MORPHOLOGICAL-STUDIES ON BETA-FESIAL5 PHASE IN AL-7SI-0.3MG ALLOY WITH TRACE ADDITIONS OF BE, MN, CR, AND CO

Although the detrimental effect of iron on the mechanical properties o f Al-7Si-0.3Mg alloy has been well established, studies on neutralizin g this effect have been very few and inconclusive. The beta-phase (FeS iAl5), seen in the interdendritic areas in the form of needles/plates, is responsible for this effect. In the present study, it has been obs erved that trace additions of Be, Cr, Mn, and Co, individually or comb ined, will tie up Fe to form new phases with altered morphologies such as Chinese scripts, polygons, stars, and irregular shapes. Cr-Fe and Mn-Fe phases are seen both in interdendritic regions and inside the a- Al dendrites, while Co-Fe phase is mostly seen inside the alpha-Al den drites. However, Be-Fe phase is observed only inside the a-Al dendrite s. Empirical formulas have been developed from wavelength dispersive a nalysis on these relatively new Fe phases with trace additions. At hig her Fe levels (0.8-1%) and with Be trace addition, individually or com bined with Mn and Cr, fractographs reveal pyramids surrounded by cells , while other trace additions such as Mn, Cr, and Co reveal brittle cl eavage fracture. All Be-added alloys, individually and combined with M n and Cr additions, appear to neutralize the detrimental effect of Fe content on mechanical properties.