Precipitation of Al3(Sc,Zr) Particles in a Direct Chill Cast 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. The average particle diameter increased from 6.0 nm to 27.3 nm, the volume fraction increased from 0.11% to 0.46%, while their number density decreased from 5.4x10(exp 15) cm-3 to 3.1x10(exp 14) cm-3 with an increase in the solution treatment temperature from 460deg C to 480 deg C and holding time from 1 hour to 48 hours. The particle size distributions were well described by normal (Gaussian) distributions but did not follow the Lifshitz-Slyozov-Wagner (LSW) theory of Ostwald ripening. To describe the observed particle size distributions, a stochastic process of particle growth was added to the normal process of growth. The kinetics of phase transformation followed the Kolmogorov-Johnson-Mehl-Avrami (KJMA) law, with the Avrami exponent m = 0.404.
Submitted for publication in Acta Materialia. Prepared in collaboration with Air Force Research Laboratory (AFRL), Wright-Patterson AFB, OH.