| Al-Si casting alloys are widely applied in automobile production and other industry fields because of their high specific strength,good castability and machining performance.Grain refinement induced by inoculation of refiners can improve mechanical properties and metallurgical quality of aluminum castings.In this thesis,an improved method to prepare novel Al-2La-1B and Al-2Ce-1B refiners suitable for Al-Si casting alloy was developed.There was neither poisoning effect between two refiners and silicon element nor negative influence on Sr-modification effect.It realized the grain refinement,defect control and properties improvement of Al-Si casting alloys.Some certain orientation relationships?ORs?calculated with E2EM model were demonstrated by TEM analysis.It revealed the refinement behavior and mechanism of LaB6/CeB6 through investigating the reliability of refining ability on alloying elements.Both Al-2La-1B and Al-2Ce-1B refiners can significantly refine Al-Si casting alloys.The nucleation capacity of CeB6 particles is higher than that of LaB6 particles.When the addition level is the same,the refining effect of Al-2Ce-1B refiner is better than that of Al-2La-1B refiner.In present work,there is an optimal value of 1.0wt%for the addition level of refiners.After adding 1.0wt%Al-2Ce-1B refiner,the mean grain size of Al-7Si-4Cu decreased from 1200?m to 240?m.The ultimate strength,yield strength and elongation of Al-7Si-4Cu as-cast specimen is 250.6Mpa,130.8MPa and 4.5%,which increased 22.6%,33.3%and 66.7%,respectively.The fluidity of the alloy was also improved.The other two alloys showed similar principle.Three different possible ORs between LaB6/CeB6 and Al can be predicted,which are?100?Al//?100?LaB6/CeB6,[001]Al//[001]LaB6/CeB6;?110?Al//?110?LaB6/CeB6,[001]Al//[001]LaB6/CeB6;?11????Al//?11????LaB6/CeB6,[011]Al//[011]LaB6/CeB6.The predicted results are perfectly confirmed through TEM analysis and prove the nucleation potency of LaB6/CeB6.After adding refiner,the undercooling required for the nucleation of?-Al was significantly reduced,the nucleation rate was obviously increased,and the grain size of the alloy was remarkably reduced after solidification.The refining mechanism of Al-2La-1B/Al-2Ce-1B refiner is the direct nucleation of primary aluminum on the surface of LaB6/CeB6 particle.In this case,the lattice mismatch and interface state between two phases obviously influence the refining behavior.The lattice parameter of?-Al alloying elements with larger atomic radius is larger than pure Al,e.g.Mg,Sr and Y.As a result,the lattice mismatch between LaB6/CeB6 and Al decreases,the interface energy between LaB6/CeB6 and Al is smaller during nucleation,the undercooling required for the nucleation of primary aluminum on LaB6/CeB6 surface is lower.While it shows inverse effect after alloying elements with smaller atomic radius,e.g.Cu,Fe and Zn.The results of high resolution transmission electron microscopy showed that the arrangement rules of Al atoms at the interface between LaB6 and?-Al in Al-2.0Mg alloy were regular.There is a‘transition zone'of Al atoms at the two-phase interface of Al-2.1Cu alloy,and the expansion of Al atom spacing and dislocation occur in the‘transition zone'.When atoms are arranged in regular order,they are in the lowest energy state.Compared with the interface between LaB6 and Al matrix in Al-2.1Cu alloy,the interface energy between LaB6 and Al in Al-2.0Mg alloy is lower.Therefore,it required less undercooling for primary aluminum nucleation in Al-Mg alloy. |
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