Enhancement of the fracture toughness of bulk L12-based (Al+12.5 at.% M)3Zr (M=Cu, Mn) intermetallics synthesized by mechanical alloying

The microstructural evolution and the mechanical properties of L12-type bulk (Al+12.5 at.% M)3Zr (M=Cu, Mn) intermetallic compounds with a nanocrystalline structure were investigated. The (Al+12.5 at.% M)3Zr (M=Cu, Mn) powders synthesized by planetary ball milling (PBM) could be successfully consolidated into nearly pore-free bulk compacts at 580 and 620 °C without taking holding time by spark plasma sintering (SPS). Their grain sizes were in the range from 8 to 10 nm. The micro-hardness of the SPS-processed bulks was measured to be 975.8 and 983.9 Hv, respectively. On the other hand, their fracture toughness was barely ∼2 MPam1/2. It was lower than those (∼4–6 MPam1/2) of the coarse-grained (∼100 nm) bulk specimens annealed. This result indicates that a grain refinement towards the nanoscale does not have an appreciable effect on improving fracture toughness in brittle intermetallics. Thus, it was found that the fracture toughness could be enhanced by proper annealing and addition of the boron. Furthermore, the effect of grain size on the fracture toughness in nano-sized level was investigated in the bulk specimen prepared by arc melting, using mechanical alloying powders with ball-milling.