Effect Of B On The Microstructure And Property Of High Nb TiAl Based Alloy

High Nb TiAl based alloy has been considered as an advanced high-temperature structure materials because of its low density,high specific strength,good high temperature oxidation resistance and other excellent characteristics,but its low mechanical properties at room temperature and high temperature oxidation limit its widespread application.Multiple alloying is the effective way to improve comprehensive performance of high Nb TiAl based alloy.For example,V can improve the plasticity and Cr can improve the fracture toughness,Sb can improve the ability to resist high temperature and long cycle oxidation.The research about the effect B on the microstructure and property of high Nb TiAl based alloy is relatively less yet.In this paper,Ti45A18Nb-xB(at.%)(x=0,0.1,0.3,0.5,0.8,1.0)alloy is prepared by non-self-consumed tungsten electrode argon arc acuum furnace.This research mainly studies the effect of B with different addition on the macrostructure,microstructure,phase composition by SEM,EDS and XRD analysis.Thermal treatment,mechanics performance test and 900℃ 280 h cyclic oxidation experiment are conducted to figure out the effect of B on the tensile strength,tensile plasticity,compression stress,compression strain and oxidation resistance of Ti45A18Nb based alloy.Fracture and oxide film morphology are also researched by secondary electron images in SEM.The composition with the superior performance is finally obtained by assessing the structure and properties comprehensively.The obtained main results are as follows:(1)The microstructure of as-cast Ti45A18Nb-xB alloy is typical dendritic crystal which reduce with the increment of B.Boride distribute in the grain boundary and grain and its shape transform from dot to long bar.After the heat treatment of 1350℃/30min+900℃/30 min AC,the microstructure of alloy is duplex composed of γ and α2 phase,at the same time,there are many annealing twins within grain.After heat treatment of 1400℃/30 min+900℃/30 min AC,the microstructure of alloy is fully lamellar composed of γ and α2 phase,at the same time,the more B content is,the more obvious grain refinement and more smaller lamellar spacing is.After heat treatment of 1450℃/30 min+900℃/30min AC,the microstructure of alloy is still fully lamellar composed of γ and α2 phase,the change of grain size and lamellar spacing is not obvious.The change of boride morphology is not obvious after heat treatment.(2)Vickers hardness value of heat treatment and as-cast alloy increases and then decreases with the increment of B,when the B content is 0.5 at.%,there is the largest hardness values.The tensile strength,tensile ductility,compressive plasticity and compressive strength of the alloy are constantly increasing with the increment of B.(3)It is found that the oxidation resistance of the alloys enhances after cyclic oxidation at 900℃ for 280 h,the oxide particles become finer,Al2O3 content increase,the oxide layer is dense and the adhesion is increased with the increase of B.The oxidation rate of Ti45A18Nb0.5B alloy is the lowest and the thickness of oxide layer is the smallest,which shows the best oxidation resistance at high temperature.