Preparation of tungsten-particle-reinforced Zr-based bulk metallic glass composites by two-step spark plasma sintering: microstructure evolution, densification mechanism and mechanical properties

A new two-step spark plasma sintering (TSS) process with low-temperature pre-sintering and high-temperature final sintering has been successfully applied to prepare the tungsten-particle (Wp)-reinforced bulk metallic glass composites (Wp/BMGCs). Compared to normal spark plasma sintering (NS), the densification rate and relative density of Wp/BMGCs can be improved by selecting TSS with appropriate sintering pressure in the low temperature pre-sintering stage. However, the compressive strength and plastic strain of 30%Wp/BMGCs prepared by TSS are both higher than those of the samples prepared by NS. The TSS process can significantly enhance the compressive strength of 30%Wp/BMGCs by 12% and remarkably increase the plastic strain by 50%, while the trend is completely opposite for 50%Wp/BMGCs. Quasi-in situ experiments and finite element simulations reveal that uneven temperature distribution among particles during low-temperature pre-sintering causes local overheating at contact points between particles, accelerating formation of sintering neck between particles and plastic deformation of Wp. When the volume fraction of Wp is low, TSS can improve the interface bonding between particles by increasing the number of sintering necks. This makes the fracture mode of Wp/BMGCs being predominantly transgranular fracture. However, as the volume fraction of Wp increases, the adverse effects of Wp plastic deformation are becoming more and more prominent. The aggregated Wp tends to form a solid "cage structure" that hinders the bonding between particles at the interface; correspondingly, the fracture behavior of Wp/BMGCs is mainly dominated by intergranular fracture. Additionally, reducing the sintering pressure during the low-temperature pre-sintering stage of TSS has been shown to effectively decrease plastic deformation in Wp, resulting in a higher degree of densification and better mechanical properties.