Nanoscale strength distribution in amorphous versus crystalline metals

Abstract

Low-load nanoindentation can be used to assess not only the plastic yield point, but the distribution of yield points in a material. This paper reviews measurements of the socalled nanoscale strength distribution (NSD) on two classes of materials: crystals and metallic glasses. In each case, the yield point has a significant spread (10-50% of the mean normalized stress), but the origins of the distribution are shown to be very different in the two materials classes. In crystalline materials the NSD can arise from thermal fluctuations and is attended by significant rate and temperature dependence. In metallic glasses well below their glass-transition temperature, the NSD is reflective of fluctuations in the sampled structure and is not very sensitive to rate or temperature. Computer simulations using shear transformation zone dynamics are used to separate the effects of thermal and structural fluctuations in metallic glasses, and support the latter as dominating the NSD of those materials at low temperatures. Finally, the role of the NSD as a window on structural changes due to annealing or prior deformation is discussed as a direction for future research on metallic glasses in particular.

Fatigue endurance limit and crack growth behavior of a high-toughness Zr61Ti2Cu25Al12 bulk metallic glass

“…With nanoindentation testing, Schuh et al [57][58][59][60] found hardening of BMG, rather than softening, via cyclic loading in the elastic range. Sun et al [61] also presented a Bauschinger-type effect when BMGs were loaded under pure shear stress reversal in the elastic range.…”

“…Secondly, the triaxial stress state ahead of the crack tip is completely different from that under the indenter [57][58][59][60] and torsional loading [61]: tensile mean stresses dominate ahead of the crack tip, while compressive mean stresses surround the indent. As indicated by Flores et al [63,64], the former stress state increases, while the latter decreases, the free volume in BMGs.…”

Rejuvenation of metallic glasses by non-affine thermal strain

“…Strongly and weakly bonded regions have been postulated to account for ,5 nm zones of accelerated crystallization 9 . The initial yield load (in nanoindentation) shows a dispersion of values associated with structural heterogeneity 10 . Mapping of heterogeneity by atomic-force microscopy, over areas much larger than in simulation, shows variations of energy dissipation 11 and of elastic modulus (,30%) 12 with correlation lengths of 2.5-20 nm.…”

Cavitation in brittle metallic glasses – Effects of stress state and distributed weak zones

“…As mentioned earlier, the distribution of weak zones mimics the presence of intrinsic nanoscale density fluctuations in brittle metallic glasses (Packard et al, 2010;Wagner et al, 2011;Murali et al, 2011).…”

“…They traced the origin of the above cavitation behavior in the brittle MG to the presence of strong atomic density fluctuations which gives rise to distributed weak zones. In this connection, it must be mentioned that Packard et al (2010) have reported from their low load nanoindentation experiments on MGs the presence of significant spread of 10-50% in nanoscale strength distribution, which has been attributed to fluctuation in their sampled structure. Also, Wagner et al (2011) measured the indentation modulus on the nanometer scale in Pd-based MGs using atomic force acoustic microscopy and found around 30% http://dx.doi.org/10.1016/j.ijsolstr.2014.09.005 0020-7683/Ó 2014 Elsevier Ltd. All rights reserved.…”