Abstract
As-cast bulk metallic glasses are isotropic, but anisotropy can be induced by thermomechanical treatments. For example, the diffraction halo in the structure function S(Q) observed in transmission becomes elliptical (rather than circular) after creep in uniaxial tension or compression. Published studies associate this with frozen-in anelastic strain and bond-orientational anisotropy. Results so far are inconsistent on whether viscoplastic flow of metallic glasses can induce anisotropy. Preliminary diffraction data suggest that the anisotropy, if any, is very low, while measurements of the elastic properties suggest that there is induced anisotropy, opposite in sign to that due to anelastic strain. We study three bulk metallic glasses, Ce65Al10Cu20Co5, La55Ni10Al35, and Pd40Ni30Cu10P20. By using resonant ultrasound spectroscopy to determine the full elasticity tensor, the effects of relaxation and rejuvenation can be reliably separated from uniaxial anisotropy (of either sign). The effects of viscoplastic flow in tension are reported for the first time. We find that viscoplastic flow of bulk metallic glasses, particularly in tension, can induce significant anisotropy that is distinct from that associated with frozen-in anelastic strain. The conditions for inducing such anisotropy are explored in terms of the Weissenberg number (ratio of relaxation times for primary relaxation and for shear strain rate). There is a clear need for further work to characterize the structural origins of flow-induced anisotropy and to explore the prospects for improved mechanical and other properties through induced anisotropy.
Highlights
A glass is commonly expected to be isotropic, but can be anisotropic, even when formed from a liquid composed of atoms or of molecules that are not orientable
In contrast with ribbons and thin films, bulk metallic glasses (BMGs) are expected to be much closer to isotropy in their as-cast state, and this has been confirmed by Resonant ultrasound spectroscopy (RUS) measurement of the full elasticity tensor [13]
This study has focused on the effects of thermomechanical treatments on three bulk metallic glasses (BMGs), Cebased, La-based and Pd-based
Summary
A glass is commonly expected to be isotropic, but can be anisotropic, even when formed from a liquid composed of atoms or of molecules that are not orientable. Small-angle neutron scattering suggests aligned defects possibly connected with the shearing [4,5] Both X-ray and neutron scattering show that the structure function S(Q) is different in different directions in ribbons [6,7]. Resonant ultrasound spectroscopy (RUS) suggests that the ribbon elasticity is (within ~3%) isotropic in plane, but that the stiffness coefficient C33 normal to the ribbon is much greater than the coefficients C11 and C22 in the plane [10] This greater difference normal to the ribbon is consistent with structural studies [7]. In contrast with ribbons and thin films, bulk metallic glasses (BMGs) are expected to be much closer to isotropy in their as-cast state, and this has been confirmed by RUS measurement of the full elasticity tensor [13]
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