The Formation And Growth Mechanism Of Nano- Eutectic In Mg-Zn-Y Alloy With Semisolid Process

In I-phase strengthened Mg-Zn-Y alloy, higher strengths could be obtain without losing much ductility. In order to obtain ultra-high strength of I-phase strengthened Mg-Zn-Y alloy, adjusting the distribution of I-phase on the a-Mg matrix can be a feasible program. The purpose of this study is to minimize the lamellar spacing of the (I-phase+α-Mg) quasi-eutectic and to get the critical condition under which (I-phase+a-Mg) nano-eutectic could be obtained, as well as the morphology of (I-phase+α-Mg) nano-eutectic, the interface between I-phase and a-Mg and the Thermostability of nano-eutectic.The technical plan for minimize the spacing of quasi-eutectic is to re-melt the (I-phase +α-Mg) quasi-eutectic or I-phase particles under the semisolid temperature, and then to cool the samples under fast cooling rate to obtain (I-phase+α-Mg) nano-eutectic. The involving investigation is as following:1) to study the evolution of microstructure of Mg-Zn-Y alloys under isothermal holding at 540℃,2) to study the effect of cooling rate on the (I-phase+α-Mg) quasi-eutectic and to obtain the critical cooling rate of the formation of (I-phase+α-Mg) nano-eutectic,3) to investigate the morphology of I-phase in the (I-phase+α-Mg) nano-eutectic, the interface between I-phase and a-Mg and the orientation relationship under TEM,4) to observe the Thermostability of (I-phase+α-Mg) nano-eutectic.The results of study showed that the entrapped globular liquid in the cast Mg-6xZn-xY(x=0.5,1) alloy is coarsening under the isothermal holding process with both Ostwald ripening and coalescence, but the spheroidization of a-Mg solid phase is proceeding slowly. For the extruded Mg-6xZn-xY(x=0.5,1) alloy after isothermal holding, the coarsening rate of a-Mg solid grain is progressively slower over time. With the higher liquid fraction, the degree of coarsening is lower and the rate of spheroidization is quicker.The spacing of (I-phase+α-Mg) quasi-eutectic λ is inversely proportional to cooling rate v, and they have the relationship, λ=12.0771/v-0.0185. The critical condition for the formation of (I-phase+α-Mg) nano-eutectic is between cooling in 60℃ water and 40℃ water.The morphology of I-phase in (I-phase+玖-Mg) nano-eutectic is polygonal rod and the lattice structure of I-phase is primitive icosahedron with slightly distortion. The orientation relationship of I-phase and a-Mg is that (120120) II (1010)α-Mg and (111211) || (0111)α-Mg. The interface of I-phase and a-Mg is semi-coherent and (111211) || (0111)α-Mg || interface.The fine (Ⅰ-phase+α-Mg) quasi-eutectic shows good Thermostability under 350℃, and the tiny spacing did not coarsen. Consequently, the I-phase in the nano-eutectic could be broken up into little particles, avoiding getting coarsening under the wrought process.