Improving Plasticity and Toughness of Additive Manufactured Zr-Based Bulk Metallic Glass Composites by Martensite Phase Transformation

Additive manufacturing based on laser powder bed fusion (LPBF) technique offers a novel approach for fabricating bulk metallic glass (BMG) products without restriction in size and geometry. Nevertheless, the BMGs prepared by LPBF usually suffered from less plasticity and poorer fracture toughness as compared to their cast counterparts due to partial crystallization in heat-affected zones (HAZs). Since crystallization in HAZs is hard to avoid completely in LPBF BMGs, it is desirable to design a suitable alloy system, in which only ductile crystalline phase, instead of brittle intermetallics, is formed in HAZs. This unique structure could effectively improve the toughness/plasticity of the LPBF BMGs. To achieve this goal, a BMG system with a composition of Zr_47.5Cu_45.5Al₅Co₂ is adopted and subjected to LPBF. It is observed that ductile B2-ZrCu phase precipitates in HAZs, while a fully amorphous phase formes in molten pools (MPs), which alternatively distribute in the whole LPBF samples. This B2 phase reinforced BMG composite exhibits excellent mechanical properties with enhanced plasticity and toughness. Importantly, it is easy to modulate the mechanical properties by altering the amount of the B2 phase via adjusting the laser energy input. Finally, the best combination of strength, plasticity, and notch toughness is obtained in the BMG composite containing 27.4% B2 phase and 72.6% amorphous phase, which exhibits yield strength (σ_s) of 1423 MPa, plastic strain (ε_p) of 4.65%, and notch toughness (K_q) of 53.9 MPa m^1/2. The detailed microstructural investigations reveals that the improvement of plasticity/toughness are mainly due to the martensite phase transformation from the B2 phase to the Cm phase during plastic deformation (i.e.,TRIP effect). The current work provides a guide for making advanced BMGs and BMG composites by additive manufacturing.