Effect of Zr Composition and Heat-Treatment on Fracture Toughness in Zr-Cu-Al Bulk Metallic Glass

Fracture toughness tests were carried out using different Zr composition in Zr-Cu-Al based bulk metallic glasses (BMGs) and heat-treated those just under the glass transition temperature (Tg) in order to examine the effects of Zr composition and heat-treatment on the fracture toughness (KQ; fracture toughness including the values under unsatisfying the plane strain condition). Zr50Cu40Al10, Zr55Cu35Al10 and Zr60Cu30Al10 BMGs fabricated by a tilt ark casting method were used. Heat-treatments were done for 90 minutes just under the Tgs of each BMG. Tests were conducted based on ASTM E399. Compact type specimen was used (thickness=2.3, 2.0 mm, thickness : width=1 : 4). Fatigue pre-crack was induced under ΔK less than 6 MPa•m1/2 by a servohydraulic fatigue machine and the fracture toughness test was done by using the same machine. The KQ dominantly increased with increasing the Zr composition. The KQ in heat-treated BMGs decreased in comparison with that in unheat-treated BMGs in all Zr composition. In the BMG of the high Zr composition, however, the decrease was small after heat-treatment, and the KQ showed enough high values. In heat-treated BMGs, the dispersion of KQs did not become small, but fatigue pre-cracks were induced more vertical against the loading axis and less bifurcation than those in unheat-treated BMGs.