Early Stages of Precipitation Process in Al-(Mn-)Sc-Zr Alloy Characterized by Positron Annihilation
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INTRODUCTION
AL-BASED alloys-containing Sc are promising materials for high-temperature structural applications.[1–3] A number of investigations have been carried out in Al-based alloys-containing Mn, Sc, and Zr, e.g., References 4 through 8. The promising results are related to the formation of a dense and homogeneous distribution of the L12-structured Al3(Sc,Zr) precipitates.[2–7,9–11] The complex nanostructure of the Al3(Sc,Zr) phase in aluminum has been discussed in several recent articles by studies using conventional and high-resolution transmission electron microscopy (TEM).[12,13] Atom-probe tomography (APT) was also employed to measure Sc,Zr concentration and distribution.[14–16] APT has shown that Sc-rich clusters can be formed in the early stages of precipitation above annealing temperatures of ~523 K (250 C).[15] These clusters subsequently transform into the Al3Sc particles with the L12 structure. When Zr diffusion becomes MARTIN VLACH, OKSANA MELIKHOVA, TOMAS KEKULE, and HANA KUDRNOVA, Researchers, JAKUB CIZEK, IVANA STULIKOVA, and BOHUMIL SMOLA, Associate Professors, are with the Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague, Czech Republic. Contact e-mail: martin.vlach@mff.cuni.cz RYOTA GEMMA, Researcher, is with the Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. VOLKMAR NEUBERT, Professor, is with the Institut fu¨r Materialpru¨fung und Werkstofftechnik, Freiberger Strasse 1, 38678 Clausthal-Zellerfeld, Germany. Manuscript submitted July 21, 2014. Article published online 29 January 2015 1556—VOLUME 46A, APRIL 2015
significant, Zr atoms are found to segregate to Al3Sc/aAl matrix interfaces. During further annealing [~673 K (400 C)] dispersoids with complex core–shell structure consisting of an Al3Sc core embedded in an Al3(Sc,Zr) shell are developed.[4,5,14,15] Authors of the present study have already provided resistivity measurements of the Al-Sc-Zr alloy which showed that Zr-segregation to the Al3Sc/a-Al matrix interfaces might take place already at ~573 K (300 C).[17] Electrical resistivity measurement is a convenient way to follow precipitation kinetics in phase-separating systems[18,19] as electrical resistivity having its origin in the scattering of free electrons by phonons and lattice defects reflects the material phase composition very sensitively. In general, clusters can be universally abundant in Al-based alloys and are known to affect the resistivity. Many authors have attempted to explain changes of electrical resistivity at the beginning of the precipitation kinetics (e.g., References 20 through 22). Clouet et al.[22] modeled electrical resistivity evolution in Al-Sc-based alloys using cluster dynamics supposing that the excess resistivity caused by clusters is proportional to their number density. The proportionality coefficient was assumed to be constant and equal to the resistivity contribution of monomers. However, an increase in the resistivity a
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