| W and its alloys as the indispensable structural materials in the high-temperature field are widely used in aerospace,nuclear reactors,weapon equipment and other fields due to the excellent properties such as high melting point,high density,and low vapor pressure.However,the low-temperature brittleness of pure W,the so-called "recrystallization brittleness",namely,when the service temperature exceeds recrystallization temperature,the microstructure instability results in grain growth,the impurities concentration at grain boundaries increases and weakening of grain boundary,which leads to significant decrease in hardness and strength,as well as the irradiation embrittlement seriously affect its service performance.W alloy with higher recrystallization temperature and higher strength and toughness is the development trend.Introducing carbide or oxide particles into W matrix is an effective method to improve the mechanical properties of W alloy.Al2O3 is a kind of excellent reinforcement with high hardness,high wear resistance and excellent high-temperature strength.At present,there is no report on the study of Al2O3 particle dispersion reinforced W alloy.In this paper,the hydrothermal synthesis and liquid-liquid doping preparation kinetics of W-Al2O3 composite powder,sintering densification process optimization and grain refinement mechanism,hot working characteristics such as high-temperature deformation behavior,constitutive equation and hot working diagram,effect of swaging process on microstructure and mechanical properties,strengthening mechanism,microstructure thermal stability and property evolution were systematically studied.The main research results are as follows:(1)The results of hydrothermal synthesis and liquid-liquid doping preparation for W-Al2O3 composite powder show that the solution pH value,initial AMT concentration and holding time have obvious effects on grain size of WO3 precursor powder.With the increasing pH and concentration,the nucleation rate of the grains increases,and the grain size of the powder is smaller.The particle size of powder increases with the prolongation of holding time.The precursor WO3·0.33H2O and AlO(OH)were mixed uniformly by liquid-liquid doping.WO3-Al2O3 composite powder was obtained by calcination at 500?,and W-Al2O3 composite powder was successfully prepared by two-stage H2 reduction.With the increasing Al2O3 content.the average particle size of composite powder decreased from 1300 nm to 15 nm.and gradually became spherical.During H2 reduction,Al2O3 particles attached to the surface of WO3 particles reduced the formation rate of WO2(OH)2,blocked the gas phase migration process and refined the composite powder.(2)W-Al2O3 alloy was prepared by SPS and induction sintering.The sintering activity and mechanical properties were improved by Al2O3 addition.After induction sintering,the average grain size decreases from 10 to 3 ?m with Al2O3 content from 0 to 1.0 wt.%.When Al2O3 content is 0.25 wt.%,it has the highest hardness,compressive strength and fracture toughness of 386 HV,1318 MPa and 5.9 MPa·m1/2,respectively.The submicron or nano sized Al2O3 particles are dispersed in W matrix,while the larger particles tend to be distributed at W grain boundaries,and smaller particles are distributed in W grains,which can effectively inhibit the migration of W grain boundaries and slow down the growth rate of W grains.When the Al2O3 content is 0.5 wt.%and above,the particle agglomeration is serious,the pinning effect is weakened and the microstructure is uneven.(3)The stress-strain curves at different temperatures(1300?1600?)and strain rates(10?0.01 s-1)were measured by thermal simulation experiment,and the deformation microstructure evolution was analyzed.Based on Arrhenius model,the constitutive relation model between flow stress and deformation temperature,strain rate and strain was established.The hot working diagram of W-Al2O3 alloy based on power dissipation rate and instability criterion was established,and the processing parameters of rotary forging were determined.(4)After swaging,the relative density of W-Al2O3 alloy increases from 96.8 to 99.6%,and the hardness increases from 386 to 559 HV.The interface between W and Al2O3 is well bonded.During swaging,non-conforming deformation occurs,which results in an amorphous transition layer with thickness of 5-10 nm.The hardness(559 HV)and fracture toughness(21 MPa·m1/2)of W-0.25wt.%Al2O3 alloy were 30%and 52%higher than pure W,respectively.At 800?,the tensile strength is 611.1 MPa,which is 18%higher than pure W.The strengthening mechanism is grain refinement strengthening,dispersion strengthening and second phase strengthening.The initial recrystallization temperature of W-0.25wt.%Al2O3 alloy is 1600?,which is 300? higher than pure W.When the temperature is above 1400?,the hardness and fracture toughness of W-Al2O3 alloy gradually tuend to be stable,while pure W decreases sharply.W-Al2O3 alloy has better thermal stability than pure W. |
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