Microstructures,Thermal and Mechanical Properties of Pure Tungsten—A Comparison Between Selective Laser Melting and Hot Rolling

A comparative study on the influence of different manufacturing methods(selective laser melting and hot rolling)on the microstructure,mechanical and thermal behaviours of tungsten(W)was presented for the first time.The results indicated that the selective laser melting(SLM)W exhibited a finer grain sizes,a lower strength ductility,hardness and thermal conductivity compared to hot-rolled W.The main reason for this result was that the laser underwent rapid heating and cooling when it was used to melt W powder with high energy density,resulting in large internal stress in the sample after manufacturing.Subsequently,the internal stress was released,leading to the generation of microcracks at the grain boundaries,thereby affecting the performance of SLM W samples.In addition,the higher fraction of high-angle grain boundaries(HAGBs)of SLM W was found to be the key factor for intrinsic brittleness.Because the HAGBs are the preferred crack paths,which could promote crack propagation and decrease fracture energy.

Synthesis and Characterization of Fine Grained High Density La_2Mo_2O_9-based Oxide-ion Conductors

A cost-efFective technique, including nanocrystalline powder preparation using a modified Pechini method and a two-step low-temperature sintering route, was developed for the synthesis of high performance La2Mo2O9- based oxide-ion conductors. The optimum parameters of the compaction pressure, the first step and ＇the second step sintering temperatures for the synthesis of fine grained, high density and uniform La2Mo2O9- based oxide-ion conductors were determined by a series of sintering experiments. High density and uniform sintered La2Mo2O9 samples with average grain size from 0.8 to 5 μm and La1.96K0.04Mo2O8.96 sample with average grain size as small as 500 nm were synthesized by using this cost-efFective method. The impedance measurement results show that the as-fabricated La2Mo2O9-based ceramics possess much higher ionic conductivity than that obtained by solid state reaction method. It is found that in the range of 0.8-5μm the grain size of dense La2Mo2O9 samples prepared from the nanocrystalline powders has little influence on their conductivities.

Microstructural evolution and thermal fatigue damage mechanism of second-phase dispersion strengthened tungsten composites under repetitive thermal loads

In a fusion reactor,plasma-facing tungsten(W)materials inevitably suffer severe thermal shock,and the performance of W materials under repetitive high heat loads is one of the key concerns for long-term stable operation of the reactor.In this work,the microstructural evolution and thermal fatigue resistance of two representative W-0.5 wt.%ZrC(WZC)and W-1.0 wt.% Y_(2)O_(3)(WYO)composites were investigated under cyclic heat loads.Due to the intrinsic properties of ZrC and Y_(2)O_(3)particles such as coefficients of thermal expansion,particle size and distributions in W grains,the WZC composite exhibited a better thermal shock resistance than WYO.After thermal loads with the absorbed power density(APD)≥22 MW/m^(2),WYO showed obvious grain growth,Y_(2)O_(3)particles shedding and degradation of mechanical properties.While,in the case of WZC,these damage behaviors only occurred when APD≥25 MW/m^(2).Furthermore,an interesting crack mechanism in W composites was revealed due to interface debonding and progressive shedding of second-phase particles from the W matrix.The microstructures and tensile properties of the thermally loaded WZC and WYO specimens were also investigated and the correlations between the microstructure evolution and performance degradation are demonstrated.The results are useful for evaluating the thermal fatigue resistance of oxide/carbide dispersion strengthened W composites and their application in future fusion reactors.

Microstructure and mechanical properties of tungsten composite reinforced by fibre network

In this paper the tungsten-fibre-net-reinforced tungsten composites were produced by spark plasma sintering （SPS） using fine W powders and commercial tungsten fibres. The relative density of the samples is above 95%. It was found that the recrystallization area in the fibres became bigger with increasing sintering temperature and pressure. The tungsten grains of fibres kept stable when sintered at 1350℃/16 kN while grown up when sintered at 1800℃/16 kN. The composite sintered at 1350℃/16 kN have a Vickers-hardness of -610 HV, about 2 times that of the 1800℃/16 kN sintered one. Tensile tests imply that the temperature at which the composites （1350℃/16 kN） begin to exhibit plastic deformation is about 200℃-250℃, which is 400℃ lower than that of SPSed pure W. The tensile fracture surfaces show that the increasing fracture ductility comes from pull-out, interface debonding and fracture of fibres.

High damping in Fe-Ga-La alloys:Phenomenological model for magneto-mechanical hysteresis damping and experiment

Ferromagnetic high damping(FHA)alloys with a wide temperature range from-150℃to 300℃have unique application value in extreme environments.In the present work,the damping behaviors of Fe-21 Ga-xLa(x=0.12 wt.%,0.24 wt.%,0.47 wt.%,1.18 wt.%,and 2.33 wt.%La)alloys have been studied in detail,and a new phenomenological model has been proposed.With the increase of La content,the Laves phase(LaGa_(2))in the matrix increases gradually,and the resistance opposing the domain movement increases as well.Combined with the results of synchrotron radiation X-ray diffraction,neutron diffraction,and magnetic domain observation,the resistance mainly comes from three parts:the average stress related to the lattice distortion of the matrix,the average stress related to the increasing area energy of domain walls(DWs),and the ave rage stress related to the increasing demagnetization energy induced by the Laves phase.Different from the traditional method of reducing internal stress through annealing to improve the damping capacity,the proper internal stress barriers are necessary to Barkhausen jumps to dissipate energy.Therefore,proper doping to balance resistance and mobility of DWs is a reliable way to improve damping capacity.Meanwhile,for Fe-Al and Fe-Cr based Alloys,the new model also has a good fitting effect.This study provides a theoretical and experimental reference for improving the functional properties of ferromagnetic alloys.