Compressive Behavior of a Zr‐Based Metallic Glass at Cryogenic Temperatures

Abstract

Bulk metallic glasses (BMGs) possess a combination of many unique properties, including ultrahigh strength, superelasticity, high resistance to corrosion, and good fatigue characteristics. [1][2][3][4][5] Some of these materials exhibit superconductivity at low temperatures, [6] as well as show good magnetic properties. [7] Therefore, BMGs have a wide range of promising applications and have attracted considerable scientific and technological interest over the last few decades. [8][9][10][11][12][13] At present, studies of the mechanical properties of these materials in the literature are generally confined to mechanical tests at room temperature or above. [1][2][3][4][5][8][9][10][11][12][13] The mechanical behavior of BMGs at cryogenic temperatures is yet to be investigated in detail. The aim of this communication is to study the compressive behavior of a Zr-based metallic glass at 77 K. The amorphous state of the as-cast Zr 57.4 Cu 17.9 Ni 13.4 -Al 10.3 Nb 1 alloy was verified using X-ray diffraction and transmission electron microscopy (TEM). The compressive stressstrain curves for this alloy at 298 and 77 K are shown in Figure 1, and the compressive properties are summarized in Table 1. This material exhibits typical BMG features of high strength and low global plasticity. Interestingly, a comparison of curves A and B demonstrates that at the same strain rate of 2 × 10 -4 s -1 both the strength and plasticity increase at 77 K.Moreover, at 77 K, the strength increases upon increasing the strain rate, which is quite different from previously reported results that have suggested the absence of a strong strength dependence on the strain rate at room temperature. [5,14] Due to the absence of any lattice order, the dislocation-mediated deformation mechanism seen in crystalline materials is not applicable in BMGs. In the vicinity of the glass-transition temperature, T g , the deformation of the BMGs is homogeneous, while at temperatures lower than T g , the plastic deformation becomes inhomogeneous, with highly localized shear bands. [1,15] The material starts yielding at the onset of shearband formation. Although this shear localization has often been observed experimentally, the physical mechanisms related to the initiation of the shear band are still not well established. Simulation and modeling results suggest that the shearband nucleation is associated with an excess free volume and that it starts around the region of free-volume concentration with a scale of several atomic diameters. [15][16][17][18] In as-cast metallic glasses, there is plenty of non-equilibrium atomic-scale free volume, caused by the quenching process. [19] Such free volume can also be generated by the applied stresses, by squeezing atoms into spaces smaller than themselves.[20] Concentration of the free-volume zones can lead to a local increase in the inelastic strain and energy, as well as cause a concentration of the local shear stress. [16] Furthermore, changes in the free-volume concentration are determined by a diffusion-like equation...

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

“…Fig. 1a shows the tensile failure strength r f of Vitreloy 1 as a function of the environmental temperature T. The failure strength increases gradually with decreasing temperature from 300 to 77 K, which is consistent with the observations in other Zr-based metallic glasses [32][33][34][35]. However, with further decreasing temperature to 20 K, the failure strength is found not to increase any more, but starts to decrease instead.…”

Shear transformation zone volume determining ductile–brittle transition of bulk metallic glasses

“…13 225 at 77 K might also be ascribed to the decrease of the STZ volume due to low temperature, where nano-scaled "chevron" patterns rather than vein patterns on the low temperature fracture surface indicate an apparent decrease in the plasticity [7]. While for other BMGs [49][50][51][52][53][54][55] with both enhancements of yield strength and plasticity under compression when the test temperatures decrease from room temperature to liquid nitrogen temperature (77 K), a lower test temperature might introduce a brittle failure, i.e. ductile-to-brittle transition.…”

Temperature dependent dynamics transition of intermittent plastic flow in a metallic glass. II. Dynamics analysis

“…The yield strength and elastic energy (elastic modulus) resultantly increase, which is consistent with previous reports. 35,36 Simultaneously, increasing the STZ size significantly enhances the stress concentration around the STZs, 27 further enabling multiple shear band formation. As expected, the density of the shear bands increases with temperature.…”