Effect of alloying elements on the microstructure and mechanical properties of nanostructured ferritic steels produced by spark plasma sintering

Several Fe-14Cr based alloys with varying compositions were processed using a combined route of mechanical alloying and spark plasma sintering. Microstructural characteristics of the consolidated alloys were examined via transmission electron microscopy and atom probe tomography, and mechanical properties evaluated using microhardness testing. Lanthanum oxide (0.5 wt.%) was added to Fe-14Cr leading to improvement in microstructural stability and mechanical properties mainly due to a high number density of La-Cr-O-enriched nanoclusters. The combined addition of La, Ti (1 wt.%) and Mo (0.3 wt.%) to the Fe-14Cr base composition further enhanced the microstructural stability and mechanical properties. Nanoclusters enriched in Cr-Ti-La-O with a number density of 1.4 × 1024 m−3 were found in this alloy with a bimodal grain size distribution. After adding Y2O3 (0.3 wt.%) along with Ti and Mo to the Fe-14Cr matrix, a high number density (1.5 × 1024 m−3) of Cr-Ti-Y-O-enriched NCs was also detected. Formation mechanism of these nanoclusters can be explained through the concentrations and diffusion rates of the initial oxide species formed during the milling process and initial stages of sintering as well as the thermodynamic nucleation barrier and their enthalpy of formation.