Effects of consolidation temperature, strength and microstructure on fracture toughness of nanostructured ferritic alloys

Fully consolidated nanostructured ferritic alloys (NFAs) were prepared by attritor milling pre-alloyed Fe–14Cr–3W–0.4Ti and 0.3 wt% Y2O3 powders, followed by hot isostatic pressing (HIPing) at 1000 °C or 1150 °C at 200 MPa for 4 h. Transmission electron microscopy (TEM) revealed similar bimodal distributions of fine and coarse ferrite grains in both cases. However, as expected, the alloy microhardness decreased with increasing in HIPing temperature. Three point bend tests on single edge notched specimens, with a nominal root radius ρ = 0.15 mm, were used to measure the notch fracture toughness, Kρ, as a function of test temperature. The Kρ curves were found to be similar for both processing conditions. It appears that the coarser ferrite grains control cleavage fracture, in a way that is independent of alloy strength and HIPing temperature.