Toughening/antioxidation Modification And Shock Wave Synthesis Of Nb-Al High Temperature Alloys

Along with the advance of aerospace technology,the high-temperature structural materials have drawn much attention due to high temperature resistance,high specific strength,high creep strength and good oxidation resistance.Compared with Fe-Al,Ni-Al and Ti-Al superalloys,niobium-aluminum alloy with high melting point,low density and high operating temperature is regarded as one of the most promising ultra-high temperature materials.However,the brittleness,weaker high temperature strength and oxidation resistance of niobium-aluminum superalloy restrict its applications in industry.The relationship between microstructure and mechanical properties,fracture and oxidation mechanism remain to be elucidated.In view of that,the responses of niobium-aluminum superalloy to pressure and axial stretching were studied.Besides,the new structures of niobium-aluminum alloy were predicted by CALYPSO software and structural analogy method.The stability characteristics of alloy surface have been also taken into account.And we studied the feasibility of improving the oxidation resistance of the alloy by B atom substitution.In addition,Nb-Al alloy was prepared by shock wave synthesis.The results showed that there was a covalent bond in tetragonal Nb2Al due to the hybridization of Nb-4d and Al-3p,and the Nb2Al structure was mechanically stable from 0 to 100 GPa.The mechanical properties including bulk modulus B,shear modulus G,Young's modulus E,E/G and Poisson's ratio were determined using Voigt-Reuss-Hill method,showing the tetragonal Nb2Al exhibits ductile behavior under pressure.The thermal data of Nb2Al under pressure were estimated for the first time.The structural parameters of tetragonal NbAl3 and cubic Nb3Al decreased linearly with the increasing pressure.The enhanced covalence was found due to the increased hybridization between Nb-4d and Al-3p states.The elastic constants showed that the tetragonal NbAl3 had a brittle-to-ductile transition pressure at 83.2 GPa.With respect of NbAl3 to uniaxial tensions along the[100]and[101]directions,the crystal structure was changed from tetragonal to orthorhombic phase.The NbAl3 crystal has been classified as brittle material associated with its relatively large value of hardness.Combinimg with mechanical and electronic properties,we found that the decreased mechanical properties were mainly due to the weakening of covalency.A new monoclinic phase of NbAl3 was proposed by particle swarm optimization.The calculated results suggested that the known tetragonal structure was more stable than monoclinic structure,but the ductility of monoclinic phase was better than that of tetragonal phase.According to the structural analogy method,the D023,monoclinic Clml,R3m and Ll2 structures of NbAl3 were predicted.The phase transitions between monoclinic Clml and cubic Ll2,trigonal R3m and cubic Ll2 have been predicted.According to the values of B/G and Poisson's ratio,we concluded that the trigonal phase was ductile.In addition,the addition of Ti element reduced the deformation resistance of the a-axis and the c-axis of the NbAl3 alloy,and reduced the resistance to deformation and shear resistance,while its toughness was improved.The increase of metallic property and decrease of covalency appear after the terminated surfaces were formed.Besides,the work functions of Nb2Al(100)and low index surfaces of NbAl3 were calculated for the first time.For the low-index surfaces of NbAl3,the calculated atomic relaxation and surface energy suggested that the(111)surface was the most stable stoichiometric surface in both Al-rich and Nb-rich conditions.Ellingham diagrams suggested the Boron oxidizes in preference to Niobium and aluminum.Hence,we performed first principles calculations to study the stability of B at possible sites in the NbAl3(100)surface.The results predicted B favor substitution.The calculations showed that the B atom in substitutional sites attracts nearby atoms and leads to increase ionic bonding.The stability of All substitution was stronger than that of the A12 substitution site with the same layer,and the stability of two substitution sites decreased with the increase of the number of atomic layers.Therefore,the replacement of B atom could spontaneously move along the All substitution position from the inside of the crystal to the surface and form an effective antioxidant layer on the crystal surface with oxygen.In the experiment,we used the two-stage light gas gun to study the feasibility of the method of preparing niobium-aluminum alloy by impact compression.By designing and improving the sample recovery box,niobium aluminum alloy was successfully prepared at 2.52 km/s and 2.60 km/s impact speed.