KINETICS OF PRECIPITATION HARDENING IN SIC WHISKER-REINFORCED 6061 ALUMINUM-ALLOY

Ageing behaviour at 180-degrees-C of 6061 aluminium alloy SiC(w) composites, drawn from bars obtained in various extruded ratios, and 6061 aluminium alloy used as matrix, have been compared. These materials were dissolved in a salt bath at 529 and 557-degrees-C for 2 h, quenched in ice-water, and aged at 180-degrees-C in an oil bath for increasing periods. Ageing kinetics were studied with Brinell hardness measurements and differential scanning calorimetry (DSC). Various samples of the composite, deriving from bars with PHI-20, PHI-35 and PHI-50 mm in diameter, and 6061 aluminium alloy, show the same ageing mechanism; however, the ageing rates results increased for composites. While 6061 aluminium alloy shows its maximum hardness value after about 4-5 h at 180-degrees-C, the 6061 SiC(w) composites reach theirs in 2 3 h. Moreover, for composites hardness abruptly decreases after 3 h, while aluminium alloy keeps its maximum value for an ageing time as long as 6 h. Thermal analysis allows us to put together a definite DSC trace for every microstructural state. The highest hardness values are obtained as a result of the formation of a Guinier Preston (GP) needle-shaped zones, which progressively become more thermally stable with protracted isothermal treatment at 180-degrees-C. The different ageing process rates observed for composites and for the 6061 alloy are correlated with the sizes of the reinforcements. Dimensional analysis of whiskers has been performed by light scattering and scanning electron microscopy. Ordinarily the longer the average length of the whiskers in the samples, the faster the ageing process. Higher temperatures are required for composite solutions than for 6061 alloy. On the other hand, 6061-SiC(w) samples solutionized at higher temperature and then quenched sometimes show microcrack formation in the materials.