Raja K. Mishra

Microstructures of precipitation‐hardened SmCo permanent magnets

The microstructural features of isothermally aged and step aged and isothermally aged Sm(Co, Cu ,Fe, Zr)7.4 alloys are examined using electron microscopy and x‐ray microanalysis. The microstructure is seen to consist of a cellular morphology with twinned rhombohedral 2:17 phase cell interiors, 1:5 phase cell boundaries, and elongated thin platelets with a 1:3 structure. Upon prolonged aging, Zr preferentially goes into the 1:3 phase, Cu to both the 1:5 and the 1:3 phase, and Fe to the 2:17 phase. Isothermal aging at 850 °C facilitates the growth of the cells and the low‐temperature step aging facilitates the chemical partitioning of the transition metals. The role of Zr in developing the morphology and the chemical partitioning is discussed.

Microstructure and properties of step aged rare earth alloy magnets

LBL-11680 tti&B Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA P^ATF - • 8 Q H A 4 — 13 Materials & Molecular Research Division Presented at the American Physical Society 26th Annual Magnetism and Magnetic Materials Meeting, Dallas, TX, November 11-14, 1980 MICROSTRUCTURE AND PROPERTIES OF STEP AGED RARE EARTH ALLOY MAGNETS Raja K. Mishra, G, Thomas, T. Yoneyama, A. Fukuno, and T. Ojitua November 1980 Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48

Surface energy of spinel

The contributions of the Coulomb electrostatic interaction and the Born‐Mayer repulsive interaction to the surface energy MgAl2O4 are calculated. These results are compared with the surface‐energy values calculated from elastic constants. The results show that surfaces parallel to {111} planes are of lowest energy for MgAl2O4 spinel as well as some spinel ferrites. Except for qualitative evidence, most of these predictions await experimental verification, including the predictions of the composition of freshly cleaved spinel surfaces.

Electron microscopy study of the ferroelectric domains and domain wall structure in PbZr0.52Ti0.48O3

The crystallography of the domains and domain walls in lead zirconate titanate are studied using electron diffraciton and transmission electron microscopy. Both 90 ° and 180 ° domains are observed and the 90° domains are shown to be deformation twins with displacement along 〈110〉 on {110}. The thickness of the 90 ° domain wall is determined to be ≲100 A.

Effect of annealing on the microstructure of sintered Nd‐Fe‐B magnets

A transmission electron microscopy study of sintered Nd‐Fe‐B magnets after annealing above 650 °C shows the formation of a thin intergranular Nd‐rich layer extending to pockets of polycrystalline fcc Nd crystals. The grain boundary phase acts as a pinning site for the magnetic domain walls and is necessary for good magnetic hardening. On the other hand, the Nd1+eFe4B4 phase is an unavoidable product of the sintering process but need not be present for good coercivity.

Electron microscopy of some rare earth–cobalt alloy magnets

Transmission electron microscopy is being used to characterize the microstructural features in some 1 : 5 and 2 : 17 RE‐Co magnets. In 2 : 17 compounds containing Fe and Mn, lamellae of hexagonal and other polytypic regions embedded in the rhombohedral matrix are observed. It is shown that the interface between the hexagonal and rhombohedral phases can be described as a stacking fault with fault vector normal to the interfacial plane. APB’s parallel to the prism planes in the rhombohedral structure have also been identified and characterized. In contrast, no planar faults have been observed in the 1 : 5 materials containing misch metal as the major rare‐earth component. In these alloys extensive twinning and precipitation of a 2 : 17 rhombohedral phase are the major microstructural features observed. The influence of these microstructures on the magnetic properties are discussed.

The effect of vanadium and vanadium plus titanium on the magnetic and mechanical properties of FeCrCo hard magnets

Abstract The microstructure and magnetic properties of the FeCrCo hard permanent magnetic alloys with and without vanadium or vanadium plus titanium were studied using transmission electron microscopy and transmission Lorentz microscopy. Mechanical and magnetic properties were studied in parallel at different stages of the production. The three alloys 57Fe- 15Co-28(Cr, V, Ti) (all compositions are in weight per cent) used for the present investigation had 28Cr, 23Cr-5V and 23Cr-3V-2Ti respectively. It is known that the magnetic properties of these alloys are improved by a heat treatment in a magnetic field. Improvement in the magnetic remanence Br is due to elongation of the FeCo (α1) phase parallel to the magnetic field direction. Subsequent step-aging or continuous-cooling treatments cause further phase separation with increased compositional diffences between the phases without any microstructural changes and thus increase the coercive field Hc. Lorentz microscopy shows that the matrix is weakly magnetic and that the magnetic hardening is due to domain wall pinning. Mechanical testing and fracture analysis show that the alloys are very ductile in the as-quenched state but become very brittle after the aging treatments. The embrittlement is severe for the ternary alloys and is most effectively retarded by the addition of vanadium or vanadium plus titanium. It is attributed to the high chromium content of the chromium-rich (α2) phase and to the large grain size (a result of high homogenization temperature) of the ternary alloys. Since the addition of vanadium or vanadium plus titanium does not change the magnetic properties but improves the mechanical properties and makes the processing more convenient, the alloys with vanadium or vanadium plus titanium seem to be of commercial interest.

Microstructure and Magnetic Properties of Spinel Ferrites

Achieving suitable magnetic properties in ceramic ferrites through thermomechanical treatments rather than through varying the processing and fabrication parameters alone are investigated. The high temperature phase transformation in lithium ferrite (LiFe5O8) spinel and the defects in LiFe5O8 and NiFe2O4 are studied using high voltage transmission electron microscopy in an effort to characterize the microstructures. Results show that a dispersion of paramagnetic LiFeO2 particles in LiFe5O8 matrix gives rise to increased squareness of the hysteresis curve and increased coercivity. Annealing treatments of sintered ferrites remove undesirable intra‐granular {100}1/4〈110〉 cation stacking faults and improve hysteresis loop parameters.

Microstructure and magneto-acoustic oscillations of (Cu, Co) nickel ferrite

Effects of the microstructural features as revealed by scanning and transmission electron microscopy on the magnetostrictive vibration of Cu and Co doped NiFe 2 O 4 are studied. Results show that changes in the grain size are strongly coupled with the geometry of the pores and the two features are not separable. The magneto-mechanical coupling coefficient is seen to depend strongly on the grain size. However, it appears that the pore structure and not the grain size is the important microstructural parameter that affects this magnetic property. Additional evidence is presented to show the importance of the pore geometry. Effects of microstructural features such as stacking faults, dislocations, nickel phosphide precipitates on grain boundaries, etc. are shown to be secondary in importance.

Segregation in Czochralski grown calcium gallium germanium garnet single crystals

Morphology and crystallography of the formation of small segregates in Czochralski grown calcium gallium germanium garnet are studied using TEM and STEM. Based on the observations, the following model has been developed for the formation of these defects. Evaporative loss of Ge from the melt during the crystal growth and the resulting local deviations from stoichiometry lead to random nucleation of small plate‐like precipitates, rich in Ga. As these precipitates grow, strain field interaction leads first to the formation of three‐dimensional segregates, and later to their alignment along the elastically soft 〈100〉 directions. Isolated segregates are also observed. Some of the segregates contain a central void filled with oxygen gas, while some others with or without a void have one or more dislocation loops around them. These segregates have the same garnet structure but a different chemical composition; they are rich in Ga and depleted in Ge. The model suggests that segregate‐free crystals can be grown b...