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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 757; pp. 95 - 100
• Main Authors: Armstrong, Ronald W., Elban, Wayne L.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 29.05.2019; Elsevier BV
• Subjects: Ball tests; Compression tests; Crystal surfaces; Deformation; Dislocations; Hardness; Hardness tests; Indentation; Indenters; Magnesium oxide; Micromechanics; Plasticity; Sessile dislocations; Size effects; Softening; Strain hardening; Stress/strain calculations; Tensile tests; Micromechanics; Hardness; Stress/strain calculations; Strain hardening; Plasticity; Dislocations
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2019.04.090

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.