Materials Science

Synergistic Alloying

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Science  16 Jun 2006:
Vol. 312, Issue 5780, pp. 1575
DOI: 10.1126/science.312.5780.1575b

Impurities are often empirically added to a metal to improve its properties through the formation of second-phase precipitates; however, understanding and predicting the performance of different impurities can be complex. Zirconium (Zr) and scandium (Sc), for example, individually form alloys with aluminum that enhance strength and resistance to recrystallization. When Zr and Sc are added together, the net effect is significantly greater than the impact of either one alone.

Clouet et al. use a combination of atomic simulations and experiments to explain this synergism. They find that the precipitates are not uniform in composition but instead have a Sc-rich core surrounded by a Zr-rich shell. Sc diffuses more rapidly than Zr within the solid solutions and thus begins to form precipitates first, until reaching an equilibrium concentration. Lattice thermodynamics inhibit Zr from diffusing into the core, which in turn prevents the precipitates from coarsening. In situ small-angle x-ray scattering studies show that once enough Zr reaches the shell, an Al3Zr composition results that is resistant to further annealing. Overall, these effects lead to a higher concentration of smaller precipitates, enhancing the nucleation of the aluminum and thereby creating a stronger alloy. — MSL

Nat. Mater. 5, 482 (2006).

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