Modification of Primary Mg 2 Si in Mg-4Si Alloys with Antimony
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IN recent years, the fascinating properties and promising application of Mg–high Si alloys have attracted particular interest due to the formation of thermally stable Mg2Si phase.[1,2] The intermetallic compound Mg2Si, which has anti–CaF2-type structure with a lattice constant of a = 0.6351 nm, acts as an effective strengthening phase that possesses high melting temperature [1358 K (1085 C)], low density (1.99 9 103 kg m–3), high hardness (4.5 9 109 N m–2), and low thermal expansion coefficient (7.5 9 10–6 K–1).[3–8] A desirable as the physical traits are, Mg2Si phases are prone to develop undesirable morphologies characterized by large primary dendrites and long Chinese script eutectics in Mg–high Si alloys when produced by conventional methods such as ingot metallurgy, which would deteriorate the mechanical properties of materials.[5,6] Generally, modifying treatment by addition of minor elements into melts is a basic practice to improve the strength, toughness, and machinability of materials.[4] Traditional modifying elements such as P, Ca, Sr, Sn, and Sb are typically used to modify the Chinese script Mg2Si in Mg–low Si alloys,[9–13] among which, however, the exact effect of Sb on eutectic Mg2Si remains unclear or even debatable up until now. Some studies[11–16] have reported Sb (~0.2 to 0.5 wt pct) as an effective modifying agent in Mg–low Si alloys, which can modify the eutectic Mg2Si from coarse Chinese script morphology to fine particles or polygon shapes. Moreover, Yuan et al.,[12] Srinivasans et al.,[13] and Liao et al.[14] suggested that Sb could combine with Mg to form a Mg3Sb2 compound, which might act as a heterogeneous nucleus for eutectic Mg2Si phase during solidification.
Different from this view, Yang et al.[17] suggested that adding 0.4 wt pct Sb in AZ61-0.7Si alloys refined the Chinese script Mg2Si by increasing the undercooling degree of melts, while the modification of Mg2Si in morphology was not obvious. Besides, Quimby et al.[18] argued that the 1.5 wt pct Sb addition can neither modify the morphology nor refine the size of eutectic Mg2Si in AS21 alloy; i.e., Sb does not have a desired modification effect. As aforementioned, there still remains a great debate in the modification effect of Sb on the eutectic Mg2Si in Mg–low Si alloys, and more studies need to be performed. Nevertheless, to our best knowledge, limited research has been carried out and hence little published information is available on the influence of Sb addition on primary Mg2Si in Mg–high Si alloys.[19] Guo et al.[19] reported that only with the Sb addition increasing to 1.2 wt pct were coarse dendritic primary Mg2Si crystals effectively modified into small particles, while the morphologies cannot be effectively modified by Sb under the condition of Sb content less than 0.8 wt pct in Mg-4.8Si alloys. Simultaneously, they suggested Mg3Sb2 particles might act as nucleating substrates for primary Mg2Si crystals during solidification, and further research is needed to clarify the modification mechanism.[19] Consequently, in this study, we a
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