Observation and characterization of a twinned monoclinic phase as a product of the solid state decomposition of Nd 2 Fe
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A microtwinned monoclinic phase is observed to occur within the grains of the hard magnetic phase in Nd2Fe14B-based permanent magnet alloys. This phase seems to be the product of a displacive solid state phase transformation of tetragonal Nd 2 Fei 4 B, distinguished from Nd2Fei4B by the presence of an ordered substitution into alternating unit cells parallel to the crystallographic c-axis and by a small shear of the type (100) {Oil}. The four different orientational variants of this monoclinic phase with respect to the parent tetragonal phase are arranged into two distinct colonies of twin-related plates. The observation that Nd2Fei4B can undergo a displacive solid state transformation has broad implications for the entire field of Nd 2 Fei 4 B permanent magnets.
I. INTRODUCTION The most successful of the current generation of permanent magnet materials are based on the ternary intermetallic compound Nd 2 Fei 4 B. The crystal structure of this compound is well known1'2; its remarkable intrinsic magnetic properties have been the subject of many experimental studies.3'4 It is now routinely processed into actual permanent magnets using several different processes.5 It has generally been accepted that Nd 2 Fe 14 B is a stable line compound from 0 K to its melting temperature. Except in extreme circumstances,6 the 2:14 :1 stoichiometry among rare earth metal, transition metal, and metalloid is well maintained over the entire range of stability of this phase. Thus, once Nd 2 Fe 14 B forms, it is not expected to undergo any internal structural or metallurgical changes. The only known exceptions involve high concentrations of early group transition metals7 such as Nb or V, or some form of far-fromequilibrium processing.5 Nd 2 Fe 14 B generally forms as defect-free grains, although extended defects8 and mechanical twins9 have been reported. Nd 2 Fe ]4 B exhibits a ferromagnetic Curie temperature of 312 °C and a spin reorientation temperature of 135 K. Upon cooling through the spin reorientation temperature, the spins rotate out of a simple alignment parallel to the c-axis and form a conical distribution about the c-axis; the overall symmetry of the structure is reduced10'11 from P42/mnm to Cm. The purpose of this article is to report the observation and characterization of a new, monoclinic ^Permanent address: Center for Materials Science and Engineering, The University of Texas at Austin, Austin, Texas 78712.
1762 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 7, Jul 1992 Downloaded: 14 Mar 2015
phase that occurs within the grains of Nd 2 Fei 4 B in several different potential magnet alloys. Although the observations reported here indicate that this monoclinic phase must be relatively common, it seems to have been previously overlooked in the published literature of Nd 2 Fei 4 B magnets because it is not readily observed, except by means of transmission electron microscopy. II. EXPERIMENTAL The alloys actually examined are members of a series of samples (see Table I) prepared by several different laboratorie
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