Mechanical properties and tempering resistance of an ultrafine grained Tool Steel-PSZ composite fabricated by high energy mechanical milling and spark plasma sintering

The mechanical properties and tempering resistance of a powder metallurgical (PM) tool steel (AISI H13) reinforced with partially stabilized zirconia (PSZ) were investigated. The choice of particles reinforcement was made in view to the well-known phase transformation toughening effect in PSZ. Composite powders were produced by high energy mechanical milling and subsequently consolidated by spark plasma sintering (SPS). An ultrafine grain microstructure was achieved showing a homogenous distribution of refined PSZ particles within the severely deformed matrix. Thanks to the fast SPS consolidation, a small amount (less than 2 vol%) of reaction products were formed. The hardness of the composites was significantly higher than the unreinforced counterpart, due to the microstructural refinement, increased dislocation density, and dispersion hardening. Tempering resistance at 550 °C and 650 °C was also significantly improved. The hot compressive yield strength of the composites measured 650 °C and 450 °C was increased up to 1.8 times compared to unreinforced steel. PSZ phase transformation during hot compression contributed to increased work hardening. Fracture toughness was significantly decreased compared to the base material. However, compared to a TiC reinforced H13 with the same hardness, a marginal increase in apparent fracture toughness was observed.