Tensile properties of Zr70Ni16Cu6Al8 BMG at room and cryogenic temperatures

Abstract

The mechanical behaviour in tension of a hypoeutectic Zr 70 Ni 16 Cu 6 Al 8 Bulk Metallic Glass (BMG) was studied at room (295K) and cryogenic temperatures (150 K and 77K) using various strain rates between 10 -4 and 10 -1 s -1 . The yield strength was found to increase at lower temperatures with average values increasing by 16 %, from 1503 MPa at 295 K to 1746 MPa at 77 K. The Zr-based BMG was found to exhibit tensile plastic elongation of about 0.4% before fracture at room temperature and high strain rates (10 -1 s -1 ). Even higher tensile plasticity was recorded at low temperatures; plastic deformation was found highest at the intermediate temperature (150K) reaching remarkable plastic strains in the order of 3.9 %, while values up to 1.5% were recorded at 77 K. The lateral surface of the tensile specimens was observed in-situ during deformation using a high frame rate camera offering interesting insights with regard to the deformation mechanisms. Room temperature plasticity occurred through the formation and interaction of several nucleated shear bands before critical failure, while at intermediate and liquid nitrogen temperatures, most of the plastic deformation was accommodated through stable flow within a single shear band.

Probing heat generation during tensile plastic deformation of a bulk metallic glass at cryogenic temperature

“…3a,b ), indicate that the plastic deformation was mainly accommodated along the primary shear band. This high degree of localization is in accordance with previous reports for cryogenic tensile testing of BMG 10 , 12 , 13 , 32 , and will be discussed in further detail later.

Figure 2 Plastic part of engineering stress-strain relationship (blue line) overlaid with corresponding cumulative energy expenditure for gas evaporation (dashed red line).

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“…A crack in the surface, marked by white arrows in ( a , c ), indicates that sliding first happened along the primary shear band, before final fracture occurred along the intersection of the primary and a secondary shear band. The dark particles on the surface, primarily visible in ( a , b ), are remainders of tracer markings used for image analysis 13 . …”

“…With viscosity and temperature being intimately related, and considering these shear bands are high-friction environments, the role of temperature and heating in the failure of metallic glasses has received a fair amount of attention. Significant work has focused on the effect of ambient temperature, since cryogenic temperatures have an apparent stabilizing effect on plastic deformation 10 – 13 , as well as on the heating of the shear band itself 7 , 14 – 20 . Various methods applied to disentangle structural and caloric effects include coating the metallic glass sample with fusible coatings with known melting points 15 , 16 , high frequency infrared thermography 21 , 22 , back-calculations from estimated viscosity in shear bands 23 and theoretical approaches based on release of elastically stored energy in the sample 19 , 24 and heat release from shear deformation 25 .…”

“…Testing and application of metallic glasses at cryogenic temperatures has some important benefits. Strength and plasticity have both been found to increase 11 – 13 . Further, some groups report the concentration of plastic deformation into few, or even single, shear bands, as opposed to the normally expected multitude 10 , 13 .…”

“…Strength and plasticity have both been found to increase 11 – 13 . Further, some groups report the concentration of plastic deformation into few, or even single, shear bands, as opposed to the normally expected multitude 10 , 13 . The combination of increased plasticity and localization of deformation significantly facilitate the study of shear band dynamics at reduced temperatures.…”