Static Recrystallization of Molybdenum After Deformation Below 0.5* T M (K)
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I.
INTRODUCTION
DUE to the high melting point of 2893 K (2620 C), 97 pct of the worldwide production of technically pure molybdenum and its alloys is carried out via the powdermetallurgical (PM) route followed by thermomechanical processing.[1,2] The main fields of application of molybdenum besides high-temperature parts are electronics and coating technology. Prior to thermomechanical processing, the molybdenum powder is compacted by cold-isostatic pressing and sintered above 2073 K (1800 C). Molybdenum sinter parts exhibit a residual microporosity of approximately 5 pct, which is eliminated by deformation of at least 60 pct. Due to the limited knowledge about the microstructural and textural behavior of PM molybdenum during thermomechanical processing,[2] detailed studies employing state-of-the-art methods of investigation such as electron backscatter diffraction (EBSD)[3,4] are required. Previous studies[5,6] focused on hot-deformation and SOPHIE PRIMIG, Ph.D. Student, and HARALD LEITNER, Senior Scientist, are with the Christian Doppler Laboratory for Early Stages of Precipitation, Montanuniversita¨t Leoben, Leoben 8700, Austria, and also with the Department of Physical Metallurgy and Materials Testing, Montanuniversita¨t Leoben. Contact e-mail: sophie. [email protected] WOLFRAM KNABL and ALEXANDER LORICH, Employees, are with PLANSEE SE, Reutte 6600, Austria. ROLAND STICKLER, Emeritus Professor, is with the University of Vienna, Vienna 1090, Austria. Manuscript submitted January 23, 2012. Article published online July 10, 2012 4806—VOLUME 43A, DECEMBER 2012
subsequent static recrystallization of PM molybdenum. These studies revealed a recovery-controlled hotdeformation behavior due to the high stacking fault energy[7] and the absence of classic discontinuous dynamic recrystallization.[5,6,8,9] This is the reason for the rapid textural evolution during hot deformation, which yields two main components, i.e., h111i parallel to the loading direction (which will be termed h111i in the following) exhibiting a high Taylor factor and h001i parallel to the loading direction (which will be termed h001i in the following) with a low Taylor factor.[6] The fraction of the h001i component is further increased by bulging (i.e., strain induced boundary migration)[6,8–10] of h001i nuclei into neighboring h111i regions after a true strain of approximately u = 0.5 at 0.54*TM (K). Furthermore, subsequent static recrystallization occurs mainly by nucleation of orientation components exhibiting a low stored energy,[6,11,12] which additionally enhances the intensity of the h001i component. This happens continuously without a period of incubation since recovered subgrains generated dynamically during hot deformation act as nuclei. Apparently, the conditions for a successful nucleus (size and misorientation advantage as well as direction of growth)[8,9] are more frequently fulfilled by the h001i subgrains, which are on average the largest ones.[6,13,14] This soft component exhibits also a larger fraction of deformation-induced, m
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