Solute Segregation and Aggregation in Mg Alloys

Abstract

Extended AbstractThe addition of adequate amounts of rare-earth (RE) alloying elements can produce appreciable effects on microstructures, formability and mechanical properties of magnesium alloys. At low concentrations, it can weaken the basal texture in the asdeformed and the recrystallized conditions in wrought alloys and improve the creep resistance of casting alloys at elevated temperatures; at high concentrations, it can bring significant age hardening effects and thus improve the alloy strength. While this knowledge has been known for some years, the precise roles of RE solutes in texture weakening and the early stage of precipitation remain unclear.It has been demonstrated in recent years that the application of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) can reveal the distribution of RE solutes and the structure of precipitate phases at the atomic scale. The use of this Z-contrast imaging technique, combined with atom probe tomography or other characterization techniques such as energy dispersive X-ray spectroscopy and electron energy loss spectroscopy, has revealed the occurrence of solute segregation in high-angle grain boundaries even in solid solution single phase alloys based on the Mg-Gd system [1-4]. The segregation of RE atoms in the grain boundaries will no doubt impede the boundary migration [5] and thus change the recrystallization behaviours and perhaps even the recrystallization texture. In addition to the wellestablished solute segregation in the partially-coherent high-angle grain boundaries, a very recent study using HAADF-STEM [6] has revealed the occurrence of a periodic segregation of solute atoms in fully-coherent twin boundaries of { } D , { } D and { } D . This segregation results from the minimization of elastic strain energies associated with twin boundaries and the over-sized individual solute atoms, and it provides a strong pinning of the twin boundaries and a concomitant annealing hardening effect when the alloy is deformed under certain circumstances. Subsequent studies using first-principles density functional theory computations indicate that solute atoms of most commonly used alloying elements have a strong tendency to segregate to { } D twin boundaries [7,8], and that solute segregation can also occur in a symmetrical tilt boundary [9].

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“…It can lead to the occurrence of static strain aging (SSA) and dynamic strain aging (DSA) [6,7] depending on the deformation conditions. Besides, researchers also found that RE atoms trend to segregate at grain boundaries and have a great influence on the dynamic recrystallization (DRX) behavior of magnesium alloys [8][9][10].…”