Dislocation Reaction Mechanism for Enhanced Strain Hardening in Crystal Nano-Indentations

Abstract

Stress–strain calculations are presented for nano-indentations made in: (1) an ammonium perchlorate (AP), NH4ClO4, {210} crystal surface; (2) an α-iron (111) crystal surface; (3) a simulated test on an α-iron (100) crystal surface. In each case, the calculation of an exceptionally-enhanced plastic strain hardening, beyond that coming from the significant effect of small dislocation separations in the indentation deformation zone, is attributed to the formation of dislocation reaction obstacles hindering further dislocation movement. For the AP crystal, the exceptionally-high dislocation reaction-based strain hardening, relative to the elastic shear modulus, leads to (001) cleavage cracking in nano-, micro- and macro-indentations. For α-iron, the reaction of (a/2) <111> dislocations to form a [010] Burgers vector dislocation obstacles at designated {110} slip system intersections accounts for a higher strain hardening in both experimental and simulated nano-indentation test results. The α-iron stress–strain calculations are compared, both for the elastic deformation and plastic strain hardening of nano-indented (100) versus (111) crystal surfaces and include important observations derived from internally-tracked (a/2) <010> Burgers vector dislocation structures obtained in simulation studies. Additional comparisons are made between the α-iron calculations and other related strength properties reported either for bulk, micro-pillar, or additional simulated nano-crystal or heavily-drawn polycrystalline wire materials.

Metal Crystal/Polycrystal Plasticity and Strengths

“…Goel, Cross, Stukowski, Gamsjäger, Beake and Agrawal demonstrated that a[100] reacted dislocation line lengths formed at the earliest stages of deformation in simulations of nano-indentations made in tungsten crystals [33]. Armstrong and Elban have reported comparative plastic strain hardening behaviors in post-pop-in plastic deformation at nano-indents and in drawn wire and micro-pillar deformation tests [34].…”

Crystal Strengths at Micro- and Nano-Scale Dimensions

“…The test method was employed to probe the material anisotropic elastic deformation behavior through testing the diamond pyramid hardness of (100), (110), and (111) crystal surfaces. The research relates to reference [20] in which a comparison was made between the experiment and simulation of nano-indentations in (111) and (100) α-iron crystal surfaces. At an opposite dimensional scale, an important civil engineering connection spanning nano-to macro-size dimensions was provided by a comparison of the ultrafine steel wire measurements described for the drawn micro-wire strengths presented in Figure 2 and the comprehensive description by Ono [39] of steel wire materials employed in historical and current transportation bridge constructions.…”

“…Close examination of the deviation from the labeled d 2 dependence in Figure 1 shows that an increasing hardness applies for a smaller indentation size. Calculations of the stress-strain behaviors both at the onset of plastic yielding and follow-on nano-indentation strain hardening behaviors have been reported very recently for NaCl, MgO, and copper crystals [18], tungsten crystals [19], ammonium perchlorate, and α-iron crystals [20]. Beyond the well-established determination of very high flow stress levels for initial plastic yielding, whether gradual or of pop-in type, the plastic strain hardening behavior has also been shown to be exceptionally high.…”

Another perspective on elastic and plastic anisotropy of textured metals

“…The parameter μ φ is the atomistically resolved friction coefficient. The coefficient quantifies the slip non-planarity caused by screw-dislocations, cross-slip, dislocation bowing and other mechanisms [21–23]. Among the common features encapsulated in μ φ are the existence of non-glide shear stress and the break-down of Schmid's Law.…”