The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y₂O₃ dispersed tungsten alloy were investigated as functions of the Y₂O₃ content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y₂O₃ content increases, the sintered density increases at a given temperature. The transition temperature (T_tr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y₂O₃ particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.     

Y2O3 evolution and dispersion refinement in Co-base ODS alloys

Co-base ODS alloys are potential materials for high-temperature applications. The evolution of Y₂O₃ during mechanical alloying (MA) and subsequent annealing were studied with the main focus on the refinement of particle size by the addition of minor alloying elements. The results reveal that the MA powder is characterized by ultrafine grains and high dislocation density, which facilitate the decomposition of Y₂O₃. The decomposed Y₂O₃ thermally precipitates at about 690 °C. Ti, Zr and Ca cause the coarsening of oxide, while Hf refines the oxide particle size and improves the homogeneity. The optimum Hf content is 1.2 wt.% and the resulting particle size is reduced to 9.6 nm with the precipitation of partially coherent Y₂Hf₂O₇ oxides. Higher Al content tends to result in the formation of Al₂O₃ and Y₃Al₅O₁₂ oxides, which are of larger size. The threshold stress is predicted to be enhanced by the reduction of inter-particle spacing. The refinement effect is also verified by hardness measurements.     

Tungsten–vanadium–yttria alloys for fusion power reactors (I): Microstructural characterization

This study and a second part dedicated to the mechanical characterization provide a better knowledge of tungsten (W)–vanadium (V) alloys reinforced with yttrium oxide (Y₂O₃) particles, which have been scarcely investigated. Two W alloys (W-2 or 4 wt.% V-0.5 wt.% Y₂O₃) and a pure W material processed by powder metallurgy and consolidated by hot isostatic pressing were analysed. Along this part, the microstructure of the materials at room temperature is mainly analysed with a field emission scanning electron microscope.The densification in the compacts shows an increase with the V and Y₂O₃ additions. Porosity is reduced because of the formation of a W–V solid solution and V pools that fill the pores between the grains, although such effect is mainly observed in the W2V0.5Y alloy. The microstructure of pure W is composed of coarse polyhedral grains whereas a few coarse W grains, V pools and a nanostructured area, composed of fine W grains with dispersed Y, form the alloys. In contrast to previously studied W-4 wt.% V alloys, the V pools exhibit a reduction in the oxygen content, which prevents the formation of acicular oxide structures. Finally, the refinement of the microstructure induced by the addition of V and Y₂O₃ was analysed by electron backscattered diffraction measurements. Pure W presents high amount of grains over 1 μm (around 60% of the volume fraction) and only 2% below 100 nm. In the new alloys meanwhile, the population of micron size grains is highly reduced to less than 10% and grains smaller than 100 nm represent the 20%.     

Effect of Mo and Y2O3 additions on the microstructure and properties of fine WC-Co cemented carbides fabricated by spark plasma sintering

The effects of Mo and Y₂O₃ additions on the microstructure and properties of fine WC-6Co cemented carbides prepared by spark plasma sintering (SPS) were investigated. The microstructure, mechanical properties and corrosion behavior were analyzed by SEM, TEM, XPS, mechanical property tests and potentiodynamic polarization curve measurements. The results show that there are no significant differences in the microstructure and properties of alloys with 1.0% Y₂O₃ addition. However, the addition of Mo was beneficial for refining the WC grains. The hardness of alloys increased until the maximum value, and then followed a decreasing trend with the increase of Mo content. In addition, the relative density of alloys decreased with the increase of Mo addition, which caused to an obvious decline in fracture toughness. With the additions of Mo and Y₂O₃, the corrosion resistance of alloys improved significantly both in acid and alkaline solutions (0.1 M HCl and 0.1 M NaOH solutions). The adding amount of Mo should be controlled within a certain range and 1 wt% of Mo has the most positive effect on the comprehensive properties of WC-6Co alloys.     

Microstructural effects on the high temperature oxidation of two-phase Cu-Cr alloys in 1 atm O2

The oxidation of a Cu-Cr alloy containing about 60 wt% Cr and of two Cu-Cr alloys containing about 40 wt% Cr was studied at 700 and 800 °C in 1 atm O₂. The 60 wt% Cr alloy was prepared by powder metallurgy (PM) and had a phase particle size of 50-150 μm. One of the two alloys containing about 40 wt% Cr was prepared by mechanical alloying (MA) and had a phase grain size ranging from 10-50 nm to 200-300 nm, depending on the location, while the other was prepared by magnetron sputtering (MS) and had a phase grain size around 5-10 nm. The most important difference between the oxidation behavior of the three alloys is the formation of an exclusive chromia scale on the surface of the Cu-40 wt% Cr alloy prepared by magnetron sputtering and of a continuous chromia layer beneath an outermost layer of copper oxides on the corresponding alloy prepared by mechanical alloying, while the Cu-60 wt% Cr alloy prepared by powder metallurgy formed complex scales composed mostly of CuO, Cu₂O with some Cu₂Cr₂O₄ and Cr₂O₃. Thus, the microstructure of two-phase binary alloys has a strong effect of their oxidation behavior. In particular, a decrease of the alloy grain size favors the exclusive external oxidation of the most reactive component, reducing the corresponding critical content in the alloy. This effect is attributed to the presence of larger concentrations of rapid diffusion paths for the migration of the components in the alloy as well as to a faster dissolution of the particles of the Cr-rich phase in the copper matrix.     

Synthesis and upconversion luminescence properties of monodisperse Y2O3:Yb, Ho spherical particles

Monodispersed spherical Y₂O₃:Yb, Ho upconversion luminescence (UCL) particles with sizes of 40-200 nm are prepared using a homogeneous precipitation method. It is found that aging time, varying between 90 and 10 min, has a profound influence on the precursor size, which systematically decreases from 230 nm to 50 nm. The precursor shows poor stability when aging time is 10 min, and the stability of precursor can be improved by increasing the urea concentration. The UCL spectra of Y₂O₃:Yb, Ho with different particle sizes are investigated. The results indicate that the integrated emission intensity ratio of green to red (R_green/red) exhibits a gradual decrease from 2.7 to 0.45 when the particle size decreases from 200 nm to 40 nm, and the possible reasons are evaluated.     

Microstructural refinement and improvement of mechanical properties and oxidation resistance in EPM TiAl-based intermetallics with yttrium addition

New Y-modified TiAl-based alloys, with nominal chemical compositions of Ti–46.6Al–1.4Mn–2Mo–0.3C–xY (x=0.1, 0.33 and 0.6), were developed with the elemental powder metallurgy (EPM) method. The alloys with Y addition had higher ultimate tensile strength, elongation and oxidation resistance than the Y-free alloy. Combined solid-solution strengthening of Mn, Mo, C and Y, and the microstructural refining effect of C and Y, mainly contributed to the strength at room temperature as well as at high temperature up to 800°C, in addition to strengthening due to Y₂O₃ and carbide precipitation. The room-temperature ductility was also improved by the microstructural refinement and the removal of interstitial oxygen in the α₂ and γ phases. Plastic deformation of γ plates during tensile testing occurred through the activation of ordinary dislocations of perfect Burger's vectors. In an oxidation test in air, the Y-containing alloys showed a significantly low weight gain compared with the Y-free alloy.     

Multiscale Experimental and Numerical Approach to the Powder Particle Shape Effect on Al-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating Build-Up

Aluminum (Al) powders with spherical and irregular particle shapes were mixed with two alumina (Al₂O₃) powders with either a spherical or an angular particle shape to achieve high-performance cold-sprayed coatings onto steel. Two effects of the aluminum particle shape were observed. First, coating microstructure observation showed impinging heterogeneity depending on particle shape. Second, particle jet differences depending on particle morphology were shown by velocity maps. From the latter, SEM and XRD, three effects of the alumina particle shape were also shown, i.e., higher in-flight velocity of angular particles, fragmentation of spherical hollow particles and embedding of alumina particles with aluminum. Numerical simulation of particle impacts was developed to study the densification of Al coating due to Al₂O₃ addition through elucidation of Al-Al₂O₃ interaction behavior at the scale of the coating. Al/Al and Al/Al₂O₃ interfaces were investigated using TEM to understand coating strengthening effects due to alumina addition at the scale of the particle. As a whole, Al and Al₂O₃ particle shape effects were claimed to explain coating mechanical properties, e.g., microhardness and coating–substrate bond strength. This study resulted in specifying criteria to help cold spray users in selecting powders for their applications, to meet economic and technical requirements.     

Ductilization of Mo–Si solid solutions manufactured by powder metallurgy

Mo–1.5 at.% Si alloys with additions of either Y₂O₃ or Zr were manufactured by mechanical alloying. The Y₂O₃ particles reduced the grain size and increased the room temperature strength, but did not alleviate the brittleness of previously investigated Mo–1.5 at.% Si without Y₂O₃. Additions of Zr, on the other hand, resulted not only in a fine grain size and an extremely high bend strength (～2 GPa), but also in limited bend ductility at room temperature. Zr additions are seen to be beneficial for three reasons. First, Zr reduces the grain size. Second, Zr getters detrimental oxygen by forming ZrO₂ particles (which in turn help to pin the grain boundaries). Third, in situ Auger analysis shows that Zr reduces the concentration of Si segregated at the grain boundaries. This is thought to enhance the grain boundary cohesive strength and thus leads to the observed ductility.     

A study on microstructure development and oxidation phenomenon of arc consolidated Mo-Nb-Si-(Y) alloys

Two alloys of compositions Mo-26Nb-19Si and Mo-26Nb-19Si-0.5Y (at.%) were prepared by non-consumable arc melting. The alloys characterized using BSE, EDS and EBSD techniques confirmed the formation of two phase (Mo, Nb)_SS-(Mo, Nb)₅Si₃ microstructures. Y-rich (Y₂O₃) particles were mainly found to be present at the inter-phase boundaries creating different phase morphologies in Mo-26Nb-19Si-0.5Y alloy. Both the phases showed a preferred orientation indicating a directional growth during solidification. Oxidation tests conducted in air at 1000 and 1300 °C showed a porous oxide scale formation on the alloy surface and loss of material due to evaporation of MoO₃. The detailed SEM-EDS analysis of the oxide scale formed on these alloys in argon environment at 1000 °C, indicated the presence of Nb₂O₅ particles in the silica (SiO₂) matrix.     

Tensile properties and strengthening mechanisms of Mo-Si alloy

Pure Mo and Mo-Si alloys with different silicon content were fabricated by powder-metallurgical and thermo-mechanical processing. Tensile properties of the pure Mo and Mo-Si alloys were measured at room temperature and the fracture surface was analyzed after test. The results indicate that Si can effectively reduced the grain size and improve the yield strength of Mo-Si alloys. With the decrease in grain size, the dominant fracture morphology is changed from intergranular to transgranular. The strengthening mechanisms were discussed and the yield strength was analyzed described with respect to grain size, solid solution hardening and Mo₃Si particle strengthening. Calculations show that the yield strength of Mo-Si alloys is governed by grain size.     

Structure and properties of titanium-free maraging alloys

The changes in the structure, phase composition, and physicomechanical properties of titanium-free maraging alloys based on the Fe-15–23% Ni-(Co, Mo, V) system after heating to the single-phase α field and two-phase α + γ field have been studied. It has been established that the strengthening of N15K10M5F5-type maraging alloys is caused by the precipitation of fine particles (20–50 nm) of intermetallic phases such as the fcc Ni₃(Mo, V) phase and the Fe₂(Mo, V) Laves phase (in the N23K9M6 alloys, with the formation of the Ni₃Mo and Fe₂Mo phases). It has been shown that the two-step aging of the N15K10M5F5 alloy leads to an additional strengthening by 200–250 MPa and provides the achievement of the ultimate tensile strength σ_u=2400?2500 MPa. The high-strength N15K10M5F5 maraging alloys are obtained with two levels of the coercive force H _c: (a) semihard maraging alloys with H _c=20?50 Oe and σ_u=2100?2400 MPa; and (b) hard magnetic maraging alloys with H _c=180?230 Oe and σ_u=1500?1800 MPa. The high-strength titanium-free N15K10M5F5 and N23K9M6 maraging alloys possess many properties characteristic of structural, elastic, and magnetic alloys and are thus multifunctional materials. These alloys can be used for advanced high-tech articles and as high-strength magnetic materials.     

高Ti,Nb析出强化高强钢接头强度及焊接热影响区软化行为分析

采用混合气体（80％Ar＋20％CO2）保护焊对高Ti,Nb析出强化高强钢进行了焊接强度试验研究．结果表明,随着焊接热输入增大,接头强度有降低趋势．焊接热影响区较母材硬度降低,存在软化行为．粗晶区晶粒长大及10nm以下（Ti,Nb,Mo）（C,N）第二相粒子的溶解造成强化效果降低．未溶的（Ti,Nb,Mo）（C,N）第二相粒子固定了C,Mo元素,降低过冷奥氏体的稳定性,不能得到硬度较高的板条状马氏体或贝氏体,而形成硬度较低的粒状贝氏体．第二相强化效果的降低不能通过组织强化有效弥补,从而造成粗晶区软化．在细晶区热循环作用下,10nm以下第二相粒子粗化,使得偏离其临界强化尺寸,析出强化效果降低,造成细晶区软化．     

Effect of oxide species and thermomechanical treatments on the strength properties of mechanically alloyed Fe-17%Cr ferritic ODS materials

The effects of several oxide species, such as Y₂0₃, Zr0₂ and MgO, and the thermomechanical treatment (TMT) after the mechanical alloying (MA) process on the strength properties of Fe-17%Cr ferritic ODS (oxide dispersion strengthening) MA materials were investigated. Y20, showed the most uniform dispersion of the finest particles among me above oxides, but the microstructural evolution during the TMT had a larger effect on the strengthening of the alloys than the fine and uniform dispersion of the Y₂O₃ particles had.     

Microstructure and tensile properties of Mo alloy synthetically strengthened by nano-Y2O3 and nano-CeO2简

Nano-Y2 O3 and nano-CeO2 of different weight ratio mixed with deionizing water were doped into MoO2 powder by liquid–solid doping method. The diameter 1.80 and 0.18 mm alloy wires of Mo–0.3Y, Mo–0.3Ce, and Mo–0.15Y–0.15Ce were prepared through reduction, isostatic pressing, sintering, and drawing. Tensile properties, second phase microstructure and fracture surface appearance of wires were analyzed. The better refining effect for Mo alloy powder can be gotten after two kinds of nanoparticle oxide doped into MoO2 than only one doped. Nano-Y2 O3 and nano-CeO2 have different influences on sintering process. For nano-CeO2, the constraining effect of grain growth focuses on the initial sintering stage, nanoY2 O3 plays refining grains roles in the later densification stage. Nano-Y2 O3 is undistorted and keeps intact in the process of drawing; and nano-CeO2 is elongated and broken into parts in the drawing direction. The strengthening effect of nano-Y2 O3 and nano-CeO2 keeps the finer grains and superior tensile properties for Mo–0.15Y–0.15Ce wire.     

Influences of annealing temperature on microstructure and mechanical properties of Mo-La2O3

Influences of annealing temperature on the microstructure and mechanical properties of Mo-La₂O₃ were investigated. Effects of annealing temperature on tensile properties, fracture toughness, and microhardness are discussed. Microstructure and fracture morphology of Mo alloys are observed by optical microscope, SEM, and TEM. The results indicate that grain size increased while tensile strength, fracture toughness, and microhardness decreased with increasing annealing temperature. Larger La₂O₃ particles are located at grain boundaries or sub-boundaries, while the majority of smaller La₂O₃ particles are located within the grain. The strengthening effect is quantitatively assessed, which yielded a predicted yield strength in good agreement with measurements.     

SANS evidence for the dispersion of nanoparticles in W–1Y2O3 and W–1La2O3 processed by hot isostatic pressing

The development of a dispersion of nanoparticles in the W–1%Y₂O₃ and W–1%La₂O₃ (wt%) alloys processed by hot isostatic pressing have been investigated using small-angle neutron scattering. The analyses of the scattering data using the Beaucage unified approach reveal the presence of a bi-modal distribution of spherical scattering centers with sizes of less of 180 nm in these alloys. The mode values of these centers are found at ~ 10 and 40 nm in W–1%Y₂O₃, and at ~ 15 and 80 nm in W–1%La₂O₃. The scanning electron microscopy analyses showed the presence of small second phase particles. The contribution of the pore space to the scattering curves has been analyzed using the results obtained for pure W processed following the same procedure used for the alloys, and the porosity measurements of the samples.     

Microstructural,mechanical and shape memory characterizations of Ti–Mo–Sn alloys

As a β stabilizing element in Ti-based alloys, the effect of Mo on phase constitution, microstructure, mechanical and shape memory properties was investigated. Different compositions of Ti–xMo–3Sn alloys (where x=2, 4, 6, at.%) were prepared by arc melting. A binary composition of Ti–6Mo alloy was also prepared for comparison. Ti–xMo–3Sn alloys show low hardness and high ductility with 90% reduction in thickness while Ti–6Mo alloy shows high hardness, brittle behavior, and poor ductility. Field emission scanning electron microscopy (FESEM) reveals round morphology of athermal ω (ω_ath) precipitates. The presence of ω_ath phase is also confirmed by X-ray diffraction (XRD) in both as-cast and solution-treated and quenched conditions. The optical microscopy (OM) and FESEM show that the amount of martensite forming during quenching decreases with an increase in Mo content, which is also due to β→ω transformation. The hardness trends reinforce the presence of ω_ath too. The shape memory effect (SME) of 9% is the highest for Ti–6Mo–3Sn alloy. The SME is trivial due to ω_ath phase formation; however, the increase in SME is observed with an increase in Mo content, which is due to the reverse transformation from ω_ath and the stress-induced martensitic transformation. In addition, a new and very simple method was designed and used for shape memory effect measurement.     

Nanoprecipitation of oxide particles and related high strength in oxide-dispersion-strengthened iron–aluminium–chromium intermetallics

A series of oxide-dispersion-hardened Fe–Al–Cr iron aluminides has been prepared by mechanical alloying techniques producing compositions intermediate between binary Fe–Al and commercial Fe–Cr alloys. The very fine oxide particles form by precipitation on the {1 0 0} planes in the B2 ordered matrix, initially as a metastable face-centred cubic Y₂O₃ phase which later transforms to an Al-richer phase with monoclinic Al₂Y₄O₉ structure. The importance of this precipitation sequence in determining particle size and density is discussed. Mechanical properties are examined up to high temperatures, and strength is shown to depend mostly on the contribution of the fine particles, with also contributions from the fine grain size, and from the ordered matrix. The roles of elastic modulus variations, strongly affected by alloy composition, and of matrix strength variations in determining material strength are emphasized. Analysis of the factors determining particle nucleation rate and high-temperature strength leads to the development of predictive rules for improving microstructural refinement and strength.     

Preparation and properties of nano-SiC-based ceramic composites containing nano-TiN

Nano-SiC based ceramic composites were prepared by two-step pressureless sintering. The addition of nano-TiN and Y₃Al₅O₁₂ restrained the growth of nano-SiC crystals and resulted in high mechanical properties and uniform microstructure with the crystal size of 200-400 nm. The reinforcing mechanism of nano-SiC based ceramic composites integrated the strengthening and toughening mechanism of nano-TiN and Y₃Al₅O₁₂, such as pining effect, strengthening SiC boundaries, crack deflection, crack bridging and crack interlocking (see Fig. 1).

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