Experimentally-based micromechanical modeling of dynamic response of molybdenum

Molybdenum (Mo), a bcc metal with a melting point of 2,610 degrees C and a density of 10.22 g/cm(3), is an important refractory metal. The refractory properties of molybdenum reflect the high strength of interatomic bonding r esulting from the overlap of the 4d-orbitals and the number of bonding elec trons available (1). The melting point of molybdenum is exceeded only by those of tungsten and t antalum, among the useful high-temperature metals. This makes molybdenum es sentially a "hot strength" material. Molybdenum is ductile at room temperat ure, with a brittle-ductile transition temperature significantly lower than that of tungsten. The density of molybdenum is approximately 62% of that o f tantalum, and is approximately one half of that of tungsten, making molyb denum a good candidate for applications where high-temperature capability, weight considerations, and ductility are key issues. Molybdenum also possesses much greater specific heat than either tantalum o r tungsten, making it easier to thermally treat molybdenum to produce struc tures with low thermal stresses than most other metals. Molybdenum is resis tant to most chemical reagents except for oxidizing acids. The relatively l ow thermal neutron cross section of molybdenum also makes it suitable for n uclear applications. The unique properties of molybdenum make it an ideal m aterial for high-temperature engineering applications. Since the 1960's, mo lybdenum has also been chosen by many researchers as an ideal material to e xamine the deformation behavior of bcc metals. These studies have, however, focused on low strain-rate regimes. The deformation characteristics of bcc metals differ markedly from those ex hibited by fee metals. For example: 1) the yield strength and flow stress of bcc metals increase rapidly with d ecreasing temperature and increasing strain rate, in contrast to the much m ore modest trends exhibited by fee metals, 2) the activation volume for the deformation of bcc metals is in the range of 5-50b(3) (b = magnitude of Burgers' vector of dislocations), whereas tha t for fed metals is often 10-100 times greater, 3) the increase in the yield point with a decrease in temperature is indepe ndent of the strain-hardened state in bcc metals, whereas it is greater for the higher strain-hardened states for fee metals. The study of the deformation behavior of molybdenum is expected to lead to a more detailed understanding of the deformation mechanisms of bcc metals, in particular the refractory metals. The focus of the present work has been on the high strain-rate, high strain response of molybdenum, over a broad range of temperatures. Before presenting our experimental and the correspon ding micromechanical modeling results, we briefly review some of the existi ng relevant contributions.