The discussion centers on the comparative ductility of face-centered cubic (fcc) and body-centered cubic (bcc) crystal structures, exploring the reasons behind the observed differences in ductility despite the greater number of slip planes in bcc structures. The conversation includes theoretical considerations, material properties, and the influence of alloying elements.
Participants express differing views on the primary factors influencing ductility, with some emphasizing the importance of crystal structure while others highlight the role of alloying elements. The discussion remains unresolved regarding the relative significance of these factors.
Participants reference various materials and conditions that may affect ductility, indicating that the discussion is influenced by specific examples and contexts, which may limit the generalizability of claims made.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/solidstate.htmCrystalline structure is important because it contributes to the properties of a material. For example, it is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore, lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Additionally, cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because their symmetry provides closely packed planes in several directions. A face-centered cubic crystal structure will exhibit more ductility (deform more readily under load before breaking) than a body-centered cubic structure. The bcc lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten have the bcc form. The fcc lattice is both cubic and closely packed and forms more ductile materials. Gamma-iron, silver, gold, and lead have fcc structures. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the fcc metals.
http://www.exo.net/~jillj/activities/mechanical.pdfIn fcc metals, the flow stress, i.e. the force required to move dislocations, is not strongly
temperature dependent. Therefore, dislocation movement remains high even at low
temperatures and the material remains relatively ductile.
In contrast to fcc metal crystals, the yield stress or critical resolved shear stress of bcc
single crystals is markedly temperature dependent, in particular at low temperatures. The
temperature sensitivity of the yield stress of bcc crystals has been attributed to the
presence of interstitial impurities on the one hand, and to a temperature dependent
Peierls-Nabarro force on the other. However, the crack propagation stress is relatively
independent of temperature. Thus the mode of failure changes from plastic flow at high
temperature to brittle fracture at low temperature.
P and S are generally considered impurities in most alloys, particularly structural materials. Both can increase notch sensitivity, or conversely reduce fracture toughness, particularly at cold temperatures.Enthalpy said:Hence my claim that essentially the alloying elements (C, P, S...) determine ductility.