Structure and Magnetic properties of some Alnico Magnet alloys : Effect of cooling rate and tempering treatment

Abstract

Experiments are described which show that coercive force of iron-nickel-aluminium base alloys is conditioned by multiple precipitation controlling factors like cooling rate, ageing treatment and alloy composition. The cooling rate has been shown to be the most important variable controlling the magnetic quality of a particular alloy. The alloy develops the best magnetic properties if cooled at a particular rate called the critical cooling rate which greatly depends upon the chemical composition of the alloy. The effects of copper , cobalt, aluminum and nickel have been discussed in terns of their effect on the critical cooling rate and coercive force. The electron-microscopic study of the cooled and tempered specimens has shown that high coercivity of Alnicos is related to fine dispersion of precipitates which form as platelets on mutually perpendicular [ 100] planes of the matrix. The size and shape of these platelets depends upon the cooling rate, ageing temperature and time. Some evidence is also brought forth to show that prior to the formation of platelets the solid solution first decomposes by rejection of iron atoms to form iron rich zones. X-ray diffraction studies of differently heat treated specimens of alloy containing 12% Co and 11% Al have revealed the presence of ordered and disordered b.c.c structure in the cooled specimens and an additional f.c.c. structure similar to δ-iron in specimens tempered at and above 700°C. The transformations in this alloy are compared to transformations iron in which f.c.c. δ –iron is formed at intermediate temperatures. The other Alnico alloys containing no cobalt showed the presence of b.c.c. structures only. The paper also records the occurrence of constricted hysteresis loops in some Alnico permanent magnet alloys. Such loops hitherto have been observed in perminvar type soft magnetic materials only.