ppm).[4,5,6] Sustained low AC losses in these alloys have been documented at
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