Preparation And Properties Of Nanoscale ZrC Dispersion Strengthened Molybdenum Alloys With High Strength And Excellent Ductility

The exploration of space has moved from near-Earth orbit to more distant deep space,and the longer space residence,longer flight distance and harsh environment all put more demanding requirements on the power system of the vehicle.At present,the commonly used solar power and chemical power cannot meet the demand for high power and long-distance deep space exploration in a lightless environment.Space nuclear reactor power supply is suitable for space missions such as deep space exploration and star surface bases requiring instantaneous high power because of the advantages of good environmental adaptability,wide power range and large power to mass ratio.The severe service environment of high-power space reactors requires structural materials that can resist neutron irradiation and liquid metal corrosion at high temperature for a long time to ensure that the space reactor power supply system can operate stably for a long time.Molybdenum(Mo)and its alloys are considered as key structural candidates for space reactors because of their high melting point,high thermal conductivity and good compatibility with liquid alkali metals.However,pure Mo suffers from insufficient room-temperature plasticity,recrystallization brittleness,inadequate strength at elevated temperature and irradiation embrittlement.Therefore,it is urgent to develop high-performance Mo alloys to fulfill the requirement of future space reactor structural materials.In view of the present contradictory relationship between strength and elongation of Mo alloys and the problem of insufficient strength at high temperature,a high strength and tough Mo-ZrC alloy is prepared by powder metallurgy method based on different strengthening mechanisms,including grain boundary purification,grain boundary strengthening,and nano-ZrC dispersion strengthening as well.Combined with microstructure observation of Mo-ZrC alloy,the mechanical properties,thermal stability and high temperature liquid lithium corrosion are carefully investigated.The main research contents and results of the paper are as follows:Mo-ZrC alloys were prepared by spark plasma sintering(SPS)technique,and the effects of the sintering process and the addition of ZrC nanoparticles on the microstructure and mechanical properties of Mo alloys were investigated.A gradienttype SPS sintering process was developed for the preparation of dense Mo alloys.Pure Mo and Mo-ZrC alloys were sintered at 1600℃ for only 2 min,which leads to the density of all samples higher than 98.5%.Tensile tests and microstructural characterization results showed that the alloy samples exhibited superior mechanical properties when the ZrC addition was in the range of 0.5-1.0 wt.%.Based on the results of component optimization,Mo-ZrC alloy sheets with high strength and excellent high-temperature stability were prepared by using conventional sintering and rolling processes suitable for industrial production to further improve the densitiy and mechanical properties of the materials.Among them,the room temperature tensile strength and total elongation of Mo-0.5 wt.%ZrC alloy in the rolled state were 725 MPa and 24.1%,respectively.After vacuum annealing at 1300℃,the tensile strength of Mo-0.5 wt.%ZrC alloy sheet slightly decreased to 676 MPa,but the total elongation increased to about 32.8%.After recrystallization annealing at 1600℃,the tensile strength of Mo-0.5 wt.%ZrC plate alloy was about 507 MPa and the elongation was up to 56.4%,showing excellent room temperature plasticity.Mo-ZrC alloy rods with nanostructures were prepared by conventional pressureless sintering and spin-forging processes,and the alloy exhibits excellent mechanical properties at both room and high temperatures.The tough-brittle transition temperature was below-80℃.The yield strength and total elongation of the nanostructured Mo-0.6 wt.%ZrC alloy at room temperature were 920 MPa and 34.4%,respectively;the tensile strength and total elongation at 1000℃ were 562 MPa and 23.5%,respectively.When the test temperature increases up to 1200℃,the tensile strength of Mo-0.6 wt.%ZrC alloy can maintain about 483 MPa and the total elongation was 20.9%.The recrystallization onset temperature of the nanostructured Mo0.6wt.%ZrC alloy is about 1400℃,about 400℃ higher than that of pure Mo,and the results envideced the excellent thermal stability of nanostructured Mo alloy induced by ZrC addition.Microstructural analysis shows that the average grain size of Mo-0.6 wt.%ZrC alloy is about 0.67 μm,and most of the second phase particles are uniformly distributed inside the Mo grains with an average grain size of about 54 nm.In addition,the compatibility of Mo-ZrC alloy with liquid lithium metal was investigated,and the liquid lithium corrosion test of Mo alloy at 550 ℃ for 1000 h was carried out using a static corrosion test apparatus(STATIC-1).The experimental results showed that the surfaces of the Mo,Mo-La2O3,TZM(Mo-0.5Ti-0.1Zr)and Mo-ZrC samples after corrosion exhibited inhomogeneous in the aspect of corrosion pores and corrosion particulate matter.The mass test results of pure Mo and the related Mo alloys before and after corrosion showed that the mass loss rates of Mo,Mo-La2O3,TZM and Mo-ZrC samples were 4.4×10-3 g/m2h,3.2×10-3 g/m2h,3.3×10-3 g/m2h and 3×10-3 g/m2h,respectively.The annual corrosion depth of the material can be calculated from the mass loss rate,and the corrosion resistance of all four samples under the similar experimental conditions is evaluated as class 2(i.e.very corrosion resistant).The Mo-ZrC alloy prepared in this study has good prospects for application in high-temperature industry and space nuclear reactors,and can be also give an important guideline for the development of other high strength and ductility refractory metals.