Effect of Cooling Rate on Precipitation during Homogenization Cooling in Balanced Al–Mg₂Si Alloy

Abstract

The effect of the cooling rate on the precipitation process during homogenization cooling in a balanced Al–0.64Mg–0.32Si mass% alloy was investigated. Different cooling rates (Furnace-cooling/Air-cooling) were used. During homogenization air-cooling (∼830 K/h), optical microscopy (OM) revealed that few precipitates could be observed in the matrix; however, there were many “boundaries” formed in the grains. Transmission electron microscopy (TEM) observations confirmed that these “boundaries” were caused by heterogeneous nucleation of precipitates along the dislocations during cooling, and many precipitates only grew along the [100]_Al and [010]_Al directions. Among the precipitates, over-aged β′/Type-B, together with string-like precipitates, were found on the dislocations. For the furnace-cooled samples (20 K/h), rod/lath-like precipitates of β′ and β′/Type-B were also found along the dislocations.

Fig. 6 TEM bright field image and the selected area electron diffraction (SAED) pattern of the air-cooled sample. (a) is the magnified region of Fig. 4(c), the diffraction pattern in the lower left was taken from the marked area with dashed circle. (b) and (c) are the similar microstructures found in the matrix.