MICROSTRUCTURE OF A SPRAY-FORMED AL SIC COMPOSITE/

The microstructure of an Al-6Cu-2Mn-0.45Mg-1(Ag,Ti,V,Zr,Cr) alloy, rei nforced with 13 vol.% SiC particles, made by spray deposition has been investigated by transmission electron microscopy, high-resolution ele ctron microscopy and electron diffraction X-ray spectroscopy. Particul ar attention was focused on the influence of the reinforcement on the precipitation sequence. Instead of the expected (sss)-->GPZ+Omega-->th eta'+Omega+S-->theta+Omega+S precipitation sequence due to the high Cu /Mg ratio, there is an additional sigma precipitate which was previous ly observed in A1 alloys containing silicon, This precipitate becomes predominant at the T6 temper. The new precipitation sequence for this reinforced alloy is therefore a'+Omega-->theta'+Omega+sigma-->theta+Om ega+sigma. The precipitation of sigma phase is believed to be due to t he presence of SiC particles, and seems to be correlated with the occu rrence of large Mn-rich particles. Although expected, no S phase preci pitation is found to occur in the matrix grains, At the matrix grain b oundaries, small Al2Cu(theta) and Al2CuMg (S), as well as Mn-rich prec ipitates are found. At the SIC particle surfaces, preferentially orien tated Ag-rich and Mg-rich intermetallic precipitates are found. They c an coexist with amorphous patches containing oxygen enclosed in an irr egularly shaped Al2Cu (theta) phase remaining from large crystalline a reas which did not go into solution.