| As a lightweight material,magnesium alloys have great advantages and potential applications.However,the application of magnesium alloys is restricted by the low absolute strength,poor plasticity at room temperature,lack of effective strengthening method and other factors.Grain refinement is one of the effective ways to improve the strength of magnesium alloy.At present,more than 90%of magnesium alloy products are still obtained in the form of castings.Carbon inoculation is one of the most effective refining methods for as-cast Mg-Al alloy.However,its refinement effect is unstable,and the nuclei is susceptible to Fe?Mn?"poisoning"resulting in accelerated inoculant-fading.At present,there is no consensus on the mechanism of the influence of Fe?Mn?element on the refinement effect.Previous studies have found that Ca?Sr?can inhibit the adverse effect of Fe?Mn?on the refinement of carbon inoculation.This study intends to investigate Mg-3Al alloys containing trace Fe?Mn?elements.The influence of carbon inoculation combining with Ca?Sr?on the formation and structural evolution of carbonaceous nuclei,as well as the synergistic effect between solute elements and carbonaceous nuclei will be discussed in depth.The research results are as follows:Carbon inoculation combining with Ca?Sr?can effectively refine the Mg-3Al alloy,and the grain size is refined from 623±21?m to 127±5?m and 114±5?m,respectively.The refining effect is stable,and there is no inoculant-fading occurred after holding 80 mins.The addition of 0.2%Ca?Sr?can increase the grain refinement ratio of 10%on the basis of carbon inoculation,and has a good synergistic refinement effect.EPMA observations indicate that the Mg-3Al alloy treated by carbon combining with Ca?Sr?inoculated mainly contains Al-C,Al-Ca and Al-Sr particles.As a surface active element,Ca?Sr?increases the constitutional undercooling at the front of the solid/liquid interface and promotes the heterogeneous nucleation.These particles provide extra constitutional undercooling to the subsequent nuclei during growth and promote the nucleation of subsequent grains.Therefore,the number of effective nucleation particles is increased objectively.After the addition of the Ca,the effective nucleation particles size decreases about 2?m.The first principle calculation results show that the Ca element can significantly reduce the interface energy between Al4C3/Mg,and increase the adsorption energy of Al4C3 to Mg.This will promote the adsorption of more Mg atoms to the surface of Al4C3particles for nucleation.The refining effect of Fe on the grain of Mg-3Al alloy is not significant.The average grain refinement ratio of the Fe-containing Mg-3Al alloys refined by carbon inoculation combining with Ca?Sr?was significantly improved to about 75%.Ca?Sr?can effectively inhibit the adverse effect of Fe?Mn?on the refining efficiency and the fading of carbon inoculation.In the melt of Mg-3Al-Fe treated by carbon combining with Ca?Sr?inoculated,there are exist Al4C3 cluster particles and duplex-phase structure particle with Al13Fe4 in the core and Al4C3 in the shell.The refinement mechanisms are Al4C3??-Mg,andstructure particle??-Mg,respectively.The adsorption thermodynamics model was established to calculate and analyze the Gibbs free energy changes during the nucleation process.The results of the thermodynamic model show that the Gibbs free energy decreases with the increase of the number of adsorption layers.The formation and growth of duplex-phase structure particle and Al4C3 cluster particles lead to a reduction in the Gibbs free energy of the entire system,therefore these particles can spontaneously form and grow in the melt.In the initial stage of inoculation,the probability of the formation of two kinds of particles is almost equal.With the prolongation of holding time,the tendency of the adsorption process to form duplex-phase structure particle is more pronounced than agglomerative growth,and the smaller Al4C3 particles tend to adsorb to the surface of the Al-Fe particles.The Gibbs free energy changes of adsorbed duplex-phase structure particle of different layers were negative after the addition Ca?Sr?.The addition of Ca?Sr?reduces the resistance to the formation of duplex-phase structure particle and promotes the formation of duplex-phase structure particle.The average refining ratio of Mn element to Mg-3Al alloy is about 40%.This result indicates that Mn has a limited refining effect on Mg-3Al alloy.After carbon combining with Ca?Sr?inoculated,the grain size of Mn-containing Mg-3Al alloy can be refined to 80-140?m,and the average grain refinement ratio is more than 80%and higher than the melt containing Fe.With the addition of Ca?Sr?,the addition sequence and holding time of Mn have no effect on the grain refinement effect,and the grain size does not coarsen with the extension of holding time.There are differences in the influence of Fe and Mn on the structural of carbon nuclei.In addition to the presence of Al-C nuclei particles,duplex-phase structure particle with Al8Mn5phase in the core and pseudomorphous carbon in the shell were found in Mn-containing melts.The adsorption energy of C on the surface of Al8Mn5 and Al13Fe4 are calculated by the first principle.The results show that C is easier to adsorb on the surface of Al8Mn5.In the initial stage of nucleation of Mn containing melts,the C atom is easy to be adsorbed on the surface of Al8Mn5 for epitaxial growth.During the solidification,Mg atoms are stacked on the surface of C layer.The strain energy accumulated during the growth process will be released due to the large difference between C atoms and Al8Mn5 and Mg atoms.Influenced by the strain energy release of Al8Mn5 and Mg,the C layer exists in an pseudomorphous state with a thickness of about 100 nm.Based on the epitaxial growth model,the pseudomorphous C layer and Mg matrix are completely coherent,and the mismatch is close to 0,less than the mismatch between Al4C3/Mg.This special duplex-phase structure particle has higher nucleation efficiency than duplex-phase structure of Al13Fe4/Al4C3,which makes the grain refinement ratio of Mn-containing Mg-3Al alloy higher than that of Fe-containing Mg-3Al alloy.The solidification characteristic parameters can be obtained from the solidification cooling curve.The initial nucleation temperature Tnand the recalescence phenomenon can be used to qualitatively judge the effect of grain refinement.The initial nucleation temperature Tn of Mg-3Al increases after carbon inoculation and the recalescence phenomenon disappeared.When the nuclei is"poisoned",the initial nucleation temperature Tndecreases and recalescence phenomenon occurs.Finally,the grains become coarse and the tensile strength decreases.After carbon inoculation combining with Ca,the initial nucleation temperature Tn increases and the recalescence phenomenon disappears.The mechanical properties of the alloy are consistent with the grain refinement effect.This research has confirmed that carbon combining with Ca?Sr?inoculation can efficiently refine Mg-Al alloys and achieve"poison-free"effect.The internal mechanism of Ca?Sr?on the formation and structural stability of nuclei is revealed by the thermodynamics model.It is found that nucleation activity is positively correlated with nucleation undercooling.These results have positive guiding significance and reference value in design and development of grain refiner of magnesium alloy,promotion of carbon inoculation technology,and on-line assessment of grain refining efficiency of carbon inoculation. |
PDF Full Text Request