The upper limit of strength (the "ideal strength") has been an active subject of research and speculation for the better part of a century. The subject has recently become important, for two reasons. First, given recent advances in ab initio techniques and computing machines, the limits of strength can be calculated with considerable accuracy, making this one of the very few problems in mechanical behavior that can actually be solved. Second, given recent advances in materials engineering, the limits of strength are being approached in some systems, such as hardened or defect-free films, and their relevance is becoming recognized in others, including hard coatings, carbonitrides and diamond-cubic crystals. An elastically strained solid is always at least metastable. Given a kinetically plausible pathway, it will spontaneously transform into a sheared or brooked replica of itself or into a new phase entirely. In that sense, plastic deformation is a structural phase transformation whose onset is governed by the usual criteria. It can be nucleated (and ordinarily is) but, failing that, must commence at the limit of stability of the elastic state. This thermodynamic instability sets the upper limit of strength. The present paper defines the limits of elastic stability (which are surprisingly subtle), reviews ab initio computations for a number of metals and compounds, shows how those limits reflect the symmetry of the strained lattice, and discusses the experimental situations in which they are known or expected to be important.
展开▼
