Precipitation of Al3(Sc,Zr) particles in an Al–Zn–Mg–Cu–Sc–Zr alloy during conventional solution heat treatment and its effect on tensile properties

The effect of heat treatment on precipitation and growth of coherent nanometer-sized Al3(Sc,Zr) particles and the effect of these particles on tensile properties of a direct chill (DC) cast Al–Zn–Mg–Cu–Sc–Zr alloy were studied. The size distribution, average size, number density and volume fraction of the Al3(Sc,Zr) particles were determined as a function of the solution treatment temperature and time. An increase in the solution treatment temperature and time resulted in Al3(Sc,Zr) particles with a larger mean diameter, higher volume fraction and lower number density. The particle size distributions were described well by normal (Gaussian) distributions. The kinetics of the phase transformation followed the Kolmogorov–Johnson–Mehl–Avrami law, with the Avrami exponent m = 0.404. Room temperature tensile properties were evaluated in the as-solution treated and artificially aged conditions. The coherent nanometer-sized Al3(Sc,Zr) particles provided additional Orowan strengthening, which increased with increasing particle volume fraction and decreasing particle size, and varied from 75 to 118 MPa after different heat treatments.