The Preparation And Characterization Of TiC Particle Reinforced Tungsten Alloys Via A Precipitate-coating Method

Plasma facing materials(PFMs)are the key to the successfully operation of future fusion reactors.Due to its excellent thermal-physical properties,tungsten has been chosen as the main candidate material for PFMs.However,tungsten also exhibits serious embrittlements,such as:low temperature brittleness,recrystallization brittleness and irradiation brittleness,which limit its further applications.A lot of researches have demonstrated that the addition of TiC particles to the tungsten matrix can significantly improve the performance of tungsten in terms of lowering the ductile-to-brittle transition temperature(DBTT),increasing the recrystallization temperature,and enhancing the irradiation resistance.Employing mechanical alloying method to prepare W-TiC alloys always leads to the contamination of impurities and precipitation and aggregation of TiC particles at the tungsten grain boundaries.In this study,in order to improve the uniformity of TiC particles and reduce the contamination of impurities,a precipitate-coating method was adopted to prepare TiC particle reinforced tungsten alloys.The influences of the precipitate-coating process,the hydrogen reduction process and the content of TiC particles on the morphologies of the obtained powders,and the microstructures,the mechanical properties and the thermal conducting properties of the sintered W-TiC alloys were studied in this thesis.In addition,hot plastic working was adopted to improve the mechanical properties of the sintered W-TiC alloys.The effects of different rolling reductions on the ductility and DBTT of the tungsten alloys were studied.Moreover,the resistances of the prepared W-TiC alloys to the radiation of D plasma and He ions were studies.The main contents and obtained results are as follows:1.The combination of adding 1wt.%Polyvinylpyrrolidone(PVP)and ultrasonically processing the suspension for 30 min resulted into the higher stability and dispersibility of the TiC particles in the suspension solution.The W-TiC precursor powders with a core-shell structure i.e.,TiC cores were coated by the precipitate,were successfully achieved by the precipitate-coating process.The reduced powders which were pseudomorphic to the precursor powders still remained the core-shell structure of TiC/W.The reason for the forming of core-shell structure powders was mainly due to the heterogeneous nucleation of the precipitates and growth on the surface of TiC particles during the precipitate-coating process.2.Adding 1wt.%PVP into the TiC suspension can significantly improve the uniformity of TiC particles in the sintered matrix of W-0.1TiC alloy.Most of the TiC particles were dispersed uniformly in the tungsten grain interiors.The average size of the TiC particles was similar to the original TiC size after sintering.However,without the addition of PVP,TiC particles were distributed unevenly in the sintered W-0.1TiC alloy.Besides,the bending strength of the W-TiC alloy with the addition of PVP was improved significantly when compared to the W-0.1TiC alloy without the addition of PVP.3.W-(0-0.9)wt.%TiC alloys were prepared by the precipitation coating method and spark plasma sintering(SPS).The results revealed that the average grain sizes of the tungsten alloys decreased,and the average TiC particle sizes increased with the increase of the TiC contents.The W-0.5TiC alloy achieved the highest bending strength of 1065.72 MPa.The thennal conducting properties of the sintered W-TiC alloys were gradually reduced with the increase of the TiC content.The thermal conductivities of the alloys showed a moderate reduction when the TiC contents were lower than 0.7%,but showed a substantial reduction when the TiC contents were in the range of 0.7%to 0.9%.Moreover,another kind of W-0.5TiC alloy was also prepared by ball milling method.In its microstructure,the TiC particles were significantly grown up and were distributed mainly at the tungsten grain boundaries.It also had the lower mechanical and thermal conducting properties than did the W-0.5TiC alloy prepared by the precipitate coating method.4.Commercial pure tungsten(CPW)and W-TiC alloy(prepared by precipitate coating method)both with different rolling reductions(65%and 83%)were prepared by medium-frequency induction sintering and hot rolling.The results showed that the TiC particles in the rolled W-TiC alloys were significantly elongated along the rolling direction,and the elongated particles were mainly distributed in the tungsten grain interiors.The W-TiC alloy with 83%rolling reduction achieved the finest grain sizes,the highest bending fracture strength(1260.3 MPa),the best tensile properties and the best Charby impact properties.Its DBTT obtained from the high temperature tensile test was lower than 300 ?.The DBTT obtained from the Charpy impact test was about 450 ?,which was about 200 ? lower than that of the CPW and W-TiC alloy with 83%and 65%rolling reductions,respectively.The mechanism of strengthening and toughening of the W-TiC alloy with 83%rolling reduction was the combined effect of hot working,grain refining and particle strengthening.5.The resistances to D plasma and helium ion irradiations of the PW and W-TiC alloys were studied.The results indicated that the prepared W-TiC alloys showed higher resistance to D plasma than did the sintered PW sample.This was due to that the addition of TiC particles refined the tungsten grains and increased the sintering densities of the W-TiC alloys.Besides,the damage levels of the tungsten surfaces decreased with the increase of TiC contents from 0.1%to 0.5%.In addition,the W-TiC alloy with 83%rolling reduction showed better resistance to D plasma and helium ions than did the W-TiC alloy with 65%rolling reduction.6.Oxide(Y2O3 and La2O3)and nitride(AIN)particle reinforced tungsten alloys were also successfully prepared by the precipitate-coating method.The results showed that Y2O3,La2O3 and AIN particles can be doped into tungsten powders and form different core-shell structures.The W-0.35%AlN alloy achieved the finest grain sizes(average grain size of about 2.1 ?m)and dispersive particle sizes(average particle size of about 100 nm).In addition,the dispersive particles were distributed uniformly in the tungsten matrix and distributed mostly in the tungsten grain interiors.This also resulted in the highest mechanical and thermal conducting properties of the W-0.35%AlN alloy.However,the AIN particles transformed into Al2O3-AIN phases during the precipitate-coating process.