Some Aspects of Precipitation and Magnetic Properties of Alnico Alloys

Abstract

I. Single crystals of Alnico V and Alnico VIII alloys with 35, 40, and 42 wt.% Co and 5, 7, and 8 wt.% Ti, respectively, were investigated by x-ray analysis and electron microscopy of thin foils. It was found: (1) In Alnico V, alloy decomposition begins with the formation of randomly arranged equiaxial spherical zones. When the tempering temperature and time increase, at first very short-range ordering appears in their arrangement, and then an entire periodic modulated structure is formed. At later stages, rod-shaped particles are formed, then metastable tetragonal phases, and at least, two cubic phases. (2) In Alnico VIII-type alloys with an increased content of Co and Ti, volume changes during decomposition are significantly greater, and the role of elastic energy is decisive. So a three-dimensional periodically modulated structure forms even during the quenching process. These structure elements are mainly equiaxial precipitates which intergrow in rods in some places. During tempering, the number of rod-shaped formations increases, and continuous transformation from the modulated structure to tetragonal phases takes place. Two cubic phases form at later tempering stages. (3) Transformation from the modulated structure to tetragonal phases is not accoupanied by changes in alloy structure morphology but are seen in x-ray scattering results. The tetragonal phases have equal ``c'' periods along the cubic axis but different ``a'' periods. This is accounted for by an alloy structure morphology in which there are interconnected rod-shaped particles quasicoherent with the matrix (phase lattice periods coincide only along the c axis). In the state of optimum magnetic properties, the structure of all the investigated alloys consists of two tetragonal phases with the following parameters: α-phase Åα′-phase ÅItem Nos.Alloyacc/aacc/a1Alnico V2.8762.8720.9982.8702.8721.0012Alnico VIII2.9092.8730.9872.8552.8731.0063Alnico VIIIa2.9092.8720.9872.8502.8721.0084Alnico VIIIb2.9152.8720.9852.8522.87210.0740% aCo, 7% Ti.42% bCo, 8% Ti.(4) A three dimensional periodic modulated structure is formed at an early stage of tempering with and without a magnetic field applied. As tempering time increases, modulation disappears in the direction coinciding with the external field vector. As a result of this process, a uniaxial structure of rod-shaped particles modulated along the two perpendiculars to the field is formed. The data obtained in this work confirms the spinodal nature of solid solution decomposition in Alnico alloys. II. Microstructural analysis of Alnico VS55 single crystals revealed their dendritic structure. Some segregation was observed in the lower parts of single crystals. Electron microprobe analysis of these regions showed their chemical heterogeneity. Sharp variations of titanium, copper and iron content with respect to the basic composition of the alloy were observed. Investigation of the composition variation shows strong liquation of copper and titanium with distribution coefficients K<1. The observed chemical heterogeneity is accompanied by physical heterogeneity. X-ray topography observations were made on single crystals parallel and perpendicular to the growth direction. Maximum misorientation on the end surface is 4°, while on the surface parallel to the growth direction it does not exceed 32′. The boundaries lie between the dendrite branches. The average size of subgrains is 0.2–0.7 mm. Investigation of the fine decomposition structure α→α+α′ after standard magnet heat treatment reveals places with particles having different shape anisotropy, different orientations, and even different degrees of decomposition. Work was carried out to find the influence of copper and titanium concentration on the decomposition process, and thus on the temperature of isothermal tempering in a magnetic field, and also the influence of the value of the angle between 〈100〉 axes and the direction of magnetic field during heat treatment. Thus, the single-crystal heterogeneity and permanent-magnet properties are shown. Possible ways of removing different kinds of imperfections by changing the regimes of single-crystal growth, and by adding small quantities of some alloying elements to decrease constitutional supercooling, were investigated. The maximum magnet energy of the single crystals was larger than 10·106 G–Oe, with coercive force of 2000 Oe. Both parts of this abstract will be published in the I.E.E.E. Transactions on Magnetics as individual papers.