Exceptional crystal strain hardening determined over macro- to micro- to nano-size scales in continuous spherical indentation tests

摘要

Calculations of an order of magnitude greater strain hardening coefficient over compression or tensile test measurements are demonstrated for continuous indentation hardness measurements past "pop-in". Analyses were performed at small indentation strains for a macro-ball test on a NaCl crystal and at larger strains measured for rounded points of micro- and nano-tipped indenters in tests of MgO and copper crystal surfaces. The exceptional strain hardening is attributed to the smaller spacing and consequent interactions of the plastically-induced dislocations, including for MgO, the formation of nano-scale sessile dislocations accompanying the imposed three-dimensional deformation. The dislocation-based hardening is much greater than the smaller so-called "Indentation Size Effect (ISE)" of softening obtained at larger, constant strain, penetration depths with Berkovich-type indenters. Such ISE softening is attributed rather to the reverse effect of increasingly larger dislocation separations accompanying the greater plastic indentation depths.