Grain refinement of Al-Si alloys by Nb-B inoculation. Part I: Concept development and effect on binary alloys

The effect of Nb-B inoculation on Al-Si alloys for their grain refinement has been studied through the analysis of binary Al-xSi (where x = 1-10 wt.%) to avoid possible effects of other alloying elements. In Part I of this work the concept development of the Nb-B inoculation is discussed in detail on the basis of the theoretical and fundamental concepts employed (i.e. pro-peritectic particles formation, lattice structures and mismatch as well as analogies between the Al-Ti/Al-Nb or Ti-Si/Nb-Si binary phase diagrams). The systematic study of the addition of different level of Nb-B inoculation to pure Al permitted to determine the best addition rate. From the microstructural and thermal analysis of binary Al-xSi alloys it is found that Nb-B inoculation is highly suitable for Al-Si alloy with Si content greater than 6 wt.%. As results of the Nb-B inoculation the microstructural features of binary Al-xSi alloys (i.e. primary Al α-grains and eutectic phase) are significantly refined. Most importantly, the inoculation of Al-Si cast alloys with Nb-B is not characterised by any visible poisoning effect (i.e. formation of silicides) which is the drawback of using commercial Al-Ti-B master alloys on Al cast alloys. The effect of Nb-B inoculation on commercial Al-Si alloys (which normally include other alloying elements in their chemical composition) is assessed in Part II of this work.