Microstructural properties of electrochemically prepared Ni–Fe–W powders

A nanostructured Ni–Fe–W powder was obtained by electrodeposition from ammonium citrate electrolyte within the current density range of 500–1000 mA cm⁻² at the electrolyte temperature of 50 °C–70 °C. XRD analysis shows that the powder contains an amorphous matrix having embedded nanocrystals of the FCC solid solution of iron and tungsten in nickel, with an average crystal grain size of 3.4 nm, a high internal microstrain value and a high density of chaotically distributed dislocations. EDS analysis exhibits that the chemical composition of the Ni–24%Fe–11%W powder does not depend upon current density and electrolyte temperature due to the diffusion control of the process of codeposition of nickel, iron and tungsten.     

Preparation and characterization of sputtered Cu films containing insoluble Nb

Copper films containing various amounts of insoluble Nb (up to 24.7 at.%) were prepared by r.f. magnetron sputtering. The crystallography and microstructure of the films were investigated for as-deposited and annealed Cu(Nb) thin films. Cu(Nb) thin films are found to consist of non-equilibrium supersaturated solid solution of Nb in Cu with a nanocrystalline microstructure. X-ray diffraction and scanning electron microscope analyses revealed a reduction in the grain sizes of the films with increasing Nb content in the films leading to a grain refinement. The electrical resistivity of as-deposited and annealed Cu(Nb) thin films is found to be low for an Nb content 2.7 at.%. Significant drops in the resistivity were observed for the high Nb contents after annealing at 530 °C which may be due to grain growth and formation of Nb-bearing phase in the film. Microhardness of the films was found to increase with the Nb concentration due to the combined effects of grain refinement and the solute strengthening of Nb.     

Microstructures and properties of laser cladding NiTi alloy with W for biomedical applications

The feasibility of improving the radiopacity of NiTi by means of Nd:YAG laser cladding is investigated in the present study. Fine elementary tungsten powder was pasted on NiTi sheets and then melted using laser. The resulting microstructure was analyzed by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray (EDX) spectrometry. The results show that the solubility of tungsten in the NiTi-matrix is low and the excessive tungsten forms fine precipitates and W-rich compounds, which result in the average W content in the fusion zone up to 8 at.%. The laser-clad sample failed near the end of the plateau with a tensile strength of about 410 MPa.     

Fabrication of ultrafine powder using processing control agent,and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi,Sb)2Te3 alloys

Low-dimensional materials can significantly enhance the efficiency of thermoelectric devices for power generation and cooling applications. In the present work, ultra-fine powders of p-type (Bi, Sb)₂Te₃ alloys are fabricated through high energy ball milling using stearic acid as a process control agent (PCA). The influence of the PCA addition on powder characteristics, microstructure and thermoelectric transport properties are studied. Further, the ultra-fine powder is subjected to calcination (Cal-PCA) and subsequently consolidated all powders using spark plasma sintering (SPS). The PCA, Cal-PCA, and non-PCA powder morphological effects on the microstructure and thermoelectric properties are systematically investigated and elucidated thoroughly. The electron beam scattering diffraction (EBSD) results confirmed that the PCA sample exhibited very fine grains (average grain size of ～800 nm) compared to the non-PCA (average grain size of about 2.6 µm), while the grains were distributed randomly for all samples. Formation of fine grains and partial existence of the stearic acid (carbon and oxygen phases) in the matrix were strongly inhibiting the transport of the carriers that severely decreased the carrier mobility, reflecting the severe reduction in electrical conductivity for PCA sample compared to Cal-PCA and non-PCA. The lowest thermal conductivity (κ) of 0.745 W/mK was achieved for the PCA sample, which is 19%, 12% lower than that of non-PCA, and Cal-PCA samples. The strong reduction in κ was mainly attributed to the dramatic decrease in the phonon thermal conductivity owing to phonon scattering at numerous grain boundaries and oxide phases. The obtained high electrical conductivity with balanced thermal conductivity in Cal-PCA sample is attributed to the significant improvement in ZT of 1.1, which is 27%, and 47% higher than that of the Non-PCA sample at room temperature, and 350 K, respectively.     

Structural evolution of matrix phase for liquid phase sintered 93W-4.9Ni-2.1Fe

The structure of the matrix of a two phase heavy alloy, 93W-4.9Ni-2.1Fe (wt%), was characterized. The matrix phase was not the expected face centred cubic Ni-Fe-W solid solution with a large grain size reported in the literature. Instead, an amorphous phase containing fine grained crystals as well as intermetallic compounds having different compositions were found. The partition of tungsten from the matrix toward the tungsten phase resulted in formation of different phases in the matrix. Under furnace cooling, the matrix phase was composed of an amorphous phase for the matrix phase remote from the tungsten grain-matrix interfaces, and a strained FeNi intermetallic phase near the interfaces. For specimens solution treated at temperatures between 1000–1400 °C followed by water quenching, an intermetallic phase rich in tungsten, (Ni, Fe) W, evolved and surrounded the tungsten grains in clusters. The relative abundance of this intermetallic phase was highest for a solution treatment temperature of 1400 °C, indicating that the formation of this phase was a result of supersaturation of tungsten in the matrix phase and retarded partition of tungsten from the matrix phase to tungsten grains under a rapid cooling condition.     

Densification behavior,mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1 wt.%Nb–0.1 wt.%Ni material sintered at 1600 °C was obviously greater than 93.7% of theory density for W material sintered at 2000 °C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1 wt.%Nb–0.1 wt.%Ni powder, and the powder milled for 10 h (W10) could be sintered to near full density (99.4% of theory density) at 1600 °C. The ball milling for 15 h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.     

Interfacial reactions between Sn-3.5 Ag solder and Ni–W alloy films

Nickel based alloys are currently being investigated in an effort to develop stable barrier films between lead free solder and copper substrate. In this study, interfacial reactions between Ni–W alloy films and Sn-3.5 Ag solder have been investigated. Ni–W alloys films with tungsten content in the range of 5.0–18.0 at.% were prepared on copper substrate by electrodeposition in ammonia citrate bath. Solder joints were prepared on the Ni–W coated substrate at a reflow temperature of 250 °C. The solder joint interface was investigated by Cross-sectional scanning electron microscopy, energy dispersive X-ray spectroscopy and electron back scatter diffraction. It has been observed that a Ni₃Sn₄ layer with faceted morphology formed on the Ni–W alloy film after reflow. The thickness of the bright layer was found to decrease with the increase of tungsten content in the Ni–W film. An additional layer with a bright appearance was also found to form below the Ni₃Sn₄ layer. The bright layer was identified to be a ternary phase containing Sn, W and Ni. The bright layer is found to be amorphous and is suggested to have formed through solid state amorphization caused by anomalously fast diffusion of Sn into Ni–W film.     

Influence of Ni content on the structure and properties of Cr-Ni-N coatings prepared by direct current magnetron sputtering

Cr-Ni-N coatings were deposited on 304 stainless steel substrates using a conventional direct current magnetron reactive sputtering system in nitrogen-argon reactive gas mixtures. The influence of Ni content (0 ≦ x ≦ 20 at.%) on the coating composition, microstructure, and tribological properties was investigated by glow discharge optical spectroscopy, X-ray diffraction and transmission electron microscopy, scanning electron microscopy (SEM), nano-indentation, and pin-on-disk tests. The results showed that microstructure and properties of coatings changed due to the introduction of Ni. The ternary Cr-Ni-N coatings exhibited solid solution structures in spite of the different compositions. The addition of Ni strongly favoured preferred orientation growth of <200>. This preferred orientation resulted from the formed nano-columns being composed of grains with the same crystallographic orientation, as confirmed by SEM cross-sectional observations. The mechanical properties including the nano-hardness and reduced Young's modulus decreased with increasing Ni content. Pin-on-disk tests showed that low Ni content coatings presented higher abrasion resistance than high Ni content coatings.     

Effect of molybdenum on grain growth of W-Mo-Ni-Fe heavy alloys

The grain growth phenomena of tungsten heavy alloys with various concentrations of Mo were investigated. The early formation of a eutectic liquid phase with the alloying of Mo to W-Ni-Fe facilitated growth and spheroidization of tungsten grains in the initial stage of isothermal hold. However, when the concentration of molybdenum was so high as to leave behind non-dissolved Mo grains, coalescence of W grains around Mo grains resulted in the formation of larger W-Mo grains having Mo-rich cores. These larger grains later grew at the expense of the smaller grains and grain growth was statistically very fast. During grain growth in the later stage of isothermal hold, W had a higher potential than Mo to precipitate from the liquid phase onto the grains. This competitive behavior led to the gradual accumulation of Mo atoms in the liquid phase, which not only retarded the growth of grains, but also caused precipitation of intermetallic phases in the interfaces between the solid grains and liquid phase during cooling.     

Microstructure of nanostructured Fe40Ni38Mo4B18 alloy

Nanostructured Fe40Ni38Mo4B18 specimens with a wide range of different nanostructures can be produced by annealing the amorphous alloy in the temperature range from 700 K to 1060 K. The nanostructured Fe40Ni38Mo4B18 specimens prepared between 700 K and 760 K have a microstructure of the 4–9 nm fcc (Fe, Ni) solid solution crystallites embedded in the amorphous matrix. The nanostructured Fe40Ni38Mo4B18 specimens obtained between 780 K and 910 K show a polycrystalline microstructure of the major cubic (Fe, Ni, Mo)23B6 phase with grain sizes from 20 nm to 40 nm and the minor fcc (Fe, Ni) solid solution crystallites of about 10 nm. The nanostructured Fe40Ni38Mo4B18 specimens produced between 930 K and 1060 K represent a polycrystalline microstructure of the 12–25 nm fcc (Fe, Ni) solid solution crystallites and the 45–240 nm cubic (Fe, Ni, Mo)23B6 crystallites.     

Ce1−xNdxO2−δ/Si thin films obtained by pulsed laser deposition: Microstructure and conduction properties

Substituted Ce_1−xNd_xO_2−δ cerium dioxide thin films are obtained by pulsed laser deposition technique. The films are deposited for various deposition times and at.% Nd, on [100] Si substrates, covered by a thin native SiO₂ layer. The evolution of the cell parameters with Nd content shows that a solid solution is formed, up to x = 0.27. The thin films are homogenous in composition at a nanometer scale. The morphology of the grains does not change significantly with Nd content. The microstructure is columnar, with a preferential [100] growth direction. The width of the grains varies from 20 to 30 nm. The conductivities of the thin films are determined from impedance spectroscopy analyses, in the temperature range 200 °C to 600 °C. The experimental data are explained in the frame of the space charge layer model.     

Microstructural evolution and mechanical behavior of warm multi-axially forged HSLA steel

Detailed studies are conducted on the microstructural evolution and mechanical behavior of a high strength low alloy (HSLA) steel processed by warm multi-axial forging (MAF). After nine MAF strain steps, the initial ferrite grains of average 13-μm size reduced to submicron-sized grains with over 0.7 fraction of high angle grain boundaries. Pearlitic cementite is fragmented and refined to about 50–100 nm size particles. The microstructure evolution with respect to fraction of HAGB with increase in number of strain steps is more sluggish in HSLA steel as compared to plain carbon steel of comparable carbon content. This is ascribed to the Zener pinning effect of (Ti, Nb) carbide particles. Tensile strength and hardness values of MAFed steel increased by more than 45 and 58 %, respectively, after nine warm MAF strain steps, whereas the fracture strain was reduced from 21 to 12 %.     

Microstructural studies on nanocrystalline oxide dispersion strengthened austenitic (Fe–18Cr–8Ni–2W–0.25Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) alloy synthesized by high energy ball milling and vacuum hot pressing

In the present work, nanostructured (Fe–18Cr–8Ni–2W) austenitic base and oxide dispersion strengthened (ODS) alloy powders were produced through mechanical alloying and these nano powders were consolidated by vacuum hot pressing. The results showed that initially bcc solid solution formed in both the alloys and this transformed to fcc with continued milling. The bcc solid solution formation and the subsequent transformation to fcc were significantly faster in the ODS alloys when compared to the base alloy. In the ODS alloy, a grain size of ~25 nm is achieved within 5 h of milling. Study of variation of microhardness of mechanically alloyed powder particles with grain size showed linear Hall–Petch kind of behavior. Following vacuum hot pressing of mechanically alloyed powders, nearly fully dense (>99% of theoretical density) compacts were obtained with a grain size of ~80 nm. The bulk hardness of base and ODS alloys are ~530 and ~900 HV, respectively. These are significantly higher than the values reported in the literature so far. The enhanced strength the ODS alloy is due to increased dislocation density and presence of fine dispersoids along with the nanocrystalline grains.     

Effect of strain-induced precipitation on the low angle grain boundary in AA7050 aluminum alloy

Effects of the particles induced by strain on dynamic recrystallization and microstructure of the AA7050 aluminum alloy were investigated during hot deformation using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). Experimental results showed that partial recrystallized grains containing little sub-structure were produced during the solution treatment. Numerous particles were successfully obtained by the strain-induced precipitation during first-pass deformation at 573 K. The deformation promoted spheroidization and refinement of the precipitate particles. Then these particles pinned dislocations and grain boundaries inhibiting dynamic recrystallization during second-pass high-temperature deformation at 673 K and low angle grain boundary fraction was increased significantly to 83.8%. Furthermore, the tensile test indicated that microstructure with numerous low angle boundaries (LAGBs) and 5 μm sub-grains had increased the strength and ductility of the AA7050 aluminum alloy.     

Mechanical milling to foster the solid solution formation and densification in Cr-W-Si for hot-pressing of PVD target materials

Based on the significantly different melting points and high oxygen affinities, the fabrication of chromium-based tungsten silicides is restricted to powder metallurgical production routes. To foster particle contacts and diffusion processes between chromium and tungsten, which are known to necessitate long sintering times, mechanical alloying or milling processes prior to sintering are established. Nonetheless, due to spinodal decomposition of Cr and W, the solid solution formation is complex and yet little understood. For this reason, the influence of the mechanical milling time (0–24 h) on the crystal structure and the microstructural properties of hot-pressed 60Cr30W10Si (wt.–%) is examined. In this context, two different powders containing a different tungsten particle size (0.8 and 3 µm) were mechanically alloyed to analyze the impact on the phase formation and the particle distribution in the microstructure. It was shown that mechanical milling supported the mechanical clamping between the particles. However, the increased milling times significantly decreased the crystallite sizes of the particles and fostered the tungsten solubility in the Cr-rich (Cr, W) solid solution formed during sintering, thus supporting the densification.     

Sintering and microstructure of ultrafine yttriazirconia compacts

Sintering behaviour of ultrafine yttria-zirconia solid solution compacts, with yttria contents ranging between 4 and 10 mol % have been investigated and the microstructural developments during sintering were followed. The 4 mol % yttria-zirconia compacts sinter at 1200 ° C to a high density with a relatively fine uniform microstructure. Firing at 1400 ° C produces no improvement in densification and discontinuous grain growth develops. The lowest density values are obtained with the 10 mol % yttria-zirconia compacts, with clustering in the grains and greater porosity in the microstructure. The activation energies were determined for both 4 and 10 mol % Y₂O₃ -containing bodies in the temperature range 800 to 1000 ° C.     

Preparation of slaking resistant CaO aggregate from lightweight CaCO3 with oxide addition

CaO aggregate was sintered from reagent-grade lightweight CaCO₃ powder by the addition of 0-20% (molar ratio) MgO and ZrO₂, respectively. The results showed that the CaO derived from lightweight CaCO₃ was highly sinterable and compact CaO aggregate with relative density above 96% was obtained after sintering at 1400 °C for 2 h, but further increase of compactness was restrained due to the occurrence of abnormal grain growth. The densification of the aggregate was promoted due to the behavior of oxide addition on restraining the grain growth of CaO. With increasing the amount of oxide addition, the microstructure of CaO aggregate underwent a restructuration process. Homogeneous microstructure, with well growing MgO grains occupying most of the boundary triple points of CaO grain, formed by the addition of 20% MgO. Especially when 20% ZrO₂ was added, CaZrO₃ layer formed around CaO grains. The slaking resistance of the aggregate was appreciably improved due to the promotion of densification, the formation of CaO solid solution (while MgO added) and the modification of microstructure.     

The heat treatment effect on the structure and mechanical properties of electrodeposited nano grain size Ni-W alloy coatings

The Ni-W alloy coatings with tungsten content from 32.5 to 61.2 wt.% were prepared in this study by electro-deposition. Experimental results show that the grain size of Ni-W coatings evaluated by XRD decreased with increasing tungsten content in coatings, however, the micro-hardness increased with increasing tungsten content. As-deposited Ni-61.2 wt.%W coating has amorphous-like structure and the grain size is around 1.5 nm, after annealing at 500 °C, the hardness of the coating is promoted to 1293 Hv owing to formation of Ni₄W and NW precipitates. In addition, the heat-treated Ni-W coatings show a better wear resistance than the as-plated Ni-W coatings.     

Decomposition process in a FeAuPd alloy nanostructured by severe plastic deformation

The decomposition process mechanisms have been investigated in a Fe50Au25Pd25 (at.%) alloy processed by severe plastic deformation. Phases were characterized by X-ray diffraction (XRD) and microstructures were observed using transmission electron microscopy (TEM). In the coarse grain alloy homogenized and aged at 450 °C, the bcc α-Fe and fcc AuPd phases nucleate in the fcc supersaturated solid solution and grow by a discontinuous precipitation process resulting in a typical lamellar structure. The grain size of the homogenized FeAuPd alloy was reduced in a range of 50–100 nm by high pressure torsion (HPT). Aging at 450 °C this nanostructure leads to the decomposition of the solid solution into an equi-axed microstructure. The grain growth is very limited during aging and the grain size remains under 100 nm. The combination of two phases with different crystallographic structures (bcc α-Fe and fcc AuPd) and of the nanoscaled grain size gives rise to a significant hardening of the alloy.     

9Cr3W3Co钢高温时效脆化现象与改进方法

采用SEM进行显微组织观察,研究导致9Cr3W3Co钢时效脆化的主要因素。采用Thermo-Calc软件,计算平衡态下不同W含量(2.36%,2.63%,2.96%,3.11%,质量分数)的9Cr3W3Co钢中析出相的含量。利用TEM,SAXS和相分析等实验手段研究时效过程中的组织演变。结果表明:9Cr3W3Co钢的冲击韧度在100h时效后的迅速降低是时效过程中大量析出的富W Laves相所造成的。平衡态的Laves相含量主要由钢中的W含量决定。时效8000h后,W含量最低的钢冲击韧度最好,同时其Laves相的尺寸最小,粗化速率最低。通过降低W含量能够抑制9Cr3W3Co钢的时效脆化。