6. Age Hardening The following figure shows the Diamond Pyramid Hardness (DPH) during age hardening at 130 C for Al- Cu alloys containing 2, 3, 4 and 4.5% Cu. There are four types of precipitates that form in fcc Al. The first is Cu in fcc Al (10 nm). The second (100 nm), third (~1 micron) and fourth (> 1 micron) are CuAl2 in fcc Al. (1) For each type of the four types of precipitates, describe the following (i) The crystal structure (ii) The nature of the interface (2) What features contribute to hardness? (3) Which of the four precipitates contributes most to the hardness? Why? (4) Which of the four precipitates do you expect to have the lowest Gibbs free energy? (5) rcritical is the size of the precipitate where the transition from coherent to non-coherent takes place. Based on the information in this problem, what is the approximate size of r_critical for this age hardening example? (6) Which one of the four precipitates do you expect to lie on the *same* Gibbs free energy curve as the fcc Al? (7) Why do the precipitates evolve through these four different types of precipitates instead of forming the fourth one right away? (8) Draw a graph to illustrate your answer to (5) Vickers pyramid No. 140 SP -e" 120 e' 100- (a) 4.5% 80 Cu 60 4.0% Cu 40- 3.0% Cu Aged 130C 4.5% Cu 2.0% Cu 0.1 1 10 100 Ageing time, days 4.0% Cu 3.0% Cu 2.0% Cu