Evaluating grain size, dendritic scale, and tensile properties of a NbB-inoculated 6201 alloy using solidification rate

Previous studies investigated the ability of Nb-B inoculants to refine grain in Al alloys; however, their dendritic network was not considered. In the present study, the synergistic effect of 0.5Nb-0.1B wt.% on the dendritic microstructure scale and grain size (GS) of a 6201-base alloy with an extensive range of solidification cooling rates is investigated. It is demonstrated that 0.5Nb-0.1B wt.% refined GS for either columnar or equiaxed morphology independent of the cooling rate, with a refining capability of 13-fold reduction in GS. The secondary dendritic spacing (lambda(2)) also decreased, specifically for low cooling rates, as compared to the non-modified alloy. It is proposed that the GS to lambda(2) ratio may characterize alpha-Al cell micromorphology in the 6201 + NbB alloy, which occurs when this ratio is lower than 6. High-cooling-rate cellular arrays were observed for cooling rates higher than 5.5 degrees C/s. Although a low GS and lambda(2) result in an increased strength of the 6201 + NbB alloy, a limit for optimizing this property via grain refining appears to be reached at approximately 190 MPa for the GS ranging from 142 to 239 mu m and the lambda(2) ranging from 22.6 to 24.5 mu m.