Synthesis And Mechanical Behavior Of Ternary Layered Ceramics Fe₂AlB₂

The ternary transition metal boride MAB phase ceramics(M is the transition metal,A represents the IIIA and IVA-group elements,B represents the boron element)have a similar microstructure with the well-established MAX phase,indicating the high fracture toughness,high damage tolerance,and good thermal shock resistance of the former which the brittle ceramics do not include.As a representative of MAB phases,Fe2 Al B2 also has the potential to become a magnetic refrigeration material due to its magnetocaloric effect.Due to the insufficient preparation process and incompleted research as a result of the preparation process,the polycrystal line Fe2 Al B2 bulk is successfully synthesized by in-situ reaction/hot pressing,and the basic properties and mechanical behavior of the derived polycrystalline Fe2 Al B2 bulk are investigated in this paper.Polycrystalline Fe2 Al B2 bulk was synthesized by in-situ reaction/hot pressing from Fe,Al,and B elemental powders at 1200 oC and 30 MPa for 30 min.A little excess of Al can improve the purity of the Fe2 Al B2-basedproduct to compensate the loss of Al as a result of volatilization.The relative density of Fe2 Al B2 bulk is 96%(5.56g/cm3).Pressing is the key to improving densification and forming the bulk.Although the impurity phase was not found by X-ray diffraction(XRD)analysis,a small amount of impurity Al2O3 and Fe-Al intermetallic compounds were found in EDS.XRD of different temperature products and differential thermal analysis results show that the diffusion of B atoms into the Fe-Al intermetallic compound and the binary boride of Fe forms Fe2 Al B2.Due to the high chemical activity of Fe and B with respect to Fe B,the small B atomic volume is a faster diffusion rate,which greatly accelerates the reaction rate,and greatly reduce the material preparation time.The basic properties of the Fe2 Al B2,especially the mechanical behavior,were studied experimentally.The resistivity of Fe2 Al B2 increases with temperature increasing,and after 700 K,it shows a linear change,which shows the behavior of metal conduction.The hardness decreases with load increasing and there is no major cracks around the indentation in the load interval of 4.9-49 N,so Fe2 Al B2 exhibits a higher damage tolerance.The fracture toughness is between 5.3-5.4 MPa?m1/2.It is surprising that the Fe2 Al B2 fracture toughness test showed non-catastrophic failure characteristics,accompanied by high fracture work(90-117 J/m2)Bending strength is between 232-242 MPa,while the compressive strength is 10 times of the bending strength with 1992-2101 MPa.The hot-pressing process using uniaxial loading shows that the ceramics exhibit significant anisotropy,The ceramics in the direction perpendicular to the compression surface,have higher strength,fracture toughness and fracture worke.The thermal shock behavior of Fe2 Al B2 was studied by water quenching method.The surface morphology and damage evolution were investigated.Due to the higher fracture toughness,especially its high damage tolerance,Fe2 Al B2 did not break at all temperatures in thermal shock experiments.Residual strength started to decrease slowly after 200 oC.After the thermal shock experiment,the intergranular fracture phenomenon and the crystal extraction phenomenon were mainly occurred at the lower temperature,serious transgranular fracture occured with long cracks at higher temperatures.After the thermal shock test,the surface of the sample did not show obvious oxidation at the low temperature,and the crack was finded in the surfac e,with the temperature further increasing the oxide film generated and densed gradually,but the oxide film surface also had significant cracks.