Studies of microstructural changes upon retrogression and reaging (RRA) treatment to 8090 Al – Li – Cu – Mg – Zr alloy

Abstract

Microstructural changes upon retrogression, and retrogression and reaging (RRA) treatment to 8090 Al -Li -CuMg -Zr alloy in the peak aged (T8) temper have been studied by XRD and TEM. The variation of hardness with retrogression, and retrogression and reaging (RRA) has been measured to assess RRA behaviour. XRD studies exhibited all the phases that would be expected in the system. Retrogression primarily caused dissolution of matrix strengthening d9 precipitates in the solution. Reaging the retrogressed state caused reprecipitation of the d9 precipitates in the matrix. Retrogression and reaging (RRA) treatments retained the strength of the conventional peak aged temper, but TEM observations revealed some microstructural changes, such as growth of d, T 1 and S9 phases, reduction of the dislocation densities, and generation of more dislocation loops and helices. MST/6039

Electrochemical behaviour of retrogressed and reaged (RRA) 8090 and 1441 Al–Li–Cu–Mg–Zr alloys

“…The occurrence of d¢ (Al 3 Li) fi d (AlLi) phase transformation with the ageing process and RRA treatment results in the enhanced precipitation of d phase mostly along the grain and subgrain boundaries. The total ageing time of the RRA temper is more than twice to that of the T8 tempers [12][13][14][15][16][17][18][19][20][21][22][23]. Hence, the observation of enhanced precipitation of d phase along grain and subgrain boundaries in Figures 1b, c and Figure 2b in the RRA tempers is obvious.…”

“…the neat heat flow to the reference (high purity annealed aluminum) relative to the samples was recorded as a function of temperature. [23,24]. The TEM photomicrographs exhibiting the anodic phases of the alloys that are mostly associated with the observed electrochemical behaviour have been shown in the present paper.…”

“…Retrogressions were carried out in a small vertical tube furnace in air and the furnace temperatures were maintained within ± 2°C of the set value. The retrogression and reaging (RRA) schedule applied to the alloys, discussed elsewhere [22,23], yielded four RRA tempers of each of the alloys. Electrochemical potentiodynamic polarizations were carried out on 10 mm  10 mm coupons of peak aged T8, four RRA T77 and over aged T7 tempers on both alloys using a computer controlled MEINSBERGER potentiostat/galvanostat with inbuilt PS6 software.…”

“…The details of specimen preparation have been discussed elsewhere [22,23]. A PHILIPS CM12 TEM was used for observation of the microstructures.…”

Editorial

“…The retrogression and re-aging process has been applied to aerospace 7000 series alloys 46 and to the Al-Li AA 8090 alloy. 47 In a different approach, the normal aging treatment is interrupted and the alloy is quenched to and held at a low temperature (about 25-65uC) for an extended period before returning to the normal aging temperature. This modified heat treatment can lead to superior mechanical properties, including fracture toughness, for a range of wrought alloys 48 and has been improved further by omitting re-aging after the lower temperature treatment 49 (whether this is commercially attractive remains to be seen).…”