Morphology Control Mechanism Of Primary And Eutectic Mg₂Si In Modified Al-Mg₂Si Alloys

The development of composite successfully combines the interisic advantages of the reinforcement and base alloy,and hence significantly improves the comprehensive properties of the alloy.Owing to the excellent physicochemical characteristics of Mg₂Si,Al-Mg₂Si alloy has a bright application prospect to meet the urgent need for materials with lightweight and high performance.Modifying the morphology of Mg₂Si in Al-Mg₂Si alloy is of significant importance as the mechanical properties of the alloys are sensitive to microstructure.The modification of Mg₂Si phase in Al-Mg₂Si alloys can be achieved through chemical modification,inoculation,and rapid solidification.In recent years,studies have shown that the addition of two types of modifying elements with large electronegativity difference could form a compound,which may act as nucleus and refine primary Mg₂Si through inoculation.Besides that,the combined addition of the adsorbing and nucleating elements may achieve a simultaneous modification effects.Moreover,there are limited studies focusing on the effect of rapid solidification on primary Mg₂Si in hypereutectic Al-Mg₂Si alloys.Also,the modification effect of the element on primary Mg₂Si in Al-20Mg₂Si alloys during rapid solidification is rarely reported.In addition,researches on the refinement of the eutectic phase in hypoeutectic Al-Mg₂Si alloys are mainly concentrated on the adsorbing elements such as Sc,Sr,etc and it is still indistinct whether the adding elements are able to play other roles while refining eutectic Mg₂Si.Also,challenges still remain to obtain a round,homogeneous eutectic Mg₂Si structure through modification.As a result,in the present work,the combined modification effect of adsorbing and nucleating elements on primary Mg₂Si in hypereutectic Al-Mg₂Si alloy was discussed.The modification mechanism of modifying elements on primary Mg₂Si under subrapid solidification in hypereutectic Al-Mg₂Si alloy was uncovered.Moreover,the spheroidization of eutectic Mg₂Si in hypoeutectic Al-Mg₂Si alloy was achieved through element modification,which is essential to develop high-performance Al-Mg₂Si alloys.The results are as follows:1)The modification mechanism of Be-Sb on primary Mg₂Si was uncovered:it is found that primary Mg₂Si can nucleating on Mg₃Sb₂ particles;Sb element can substitute the Si atoms in primary Mg₂Si crystal;Be element can selectively adsorb on{100}surfaces of primary Mg₂Si and supress the movement of advancing steps,leading to the exposure of Mg₂Si{100}surfaces.2)The coarsing of primary Mg₂Si under subrapid solidification was investigated and the mechanism was uncovered to be the narrow constitutional undercooling zone in solid/liquid interface under high cooling rate.The modification of Sr successfully inhibits the growth of Mg₂Si during the establishment of constitutional undercooling zone,and thus prevents grain coarsing of primary Mg₂Si.With the increase of cooling rate,Sb element exibits a higher probability to modify Mg₂Si through the substitution of Si in Mg₂Si crystal rather than heterogeneous nucleation.3)The spheroidization and over-modification mechanism of Sb were uncovered:Sb element inhibits the growth of eutectic Mg₂Si by substituting Si in eutectic Mg₂Si crystal and adsorbing at the advancing steps along the fast growth direction of eutectic Mg₂Si phase,which hinders the growth of eutectic Mg₂Si and results in the microstructure refinement;when the addition of Sb increases to 1.0 wt.%,the formation of large Mg₃Sb₂ particles consumes Sb atoms,causing the eutectic Mg₂Si particles in Al-11Mg₂Si alloy grow coarse again.4)The refinement of eutectic Mg₂Si in Al-11Mg₂Si is due to the adsorption of Ca and Sb atoms on Mg₂Si surfaces,which inhibits eutectic growth in the fast growth direction.The transition from hypoeutectic to hypereutectic structures was observed when varying Ca-Sb contents from 0 to 1.0 wt.%.The mechanism is also uncovered:Ca and Sb atoms can form heterogeneous nuclei for primary Mg₂Si,which greatly promotes the formation of primary Mg₂Si.Moreover,the segregation of Ca and Sb increases the constitutional undercooling during the early stage of Mg₂Si formation,which also promotes the formation of primary Mg₂Si.5)The spheroidization of eutectic Mg₂Si in Al-11Mg₂Si modified with Sb and Ca-Sb,respectively significantly improves the tensile properties of the alloy.With 0.2 wt.%Sb modification,the ultimate tensile strength increased from 183 MPa to 208 MPa and the elongation rose from 1.5%to 9.7%comparing to the unmodified counterpart.The significant improvement in tensile properties was due to the effective spheroidization of eutectic Mg₂Si after Sb modification.