Effect of preliminary high-energy ball milling on the structural-phase state and microhardness of Ni3Al samples obtained by spark plasma sintering

The study of the influence of the duration of preliminary high-energy ball milling on the features of the structural-phase state and the level of microhardness of consolidated Ni₃Al samples obtained by the method of spark plasma sintering has been carried out. It was found that the inhomogeneous state of the precursor from the 3Ni-Al powder mixture in the case of preliminary ball milling of a short duration (1 min) is a cause of the formation of an inhomogeneous structural-phase state of the consolidated Ni₃Al sample. An increase in the duration of high-energy ball milling provides a homogeneous phase composition, promotes the refinement of the grain structure and an increase in the microhardness values of the obtained Ni₃Al samples. The main factors determining the processes of structural-phase transformation during the formation of Ni₃Al under the conditions of spark plasma sintering, depending on the preliminary high-energy ball milling, are revealed. It is shown that grain boundary strengthening is the one of the effective mechanisms for increasing the strength of the material under study.     

Morphology,structural-phase state and microhardness of a multicomponent non-equiatomic W-Ta-Mo-Nb-Zr-Cr-Ti powders mixture depending on the duration of ball milling

The features of the transformation of structural-phase states of a non-equiatomic multicomponent system based on refractory metals W-Ta-Mo-Nb-Zr-Cr-Ti after high-energy ball-milling of various durations are studied. Three main stages have been identified that differ in the morphology of the powder mixture, the distribution of components, phase compositions and the character of the change in microhardness. It was shown that already at the initial stage of treatment, two phases are formed, one of which (BCC-1) is a solid solution based on BCC (body-centered cubic) refractory metals, and the other (BCC-2) is a solid solution enriched in Zr, Cr and Ti. It has been suggested that the BCC-2 phase is a precursor state for the Laves phases that form during subsequent SPS (spark plasma sintering). A change in morphology and structural-phase composition is accompanied by a microhardness increase from 4.03 ± 1.12 GPa after 1  min to 8.93 ± 1.74 GPa after 15.5  min of treatment.     

Formation mechanism of titanium silicide by mechanical alloying

The syntheses of five titanium silicides (Ti₃Si, TiSi₂, Ti₅Si₄, Ti₅Si₃, and TiSi) by mechanical alloying (MA) have been investigated. Rapid, self-propagating high temperature synthesis (SHS) reactions were involved in producing the last three materials during room temperature high-energy ball-milling of elemental powders. Such reactions appeared to occur through ignition by mechanical impact in the fine powder mixture formed after a critical milling period. From in-situ thermal analyses, each critical milling period for the formation of Ti₅Si₄, Ti₅Si₃, and TiSi was observed to be 22, 35.5 and 53.5 minutes, respectively. However, the formation of Ti₃Si and TiSi₂ did not occur even after 360 minutes of milling of as-received Ti and Si powder mixture, due to the lack of homogeneity of the powder mixture. Other ball-milling procedures were employed for the syntheses of Ti₃Si and TiSi₂ using different sizes of Si powder and milling medium materials. Ti₃Si was synthesized by milling a Ti and 60 minutes premilled Si powder mixture for 240 minutes. -TiSi₂ and TiSi₂ were produced by high energy partially stabilized zirconia (PSZ) ball-milling for 360 minutes in a steel vial followed by jar-milling of a Ti and 60 min premilled Si powder mixture for 48 hr. The formation of Ti₃Si and TiSi₂ occurs through a slow solid state diffusion reaction, and the product(s) and reactants coexist for a certain period of time. The formation of titanium silicides by MA and the reaction rate appeared to depend on the homogeneity of the powder mixture, milling medium materials, and heat of formation of the product involved.     

Spark plasma sintering of silicon nitride using nanocomposite particles

The spark plasma sintering (SPS) of silicon nitride (Si₃N₄) was investigated using nanocomposite particles composed of submicron-size α-Si₃N₄ and nano-size sintering aids of 5 wt% Y₂O₃ and 2 wt% MgO prepared through a mechanical treatment. As a result of the SPS, Si₃N₄ ceramics with a higher density were obtained using the nanocomposite particles compared with a powder mixture prepared using conventional wet ball-milling. The shrinkage curve of the powder compact prepared using the mechanical treatment was also different from that prepared using the ball-milling, because the formation of the secondary phase identified by the X-ray diffraction (XRD) method and liquid phase was influenced by the presence of the sintering aids in the powder compact. Scanning electron microscopy (SEM) observations showed that elongated grain structure in the Si₃N₄ ceramics with the nanocomposite particles was more developed than that using the powder mixture and ball-milling because of the enhancement of the densification and α-β phase transformation. The fracture toughness was improved by the development of the microstructure using the nanocomposite particles as the raw material. Consequently, it was shown that the powder design of the Si₃N₄ and sintering aids is important to fabricate denser Si₃N₄ ceramics with better mechanical properties using SPS.     

Vibration milling of TiB<Subscript>2</Subscript> + TiNi powder mixtures

We have studied how the duration of the vibration comilling of a 80 vol % TiB₂ + 20 vol % TiNi powder mixture influences the particle size, morphology, and fine-structure parameters of its components. At a milling time of 60 h, we obtained a mixture containing 27 vol % nanoparticles, in which the cubic TiNi phase had a crystallite size of 1.1 nm. We believe that vibration-milled TiB₂ + TiNi mixtures are potentially attractive for the fabrication of composite materials by powder metallurgy methods.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Preparation of spherical particles of Ba2YCu3O7−x superconductor by plasma spraying

Ground angular powder of pre-synthesized Ba₂YCu₃O_7–x was introduced into a high-temperature plasma jet flame with argon as the plasma gas. Morphological changes in the particles and in microstructural features, as well as phase changes, were observed on the plasma-sprayed powder. Spherical particles with various surface topographies were formed: smooth surface with fine grain structure; spiky surface; and polygonal surface structure. X-ray phase analysis showed that the plasma-sprayed powder was a mixture of solidified crystalline and amorphous phases from the liquid state, while the metallic composition of the starting powder was retained. The plasma-sprayed particles were subsequently annealed for 20 to 80 h in air at temperatures up to 1123 K. The superconductor phase was recovered by this heat treatment, while the particles became porous with morphological changes in surface topography.     

Phase evolution and microstructure characteristics of Mo-based Tb2O3-Dy2O3 composites synthesized by ball milling and sintering

Mo-based Tb₂O₃-Dy₂O₃ composites used as neutron absorbers in nuclear power reactor were synthesized by powder metallurgy. The comparative studies of Mo-based Tb₂O₃ and Mo-based Dy₂O₃ composites were carried out to deeply understand the phase evolution and microstructure characteristics of Mo-based Tb₂O₃-Dy₂O₃ composites. Ball milling induced terbium oxide and dysprosium oxide in the powder mixtures to be first fined, nano-crystallized, amorphized and finally dissolved into Mo matrix to form the supersaturated nanocrystalline solid solution that was driven by mechanical work, not by negative heat of mixing. Mo lattice parameter increased with increasing ball-milling time, opposite for Mo grain size. A phase transformation of Dy₂O₃ crystal from cubic to monoclinic and then to amorphous was observed during ball milling. The microhardness of sintered bulks was first increased and then decreased with increasing sintering time. The maximum value was obtained at the bulks sintered for 8?h. The microhardness and bulk density were increased with increasing sintering temperature before 1600?°C. The mechanism of ball milling and sintering was also discussed.     

Structural and microstructural changes in monoclinic ZrO2 during the ball-milling with stainless steel assembly

High-energy ball-milling of monoclinic ZrO₂ was performed in air using the planetary ball mill with a stainless steel milling assembly. Structural and microstructural changes during the ball-milling were monitored using X-ray powder diffraction, Raman spectroscopy, Mössbauer spectroscopy, field emission scanning electron microscopy and energy dispersive X-ray spectrometry. The results of line broadening analysis indicated a decrease in the crystallite size and an increase in the microstrains with the ball-milling time increased up to ∼150 min. The results of quantitative phase analysis indicated the presence of a very small amount of tetragonal ZrO₂ phase in this early stage of ball-milling. The onset of m-ZrO₂ → t-ZrO₂ transition occurred between 10 and 15 h of ball-milling, which resulted in a complete transition after 20 h of ball-milling. Further ball-milling caused a decrease of the t-ZrO₂ lattice parameters followed by a probable transition into c-ZrO₂. It was concluded that the stabilization of t- and c-ZrO₂ polymorphs at RT can be attributed to the incorporation of aliovalent cations (Fe²⁺, Fe³⁺ and Cr³⁺) introduced into the sample due to the wear and oxidation of the milling media.     

碳纳米管与铝合金基体材料的混合工艺研究

为改善碳纳米管在铝合金基体中的分散性和发挥其增强作用,分别采用湿混、球磨以及湿混后球磨的方式将碳纳米管与铝合金粉末进行混合,再经真空烧结制备出碳纳米管增强铝合金复合材料.不同混合工艺的对比试验结果表明:碳纳米管于液相环境下被均匀分散并吸附于铝合金颗粒表面,但在烧结过程中易再次发生团聚;而较长时间的机械球磨会对碳纳米管结构造成一定程度的破坏.相比下,液相分散与机械球磨结合的方式提高了碳纳米管的分散程度和缩短了球磨时间,碳纳米管增强铝合金材料(3%CNTs/5083Al)的抗拉强度达620 MPa.     

Effect of WC nanoparticle size on the sintering temperature,density, and microhardness of WC-8 wt % Co alloys

Using X-ray diffraction, scanning electron microscopy, and density measurements, we have studied the effect of WC particle size (20 to 150 nm) on the optimal sintering temperature of the WC-8 wt % Co alloy and the effect of sintering temperature (800–1600°C) on its phase composition, density, and microhardness. The results indicate that, during sintering of the starting powder mixture, the first to form is the ternary carbide phase Co₆W₆C. At sintering temperatures of 1100°C and above, this phase reacts with carbon to form Co₃W₃C. Sintering above 1000°C leads to the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co〈WC〉, along with the ternary carbide phases. The density and microhardness of the alloy have been measured as functions of sintering temperature. The use of WC nanopowder has been shown to reduce the optimal sintering temperature of the WC-Co alloy by about 100°C.     

Effect of sintering temperature on the phase composition and microhardness of WC-8 wt % Co cemented carbide

The effect of sintering temperature (800–1600°C) on the phase composition, density, and microhardness of WC-8 wt % Co cemented carbide has been studied using x-ray diffraction, scanning electron microscopy, optical microscopy, and density measurements. The results indicate that, during sintering of the starting powder mixture, containing not only WC and Co but also the lower carbide W₂C and free carbon, W₂C reacts with cobalt metal to form Co₃W. At sintering temperatures from 900 to 1200°C, the reaction intermediate is the ternary carbide phase Co₆W₆C. During sintering at 1300°C, this phase reacts with carbon to form Co₃W₃C. Sintering at 1000°C and higher temperatures is accompanied by the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co(WC). The density and microhardness of the sintered samples have been measured as functions of sintering temperature, and the optimal sintering temperature has been determined.     

Mechanically alloyed Ni50Ti50 and its transformation by thermal treatments

Amorphous powders have been obtained by mechanical alloying (MA) equiatomic powder mixtures of nickel and titanium. The amorphous phase thus formed decomposes upon heating first into the cubic B2 NiTi intermetallic compound; however, further heating promotes the precipitation of the intermetallics Ni₃Ti and NiTi₂. These transformations are shown to occur also in mechanically ground (MG) NiTi wire, but not in this same material exempted from ball-milling processing. It is suggested that this unique behaviour is brought about by the particular structural features of the MA or MG powders, which promote the otherwise sluggish decomposition of B2 NiTi.     

形核密度对AlON粉体合成及其透明陶瓷制备的影响

以α-Al₂O₃为原料, 采用碳热还原氮化法合成AlON粉体, 利用活性炭和亚微米碳粉改变球磨后一次粉体(α-Al₂O₃和C混合粉体)的形核密度, 并研究形核密度对AlON粉体相组成、形貌及其透明陶瓷透光性的影响。结果表明, 形核密度不同的一次粉体在1750℃保温60 min均能合成纯相AlON粉体, 但是所合成的两种AlON粉体形貌和性能差异较大。高形核密度下(添加活性炭)合成的AlON粉体形貌不规则、结构疏松且晶粒较小, 并易于球磨获得细颗粒粉体(~0.93 μm); 而低形核密度下(添加亚微米碳粉)合成的AlON粉体整体形貌呈近球形, 晶粒发育较完整, 且尺寸较大, 该粉体球磨后颗粒尺寸较大(~2.13 μm)。因此, 形核密度是影响AlON粉体形貌、结构特征和破碎性的主要因素。研究结果表明, 高形核密度粉体合成的AlON粉体具有更好的烧结活性, 它在1880℃保温150 min获得的透明陶瓷最大红外透过率达76.5% (3 mm厚), 比低形核密度粉体制备的透明陶瓷提高48.3%。因此, 以α-Al₂O₃为原料时, 提高形核密度有利于制备颗粒较小的高活性AlON粉体, 该粉体适合制备高透过率AlON透明陶瓷。     

Role of intensive milling in mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research a nano-composite structure containing of an intermetallic matrix with dispersed Al₂O₃ particles was obtained via mechanical activation of TiO₂ and Al powder mixture and subsequent sintering. The mixture has been milled for different lengths of time and then as a subsequent process it has been sintered. Phase evolutions in the course of milling and subsequent sintering of the milled powder mixture were investigated. Samples were characterized by XRD, SEM, DTA and TEM techniques.The results reveal that the reaction begins during milling by formation of Al₂O₃ and L1₂ Al₃Ti and further milling causes partial amorphization of powder mixture. DTA results reveal that milling of the powder mixture causes solid state reaction between Al and TiO₂ rather than liquid–solid reaction. Also, it was observed that the exothermicity of aluminothermic reduction is reduced by increasing the milling time and the exothermic peak shifts to lower temperatures after partial amorphization of powder mixture during milling. Phase evolutions of the milled powders after being sintered reveal that by increasing the milling time and formation of L1₂ Al₃Ti in the milled powder, intermediate phase formed at 500 °C changes from D0₂₂ Al₃Ti to Al₂₄Ti₈ phase.     

Detonation spraying behaviour of refractory metals: Case studies for Mo and Ta-based powders

In thermal spraying of refractory metal powders, two major issues need to be solved: particles of materials having high melting temperatures should be heated to reach a semi-molten/molten state or temperatures close to the melting point, while oxidation of the metals should be prevented. It has long been believed that it is rather difficult, if not impossible, to produce high-quality refractory metal coatings by detonation spraying. In this work, we demonstrated the capability of the detonation spraying method to produce tantalum-based and molybdenum coatings of low porosity. Using a computer-controlled detonation spray (CCDS2000) facility, the detonation spraying behaviour of a molybdenum powder and a partially oxidized tantalum powder was studied. Spraying was conducted onto steel substrates using an acetylene-oxygen mixture with O₂/C₂H₂?=?1.1. The spraying process was studied by means of analyzing the splat morphology and calculating the particle temperatures and velocities. According to the X-ray diffraction phase analysis, the metals did not experience oxidation during the deposition. Rather, partial reduction of the oxide phase contained in the Ta-based powder occurred during spraying.     

On sinterability of Cu-coated W nanocomposite powder prepared by a hydrogen reduction of a high-energy ball-milled WO3-CuO mixture

Cu-coated W nanocomposite powder was prepared by a combination of high-energy ball-milling of a WO₃ and CuO mixture in a bead mill and its two-stage reduction in a H₂ atmosphere with a slow heating rate of 2 °C/min. STEM-EDS and HR-TEM analyses revealed that the microstructure of the reduced W–Cu nanocomposite powder was characterized by ~50-nm W particles surrounded by a Cu nanolayer. Unlike conventional W–Cu powder, this powder has excellent sinterability. Its solid-phase sintering temperature was significantly enhanced, and this led to a reduction in the sintering temperature by 100 °C from the 1,200 °C required for conventional nanocomposite powder. In order to clarify this enhanced sintering behavior of Cu-coated W–Cu nanocomposite powder, the sintering behavior during the heating stage was analyzed by dilatometry. The maximum peak in the shrinkage rate was attained at 1,073 °C, indicating that the solid-phase sintering was the dominant sintering mechanism. FE-SEM and TEM characterizations were also made for the W–Cu specimen after isothermal sintering in a H₂ atmosphere. On the basis of the dilatometric analysis and microstructural observation, the possible mechanism for the enhanced sintering of Cu-coated W composite powder in the solid phase was attributed to the coupling effect of solid-state sintering of nanosized W particle packing and Cu spreading showing liquid-like behavior. Homogeneous and fully densified W–20 wt% Cu alloy with ~180 nm W grain size and a high hardness of 498 Hv was obtained after sintering at 1,100 °C.     

On the features of mechanochemical synthesis of Y-Cu equiatomic powder mixture

Structural features of nanocomposite YCu powder material obtained by mechanical alloying of the equiatomic Y-Cu elemental powder mixture in a high-energy ball mill have been characterized by X-ray diffraction, and scanning electron microscopy. It has been shown that YCu compound with a full ordered CsCl-type structure along with YCu₂ intermetallic have formed at early stage of the milling process (10?min) while a long-time processing product contains these phases as well as Y₂O₃ oxide. The nanoindentation measurements of the bulk samples compacted after 60 and 120?min of milling have revealed that the mean hardness values H for these samples are higher than those for conventional bulk YCu intermetallic obtained by arc melting. The main reasons for hardness increase are fine-grained microstructure and reinforcement of material by Y₂O₃ particles uniformly distributed throughout the volume.     

Magnetic and structural study of mechanochemical reactions in the Al-Fe3O4 system

The solid state reaction between Al and Fe₃O₄ (magnetite) using mechanochemical activation of powder mixtures under Ar atmosphere is studied. The phase evolution during the reaction is analyzed by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). At 37 minutes of high-energy ball-milling the disappearance of reactive phases and the production of -Fe, FeAl₂O₄ and -Al₂O₃ is observed, together with significant changes in the magnetic behavior of the system. The composition and properties of samples heated up to 1200°C are also investigated. The behavior of the saturation magnetization M _s is interpreted on the basis of the formation of a variable composition spinel phase Fe [Al _x Fe_2–x ] O₄ with 0 x 2 and a canting effect due to the presence of Al³⁺ ions in the spinel structure.     

In situ study on the formation of FeTe

The formation of the FeTe compound from a mixture of Fe and Te powders was studied in situ by means of high-energy synchrotron X-ray diffraction. FeTe does not form directly from the starting elements; instead, FeTe₂ forms as an intermediate product. During a 2 °C/min heating ramp, Te first reacts between 200 and 350 °C with a part of the Fe powder to form FeTe₂, which then further reacts between 350 and 530 °C with the remaining Fe to yield FeTe. This phase formation path is simpler than in the case of FeSe and the differences are discussed in terms of the equilibrium phase diagrams of these two systems.