Diffusion Reactions in the Aging of 4-79 Molybdenum Permalloy

Measurements of coercive force have been used to follow the low-temperature aging of 4-79 molybdenum Permalloy twistor wire. Twistor wire samples were annealed for varying periods of time at temperatures ranging from 119 to 300°C. This treatment resulted in decreases in coercive force with time which indicated the presence of three distinct reactions. The first reaction, associated with a stress relief anneal, was too rapid to measure. A second reaction, characterized by an exponential decrease in coercive force over the 119–200°C range, was found to fit first-order behavior at 119°C. Its energy of activation was found to be 35.8 kcal/mole or about 1.6 ev. This is the same value as that obtained by Clarebrough from release of energy studies below 300°C as the energy for the disappearance of vacancies in plastically deformed nickel. The 4-79 diffusion reaction appears to result from the same cause. From 119 to 300°C, evidence of a rapid third reaction was observed. Polygonization, as found in an x-ray examination of cold-worked 4-79 alloy heated at 400°C, appears to explain the development of a second easy direction of magnetization in the material.