Lai-Ma Luo

Mechanical properties and microstructural change of W-Y2O3 alloy under helium irradiation.

A wet-chemical method combined with spark plasma sintering was used to prepare a W–Y2O3 alloy. High-temperature tensile tests and nano-indentation microhardness tests were used to characterize the mechanical properties of the alloy. After He-ion irradiation, fuzz and He bubbles were observed on the irradiated surface. The irradiation embrittlement was reflected by the crack indentations formed during the microhardness tests. A phase transformation from α-W to γ-W was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Polycrystallization and amorphization were also observed in the irradiation damage layer. The W materials tended to exhibit lattice distortion, amorphization, polycrystallization and phase transformation under He-ion irradiation. The transformation mechanism predicted by the atomic lattice model was consistent with the available experimental observations. These findings clarify the mechanism of the structural transition of W under ion irradiation and provide a clue for identifying materials with greater irradiation resistance.

Effects of zirconium element on the microstructure and deuterium retention of W–Zr/Sc2O3 composites

Dense W and W–Zr composites reinforced with Sc2O3 particles were produced through powder metallurgy and subsequent spark plasma sintering (SPS) at 1700u2009°C and 58u2009MPa. Results showed that the W–1vol.%Zr/2vol.%Sc2O3 composites exhibited optimal performance with the best relative density of up to 98.93% and high Vickers microhardness of approximately 583 Hv. The thermal conductivity of W–Zr/Sc2O3 composites decreased initially and then increased as the Zr content increased. The moderate Zr alloying element could combine well with Sc2O3 particles and W grains and form a solid solution. However, excess Zr element leads to agglomeration in the grain boundaries. W–1vol.%Zr/2vol.%Sc2O3 composite had a good deuterium irradiation resistance very closing to pure tungsten compared with the other Zr element contents of composites. Under 500u2009K, D2 retention and release of them were similar to those of commercial tungsten, even lower between 400u2009K to 450u2009K. Pre-irradiation with 5u2009keV-He+ ions to a fluence of 1u2009×u20091021 He+/m2 resulted in an increase in deuterium retention (deuterium was implanted after He+ irradiation), thereby shifting the desorption peak to a high temperature from 550u2009K to 650u2009K for the W–1vol.%Zr/2vol.%Sc2O3 composite.

Microstructure and performance of rare earth element-strengthened plasma-facing tungsten material.

Pure W and W-(2%, 5%, 10%) Lu alloys were manufactured via mechanical alloying for 20u2009h and a spark plasma sintering process at 1,873u2009K for 2u2009min. The effects of Lu doping on the microstructure and performance of W were investigated using various techniques. For irradiation performance analysis, thermal desorption spectroscopy (TDS) measurements were performed from room temperature to 1,000u2009K via infrared irradiation with a heating rate of 1u2009K/s after implantations of He+ and D+ ions. TDS measurements were conducted to investigate D retention behavior. Microhardness was dramatically enhanced, and the density initially increased and then decreased with Lu content. The D retention performance followed the same trend as the density. Second-phase particles identified as Lu2O3 particles were completely distributed over the W grain boundaries and generated an effective grain refinement. Transgranular and intergranular fracture modes were observed on the fracture surface of the sintered W-Lu samples, indicating some improvement of strength and toughness. The amount and distribution of Lu substantially affected the properties of W. Among the investigated alloy compositions, W-5%Lu exhibited the best overall performance.

Microstructure and tribological properties of laser clad Ni-Ag/Tic composite coating

Ni-Ag/TiC composite coating was prepared on the 45 steel substrates by means of laser cladding. Microstructure and wear properties of composite coatings were analyzed using optical microscopy, field emission scanning electron microscopy and wear machine. The experimental results show that defects, such as cracks and pores, do not occur in the laser-cladded Ni-Ag/TiC composite coating and 45 steel substrate, and they present good metallurgical bonding between them. Compared with Ni/TiC composite coating, microhardness values of the two coatings do not present evident differences. The wear experiment result shows that Ni-Ag/TiC composite coated with Ag possesses low friction coefficient and good wear resistance compared with Ni/TiC composite coating.

PREPARATION OF ULTRAFINE W–Cu COMPOSITE POWDER USING ULTRASONIC-ASSISTED ELECTROLESS PLATING

W–Cu ultrafine/nanocomposite powders have high sintering activity, so ultrafine/nanotechnology of W–Cu composite powders is one of the main methods to obtain fully dense, high-performance W–Cu composite materials. Cu-coated ultrafine W composite powders were synthesized by ultrasonic-assisted electroless plating process with non-noble metal activation pretreatment at room temperature in this paper. The growth mechanism of Cu layers and surface morphologies and composition of initial ultrafine W powders, pretreated W powders and Cu-coated W powders were analyzed by field emission scanning electron microscopy (FE-SEM), and energy dispersion spectrometry (EDS). The results show that the uniformly Cu coated W composite powder is successfully synthesized without conventional sensitization and activation steps by ultrasonic-assisted electroless plating at room temperature. The Cu layers on the ultrafine W powders had cell structure with dense, uniform distribution. The growth mechanism of Cu layers appears as follows: the surfaces of pretreated W powders appear linear-like and lamellar-like surface defects which act as activated sites. The reactants in the plating solution were adsorbed on catalytic activity surfaces of powders and happened oxidation–reduction reaction. The growth and aggregation mechanisms of Cu particles after nucleation are stripy Cu-cells grew up, bend, bifurcated, and aggregated, then wounding into a cellular structure, like wrapping wool clusters in the life. Finally, Cu cells grow up and merge into a layer.

Numerical Simulation and Experimental Research of the Hydrostatic Extrusion Process of Pure Tungsten

A finite element model (FEM) of the hydrostatic extrusion (HE) process with pressure load model is established in this paper. On this basis, the hot hydrostatic extrusion process of sintered pure tungsten under different temperatures (T), extrusion ratios (R) and die angles (α) are simulated by introducing the Johnson-Cook constitutive relation for material flowing behavior. The simulation results show that there is a negative correlation between the extrusion pressure (P) and T, a positive correlation between P and lnR. And during the increase of the α value, the P value decreases first and then increases. The die angle corresponding to the minimum P is different under different extrusion ratio. Furthermore, a tendency of internal cracking during the process can be found when the R value is relatively small. Based on the simulation results, the experiment of hot hydrostatic extrusion of sintered pure tungsten is carried out. The microhardness, microstructure and mechanical properties of the tungsten before and after the process are investigated. The results show that after the hot hydrostatic extrusion, the grain size of the tungsten is subdivided, the hardness is improved obviously and the mechanical properties is remarkable.

Chemical Synthesis and Oxide Dispersion Properties of Strengthened Tungsten via Spark Plasma Sintering

Highly uniform oxide dispersion-strengthened materials W–1 wt % Nd2O3 and W–1 wt % CeO2 were successfully fabricated via a novel wet chemical method followed by hydrogen reduction. The powders were consolidated by spark plasma sintering at 1700 °C to suppress grain growth. The samples were characterized by performing field emission scanning electron microscopy and transmission electron microscopy analyses, Vickers microhardness measurements, thermal conductivity, and tensile testing. The oxide particles were dispersed at the tungsten grain boundaries and within the grains. The thermal conductivity of the samples at room temperature exceeded 140 W/m·K. The tensile tests indicated that W–1 wt % CeO2 exhibited a ductile–brittle transition temperature between 500 °C and 550 °C, which was a lower range than that for W–1 wt % Nd2O3. Surface topography and Vickers microhardness analyses were conducted before and after irradiations with 50 eV He ions at a fluence of 1 × 1022 m−2 for 1 h in the large-powder material irradiation experiment system. The grain boundaries of the irradiated area became more evident than that of the unirradiated area for both samples. Irradiation hardening was recognized for the W–1 wt % Nd2O3 and W–1 wt % CeO2 samples.

Influence of Niobium Content on The Microstructure and Properties of W–Nb/TiC Composites Prepared by Conventional Sintering

W–Nb/TiC composites were prepared by mechanical milling and conventional sintering. Field-emission scanning electron microscopy, high-resolution transmission electron microscopy, microhardness, and Charpy impact test analyses were used to characterize the samples. Results showed that W–1wt%Nb/TiC exhibited the maximum relative density (94%) and impact energy (116 KJ/m2) and that (Nb, Ti) C solid solution and Nb2C formed in the W–Nb/TiC composites. However, the density and impact energy decreased when the Nb content exceeded 1 wt% because the second-phase particles agglomerated at the grain boundaries (GBs) with increased Nb content. This agglomeration caused stress concentration of the GBs, which easily initiated cracks. The microhardness did not change significantly with the increased Nb content of the W–Nb/TiC composites.

Microstructure and properties of W-TiC/Cf composites prepared by spark plasma sintering

W-TiC/(0.5-2)wt.%Cf composites were successfully synthesized by spark plasma sintering. FE-SEM and HRTEM analysis, tensile test were used to characterize these samples. The grain size, relative density of the W-TiC/0.5wt.%Cf and W-TiC/2wt.%Cf samples were 4xa0~xa05xa0μm and 98.3%, 97.62%, respectively. The Vickers micro-hardness of W-TiC/0.5wt.%Cf and W-TiC/2wt.%Cf samples are 688.36 and 1060.77Hv, respectively. Tensile strength values of W-TiC/2wt.%Cf samples were higher than others, reached 246.9xa0MPa.