Toughening additive manufactured Zr-based bulk metallic glass composites by martensite phase transformation

Additive manufacturing technology based on laser powder bed fusion (LPBF) 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 increase the toughness/plasticity of the LPBF BMGs. To achieve this goal, a quaternary BMG system with a composition of Zr47.5Cu45.5Al5Co2 is adopted and subjected to LPBF. It is found that nearly a single phase of B2-ZrCu is precipitated in HAZs, while a fully amorphous phase is formed in molten pools (MPs). This B2 phase reinforced BMG composite exhibits excellent mechanical properties with enhanced plasticity and toughness. Furthermore, 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 (σs) of 1423 MPa, plastic strain (εp) of 4.65%, and notch toughness (Kq) of 53.9 MPa m1/2. Furthermore, a notable strain-hardening is also observed. The improvement of plasticity/toughness and appearance of strain-hardening behavior are mainly due to the martensite phase transformation from the B2 phase to the Cm phase during plastic deformation (i.e., the phase transformation-induced plasticity effect). The current work provides a guide for making advanced BMGs and BMG composites by additive manufacturing.