The effects of bismuth and calcium additions on the Si particle characteristics in Sr-modified 319 alloys (modified with 80 ppm Sr), with and without 0.4 wt percent Mg addition, were studied using thermal analysis, optical microscopy, image and SEM/EDX analyses. Bismuth additions were made in the range 50 to 9000 ppm, and calcium additions in the range 50 to 200 ppm. The effects of Bi and Ca were monitored through (i) correlation of the eutectic temperature, and (ii) correlation of the microstructural changes with the trace element additions, (iii) evaluation and quantification of the eutectic Si particles, and (iv) identification of the different phases that precipitated as a result of the additions. It was found that the modification effect of Sr continuously diminished with Bi addition up to -3000 ppm Bi, following which, further Bi addition led to the modification of the Si particles due to the presence of Bi. In the Ca-containing alloys, it was found that a coarse eutectic Si structure resulted with Ca additions of 50 ppm, due to the formation of Al_x(Ca,Sr)Si_y compounds. Increased Ca additions (up to 200 ppm) did not alter the Si particle size. The Al_x(Ca,Sr)Si_y phase particles appeared in rod-like form in the Sr-modified alloys and in plate-like form in the 0.4 wt percent Mg-containing alloys. The presence of MgO, Al_2O_3, and AlP particles at the center of the plate-like Al_x(Ca,Sr)Si_y phase appear to act as nucleants for the precipitation of the latter, aiding its growth through an impurity induced twinning mechanism.
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