In Mg2(Si,Sn)-based alloys, compositional gradient layered microstructure (CGLM) has been reported as an effective factor in enhancing thermoelectric performance, primarily due to reduced lattice thermal conductivity. However, it is quite difficult to control CGLM, since Mg loss via evaporation and oxidation during the melting and solidification processes tends to gradually shift the composition of the liquid phase away from the nominal composition of Mg2(Si,Sn) toward the Mg-poor side of the phase diagram. This leads to the formation of Si and Sn phases as the final products of non-equilibrium solidification. Hence, compensation for Mg loss is necessary to facilitate solidification being completed within the Mg2Si–Mg2Sn pseudo-binary system under a proper condition of solidification. Consequently, an alloy consisting almost entirely of CGLM (hereafter denoted as Full CGLM), achieved by 10 at% excess Mg compensation in this work, comprises six layers, starting from the primary solidification of Mg2Si phase as a core part and extending to the outer shell layer of Mg2Sn phase formed via solidification after the peritectic reaction. The core of the primary phase is considered to be a single crystal grain, while each surrounding layer of CGLM is composed of Mg2(Si,Sn) polycrystalline solid solutions exhibiting widely varied Si/Sn ratios following the solidus line of the Mg2Si–Mg2Sn phase diagram. When the solidification route, presented by a monovariant line on the liquidus surface projection of the Mg–Si–Sn ternary phase diagram, deviates from the pseudo-binary edge toward the Mg-rich side, corresponding to overcompensation for Mg loss, solidification terminates with the formation of eutectic Mg and Mg2Sn phases as the final products. The single colony size of CGLM decreases as the cooling rate of solidification becomes higher, since the nucleation of the core Mg2Si is enhanced. Ultimately, the Full CGLM alloy exhibits a significantly low lattice thermal conductivity of 1.22 W m−1 K−1 at 573 K, which is attributed not only to the solid solution effect of Mg2(Si,Sn) phases with a large volume fraction of CGLM but also to a larger interfacial area with a high density of misfit dislocations induced by the large differences in lattice parameters between Mg2(Si,Sn) grains according to the widely spread Si/Sn ratios.
Zhang et al. (Fri,) studied this question.