On the recrystallization behavior of technically pure molybdenum

In the last decades the amount of interstitial impurities in the raw material used for powder metallurgical production of molybdenum has been reduced significantly. The quality of the production process has been adapted to the latest technological standards and, consequently, the properties of technically pure molybdenum have changed accordingly. For processing pure molybdenum and to predict the resulting mechanical properties, the recrystallization behavior, which strongly depends on the concentration of the prevailing impurities, e.g. carbon, nitrogen and oxygen, is decisive. A reliable recrystallization diagram of molybdenum was published in 1965, which does not accurately describe the recrystallization behavior of the current quality of technically pure molybdenum. Therefore, in the present investigation a diagram which reflects the static recrystallization behavior of molybdenum containing low concentrations of interstitial impurities was established. The recrystallization behavior has been monitored on differently deformed and subsequently heat-treated samples by means of hardness testing, light microscopy and scanning electron microscopy employing electron channeling contrast imaging (ECCI) and electron back scatter diffraction (EBSD). Especially ECCI and EBSD investigations offer the possibility to analyze the change of microstructure with regard to recovery and recrystallization effects, e.g. the evolution of subgrains. The quantity of the impurities was determined by standard chemical analysis methods. As a bcc metal, molybdenum exhibits a high stacking fault energy. Thus, the recrystallization behavior is strongly dominated by concurrent recovery processes, which deviate from that of fcc metals showing a comparably low stacking fault energy. A revised recrystallization diagram for technically pure powder metallurgically processed molybdenum is presented.