Temperature-dependent Ta hydride formation for recycling of Ta scraps: Experimental and thermodynamic investigations

We report on the theoretical and experimental investigations about the Ta-hydride formation depending on the temperature for recycling of Ta scraps. The structural investigations based on scanning electron microscope and X-ray diffraction (XRD) showed that the amount of hydrogen incorporated into the Ta matrix varied with hydridation temperature. The XRD measurement showed that the H/Ta mole ratio in Ta-hydride increased with increasing the hydridation temperature up to 700 °C and then decreased with increasing the temperature furthermore. Depending on the hydridation temperature, various phase of Ta-hydride, such as TaH_0.93 and Ta₂H were formed and this hydride process was verified by thermodynamic analysis.     

Microstructural characterization and amorphous phase formation in Co40Fe22Ta8B30 powders produced by mechanical alloying

In this work, microstructural evolution and amorphous phase formation in Co₄₀Fe₂₂Ta₈B₃₀ alloy produced by mechanical alloying (MA) of the elemental powder mixture under argon gas atmosphere was investigated. Milling time had a profound effect on the phase transformation, microstructure, morphological evolution and thermal behavior of the powders. These effects were studied by the X-ray powder diffraction (XRD) in reflection mode using Cu Kα and in transmission configuration using synchrotron radiation, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results showed that at the early stage of the milling, microstructure consisted of nanocrystalline bcc-(Fe, Co) phases and unreacted tantalum.Further milling, produced an amorphous phase, which became a dominant phase with a fraction of 96 wt% after 200 h milling. The DSC profile of 200 h milled powders demonstrated a huge and broad exothermic hump due to the structural relaxation, followed by a single exothermic peak, indicating the crystallization of the amorphous phase. Further XRD studies in transmission mode by synchrotron radiation revealed that the crystalline products were (Co, Fe)_20.82Ta_2.18B₆, (Co, Fe)₂₁ Ta₂ B₆, and (Co, Fe)₃B₂. The amorphization mechanisms were discussed in terms of severe grain refinement, atomic size effect, the concept of local topological instability and the heat of mixing of the reactants.     

Difference between mechanical alloying and mechanical disordering in the amorphization reaction of Al50Ta50 in a rod mill

Amorphous Al₅₀Ta₆₀ alloy powders have been synthesized by mechanical alloying (MA) from elemental powders of aluminium and tantalum, and mechanical disordering (MD) from crystalline intermetallic compound powders of AlTa respectively using the rod milling technique. The mechanically alloyed and the mechanically disordered alloy powders were characterized by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, differential thermal analysis, differential scanning calorimetry and chemical analysis. The results have shown that the crystal-to amorphous transformation in the MD process occurs through one stage, while the crystallineto-amorphous formation in the MA process occurs through three stages. At the early and intermediate stages of the MA time, heating the alloy powders to 700 K leads to the formation of an amorphous phase by a solid-state amorphizing reaction. At the final stage of the MA time, the amorphous phase is crystallized through a single sharp exothermic peak. Contrary to this, amorphous alloy powders produced by MD are crystallized through two broad exothermic peaks.     

Reduced-temperature processing and consolidation of ultra-refractory Ta4HfC5

TaC, HfC, and WC powders were subjected to high-energy milling and hot pressing to produce Ta₄HfC₅, a composite of Ta₄HfC₅ + 30 vol.% WC, and a composite of Ta₄HfC₅ + 50 vol.% WC. Sub-micron powders were examined after four different milling intervals prior to hot pressing. XRD was used to verify proper phase formation. SEM, relative density, and hardness measurements were used to examine the resulting phases. Hot pressed compacts of Ta₄HfC₅ showed densification as high as 98.6% along with Vickers hardness values of 21.4 GPa. Similarly, Ta₄HfC₅ + 30 vol.% WC exhibited 99% densification with a Vickers hardness of 22.5 GPa. These levels of densification were achieved at 1500 °C, which is lower than any previously reported sintering temperature for Ta₄HfC₅. Microhardness values measured in this study were higher than those previously reported for Ta₄HfC₅. The WC additions to Ta₄HfC₅ were found to improve densification and increase microhardness.     

Effects of ma processing variables on the fabrication of nanocrystalline Al3Nb powders

In the present study, elemental powder mixtures of Al-25at.%Nb were mechanically alloyed in an attritor in order to examine the relationships between MA processing parameters and the structural characteristics of Al₃Nb nanocrystalline powders. Homogeneous Al₃Nb powder particles can be obtained with the following processing variables: The addition of 3wt.% stearic acid as process control agent (PCA), the milling speed of 400 rpm and a MA milling time over 40 hours. The partial formation of Al₃Nb and Nb₂Al intermetallics in the matrix was detected by XRD analysis in as-milled powders. The phase transformation and microstructure evolution in Al₃Nb powders according to heat treatment was analyzed and discussed using TEM analysis.     

Synthesis of Mo-Si-B intermetallic compounds with continuous α-Mo matrix by pulverization of ingot and hydrogen reduction of MoO3 powders

The fabrication of the intermetallic phase T2-Mo₃Si with continuous matrix of α-Mo was attempted with the combination process of high energy ball milling, pulverization of arc-melted ingot, addition of Mo by hydrogen reduction of MoO₃ and spark plasma sintering processes. High energy ball milling or arc melting of Mo-16.7Si-16.7B (at %) powders were performed to obtain to intermetallic phase T2 and Mo₃Si. The Mo phase of 57 vol% distributed intermetallic compound powders were prepared by hydrogen reduction of MoO₃ and further mixing of elemental Mo powders. X-ray diffractometry analysis revealed that the intermetallic phase T2-Mo₃Si can be produced by the pulverization process of arc-melted ingot. Hydrogen reduction of 1 vol% MoO₃ mixed intermetallic powder followed by further addition of Mo powders was a more adequate method enabling the homogeneous distribution of the Mo phase than that of added MoO₃ powders with total amount. The powder mixture was successfully consolidated by spark plasma sintering yielding a sound microstructure comprising the intermetallic phase T2-Mo₃Si uniformly distributed in a continuous matrix of α-Mo.     

Neutron diffraction study in amorphous Ni30Ta70 alloy powders by mechanical alloying

A mixture of elemental Ni and Ta powders with an atomic ratio of 3∶7 was subjected to mechanical alloying (MA). An amorphous Ni₃₀Ta₇₀ alloy was formed after 80 hrs of milling, the amorphization by rapid quenching technique of which has not been reported. The atomic structural changes were observed by neutron diffraction in the amorphization process during MA. The radial distribution function RDF(r) shows that peaks of fcc-Ni and bcc-Ta crystal broaden first and gradually approach those characteristic of an amorphous phase with increasing MA time. A local atomic environment around Ni and Ta atoms was studied by analyzing the first peak in the total pair distribution function g(r) after the completion of amorphization. We reach our conclusion from this analysis that the amorphization in the Ni₃₀Ta₇₀ alloy takes place by the penetration of smaller Ni atoms into the bcc-Ta lattice.     

Formation and magnetic properties of nanocrystalline Fe60Co40 alloys produced by mechanical alloying

The Fe₆₀Co₄₀ alloys were prepared by mechanical alloying of the Fe and Co powders using a high-energy ball mill. They were studied with respect to phase formation and magnetic properties using x-ray diffraction, scanning electron microscopy, and measurements of coercivity and remanence (B _r). The evaluation of Fe lattice parameters during milling showed that formation of a body-centered cubic solid solution occurred by mechanical alloying after 12 h of milling. Intensive milling of Fe-Co powders results in a nonequilibrium microstructure characterized by grain refinement to a minimum of 10–13 nm and the introduction of internal strain up to 0.5%.     

Synthesis of α-Mo-Mo5SiB2-Mo3Si nanocomposite powders by two-step mechanical alloying and subsequent heat treatment

A two-step mechanical alloying process followed by heat treatment was developed as a novel approach for fabrication of Mo-12.5 mol%Si-25 mol%B nanocomposite powders. In this regard, a Si-43.62 wt.% B powder mixture was milled for 20 h. Then, Mo was added to the mechanically alloyed Si-B powders in order to achieve Mo-12.5 mol%Si-25 mol%B powder. This powder mixture was further milled for 2,5,10 and 20 h. All of the milled powders were annealed at 1100 °C for 1 h. After first step of milling, a nanocomposite structure composed of boron particles embedded in Si matrix was formed. On the other hand, an α-Mo/MoSi₂ nanocomposite was produced after second step while no ternary phases between Mo, Si and B were formed. At this stage, the subsequent annealing led to formation of α-Mo and Mo₅SiB₂ as major phases. The phase evolutions during heat treatment of powders can be affected by milling conditions. It should be mentioned that the desirable intermetallic phases were not formed during heat treatment of unmilled powders. On the other hand, α-Mo-Mo₅SiB₂-Mo₃Si nanocomposites were formed after annealing of powders milled for 22 h. With increasing milling time (at the second step), the formation of Mo₃Si during subsequent heat treatment was disturbed. Here, an α-Mo-Mo₅SiB₂-MoSi₂ nanocomposite was formed after annealing of 30 and 40 h milled powders.     

Thermite reduction of Ta2O5/SiO2 powder mixtures for combustion synthesis of Ta-based silicides

Tantalum silicides (including TaSi₂, Ta₅Si₃, Ta₂Si, and Ta₃Si) were prepared by solid state combustion of the Ta-Si reaction system involving thermite reduction of Ta₂O₅ and SiO₂. The thermite-based combustion is self-sustaining and contributes to the in situ formation of tantalum silicides along with Al₂O₃. The combustion front temperature and propagation velocity increased with the extent of thermite reactions for the systems adopting the thermite mixture of Al-Ta₂O₅, while both of them decreased for those using Al, Ta₂O₅, and SiO₂ as the thermite reagents. Among four silicide compounds, a better degree of phase evolution was observed for TaSi₂ and Ta₅Si₃ when compared to that of Ta₂Si and Ta₃Si. The XRD analysis indicated the presence of a small amount of Ta₅Si₃ in the TaSi₂-Al₂O₃ composite. On the formation of Ta₅Si₃ with Al₂O₃, the minor phase was Ta₂Si for the Al-Ta₂O₅-containing system. In addition to Ta₂Si, an intermediate phase TaSi₂ was detected when the Al-Ta₂O₅-SiO₂ mixture was used. However, combustion yielded comparable amounts of Ta₂Si and Ta₅Si₃ in the synthesis of the Ta₂Si-Al₂O₃ composite. Moreover, instead of forming Ta₃Si the reaction produced Ta₂Si as the dominant phase along with unreacted Ta.     

Mechanical and Tribological Behavior of Ni(Al)-Reinforced Nanocomposite Plasma Spray Coatings

The mechanical and tribological behavior and microstructural evolutions of the Ni(Al)-reinforced nanocomposite plasma spray coatings were studied. At first, the feedstock Ni(Al)-15 wt.% (Al₂O₃-13% TiO₂) nanocomposite powders were prepared using low-energy mechanical milling of the pure Ni and Al powders as well as Al₂O₃-13% TiO₂ nanoparticle mixtures. The characteristics of the powder particles and the prepared coatings depending on their microstructures were examined in detail. The results showed that the feedstock powders after milling contained only α-Ni solid solution with no trace of the intermetallic phase. However, under the air plasma spraying conditions, the NiAl intermetallic phase in the α-Ni solid solution matrix appeared. The lack of nickel aluminide formation during low-energy ball milling is beneficial hence, the exothermic reaction can occur between Ni and Al during plasma spraying, improving the adhesive strength of the nanocomposite coatings. The results also indicated that the microhardness of the α-Ni phase was 3.91 ± 0.23 GPa and the NiAl intermetallic phase had a mean microhardness of 5.69 ± 0.12 GPa. The high microhardness of the nanocomposite coatings must be due to the presence of the reinforcing nanoparticles. Due to the improvement in mechanical properties, the Ni(Al) nanocomposite coatings showed significant modifications in wear resistance with low frictional coefficient.     

Structure,phase composition and magnetic properties of Fe–Si–C system due to mechanical activation of Fe–Si alloy in organic liquids

Fineness, structure, phase composition, and magnetic properties of the powders produced by mechanical milling of Fe₈₀Si₂₀ in liquid hydrocarbon environment (heptane, heptane with oleic acid additive) in planetary ball mill have been studied. Under milling the alloys are saturated with the products of organic liquid decomposition – C, O, H, and then metastable amorphous and carbide phases form. The particle size decreases down to 0.1–0.2 μm. Isochronous (775 K, 1 h) annealing leads to the formation of the Fe₈Si₂C iron silicon carbide, as well as Fe₃Si and C – for milling in heptane, and Fe₃C and SiO₂ – for milling with oleic acid additive. The magnetic properties of the powders depend both on the milling time and milling environment, and on the annealing temperature. An increase in the coercivity from 13 up to 226 Oe correlates with the amount of the synthesized Fe₈Si₂C. It is ferromagnetic (T_c = 788 K) and temperature-resistant up to 870 K.     

Characterization and Formation Mechanism of Ni3Si–Al2O3 Nanocomposite Prepared by Mechanochemical Reduction Method

In this present work, Ni₃Si–Al₂O₃ nanocomposite powders were synthesized by mechanical milling using NiO, Si and Al as raw materials. The phase transformation, formation mechanism and microstructure evolution of the powders during mechanical milling were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), transition electron microscopy (TEM) and microhardness measurements. Results showed that the Ni₃Si, Al₂O₃ and Ni₃₁Si₁₂ phases formed after 5 h of milling with a rapid mechanically induced self-propagating synthesis mode. The average grain size and internal strain of Ni₃Si and Al₂O₃ after 30 h of milling were (16.8 nm, 1.27%) and (19.6 nm, 0.94%), respectively. The maximum microhardness value of 813 HV was obtained in the 30 h milled powder. The relationship between the hardness and grain size of the powders satisfies the Hall–Petch relationship. Ni₃Si–Al₂O₃ nanocomposite powders are very stable during heating at 950 °C. By annealing of the milled powders leads to grain growth, internal strain and microhardness of Ni₃Si powder decrease and transformation of disordered structure to an ordered state. A long-range ordering parameter (LRO) of 0.97 for the ordered Ni₃Si can be achieved after annealing at 950 °C for 2 h.

     

Synthesis of nano-structured La0.8Ba0.2MnO3 perovskite via a mechano-thermal route

In this study, barium-doped lanthanum manganite, La_0.8Ba_0.2MnO₃, was synthesized via a mechano-thermal route employing high energy ball milling and subsequent heat treatment. The structural evolution, morphology and thermal behaviour of the powders were evaluated using XRD, FESEM, and DTA/TGA, respectively. DTA/TGA results showed that the calcination temperature of the carbonates significantly decreased by increasing the milling time. The results revealed that single phase perovskite was formed at 900 °C in a milled sample for 2 h and this temperature decreased to 600 °C by increasing the milling time to 30 h. The mean crystallite size also decreased from 32 to 20 nm by increasing the milling time from 2 to 30 h. The reaction sequence of La_0.8Ba_0.2MnO₃ formation via the mechano-thermal route is proposed using XRD and DTA/TGA results. FESEM micrographs showed that the mean particle size of the perovskite phase is increased slightly from 30 to 40 nm by increasing the heat treatment temperature from 600 to 900 °C.     

Kinetics and formation mechanism of amorphous Fe52Nb48 alloy powder fabricated by mechanical alloying

A single phase amorphous Fe₅₂Nb₄₈ alloy has been synthesized through a solid state interdiffusion of pure polycrystalline Fe and Nb powders at room temperature, using a high-energy ball-milling technique. The mechanisms of metallic glass formation and competing crystallization processes in the mechanically deformed composite powders have been investigated by means of X-ray diffraction, Mössbauer spectroscopy, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. The numerous intimate layered composite particles of the diffusion couples that formed during the first and intermediate stages of milling time (0–56 ks), are intermixed to form amorphous phase(s) upon heating to about 625 K by so-called thermally assisted solid state amorphization, TASSA. The amorphization heat of formation for binary system via the TASSA, ΔH_a, was measured directly as a function of the milling time. Comparable with the TASSA, homogeneous amorphous alloys were fabricated directly without heating the composite multilayered particles upon milling these particles for longer milling time (86 ks–144 ks). The amorphization reaction here is attributed to the mechanical driven solid state amorphization. This single amorphous phase transforms into an order phase (μ phase) upon heating at 1088 K (crystallization temperature, T_x) with enthalpy change of crystallization, ΔH_x, of −8.3 kJ mol⁻¹.     

Effects of Micro and Macroalloying on the Formation and Thermal Stability of Nanocrystalline L12-Al3V

Structural modification of nanocrystalline trialuminde Al₃V by microalloying with Ti and macroalloying with Zr was studied. For micro and macroalloying, the Al₇₅(V _x Ti_1?x ) x = 0-25 and Al₅ZrV₂ were synthesized by mechanical alloying of elemental blends of the nominal composition. The structural changes of powder particles during mechanical alloying were investigated by x-ray diffraction patterns. Microstructure of powders was characterized by scanning electron microscopy and the hardness of powder particles was determined by Vickers microhardness measurement. It was found that microalloying of trialuminides by Ti led to the formation of Al₃(V,Ti) with stable tetragonal structure (DO₂₂) in different amounts of Ti after 40 h milling. Thermodynamic calculations, which were based on Miedema model, showed that the higher negative formation enthalpy of intermetallics in Al-V-Ti system induced refinement of trialuminides. In addition, it was found that the hardness of compounds was increased by enhancement of Ti in trialuminide composition. For Al₅V₂Zr compound, the metastable L1₂ phase was formed after 20 h milling. Thermal analysis of the synthesized compound showed an exothermic peak around 550 °C, which was related to the partial transition of L1₂ structure to DO₂₂. Moreover, ordering of L1₂ phase took place due to increasing temperature.     

Fabrication and Properties of Thermal Sprayed AlSi-Based Coatings from Nanocomposite Powders

AlSi-based nanocomposite powders (where nanoparticles were TiO₂, ZrO₂, and Al₂O₃ and the amount of reinforcement was 2.5, 5, and 10 wt.%) were made by ball milling and then thermal sprayed using low velocity oxy-fuel technique. The AlSi-based nanocomposite powders had nanosized ceramic reinforcement adhered to the surface of the powders after ball milling. The AlSi-based coatings had the typical thermal spray microstructure where lamellae, oxide layers, unmelted particles, and pores could be seen. Submicron second phase in the form of agglomerates, molten splats, or unmelted particles between AlSi lamellae could be observed as well. Hardness and porosity of the coatings increased when more ceramic second phase particles (harder than AlSi) were added. Sliding wear tests were carried out in pin-on-disk geometry. The wear tracks of AlSi and AlSi-based coatings show plastic deformation as the main material removal mechanism during the sliding wear test. The sliding wear rate of the coatings decreased as more second phase ceramic particles were added. It was due to an increase in the hardness and a decrease in the friction coefficient of the coatings.     

Formation of NiAl–Al2O3 intermetallic-matrix composite powders by mechanical alloying technique

This study investigated the feasibility of preparing intermetallic-matrix composite powders (NiAl/Al₂O₃) by mechanical alloying of Ni, Al and Al₂O₃ powder mixtures with various compositions of (NiAl)_x(Al₂O₃)_100–x. The as-milled powders were examined by X-ray diffraction, scanning electron microscopy, and differential thermal analysis. The formation of NiAl phase was noticed after 5 h of milling. Intermetallic-matrix composite powders (NiAl/Al₂O₃) were prepared successfully at the end of milling for (NiAl)_x(Al₂O₃)_100–x (x=79, 66, and 49), but no alumina phase was detected for (NiAl)₉₅(Al₂O₃)₅. It is suspected that the additions of alumina hampered the cold welding and fracturing process. The thermal analysis of (NiAl)_x(Al₂O₃)_100–x powders after 1 h of milling revealed that the transition temperature of NiAl phase increased with increasing amount of Al₂O₃ additions.     

纳米Mg2Sn增强镁基复合材料组织与性能

利用纳米Sn粉高的表面活性,通过微米Mg粉与纳米Sn粉的机械合金化高效合成了含原位纳米Mg₂Sn相的复合粉末,将所得复合粉末热压烧结,获得高性能纳米Mg₂Sn增强镁基复合材料。对比研究了不同机械合金化时间对镁基复合材料组织、性能的影响,结果表明：随着机械合金化时间的延长,由纳米Mg₂Sn相组成的团簇尺寸不断减小,分布更加均匀,烧结态Mg₂Sn/Mg复合材料的各项力学性能也得到不断提高。     

Synthesis of nanocrystalline Bi2Te3 via mechanical alloying

Bi₄₀Te₆₀ thermoelectric compound was fabricated via mechanical milling of bismuth and tellurium as starting materials. Effect of the milling time and heat treatment temperatures were investigated. In order to characterize the ball milled powders, the X-ray diffraction (XRD) was used. Thermal behavior of the mechanically alloyed powders was studied by differential thermal analysis (DTA). The morphological evolutions were studied by scanning electron microscopy (SEM). Results showed that the nanocrystalline Bi₂Te₃ compound was formed after 5 h of milling. Further milling (25 h) and heating to 500 °C showed that the synthesized phase was stable during these conditions. Nanocrystalline Bi₂Te₃ with 9–10 nm mean grain size and flaky morphology (lamellar structure) was obtained at the end of milling.