The Effect of the Rate of Cooling from High-Temperature Single-Phase Region on the Microstructure and Magnetic Propertie
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AlNi, a family of magnetic alloys is composed primarily of Al, Ni, and Fe, were the first modern permanent magnets. Their coercivity originates from the shape anisotropy of single domain Fe-rich particles (b phase) which are embedded in a non-magnetic Ni-Alrich matrix (b2 phase). This microstructure is obtained through decomposition of solid solution within miscibility gap into two isomorphic bcc-based phases that are b and b2 phase, respectively. Two types of heat treatment (HT) are used to increase the coercive force of as-cast AlNi alloys: HT-I involves cooling from high temperature single-phase region at a critical rate to room temperature. HT-II involves water quenching of the homogenized alloy and the subsequent aging within miscibility gap. It was considered that decomposition of solid solution during both types of HT takes place by spinodal decomposition (SD) mechanism. At the same time, VLADIMIR P. MENUSHENKOV, PhD, Head of Scientific Laboratory of Permanent Magnets, MIKHAIL V. GORSHENKOV, PhD, Scientist, ELENA S. SAVCHENKO, PhD Student, and DMITRY G. ZHUKOV, PhD, Head of International School of Microscopy, are with the National University of Science and Technology ‘‘MISIS’’, Leninskii Prospect 4, 119049 Moscow, Russia. Contact e-mail: [email protected] Manuscript submitted June 25, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
X-ray diffraction studies of phase transformation in homogenized AlNi alloy during aging have shown that the decomposition starts from the formation of randomly located uniaxial coherent zones, which serve as nuclei of precipitated phases. As the aging time increases, a tendency to the arrangement of coherent precipitates mainly along the h100i direction appears simultaneously with the growth of spherical zones. This process leads to the formation of the periodic modulated structure.[1–5] Electron microscopic studies of the precipitates morphology formed in aged AlNi alloys are rather conflicting. The microstructure of aged alloy is characterized by both equiaxed b-phase precipitates and alternating b- and b2 phase lamellas.[6,7] But no investigations of the microstructure transformations in AlNi alloys during cooling at a critical rate from the singlephase region have been carried out yet. Recent studies of AlNiCo magnets[8–10] gave new results on spinodal decomposition products, ordering, and chemical compositions of b and b2 phases. But the question about the temperature and mechanism of ordering of solid solution during crystallization of AlNi alloys remain to be clarified. It has previously been demonstrated that the solid solution ordered by type B2 at temperatures above the miscibility gap. This fact is indicated by the presence of superlattice reflections of B2 phase in X-ray and electron diffraction patterns of the water-quenched alloys. Actually, these superlattice
reflections might correspond to the b2 phase which is present in zone microstructure consisting in the mixture of b and b2 precipitates. It is known that the presence of antiphase domains (APD) in micr
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