Effects of heating rate on the hardness and microstructure of Al-Cu and Al-Cu-Zr-Ti-V alloys

Effects of heating rate on hardness and microstructure of Al-Cu and Al-Cu-Zr-Ti-V alloys were investigated by means of scanning electron microscope (SEM), electro-probe microanalyzer (EPMA) and transmission electron microscopy (TEM). The cast alloys were heat treated with a heating rate of 50 °C/h and 300 °C/h, respectively. The results show that the hardness and microstructure of both alloys can be controlled by changing heating rate. As compared with the lower heating rate, both the two alloys heated with higher heating rate exhibit a significant improvement in hardness and a decrease in the holding time for obtaining peak hardness. The heating rate is also closely relative to the size and distribution of Al3Zr precipitates. TEM statistic size of the precipitates shows the higher heating rate not only gives a larger mean size of precipitates, but also significantly activates the nucleation sites on the dendrite edge. The influences of heating rate in hardness and microstructure are attributed to the higher heating rate causing more defects and stronger mobility of solute or vacancies in the matrix.