Effect of Trace Additions (Be, Cr, Mn and Co) on the Mechanical Properties and Fracture Toughness of Fe-containing Al-7Si-0.3Mg Alloy

Abstract

The detrimental effect of iron impurity on the mechanical properties and fracture toughness of Al-7Si-0.3Mg alloy has been widely reported. It is also quite evident that Fe forms the intermetallic compound β-FeSiAl₅, which exists as needles/plates in the interdendritic regions, thereby lowering the mechanical properties. However, very little is known about the methods of counteracting this detrimental effect. The present study has shown that trace additions of Be, Cr, Mn and Co, individually or combined, tie up the iron to form new phases with altered morphology which significantly influence the mechanical properties. Among the trace additions, Be is found to be most effective in neutralising the detrimental effect of Fe on tensile properties. Hardness, tensile and yield strengths, ductility, dynamic fracture toughness (K_d and, K_jd remain constant with an increase in Fe from 0.1 to 0.93% where Be additions are present. Be additions in combination with Mn and/or Cr are also found to nullify the detrimental effect of Fe. However, Be and Mn, when added together, seem to be most effective in providing better tensile properties, than does a Be addition alone or with other additions. The probable reason for good mechanical properties, even in the case of higher-Fe-containing alloys containing a Be addition can be explained as follows: The Be-Fe phases are present inside the α-Al dendrites and act as a preferential site for crack nucleation rather than the Si particles. However, these nucleated cracks are arrested when they approach the α-Al dendrites.