Complex precipitation pathways in multicomponent alloys

Abstract

One usual way to strengthen a metal is to add alloying elements and to control the size and the density of the precipitates obtained. However, precipitation in multicomponent alloys can take complex pathways depending on the relative diffusivity of solute atoms and on the relative driving forces involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle calculations combined with various complementary experimental approaches working at different scales reveal a strongly inhomogeneous structure of the precipitates: owing to the much faster diffusivity of Sc compared with Zr in the solid solution, and to the absence of Zr and Sc diffusion inside the precipitates, the precipitate core is mostly Sc-rich, whereas the external shell is Zr-rich. This explains previous observations of an enhanced nucleation rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much higher resistance to Ostwald ripening, two features of the utmost importance in the field of light high-strength materials.

Using cluster dynamics to model electrical resistivity measurements in precipitating AlSc alloys

“…The interface free energies they deduced from their resistivity measurements (230 mJ.m −2 between 400 and 450 • C) is higher than the one used in our simulations (119 ≥ σ ≥ 105 mJ.m −2 between 200 and 500 • C) which was deduced from the atomic diffusion model of Ref. 15,18. This is in agreement with the fact that incoherent precipitates should have a higher interface free energy than coherent ones.…”

“…Good predictions were obtained with this experimental value. As for the solute diffusion coefficients and the interface free energies, they were directly deduced [11] from an atomic model previously developed for Al-Zr-Sc alloys [15,18]. The good agreement obtained between our resistivity simulations and experimental data allows then us to stress the correctness of the Al 3 Sc interface free energies used in our simulations.…”

“…The time evolution of the cluster size distribution is governed by a master equation which input parameters are the solute diffusion coefficient and the precipitate interface free energy. For Al-Sc alloys, these needed parameters were directly deduced from an atomic diffusion model [15,16,18] previously built for Al-Zr-Sc alloys. It should be stressed that, although none of these input parameters were intentionally fitted, cluster dynamics managed to reproduce reasonably experimental data on the time evolution of the precipitate size distribution.…”

Characterising precipitate evolution in multi-component cast aluminium alloys using small-angle X-ray scattering

“…For example, a uniform precipitate composition produces a different Porod profile compared to a core-shell chemical composition variation and can be identified by the overall profile of the Porod plot [28,29,32]. In conjunction with the Q parameter we calculate Porod radius (Rp) with the following relationship:…”

“…those formed from Al and the impurity elements Fe and Si) [15][16][17][18][19][20][21][22][23][24][25][26][27]. Work on alloys that form a low volume fraction (~1 vol%) of trialuminide precipitates (dispersoids) has been limited to high purity Al-Zr-Sc alloys with ppm levels of Fe and Si [28,29], but not on casting alloy systems.…”