| With the development of the aviation industry,the requirements for applicable temperature and mechanical properties of high temperature structural materials increase.Intermetallic Ni3Al and NiAl have some excellent properties,such as high melting point,low density and excellent oxidation resistance,which make it an attractive candidate for the next generation of high-temperature structural materials.However,low room-temperature toughness and poor elevated-temperature strength limited its commercial application.Appling static magnetic field during material processing could change the substance transfer in the system.The introduction of magnetic field to directional solidification has been accepted as an effective way to control the microstructure evolution.In the present work,Ni3Al and NiAl based alloys were directionally solidified under a static magnetic field.The effect of magnetic field on the microstructure and mechanical properties were investigated.The major research results of this paper are as follows:1.The effect of magnetic field on the microstructure and mechanical properties of directionally solidified two-phase γ/γ’Ni3Al-based alloy,Ni-21.5Al-0.4Zr-0.1B(at%),was investigated.The experimental results indicated that the application of a high magnetic field caused the deformation of dendrites and occurrence of columnar-to-equiaxed transition(CET).The magnetic field tended to orient the<001>crystal direction of the equiaxed grains along the magnetic field.Further,the field modified the dendrite morphology and dendritic spacing.The tertiary dendrite at the periphery of the sample grew asymmetrically.The thermoelectric magnetic effects were analyzed by 3D numerical simulations.The results showed that the maximum value of thermoelectric magnetic force(TEMF)localized in the vicinity of the secondary dendrite arm root,which should be responsible for the dendrite break and CET.Based on the high temperature creep mechanism,a simple model was proposed to describe the magnetic field intensity needed for CET:B>kG-1.5R1.25.The model is in good agreement with the experiment results.The modification of dendrite morphology and dendritic spacing should be attributed to the transportation of solute caused by thermoelectric magnetic convection(TEMC).Moreover,the magnetic field caused the increase of tensile strength and decrease of elongation,which should be attributed to the combined action of fine-grain strengthening and work hardening.2.The effect of magnetic field on the microstructure and mechanical properties of directionally solidified two-phase β/γ’Ni3Al-based alloy,Ni-26.5Al-0.4Zr-0.1B(at%),was investigated.Results revealed that the primary β-NiAl dendrites were broken by the magnetic field,while the γ’ phase in Ni-27A1 kept directional growth,resulting in a novel two-phase composite consisting of single crystal matrix and equiaxed dendritic fragments.Comparing with the traditional structure with dendritic array,the new structure has better mechanical properties.The tensile strength and elongation are increased by 6.9%and 70.4%,respectively.Influence of magnetic field on the interdendritic γ’ structure formed by different phase transition process was studied.Theγ’ phase formed by peritectic reaction between the liquid and γ phase nucleated and grew around the y dendrites.When the magnetic field was applied,the γ dendrites were broken and the γ’ phase grew on the dendritic fragments,resulting in two-phase equiaxed grains.While the γ’ phase formed by eutectic reaction nucleated and grew independently from the β phase.Although the β dendrites were broken into pieces,theγ’ phase kept growing directionally under the magnetic field.3.The effect of magnetic field on the microstructure and mechanical properties in directionally solidified NiAl-Cr(Mo)-Hf eutectic alloy was investigated.Results indicated that the application of magnetic field caused the degeneration of regular lamellar eutectic structure at low withdrawal rates.The Cr(Mo)plates tended to spheroidization and coarsening during the cooling process.This is due to the decreased thermal stability caused by TEMF acting on the eutectic during solidification.In addition,the magnetic field modified the distribution of Hf solute,decreased the volume fraction of intercellular Heusler phase and increased the Hf concentration in NiAl matrix.The reasons are twofold,first,the TEMC brought a portion of Hf solute into the liquid phase,decreased the dendritic segregation;second,the high-density crystal defect promoted the counter-diffusion of Hf atom in solid phase,made more Hf atoms soluted into the matrix.Due to the redistribution of Hf solute,the mechanical properties of NiAl-Cr(Mo)-Hf eutectic alloy directionally solidified under magnetic field increased.Both the tensile strength and elongation were improved.4.The effect of transverse magnetic field on the microstructure and mechanical properties in directionally solidified NiAl-Cr(Mo)-0.2Si near-eutectic alloy was investigated.The results showed that the microstructure evolved from planar eutectic to primary NiAl dendrites + cellular eutectic and then to dendritic eutectic with the increasing withdrawal rate.To eliminated the dendrites at the medium withdrawal rates,transverse magnetic field was applied during the solidification.The field decreased the volume fraction of primary dendrites rapidly and the residual dendrites are partial to the periphery of the sample.The results should be attributed to the radial solute segregation caused by the TEMC.The solute segregate behavior is affected by the direction of TEMC and the solute distribution coefficient,independent of the density of solute.The existence of primary dendrites would seriously deteriorate the fracture toughness.By applying appropriate transverse magnetic field intensities and withdrawal rates,nearly fully eutectic structure was achieved.The fracture toughness of the alloys could reach 17.30 MPa·m1/2,which was about 3 times higher than that of alloys with NiAl dendrites.The fracture toughness is comparable to that of directionally solidified NiAl-31Cr3Mo eutectic alloy without Si addition.Therefore,the harm of impurity element could be eliminated by applying transverse magnetic field. |
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