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Type Marquis, E. A.; Riesterer, J. L.; Seidman, D. N.; Larson, D. J.
  Publication Mg Segregation at Coherent and Semi-Coherent Al/Al[sub:3]Sc Interfaces Volume Journal Article
Pages 2006
  Abstract Microscopy and Microanalysis  
  Corporate Author Microsc. Microanal.  
Publisher 12  
Editor S2
  Summary Language 914-915 Series Editor LEAP; Al-Sc  
Abbreviated Series Title Aluminum alloys containing Sc are promising materials for high-temperature structural applications due to the high strengthening effect of the Al3Sc (L12 structure) precipitates [1]. Further improvements in strength and nanostructural stability of Al-Sc based alloys are achieved by adding alloying elements, such as Mg, as solid-solution strengtheners. It is important to understand the effects of Mg in order to control not only the specific contribution of Mg to the properties of Al-Sc alloys (strengthening effect and creep resistance) but also the changes in the nanostructure. From previous work, it is known that Mg tends to segregate to the coherent Al/Al3Sc interface due to positive interactions between Mg and Sc atoms [2]. This paper reports measurements of Mg segregation at the Al/Al3Sc interface and compares the segregation level between coherent and semicoherent Al/Al3Sc interfaces. A cast Al-2 wt.% Mg-0.2 wt.% Sc alloy was annealed at 618C in air for 24 hours, quenched into cold water, and then aged in air at 300C for 24 hours. One sample was subsequently aged at 400oC for 240 hours. Three-dimensional atom probe (3DAP) microscopy tips were obtained by a double electro-polishing technique. Field evaporation was performed at 30 K with a pulse fraction of 20 % at a frequency of 200 kHz using a LEAP microscope. Transmission electron microscopy (TEM) imaging was performed on a JEOL 1200 microscope. During aging at 300oC, Al3Sc precipitates are formed with a high number density (~2 1022 precipitates/m3), which is advantageous for random 3DAP microscope observations. The average radius of the precipitates is 2 nm and the interface is coherent (Fig. 1). After aging at 400oC, however, the average radius of the precipitates is ~19 nm and dislocations loops are observed at the matrix/precipitate interface. The number density of precipitates has also decreased dramatically (~1019 precipitates/m3) and is no longer sufficient for random atom probe observations. Atom probe tips were therefore observed by TEM to confirm the semi-coherent nature of the Al/Al3Sc interface and to determine the position of the precipitates with respect to the tip apex. Micro-polishing was used to position precipitates to within ~100 nm of the apex. A TEM image of a tip is shown in FIG.2. Al3Sc precipitates are visible with dislocations at the matrix/precipitate interfaces. Coherency loss may occur when the precipitate diameter is larger than the spacing between the misfit dislocations. This spacing is of the order of α/ε, where ε = 0.62% is the lattice parameter misfit between the α-Al matrix containing 2.2 at.% Mg and the Al3Sc phase [3,4], and α = 0.20 nm is the spacing between {200} planes. The calculated equilibrium dislocation spacing is therefore 32 nm, in good agreement with the presence of interfacial dislocations for precipitates with diameter of ~38 nm. Comparison is made between the segregation levels measured for coherent and semicoherent interfaces. The role of the interfacial dislocations will be discussed. References [1] Toporova L.S., Eskin D.G., Kharakterova M.L., Dobatkina T.B. Advanced aluminum alloys containing scandium. Amsterdam: Gordon & Breach; 1998. [2] Marquis E.A., Seidman D.N., Asta M., Woodward C., Ozolins V., Phys. Rev. Letters 91 (2003) 036101 1-3 [3] Hatch J.E. Aluminum: properties and physical metallurgy. Metals Park (OH): ASM; 1984.
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