Preparation And Properties Of Layered Ternary Borides In Cr-Al-B System Of MAB Phases

The intrinsic brittleness and poor oxidation resistance of binary transition metal borides are main obstacles that impede their applications in extreme environments.In order to overcome the inherent brittleness and improve their toughness and oxidation resistance,an effective way is to intercalate layers of group III_A or IV_A elements,e.g.Alor Si,into the orthorhombic binary transition metal borides sublattices to form nanolaminated ternary transition metal borides called MAB phases,wherein M is a transition metal,A is a group III_A or IV_A element,and B is boron.Up to now,the known MAB phase compounds mainly include Cr₂AlB₂,Cr₃AlB₄,Cr₄AlB₆,Mn₂AlB₂,Fe₂AlB₂,MoAlB and WAlB.However,researches on compounds in Cr-Al-B system,which plays a representative role and contains the most variants,are relatively rare due to the difficulty in preparation of single-phase compounds.Based on the above background,the preparation process,structure and properties of materials in Cr-Al-B system of MAB phases are systematically studied in this work.In this thesis,binary CrB was first prepared,and then the new kind of ternary transition metal borides(Cr₂AlB₂ and Cr₄AlB₄)in Cr-Al-B system were successfully prepared by introducing Alinto CrB.The mechanical properties and oxidation behavior of this new kind of ternary borides were studied in detail.To reveal their prospect in functional materials,the electromagnetic wave absorption properties were also investigated.The main research contents and conclusions are listed as follows:(1)Phase pure CrB powders and bulk with relative density of?97%were prepared by a simple and efficient high-temperature solid phase in-situ reaction method,using B and Crpowders as raw materials.Through Rietveld structure refinement of the powder XRD data,a new set of diffraction data are presented for CrB which make up for the deficiency of the existing data in ICDD PDF^#32-0277(CrB).The elastic modulus,bending strength,fracture toughness,Vickers hardness,and electrical resistivity of CrB bulk are 476±1 GPa,418±35 MPa,5.7±0.4 MPa·m^1/2,19.3±0.2 GPa,and 19.3±0.6??·cm,respectively,indicating that binary CrB is a brittle ceramic material with high modulus,high hardness,high strength and metallic electrical conductivity.(2)Pure phase Cr₂AlB₂ powder was prepared from CrB and Alpowders.A mature preparation process of pure phase Cr₂AlB₂ powder was obtained.The oxidation behavior and thermal stability of Cr₂AlB₂ powders were systematically investigated up to 1200°C.Cr₂AlB₂ is thermally stable up to 900°C in argon,partially decomposes into Cr₄AlB₄ and Alat 950-1070°C,and partially decomposes into CrB and Alat1090-1200°C.Revealing the decomposition mechanism of Cr₂AlB₂ is helpful to better understand its oxidation behavior.The initial oxidation temperature of Cr₂AlB₂ powders is 680°C in air.Oxidation kinetics of Cr₂AlB₂ powders exhibits an initial rapid linear oxidation followed by a slow parabolic oxidation behavior.A protective Cr₂O₃(700-750°C)or Al₂O₃(800-1200°C)scale is formed,which makes Cr₂AlB₂ exhibit good oxidation resistance from medium to high temperatures.The oxidation mechanism of Cr₂AlB₂ powders at different temperatures was revealed.Thermodynamic calculation results associated with the oxidation reactions and the phase stability are consistent with the experimental results,which support the experimental conclusions.(3)The electromagnetic(EM)wave absorbing properties of Cr₂AlB₂ powders were investigated in 2-18 GHz.Coupling of magnetic and dielectric loss enables Cr₂AlB₂powders with good absorption performance.The minimum reflection loss(RL)is-44.9d B(8.5 GHz,2.7 mm)and the optimal effective absorption bandwidth(E_AB)is 4.4 GHz(13.0-17.4 GHz,1.6 mm).Good high-temperature oxidation resistance makes Cr₂AlB₂powders oxidized at 750-1000°C retain good EM wave absorbing properties.With an absorber thickness of 1.5-4.0 mm,the RL value of less than-10 d B(90%absorption)can be obtained for both the Cr₂AlB₂ powders and the oxidized Cr₂AlB₂ powders in a broad frequency range of 5.0-18.0 GHz,which basically covers the microwave frequencies of the C(4-8 GHz),X(8-12 GHz),and Ku(12-18 GHz)bands.These results indicate that Cr₂AlB₂ is an oxidation-resistant high performance wave-absorbing material.(4)2D-CrB nanocrystals are successfully prepared by completely etching out Alatomic layers from Cr₂AlB₂ using dilute HCl as etchant.This is the first reported fully etched two-dimensional transition metal boride(MBene).A system of materials preparation from CrB to Cr₂AlB₂ and then to 2D-CrB is well established.(5)A new MAB phase compound Cr₄AlB₄ was discovered in Cr-Al-B system.Its crystal structure was determined by a combination of first-principles calculations,XRD structure refinement and transmission electron microscopy analysis,and the complete powder diffraction data was also obtained.Cr₄AlB₄ belongs to orthorhombic crystal system with Immm space group,the lattice parameters are a=2.9343(6)(?),b=18.8911(0)(?),c=2.9733(7)(?),and the atomic position are Cr1 4g(0,0.2936(5),0),Cr2 4h(0.5,0.5859(7),0),Al2b(0,0.5,0.5),B1 4h(0,0.3839(8),0.5),B2 4g(0.5,0.6646(2),0.5),and theoretical density is 5.60 g/cm³.(6)Cr₄AlB₄ bulk with relative density of?94%was prepared by an in-situ self-decomposing spark plasma sintering process.The bending strength of Cr₄AlB₄ is up to 841±15 MPa,and the fracture toughness is as high as 13.5±0.7 MPa·m^1/2,and the electrical resistivity is 43.0±1.9??·cm.These results indicate that ternary Cr₄AlB₄ is a good damage tolerant ceramic material with high strength,high toughness,and metallic electrical conductivity.The strengthening mechanism is revealed from the perspective of mechanics,that is,the partial texture and layered structure of Cr₄AlB₄ grains make its main fracture form be the layered transgranular fracture with crack echelon through the grain layers,while the layered transgranular fracture needs to overcome the intra-layer strong covalent bonding of Cr₄AlB₄ grain layers,which is the reason for its high strength.The toughening mechanism was revealed from the perspective of energy,that is,the crack deflection induced by layered fracture of Cr₄AlB₄ grains,particle pullout,and the ductile fracture of residual metal Al,dissipate more fracture energy,which is the reason for its high toughness.At the same time,Cr₄AlB₄/Cr₂AlB₂ composite with relative density of?95%was prepared by an in-situ self-decomposing hot pressing sintering process.The bending strength,fracture toughness,and electrical resistivity of Cr₄AlB₄/Cr₂AlB₂ composite repectively is 669±30 MPa,10.9±0.8 MPa·m^1/2,and36.5±0.8??·cm,indicating that Cr₄AlB₄/Cr₂AlB₂ is also a kind of damage tolerant ceramic material with high strength,high toughness,and metallic electrical conductivity.The high strength and high toughness of ternary Cr-Al-B ceramics are essentially induced by the layered structure characteristic and partial texture of grains.Compared with binary CrB,ternary layered Cr-Al-B ceramics exhibit higher strength and toughness,and better damage tolerance.Results have shown that MAB phase Cr-Al-B ceramics,as a nascent group of layered ternary transition metal borides,exhibiting high strength and high toughness,metallic electrical conductivity,good oxidation resistance and good microwave absorbing properties,are promising structural and functional ceramic materials.