Size-dependent failure of the strongest bulk metallic glass

Upon reducing the sample size into micrometer scale, an obvious brittle-to-ductile transition accompanied by a drastic change of failure mode from shattering to shear-banding was observed when compressing the brittle but strong Co55Ta10B35 bulk metallic glass (BMG). The shattering failure under macroscopic compression is dominated by splitting cracking, which completely differs from shear banding and originates from extrinsic defects like inclusions. To reveal the critical conditions for shear-banding and splitting cracking, various micropillar specimens with intentionally introduced holes as extrinsic defects were tested, and the stress distributions at the failure moment were analyzed with finite element simulation. The shear plane criterion was found to be quite effective to estimate the nominal stress required for the failure dominated by shear-banding. However, brittle splitting cracking does not occur although the maximum tensile stress reaches the critical value, which is different from traditional brittle solids. To initiate splitting cracking, a high-tensile-stress region over a critical distance, which depends on defect size and fracture toughness of the BMG, is required. The critical conditions for shear failure and splitting cracking demonstrated in this approach can be used to estimate the failure conditions of various BMG components with complex geometries in a wide range of length scales, and to design tough composites based on brittle BMGs. As an example, a design criterion to avoid brittle splitting fracture of porous BMG materials is proposed. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.