Effect of Quench Rate on PFZ Width in 7XXX Aluminum Alloys

The effect of quench rate on the width of precipitate free zone was examined in aluminum alloys 7075 and 7050 using the Jominy End Quench test. Samples were heat treated at 493°C for two hours, then quenched according to ASTM A255. Following quenching, the specimens were naturally aged for 1 hour, and ramped to 122°C at 3.4°C/min. The specimens were aged at 122°C for 6 hours, followed by aging at 177°C for 8 hours. The specimens were then quenched from the second stage aging temperature. Sections were taken from the Jominy bar and examined using a Phillips EM430 TEM at 300kV. Specimens were examined for high angle grain boundary precipite free zones, and the width determined. A relationship relating PFZ width to quench rate was determined and confirmed an inverse square-root dependence on quench rate. It was determined that at quench rates greater than 60°C/sec., vacancy depletion dominated. At slower quench rates, it was determined that solute depletion dominated the precipitate free zone. The critical vacancy concentration for precipitation was established as a function of quench rate.