Spatial texture distribution in thermomechanically deformed 2–14–1-based magnets

Abstract

Quantifying the relationship between crystallographic texture and magnetic properties is highly desirable for engineering high-energy-product magnets. Such an undertaking is a challenge for melt-quenched magnets formed from Nd2Fe14B-based alloys because they possess a nanoscale microstructure complicated by the presence of multiple phases, both ferromagnetic and paramagnetic. Procedures for the evaluation of texture in permanent magnets often rely upon magnetic remanence measurements; however, such determinations are based on the use of the Stoner–Wohlfarth model, and are applicable only to assemblies of noninteracting particles, which nullifies their use in texture determinations of “exchange-spring” magnets. To overcome these inherent experimental difficulties, alternative techniques for the measurement of the bulk texture of permanent magnets are explored in this work. Crystallographic alignment was investigated in both neutron and hard x-ray diffraction experiments. Transmission synchrotron x-ray diffraction measurements of the rocking curve width as a function of both the vertical and horizontal positions were performed on a series of thermomechanically processed, melt-quenched magnets based on the Nd2Fe14B composition. The results of the hard x-ray diffraction experiments were verified by neutron diffraction work. A simple model based on a squared-Lorentzian distribution is used to obtain a quantitative estimation of the population of misoriented grains for a given rocking curve half-width. It is deduced that even the most deformed magnets still possess a significant degree of particle misalignment; over 50% of the constituent grains have a misorientation of at least 15° from the overall axial direction at the center of the magnet. The spatial distribution of the rocking curve widths indicates that the development of the c-axis texture is initiated in the center region of the magnet and progresses to the perimeter with increasing deformation. Samples with lower deformation levels possess more uneven texture distributions than do those with higher deformation levels, and the degree of orientation seems to develop rapidly near a deformation level of 50%. The results of this work are utilized to produce a phenomenological model, consistent with the findings of past researchers, of deformation based on mass transport under applied stress via a liquid grain-boundary phase. It is proposed that the micromagnetic structure consists primarily of relatively large clusters of exchange-coupled grains that result in lower-coercivity areas within the magnet. Recommendations are included for improving the production and processing of thermomechanically deformed melt-quenched magnets.

Anisotropic Mechanical Properties of Pr(Co,In)5-type Compounds and Their Relation to Texture Formation in Die-upset Magnets

“…It is known that die-upset magnets from a mechanically-milled Pr(Co,In) 5 type alloy have a peculiar texture; the easy magnetization axis (EMA) is perpendicular to the pressing direction (PD) [4,5]. This texture is quite different from that in die-upset Nd-Fe-B-type alloys, in which the EMA is parallel to the PD [6,7] (Fig. 1).…”

“…1). The texture in die-upset Pr(Co,In) 5 -type magnets is thought to have developed by a different mechanism from that (stress-induced preferential grain growth via dissolution and precipitation [6,7]) operating in the standard melt-spun Nd-Fe-B-type alloy, and is thought to be linked closely to the anisotropic mechanical properties of Pr(Co,In) 5 -type hexagonal compounds. In the present study, the mechanical hardness of the Pr 17 Co 82 In 1 crystal was measured along different crystallographic directions, and the results were co-related to texture formation in die-upset Pr(Co,In) 5 -type magnets.…”

The Coercivity – Remanence Tradeoff in Nanocrystalline Permanent Magnets

“…In the second scheme--the anisotropic case -the easy axis of each grain was chosen to lie in the plane of the slice from a Gaussian distribution with a standard deviation of 100 about the y-direction. These easy axis distributions simulate, in two dimensions, the experimental easy axis distributions found in hot-pressed and optimally die-upset magnets respectively [6]. In the isotropic scheme each of the 18 grains was 100 tnm by 100 nm in size.…”

“…Fig. t shows the correlated, but non-monotonic, relationship between the coercivity and the degree of crystallographic alignment, as quantified by the half-width at halfmaximum (HWHM) value of a x-ray diffraction rocking curve for a series of Nd 2 Fel 4 B-bascd die-upset samples with nominal composition Ndl 3 ., (Fe0., C.o1 .s)O).3Bs.3Gao.5 that were deforned to varying degrees [6]. The level of deformation (%) as given in Fig.…”

Microstructure of Nd-Rich Grain Boundary Phase in Die-Upset Nd-Fe-Co-Ga-B Magnet

“…[1][2][3][4][5] As the remanence of the die-upset magnets is not so high, it is strongly expected for improvement of the c-axis alignment of the Nd 2 Fe 14 B grains. Lewis et al 6) reported by neutron and hard X-ray diffraction experiments that approximately 20% of the crystallites are misoriented by at least 30 from ideal alignment. Mishra et al 4) reported by transmission electron microscopy (TEM) that total volume fraction of the misaligned grains is estimated to be 15% of the whole magnet.…”