Evolution of core‐shell precipitates in Al‐Sc‐Zr

A. TOLLEY; V. RADMILOVIC; U. DAHMEN

BARILOCHE

Congreso; CONGRESO SAM/CONAMET 2018; 2018

ASOCIACION ARGENTINA DE MATERIALES

Low concentration additions of Sc in Al alloys result in the formation of Al3Sc precipitates that increase the resistance to recrystallization and strength. The precipitates coarsen at a very slow rate below 300ºC, resulting in good creep resistance (1). Ternary additions of Zr that substitute for Sc in the Al3Sc structure reduce the coarsening rate even further (2). The precipitates in Al-Sc-Zr have been shown to have a core-shell structure with an Al3Sc core surrounded by an Al3(Sc,Zr) shell (3,4). In this work the evolution of core-shell precipitates during high temperature annealing in an Al-Sc-Zr alloy was investigated using Z contrast High Angle Annular Dark Field (HAADF) images obtained in STEM mode in transmission electron microscopy. An Al-0,37%Sc-0,12%Zr (at%) alloy was prepared in a vacuum arc furnace. It was homogenized for 24 h at 640ºC followed by quenching in water at 0ºC. Specimens were aged at 450ºC for up to 1000h in air. HAADF STEM images were obtained with a Tecnai F20 UT microscope at the NCEM, operated at 200 kV. Figure 1 shows HAADF STEM images of core-shell precipitates after different aging times. The precipitates´ morphology was cuboidal with facets parallel to the {100} planes. The larger sized precipitates show protrusions along the directions. The shell was very thin in the early stages of annealing and presented a constant thickness around the core. Up to 69h aging, the shell was found to grow slowly with approximately constant thickness while the mean precipitate size increased during the coarsening process. However, after 1000 hours aging, the core maintained a cuboidal shape with the mentioned protrusions in large sized particles, but the shell adopted a spherical outer shape, exhibiting a non-uniform thickness.Detailed measurement of the core size, shell thickness and total size of the precipitates was carried out from the HAADF images. Figure 2 shows a plot of the shell volume as a function of the core volume for different ageing times. It can be seen that both the shell volume and core volume grow with aging time and that the shell volume is proportional to core volume. While the formation of the core-shell structure can be accounted for by the slower migration of Zr during the early stages of aging (5), the long aging evolution suggests that energy minimization effects must be included to explain the stability of the core-shell structure in this alloy.