Effect Of Magnetic Field On Crystallography And Microstructure Of As-cast Al-based Binary Alloys

In this work, hypereutectic Al-3.31wt.%Fe, hyperperitectic Al-1.36wt.%Zr and hypereutectic Al-95.54wt.%Zn alloys are solidified in non-directional ways without and with high magnetic field, and two isomorphous Al-9.8wt%Zn and hypoeutectic Al-0.24wt%Fe alloys are semi-continuously cast without and with conventional direct current (DC) magnetic field. The effects of the magnetic field on the crystallography and microstructures of these alloys are experimentally and theoretically studied with EBSD technique.Without a high magnetic field, a crystallographic feature investigation on the primary Al3Fe, Al3Zr and αZn solid solution phase in the respective Al-3.31wt.%Fe, Al-1.36wt.%Zr and Al-95.54wt.%Zn was carried out. For the primary Al3Fe crystals, they are bar-shaped and possess two preferred crystallographic extension directions (<010> or<011>). Compound twin and type I twin are found in them (their complete twinning elements have been determined). Type I twin causes the crystals to bend. The Al3Fe crystals are in forms of monoclinic or triclinic prisms. The crystallographic planes have been identified as (100) and (001) for the monoclinic prism in the extension direction and (001),(001) and (02-1) for the triclinic prism. The formation of the twins is attributed to the (100) and (001) planes exposed to the Al melt and then the stacking faults during the growth. For the primary Al3Zr crystals, they are tabular and display two forms of longitudinal cuts-octagonal (smaller sized) and rectangular (larger sized). The faceted planes binding the crystals are determined as{001},{101} and{111} for the smaller sized crystals and only{001} and{101}) for the larger sized ones. One compound twin with two variants is found in the dendritic crystals (the complete twinning elements are determined). The tabular shape is attributed to the low roughness of{001} planes and thus the low migration rate. The disappearance of{111} planes in the larger sized crystals result from their high roughness and thus the high migration rates. The twin formation is realized by lattice shear plus local atom stacking fault accompanied by local atomic reshuffling. For the primary αZn solid solution cystals, it is found that they are dendritic and composed of a short primary trunk growing along <0001> and six long secondary arms growing along<10-10>. The dendritic morphology is related to the liquid/solid interfacial energy anisotropy.The applied magnetic field shows little influence on the above crystallographic features of these two crystals but exhibits remarkable influence on some other precipitation behaviors of them. A uniform high magnetic field tends to eliminate the gravity segregation of the primary Al3Fe crystals by the magnetic viscosity force, whereas a positive gradient one moves them towards the maximum filed intensity zone to segregate at the other end of the specimen by the magnetization force. However, the distribution of primary Al3Zr crystals is not strongly affected by the magnetic field due to their high density-most of them are segregated at the lower part of the ingot. The magnetic torque induced by the magnetic anisotropy tends to rotate the paramagnetic primary Al3Fe and Al3Zr crystals in the deposit layers and thus creates strong preferential crystallographic orientation in the field direction. However, the alignment of the long bar-shaped Al3Zr crystals and dendritc ones is disturbed by the gravity force and the inconsistence of the preferred direction between the branches and trunks, respectively. The magnetic field also directly or indirectly induces bifurcation and cracking of the Al3Fe through the thermoelectric magnetic force and the shrinkage coefficient difference between the Al3Fe and the Al matrix. Crystal detachment plus magnetization energy and solute diffusive suppression enhances the nucleation of the primary Al3Fe The interaction between the increased crystals by the increased Zr content weakens the alignment tendency of the Al3Zr crystals in the deposit layer in the field direction. For the primary αZn solid solution phase, the distribution rule is similar to that of the primary Al3Fe phase, whereas the alignment rule is similar to that of the primary Al3Zr phase. With a uniform high magnetic field, they tend to be distributed homogeneously in the specimen and align with the long axies parallel to the field direction. However, the orientation rule is different from the previous two primary phases:no specific crystallographic direction appears in the field direction, but<0001> is always in the perpendicular one (they are randomly orientated in the plane perpendicular to the field direction). This is related to the diamagnetism of Zn. During the semi-continuous casting, the conventional DC magnetic field transfers the mixture of equiaxed and columnar grains in Al-9.8wt%Zn to twinned feathery ones, and the columnar grains in Al-0.24wt%Fe to twinned lamellas. Both transformations are accompanied by a change of growth directions from<100> to<110> and finally result in the formation of strong crystallographic texture and CSL∑3boundaries. The transformations are attributed to the modification of the solidification conditions at the liquid/solid front by the Lorentz force induced by the magnetic field.The experimental examination and theoretical study of the effect of the magnetic field on the crystallography and microstructure of the aluminum alloys during solidification carried out in this work contributes to the enrichment of knowledge of Electromagnetic Processing of Materials (EPM) and are of both theoretical significance and technical interest. The positive results on gravity segregation control and texturation of aluminum alloys through the application of a magnetic field could offer useful guidance for practical materials processing techniques.