Cooling-rate induced softening in a Zr50Cu50 bulk metallic glass

Contrary to the cooling-rate induced hardening observed in crystalline metals, the authors report here an unexpected surface softening in rapidly solidified Zr50Cu50 bulk metallic glass. A soft layer ∼500μm thick was detected near the surface with both hardness and elastic modulus increasing from the surface to the interior. To understand the reason for this, a correlation between cooling rate and defect concentration was derived. Defect concentration was found to increase as the cooling rate increased, suggesting that surface softening may be the result of freezing-in of excess defects, induced by a faster cooling rate near the surface compared to the interior.