| Porous metal materials have poor acid/alkali corrosion resistance and oxidation resistance at high temperature,and porous ceramic materials have high brittleness.Fortunately,intermetallic compounds have the advantages of both metals and ceramics.The Mo3Si-MosSi3-Mo5SiB2(short for MSB)intermetallics has been received extensive attentions as high-temperature structural material in recent years because of its ultra-high melting point,good high temperature strength and creep resistance,excellent oxidation resistance at high temperature and acid/alkali corrosion resistance.At present,there is no report on the study of porous Mo-Si-B intermetallics.In order to control the porosity and pore structure of porous MSB intermetallics all-round to meet the needs of various applications,porous MSB intermetallics was synthesized by in-situ reaction based on the pressureless sintering and SPS+HT(Spark Plasma Sintering+Homogenization Treatment)process.The influence laws of preparation process and its parameters on porosity,pore morphology,pore size and specific surface area et al were studied.The pore formation mechanism of in situ reaction synthesis,the effect of heating rate on the evolution of pore structure,and the formation mechanism of gradient porous structure were revealed.As a result,the goal of controllable preparations all-round on porous MSB intermetallics has been achieved.The effects of pore characteristics on mechanical properties and oxidation behavior at high temperature were studied.Taking 3D-reticulated reinforcement as the application object,the interpenetrating MSB/Cu composite was prepared by pressureless infiltration method.And the microstructure and mechanical properties of the composite were characterized.The following innovative results have been achieved:(1)Porous MSB intermetallics was prepared by in-situ solid-solid diffusion reaction synthesized via pressureless sintering from Mo,Si and B elemental powders.With the increase of sintering temperature,MSB skeleton is formed through solid-solid reaction at 1000～1200℃,the pore morphology is changed from particle accumulation to 3D-reticulated structure at 1200～1500℃.With the prolongation of sintering time and the increase of pressing pressure,the porosity,pore size and specific surface area are decreased.By adjusting the sintering time and pressing pressure in the range of 1-4 h and 50～650 MPa,respectively,the total porosity,pore size and specific surface area are controlled in 52.3～65.3%,1.80～2.48μm and 0.183～0.263 m2/g,respectively,while the phase composition and pore morphology characteristics remain unchanged.The pores are derived from green-body gaps,volume expansion and volume difference of phase transition,which is unlike the pore-forming mechanism of traditional powder metallurgy,and the Kirkendall pore-forming effect is not significant.And they contribute 52.7%,30.9%and 16.4%,respectively,to the maximum open porosity.The double-pore structural MSB intermetallics was obtained by using the NH4HCO3 as pore-forming agent.With the increase of pore-forming agent volume content in the range of 0-60%,the volume proportion of big pores produced by pore-forming agent increased from 2.3%to 69.4%,the porosity increases significantly,and the specific surface area is monotonically decreased.With the increase of pore-forming agent size in the range of 48～230 μm,the size of big pores increases significantly,the volume proportion of small pores and big pores are 40%and 60%,respectively,the porosity increases slightly,and the specific surface area decreases slightly.(2)Porous MSB intermetallics with a wide range of adjustable porosity and pore structure was prepared by SPS+HT from Mo,Si and B elemental powders.The phase transition in SPS process is dominated by solid-liquid reaction,followed by solid-phase diffusion at high temperature.Finally,the skeleton of three intermetallic compounds(Mo3Si,Mo5Si3 and Mo5SiB2)is obtained.HT is fine tuning the pore structure without changing porosity.By adjusting the sintering temperature(1250～1700 ℃),sintering pressure(3～30 MPa)and heating rate(50～250℃/min)of SPS process,the main pore characteristics of porous MSB intermetallics such as total porosity,average pore size,average skeleton size and specific surface area are can be controlled in range of 19.9～65.0%,0.91～16.2.0 μm,3.15～25.15 μm,and 0.034～0.225 m2/g,respectively.The heating rate is a key factor to determine the evolution of porous MSB intermetallics by reaction synthesis in SPS.When the heating rate is slow,the porous structure with small size is obtained through the evolution by mechanical occlusal of elemental powder,solid-liquid reaction and high temperature diffusion.When the heating rate is fast enough,the porous structure with large size is obtained through the melting-restructuring evolution mechanism.Which breaks the traditional dependence of the pore structure size of the porous material on powder size(3)The discontinuous gradient porous MSB intermetallics was obtained by adjusting the powder size and precompression in green-body of SPS.The porosity,pore size and skeleton size decreased with the reducing of powders particle size.With the rising of precompression stress,the porosity and pore size decreased,while the skeleton size increased.The axally symmetric continuous gradient porous MSB intermetallics was fabricated through combining a slightly continuous gradient porosity in the SPS green-body and the appropriate SPS heating rate or pressure.It solves the problems such as shape limitation,small gradient ranges of pore size and porosity of continuously graded porous materials.The gradient range is increased with the increase of heating rate,the phase composition and porosity are constant along the gradient direction,while the pore and skeleton size are rised.The gradient range is decreased with the increase of pressure.The gradient of pore structure changes non-monotonic along the gradient direction:the porosity decreases gradually,the pore size increases first and then decreases,and the skeleton size increases gradually.The formation mechanism of discontinuous gradient pore structure of porous MSB intermetallics is by retaining the discontinuous gradient pore characteristics in the green-body,and the formation mechanism of continuous gradient pore structure of porous MSB intermetallics is by enlarging the continuous gradient pore characteristics of the green-body vis melting-restructuring evolution mechanism.(4)The compressive strength of SPS+HT porous MSB intermetallics at room temperature monotonically decreases with the increase of the total porosity,pore size,and gradient of pore size,while the compressive strength increases with the rising of pore morphology factor J.Transgranular cleavage fracture is shown for the skeleton in the compressive specimen of porous MSB intermetallics,which mostly occurs at the minimum cross-section of the skeleton.When the porosity is higher than 53.0%,the room temperature compressive stress-strain curve is divided into three stage:elastic deformation stage,"yield" stage and fracture stage.The high temperature oxidant behavior of porous MSB intermetallics depends on the composition and viscosity of the antioxidant film.The overall oxidation occurs at 1000℃,and the porous MSB intermetallics was finally oxidized to closed-cell oxides which consist of SiO2 and MoO2.On the contrary,the oxidation is confined to the surface layer at 1300℃,the oxide layer is dominated by SiO2,MoO2 and Mo from the surface to the inside,.(5)The interpenetrating MSB/Cu composite with high density of 99%was prepared by pressureless infiltration of pure copper into the reticulated porous MSB reinforcement.The phase composition and 3D-reticulated structure of MSB intermetallics are not changed during the infiltration process,and the MSB and Cu form an entwined three-dimensional network structure which is combined by mechanical interlocking.With the porosity of MSB intermetallics increases from 29.8%to 60.2%:the hardness at room temperature decreases from 536 HV1 to 200 HV1,the compressive strength decreases from 1246 MPa to 907 MPa,and the flexural strength increases to 501 MPa and then decreased slightly. |
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