Cryogenic-temperature-induced transition from shear to dilatational failure in metallic glasses

Direct atomic-scale evidence for shear–dilatation correlation in metallic glasses

“…For the present 'ductile' CuZr glass with smaller G/B ratio, the correlation is relatively weaker which indicates that shear could be the dominating deformation mode [35]. While it is most likely to find brittle behaviors in stronger shear-dilatation correlation glass (with bigger G/B ratio) since the great propensity to nucleate cavitation (or TTZ) via shear-induced dilatation, which has been proposed as a brittle fracture mechanism in MGs [33,35,31,18,17,34]. as demonstrated by Fig.…”

“…So that local structural dilatation [19,30,31], density change [27,28], and even nanovoids [32,33,21,34] have been observed within the shear bands of MGs. Although the concept of shear-dilatation correlation is widely accepted in the glass community, and various experiments and simulations have indicated STZ and cavitation as important deformation and fracture mechanisms of glassy alloys [33,35,31,18,17,34], there still lacks a direct microscopic evidence and no quantitative relationship established for such an intimate correlation. Whereas a direct observation and derivation of such a quantitative correlation [19] down to atomic-scale is usually an extreme challenging task in experiments [36].…”

“…Several models have been proposed to rationalize the inelastic deformation of MGs, which include free volume [6], shear transformation zone (STZ) [7][8][9], cooperative shear model (CSM) [10,11], flow unit [12], vibrational soft spot [13], shear-diffusive transformation [14,15], and our recently proposed tensile transformation zone (TTZ) [16][17][18]. All of these models are based on the local structural rearrangement of atoms via the interplay of atomic-scale shear and dilation/contraction in MGs [19][20][21][22][23][24].…”

Wavelike fracture pattern in a metallic glass: a Kelvin–Helmholtz flow instability

“…Uniaxial tension tests were performed with an Instron material testing machine under a strain rate of 10 −4 s −1 at room temperature (300 K), 221 K, 152 K and liquid nitrogen temperature (77 K). At all temperatures, the Vitreloy 1 glass displays an elastic-brittle failure without appreciable macroscopical plasticity [15]. The fracture angles fall in the range of 55°-61°, demonstrating that the fracture is dominated by shear stress but affected by tensile normal stress [16][17][18][19].…”

“…We believe that wavelike flow patterns could be observed in other situations of metallic glasses [43], so long as the underlying K-H instability mechanism becomes activated. Finally, it must be pointed out that the compressive stress on the shear plane usually restrains the free volume creation [44] or cavitation [15,18], and the resulting nucleation of local sub-cracks. We, therefore, expect that the wavelike fracture pattern can be rarely observed in metallic glasses under uniaxial compressions.…”