A Quantitative Study of Precipitation of Metastable Phases in an Al-1.94 at%Cu Alloy during Isothermal Aging at 373 K

Abstract

Precipitation behavior of metastable phases in an Al 1.94atCu alloy during isothermal aging at 373 K was investigated by means of Vickers microhardness tests, DSC measurements and TEM observations. The size distribution of the precipitates was quantitatively investigated based on the TEM, HRTEM and HAADF STEM images, and statistical parameters that fit the precipitate size distribution were determined under a log normal distribution approximation. We have successfully estimated the volume fraction of copper in precipitates, and found that the G.P.(II) formation results in increases of volume fraction of metastable particles, mean size and hardness.

Competing nucleation of single- and double-layer Guinier–Preston zones in Al–Cu alloys

“…2 c, the GP2 zones can directly nucleate and grow without the initial growth of GP1 zones. The diameter of a GP1 cluster at n = 101.0 is approximately 4 nm, which is consistent with the average experimental values of GP1-zone diameters 7 , 11 , 13 – 15 , 23 , 24 . This implies that the saturation of the size of GP1 zones with aging time is associated with the crossover of the free energies of formation at , above which GP zones start to grow again as GP2 zones.…”

“…5 . The large and small plots represent the structures first detected during the aging process 7 , 11 , 15 and the solvi of the GP zones 39 – 42 , respectively, in Al–Cu alloys. It is worth noting that our predicted curve clearly marks the boundary between the GP1 and GP2 nucleation.…”

“…During the growth of GP zones, the diameter of the zone with the GP1 structure remains quite constant across the plateau of the two-stage hardness curves, and then increases rapidly at the end of the plateau as the GP1 structure changes to the GP2 structure. The average zone diameter was reported as 4–5 nm in an Al-1.7 at %Cu alloy at room temperature 11 , 22 and 6–7 nm in an Al-1.94 at %Cu alloy at 373 K 15 . The tendency for the saturation of the zone size has been recognized by many researchers 7 , 11 , 13 – 15 , 23 , 24 , although the measured zone diameters are somewhat scattered within a range below 8 nm.…”

“…The average zone diameter was reported as 4–5 nm in an Al-1.7 at %Cu alloy at room temperature 11 , 22 and 6–7 nm in an Al-1.94 at %Cu alloy at 373 K 15 . The tendency for the saturation of the zone size has been recognized by many researchers 7 , 11 , 13 – 15 , 23 , 24 , although the measured zone diameters are somewhat scattered within a range below 8 nm. To the best of our knowledge, no conclusive explanation exists for the saturation of the GP1 zone size and its abrupt growth after transformation to the GP2 structure.…”