Study On The Effect Of Static Magnetic Field On Directional Solidification Structure And Mechanical Property Of Two-phase NiAl/Ni₃Al Based Alloy

Because of the excellent mechanical properties,Ni-Al intermetallic compounds are widely considered as a new generation of aero-engine blade materials which are expected to replace traditional nickel-based superalloys.Among them,the?/??dual-phase Ni₃Al alloy has been successfully applied in the aerospace field.Although the?/??dual phase Ni₃Al alloy prepared by increasing the Al content can obtain the advantages of density and high temperature strength,it is difficult to solve the problem of room temperature brittleness due to the introduction of?-Ni Al.For the casting alloy,the solidification structure plays a decisive role in the final mechanical properties.The application of external magnetic field can effectively control the microstructure and thus improve the mechanical properties.In this work,Al-rich Ni₃Al based intermetallic compounds were used as the research object.The influence of magnetic field on the microstructures and mechanical properties of?/??dual-phase Ni₃Al alloy during directional solidification were studied by applying longitudinal magnetic fields with different intensities.The microstructures,crystal orientation and mechanical properties of the alloy under different conditions were systematically analyzed by means of metallographic microscope(OM),X-ray computed tomography(XCT),scanning electron microscope(SEM),electron backscatter diffraction(EBSD),high-resolution transmission electron Microscope(HRTEM),room/medium/high temperature tensile test,numerical simulation,hence revealing the regulation of the directional solidification structure of the alloy under the constant magnetic field and the mechanical properties of the alloy with different microstructures were also studied,the main conclusions are as follows:(1)The effect of magnetic field on directionally solidified microstructures of the alloy at low drawing rate were studied.It is found that the Ni Al dendrite were broken by the magnetic field at a speed of 10?m/s,which makes it exist in the form of particles on the Ni₃Al matrix;Furthermore,it is found that the fractured Ni Al particles maintain a certain<001>preferred orientation at low magnetic field,and the orientation of Ni₃Al matrix is not affected by magnetic field,forming a novel composite structure composed of unidirectional Ni Al particles and single crystal Ni₃Al matrix.However,at high magnetic field,the orientation of Ni Al particles is disordered,forming a composite structure composed of randomly oriented Ni Al particles and Ni₃Al matrix.(2)The effect of magnetic field on directionally solidified microstructures of the alloy at high drawing rate were studied.It is found that the magnetic field is not enough to break the dendrite when the drawing speed increased to 50?m/s and 100?m/s,but the magnetic field can significantly promote the growth of high-order dendrite,thus forming a dendrite network in space;Furthermore,it is found that the effect of magnetic field on the orientation of Ni Al dendrite is weakened when the drawing speed is higher,and it can't induce the columnar-to-equiaxed transformation(CET).(3)The room temperature mechanical properties of the alloy with different directionally solidified structures were studied.It is found that the novel composite structure composed of unidirectional Ni Al particles and single crystal Ni₃Al matrix induced by the low magnetic field can increase the tensile strength and elongation by19.5%and 78.3%respectively,and the strength and plasticity are improved at the same time,which is due to the improvement of Ni Al phase distribution and the special interface structure of?/??;At the same time,on the premise of avoiding the interference of transverse grain boundary in high magnetic field,it is found that the composite structure composed of randomly oriented Ni Al particles and Ni₃Al matrix can also play a positive role in the mechanical properties of the alloy.(4)In addition,the high temperature mechanical properties of the novel composite structure were studied.It is found that the tensile strength of the alloy at 650?is significantly higher than that at room temperature,which is due to the abnormal strength temperature characteristics of Ni₃Al;At 1000?,the high temperature strength of the alloy is significantly improved by the composite structure without decreasing the elongation.This is due to the increase of interface area of Ni Al after crushing,which increases the density of dislocation network and thus hinders the movement of dislocation.