Plastic flow between Bridgman anvils under high pressures

The shear strength of materials after extensive plastic flow under a superimposed high hydrostatic stress component is most conveniently studied by means of Bridgman opposed anvils between which thin disk-shaped samples are sheared through relative rotation. A newly designed apparatus of this type permits, for the first time ever, the monitoring of sample thickness during shearing and thus the obtaining of averaged shear stress/shear strain curves. Such relations are needed for the better understanding of geological processes, behavior under shock or explosive impact, and of the surface layers during friction and wear, among others. A semiquantitative analysis shows that nonuniform pressure distribution in the samples cannot significantly falsify the results. It is concluded that distributed shearing is always accompanied by sample thinning and that, conversely, slippage between anvils and specimens is indicated whenever the sample stops thinning during rotation. Such slippage can be greatly reduced by raising the friction coefficient through etching of samples and/or anvils, but it apparently occurred undiscovered in previous studies, partly after initial distributed shearing. Further, in previous results slippage between directly contacting anvils was frequently mistaken for sample shearing. Additionally, the previously neglected sample material being extruded during shearing can falsify results, as can “turbulent flow” initiated at sample perforation. Correspondingly all prior data gained with Bridgman apparatuses are suspect. Present best results indicate (i) that in metals ordinary dislocation glide but apparently with strongly increased Peierls–Nabarro stresses continues to the highest pressures studied, (ii) that independent of thermal activation workhardening may cease at high strains, and (iii) that “turbulent flow” resulting through sample perforation or when the sample thickness decreases below a critical value, can give rise to mechanical alloying. The majority if not all of the data by Bridgman as well as Vereshchagin et al. probably involved such mechanical alloying. When plotted versus pressure in units of shear moduli, the apparent coefficients of friction of the five cubic metals examined so far follow nearly the same curve but they are lower for metals of lower symmetry.