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 resulting from the overlap of the 4d-orbitals and the number of bonding electrons available (1). The melting point of molybdenum is exceeded only by those of tungsten and tantalum, among the useful high-temperature metals. This makes molybdenum essentially a "hot strength" material. Molybdenum is ductile at room temperature, with a brittle-ductile transition temperature significantly lower than that of tungsten. The density of molybdenum is approximately 62% of that of tantalum, and is approximately one half of that of tungsten, making molybdenum 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 or tungsten, making it easier to thermally treat molybdenum to produce structures with low thermal stresses than most other metals. Molybdenum is resistant to most chemical reagents except for oxidizing acids. The relatively low thermal neutron cross section of molybdenum also makes it suitable for nuclear applications. The unique properties of molybdenum make it an ideal material for high-temperature engineering applications. Since the 1960's, molybdenum has also been chosen by many researchers as an ideal material to examine 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 exhibited by fee metals. For example: 1) the yield strength and flow stress of bcc metals increase rapidly with decreasing temperature and increasing strain rate, in contrast to the much more 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 that for fed metals is often 10-100 times greater, 3) the increase in the yield point with a decrease in temperature is independent 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 corresponding micromechanical modeling results, we briefly review some of the existing relevant contributions.