Technical Notes - Effect of Cold Work and Annealing on the Thermoelectric Power of Molybdenum

THE properties of the metal molybdenum have been the subject of recent investigations1,2 = which have included studies of the changes that occur in the metal when it is annealed after having been cold worked. Most of the data which have been reported have pertained to only one aspect of annealing, that is, the conditions under which recrys-tallization has been observecl. The process of recovery which may precede recrystallization docs not appear to have been investigated, and it is the purpose of this note to report observations on the recovery process in cold-worked molybdenum as determined by changes ill the thermoelectric power which occur as the metal is annealed. The measurements were made on commercially pure molybdenum wires which, after annealing at 1035°C in dried hydrogen for 1 hr, were swaged to give a series of reductions up to a maximum of 84.5 pct. The variously cold-worked wires were then annea1c.d in dried hydrogen for periods of 16 hr at progrc:ssively highel. temperatures from 200" to 1000 c with a final annealing treatment of 70 hr at. 1100°C. After each annealing treatment the ther.ma1 emf's were determined against a length of original unwo~,ked material that had received the initial hydrogen anrit,aling treatment. The difference in temperature between the hot and cold junctions was maintained at about 75'C. and. the thermal emf's were measured with a potentiometer that could be read. to 0.01 microvolt. Fig. 1 shows the thermoelectric power. (dE/dT) in microvolts per C between annclalcd and cold-worked molybdenum as a function of the degree of cold work. The effect of annealing is shown in Fig. 2. The general shape of the curves in both figures is similar to results which have been obtained for copper." It was observed that after annealing at temperatures as low as 200°C recovery was well under way. The rapid changes irt thermoelectric power observed near 900 C are probably associated with recrystallization, since such a relation has been observed with copper. Beyond 900°C the thermal emf's changed in polarity. indicating that from this point or1 the specimens were more highly annealed than the standard. Annealing treatments could not be given at temperatures higher than 1100°C due to limitations of the furnace, and it is evident that, since equilibrium values of thermoelectric powers were far from having been attained, temperatures considerably higher are required to fully anneal molybdenum. References Robert M. Parke: Molybdenum, A New High-Temperature Material. Metal Progress (1951) 60, pp. 81-96. * R. A. Long, K. C. Dike, and H. R. Bear: Some Properties of High-Purity Sintered Wrought Molybdenum Metal at Temperatures up to 2400 °F. NACA TN 2319 (March 1951): G. W. Brindley: A Thern-io-Electric Study of the Cold-Rolling and Heat-Treatment of Copper, Report of a Conference on the Strength of Solids. Physical Society, London (1948) pp. 95-106.