PRECIPITATION OF Al3Zr DISPERSOIDS DURING HOMOGENIZATION OF Al-Zn-Cu-Mg-Zr ALLOYS

Dispersoids of Al3Zr in Al-Zn-Cu-Mg-Zr alloys are important as they pin grain boundaries and inhibit recrystallization during extrusion and solution heat treatment. A high volume fraction,V-Al3Zr, and low dispersoid radius, r(Al3Zr), give V-Al3Zr/r(Al3Zr) values above a critical value which are essential to prevent recrystallization. Precipitation of Al3Zr during homogenization is studied for different homogenization schedules. A 1D finite difference model based on classical nucleation and growth theory for a multicomponent system is developed to simulate precipitation of Al3Zr dispersoids. The 1D domain represents one half of a secondary dendrite arm spacing with initial concentration gradients based on Scheil type solidification. Single-step homogenization, slow heating to homogenization temperatures, and two-step homogenization are evaluated for maximum dispersoid number density, maximum volume fraction and minimum dispersoid radius. V/r ratios along the length of the SDAS are compared. Two step homogenization (10hrs at 420 degrees C and more than 10hrs at 475 degrees C) provides better V/r ratios across the grain and provides the maximum recrystallization resistance.