Cr-Al Spinel phase formation in alumina dispersed 316 L stainless steel processed by spark plasma sintering

Phase transformation of oxide phase in oxide dispersion strengthened (ODS) 316 L stainless steel alloys was observed during spark plasma sintering (SPS).The composites were prepared with two different compositions of 0.33 wt% Al₂O₃ and 1wt% Al₂O₃. The alumina particles were located at grain boundaries mixed with micrometer sized steel debris from milling after attrition milling. The alumina particles transformed to a Cr-Al spinel phase dominantly with Cr rich composition surrounded by an amorphous silica phase during SPS process in both sintered composites. Both Cr component of Cr-Al spinel phase and Si in silica could diffuse from the 316 L steel during the spark plasma sintering process. The lattice parameter of the spinel phase is 8.36Å independent of the local cation composition variation. The lattice parameter of the spinel phase is relatively large among synthetic Cr-Al spinels which implies that octahedral sites of spinel structure are mainly occupied by Cr³⁺ cations replacing a portion of Al. The finding that the transformation occurs in presence of amorphous silica is consistent with literature describing both geological occurrence of chromite and phases with spinel structure in annealed glass composites in the presence of silica phase. The phase transition may be also promoted by local temperature increase at the grain boundaries of steel during the spark plasma sintering.