Heterogeneous Nucleation Behavior Of Metallic Materials In The Steady Magnetic Field

As a common physical phenomenon,the solidification process widely exists in the nature and industrial production.The solidification process largely determines the microstructure and composition distribution of industrial products,thereby affecting product performance.As the initial stage of the solidification process,the nucleation process has a non-negligible effect on the phase composition,grain size and distribution of the solidification structure.In recent years,the steady magnetic field has become a very important means of controlling the solidification process owing to its various effects influencing on the solidification structure.Among these effects,the effect of the steady magnetic field on the nucleation is very worthy of attention.However,the current researches on the nucleation of metal melts in the steady magnetic field are very limited.Most of the studies focus on the experimental phenomena only providing qualitative explanations,which are lack of experimental verifications and not convincing.Meanwhile,the most physical quantities observed in these reseaches are in macro-scale,such as temperature,which are difficult to reveal the micro-scale heterogeneous nucleation and the mechanism of the effect of the steady magnetic field.The present work aims to investigate the influence of the steady magnetic field on the heterogeneous nucleation and solidification process of metal melts.Nonferromagnetic metals such as pure A1 and Al-Cu alloys are selected and different experiment methods are employed to control the nucleant during the solidification.The main results are as follows:Firstly,the undercooling behaviors of pure metals,i.e.,pure Al,pure Sn and pure Zn,and three Al-Cu alloys,i.e.,Al-4.5wt%Cu,Al-26wt%Cu and Al-45wt%Cu,in a steady magnetic field were investigated by differential thermal analysis.There was no obvious change in melting temperatures of pure metals in the steady magnetic field and through the thermodynamic estimation,the transformation temperature of liquid-solid phase transformation of the non-ferromagnetic metal was found to be almost unvaried under the action of a steady magnetic field.The nucleation temperatures of three pure metals and three Al-Cu alloys were found to decrease and correspondingly,their undercoolings increased in the steady magnetic field,implying that the nucleations of all pure metals and alloys were suppressed in the steady magnetic field,regardless of cooling rate,magnetism and crystal structure of the nucleated phases.From the nucleation kinetics analysis,the parameters in the nucleation,for instance,the interfacial energy between the liquid phase and nucleus and/or contact angle,can be influenced in the steady magnetic field,while the mechanism of nucleation is unaffected.Secondly,the nucleation and growth of primary α-Al phase in 2024Al alloy melt inoculated with Al5TiB master alloy in a steady magnetic field were studied by differential scanning calorimeter.It was found that the nucleation temperature of primary α-Al phase decreased in the steady magnetic field,i.e.,the undercooling was enhanced.The micro structure of primary α-Al phase in the steady magnetic field showed an enhanced degradation in grain refinement and a cellular-dendritic transition of equiaxed primary a-Al grains.According to the free growth model,the enhanced degradation in grain refinement and increase in undercooling in the steady magnetic field can be ascribed to the modified liquid/nucleus interfacial energy and the delay of formation of critical nucleus due to the retarded migration rate of atoms in the liquid phase.The cellular-dendritic transition of primary α-Al grains is attributed to the modified constitutional undercooling at the solid/liquid interface,which results from the suppressed nucleation of primary α-Al grains and the change in solute distribution by the damped convection and retarded diffusivity in the SMF.The critical growth velocity of cellular-dendritic transition is also suppressed due to the retarded diffusivity in the SMF,resulting in the dendritic morphology of primary α-Al grains.Additionally,the increase in growth dimension and the modified solid/liquid interfacial free energy under the SMF are also responsible for the enhanced constitutional undercooling and the cellular-dendritic transition.Finally,the nucleations of pure Al and Al-4.5wt%Cu alloy triggered by the singlecrystal Al2O3 substrate with a certain orientation in the steady magnetic field were investigated.The nucleation temperatures of pure Al and Al-4.5wt%Cu alloy melts were decreased and their undercoolings were increased when the steady magnetic field was applied.The good lattice matchings of(110)Al/(1010)Al2O3 and(311)Al/(1010)Al2O3 were shown at Al/Al2O3 and α-Al/Al2O3 interfaces without the steady magnetic field,respectively.However,(110)and(311)Al planes in pure Al and primary α-Al phase were deviated about 10°from the interface in the steady magnetic field and thereby,the lattice misfits increased.The increase in undercooling and the change in lattice matching can be attributed to the repulsive effect of Al atoms in the ordered layer induced by the magnetic dipole-dipole interaction in the steady magnetic field.The repulsive effect results in the increase in atom distance and the change in atom arrangement in the ordered layer,which directly influences the lattice matching between the nucleus and substrate.Besides,the increase in interfacial energies of ordered layer/substrate and ordered layer/melt can be induced by the varied atom arrangement in the ordered layer.Therefore,the nucleation where the ordered layer acts as the nucleus is suppressed due to the increased nucleation barrier and a larger undercooling is needed to initiate the nucleation.