Reactive sintering mechanism of Ti + Mo<Subscript>2</Subscript>C and Ti + VC powder compacts

TiC reinforced Ti-matrix composites have been synthesized successfully by reactive sintering of Ti-1.5%Fe-2.25%Mo (wt%) powder compacts with addition of Mo₂C and VC particles. The reactions for the formation of TiC particles start at 600 °C, but the distribution of TiC particles and the densification behavior in the two compacts are significantly influenced by the metal carbides (Mo₂C or VC). The compact with addition of Mo₂C has a relative density of 98% after sintering at 1300 °C for 1.5 h, but TiC particles are agglomerated in the Ti matrix. The compact with addition of VC has a relative density of about 91% after sintering at 1300 °C for 1.5 h, but TiC particles distribute more homogenously in the Ti matrix. Different TiC particle distribution and densification behaviors are attributed to the reaction rates between Ti and metal carbides and the subsequent diffusion process.     

High temperature behaviour of a Ti-6Al-4V/TiCp composite processed by BE-CIP-HIP method

The high temperature behaviour of a Ti-6Al-4V/TiC_p composite (10% Vol. of TiC) was investigated. A composite produced by Dynamet Technology according to the blended-elemental-cold-hot isostatic pressing (BE-CHIP) method was used. The stress-strain properties of the material were tested at 25, 200, 400, 500, 600 and 800°C. Composite specimens were aged in air at 500 and 700°C or under vacuum at 500, 700 and 1050°C, for periods ranging between 100 and 500 hours. The thermal stability of the matrix/ceramic interfaces was studied by using scanning electron microscope, electron probe microanalysis and x-ray diffraction. Carbon diffusion from the ceramic particles towards the composite matrix occurred (very likely already during the composite fabrication) because the metal matrix of all the composite samples (either in the as received or thermally treated conditions) showed a high content of carbon (more than 1% at.). However, the thermal treatments carried out at both 500 and 700°C under vacuum did not result in a ceramic-metal reaction. In spite of this, the formation of an ordered phase of formula Ti₂C can be inferred. Long period aging under vacuum at 700°C (500 h) did not lower the composite tensile strength. On the other hand, above 500°C in air the titanium matrix rapidly underwent oxidation, which gave rise to the formation of a thick surface reaction layer; this confirms that the composite material cannot be used above this temperature. Furthermore, the thermal treatment performed at 1050°C (under vacuum) resulted in a strong composite microstructure modification: the formation of new mixed carbides of Al and Ti was observed.     

Microstructure evolution and oxidation behaviour of Tif/TiAl3 composites

In this paper, Ti_f/TiAl₃ composites were fabricated by infiltration–in situ reaction method and its oxidation behaviours were investigated by cyclic oxidation testing at 700 °C, 800 °C and 900 °C. The microstructure evolution and oxidation of Ti_f/TiAl₃ composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray diffraction (EDX). The reaction between Ti₃Al particles and Al was more violent than that of Ti fibres and Al. Ti₃Al/Al reaction consumed a large amount of Al and inhibited the reaction of Ti fibres indirectly. Reactant of Ti fibres was TiAl₃ at 700 °C, and four reaction layers surrounding Ti fibre (Ti₃Al, TiAl, TiAl₂ and TiAl₃ from inner to outside) were observed above 800 °C. The thickness of the total reaction layers increased little with temperature and time, while the thickness of inner reaction layers increased remarkably. A model corresponding to the microstructure evolution process was drawn schematically. Oxidation resistance of Ti_f/TiAl₃ composites decreased with increasing of temperature, and changed from cubic law at 700 °C to parabolic law at 900 °C. The oxidation weight gain of Ti_f/TiAl₃ composite was dominated by the exposed Ti fibres. Due to outward diffusion of Ti and Al element, the oxide of Ti fibre at 900 °C changed to mushroom-shape. Fortunately, when TiAl₃ was oxidized, a thin and continuous Al₂O₃ layer was formed, protecting matrix from further oxidation.     

Combustion synthesis/quasi-isostatic pressing of TiC–NiTi cermets: processing and mechanical response

TiC–NiTi composites were produced by a technique combining self-propagating high-temperature synthesis (SHS) of elemental powders of Ni, Ti, and C with densification by quasi-isostatic pressing (QIP). In order to create a one-step synthesis/densification process, the Ti + Ni + C reactant material was surrounded in a bed of graphite and alumina particulate before initiation of the combustion reaction. The sample was ignited within the particulate and subjected to a uniaxial load immediately after passage of the combustion wave. The constitutive response, composition and resulting structures of the composites with varying volume fractions of NiTi are characterized. Powder mixtures prepared anticipating the formation of stoichiometric TiC result in the formation of composites with a eutectic matrix of Ni₃Ti and NiTi. This titanium impoverishment of the matrix is consistent with the formation of nonstoichiometric TiC _x during the combustion reaction. The Ni₃Ti phase can be suppressed by anticipating the formation of TiC_0.7 and adjusting the chemical content of the reactant mixture to include additional titanium. These cermets combine the high hardness of the ceramic phase with the possible shape memory and superelastic effects of NiTi.     

X-ray diffraction and high resolution transmission electron microscopy characterization of intermetallics formed in Fe/Ti nanometer-scale multilayers during thermal annealing

Intermetallics formation in the Fe/Ti nanometer-scale multilayers magnetron-sputtering deposited on Si(100) substrate during thermal annealing at 623-873 K was investigated by using small and wide angle X-ray diffraction and cross-sectional high-resolution transmission electron microscopy. The Fe/Ti nanometer-scale multilayers were constructed with bilayer thickness of 16.2 nm and the sublayer thickness ratio of 1:1. At the annealing temperature of 623 K, intermetallics FeTi were formed by nucleation at the triple joins of α-Fe(Ti)/α-Ti interface and α-Ti grain boundary with an orientational correlation of FeTi(110)//α-Ti(100) and FeTi[001]//α-Ti[001] to adjacent α-Ti grains. The lateral growth of intermetallics FeTi which is dependent on the diffusion path of Ti led to a coalescence into an intermetallic layer. With an increase in the annealing temperature, intermetallics Fe₂Ti were formed between the intermetallics FeTi and the excess Fe due to the limitation of Fe and Ti atomic concentrations, resulting in the coexistence of intermetallics FeTi and Fe₂Ti. It was found that the low energy interface as well as the dominant diffusion path constrained the nucleation and growth of intermetallics during interfacial reaction in the nanometer-scale metallic multilayers.     

Understanding the reaction mechanism of in-situ synthesized (TiC–TiB2)/AZ91 magnesium matrix composites

Magnesium matrix composites reinforced with a network of TiC and TiB₂ compounds have been successfully synthesized via an in-situ reactive infiltration technique. In this process, the ceramic reinforcing phases, TiC and TiB₂, were synthesized in-situ from the starting powders of Ti and B₄C without any addition of a third metal powder such as Al. The molten AZ91 magnesium alloy infiltrates the preform of 3Ti–B₄C by capillary forces. Furthermore, adding different weight percentages of MgH₂ powder to the 3Ti–B₄C preforms was used in an attempt to increase the Mg content in the fabricated composites. The results reveal a relatively uniform distribution of the reinforcing phases in the magnesium matrix with very small amounts of residual Ti, boron carbide and intermediate phases when they are fabricated at 900 °C for 1.5 h using a 3Ti–B₄C preform with 70% relative density. On the other hand, after adding MgH₂ to the 3Ti–B₄C preform, TiC_x and TiB₂ formed completely without any residual intermediate phases with the formation of the ternary compound (Ti₂AlC) at the expense of TiC. The percentage of reinforcing phases can be tailored by controlling the weight percentages of MgH₂ powder added to the 3Ti–B₄C preform. The results of the in-situ reaction mechanism investigation of the Ti–B₄C and Mg–B₄C systems show that the molten magnesium not only infiltrates through the 3Ti–B₄C preform and thus densifies the fabricated composite as a matrix metal, but also acts as an intermediary making the reaction possible at a lower temperature than that required for solid-state reaction between Ti and B₄C and accelerates the reaction rate. The investigation of the in-situ reaction mechanism with or without the addition of MgH₂ powder to the 3Ti–B₄C preforms reveals similar mechanisms. However, the presence of the MgH₂ in the preform accelerates the reaction resulting in a shorter processing time for the same temperatures.     

High temperature fracture behavior of tungsten fiber reinforced copper matrix composites under dynamic compression

W_f/Cu₈₂Al₁₀Fe₄Ni₄ composite was fabricated by flow casting method. Dynamic compression tests with strain rate of 1600 s⁻¹ at 20 °C, 200 °C, 400 °C and 600 °C were finished by means of Split Hopkinson Pressure Bar (SHPB). The results showed that the composites possessed obvious high temperature softening behaviors. The damages of W_f/Cu₈₂Al₁₀Fe₄Ni₄ composites all occurred within the tungsten fibers when compressed at 20 °C, 200 °C and 400 °C, indicating that the interface strength of the composites was high. While the damages of the composites occurred either in the tungsten fibers or in the matrix at 600 °C, in addition, the melt of matrix alloy also occurred. Microstructure of the composites after dynamic compressing at 600 °C was analyzed by transmission electron microscope (TEM), observation revealed that there were a lot of high-density dislocations, stacking faults and twins existing in the matrix. It was also found that the precipitated phase in the matrix played the role of the second phase strengthening.     

Microstructure and mechanical properties of an Al–TiC metal matrix composite obtained by reactive synthesis

A metal matrix composite has been obtained by a novel synthesis route, reacting Al₃Ti and graphite at 1000 °C for about 1 min after ball-milling and compaction. The resulting composite is made of an aluminium matrix reinforced by nanometer sized TiC particles (average diameter 70 nm). The average TiC/Al ratio is 34.6 wt.% (22.3 vol.%). The microstructure consists of an intimate mixture of two domains, an unreinforced domain made of the Al solid solution with a low TiC reinforcement content, and a reinforced domain. This composite exhibits uncommon mechanical properties with regard to previous micrometer sized Al–TiC composites and to its high reinforcement volume fraction, with a Young’s modulus of ∼110 GPa, an ultimate tensile strength of about 500 MPa and a maximum elongation of 6%.     

Ablation property of a C/C-Cu composite prepared by pressureless infiltration

Using Ti addition to improve the wettability between molten Cu and a C/C composite with a density of 1.60 g·cm^− 3, a C/C-Cu composite was obtained by pressureless infiltration. The Cu filled the pores of the C/C composite, while elemental Ti, being distributed between the carbon and Cu, reacted with carbon to form TiC which covered the carbon. During the ablation process under an O₂-C₂H₂ flame, the ablation of the infiltrated Cu and TiC in the composite was prior to the mechanical denudation of the C/C composite. The composite showed a better ablation performance in terms of short-term anti-ablation than a widely used C/C composite.     

The origin of hematite nanowire growth during the thermal oxidation of iron

The oxidation of Fe in pure oxygen between 400 °C and 600 °C has been investigated in order to obtain insight into the mechanism of the spontaneous formation of α-Fe₂O₃ nanowires. By varying the oxidation temperature, Fe can be oxidized to form Fe₂O₃/Fe₃O₄/FeO/Fe or Fe₂O₃/Fe₃O₄/Fe layered structure, followed by hematite nanowire growth on the outer layer of hematite (Fe₂O₃). It is observed that Fe₂O₃ nanowires have a bicrystal structure and form directly on the top of the underlying Fe₂O₃ grains. It is shown that the compressive stresses generated by the volume change accompanying the Fe₂O₃/Fe₃O₄ interface reaction stimulate Fe₂O₃ nanowire formation and that the Fe₂O₃ nanowire growth is via surface diffusion of Fe cations supplied from the outward grain boundary diffusion through the Fe₂O₃ layer. This principle of nanowire formation may have broader applicability in layered systems, where the stress gradient in thin layers can be introduced via solid-state interfacial reaction or other means.     

The mechanism of thermal explosion (TE) synthesis of TiC-TiB2 particulate locally reinforced steel matrix composites from an Al-Ti-B4C system via a TE-casting route

TiC-TiB₂ particulate locally reinforced steel matrix composites were fabricated by a novel TE-casting route from an Al-Ti-B₄C system with various B₄C particle sizes. The formation mechanism of TiC and TiB₂ in the locally reinforced regions was investigated. The results showed that TiC and TiB₂ are formed and precipitated from Al-Ti-B-C melt resulting from the dissociation of B₄C into Al-Ti melt when the concentrations of B and C atoms in the Al-Ti-B-C melt become saturated. However, in the case of coarse B₄C powders (≥40 μm) used, the primary reaction in the Al-Ti-B-C melt is quite limited due to the poor dissociation of B₄C. The poured steel melt infiltrates into the primary reaction product and thus leads to the formation of Al-Fe-Ti-B-C melt, thanks to the favorable reaction of molten Fe with remnant B₄C, and then TiC and TiB₂ are further formed and precipitated from the saturated Al-Fe-Ti-B-C melt. The relationship between the mechanisms of thermal explosion (TE) synthesis of TiC and TiB₂ in the electric resistance furnace and during casting was proposed.     

High-temperature properties of extruded titanium composites fabricated from carbon nanotubes coated titanium powder by spark plasma sintering and hot extrusion

Pure titanium matrix composite reinforced with carbon nanotubes (CNTs) was prepared by spark plasma sintering and hot extrusion via powder metallurgy process. Titanium (Ti) powders were coated with CNTs via a wet process using a zwitterionic surfactant solution containing 1.0, 2.0 and 3.0 wt.% of CNTs. In situ TiC formation via reaction of CNTs with titanium occurred during sintering, and TiC particles were uniformly dispersed in the matrix. As-extruded Ti/TiCs composite rods were annealed at 473 K for 3.6 ks to reduce the residual stress during processing. After annealing process, the tensile properties of the composites were evaluated at room temperature, 473, 573 and 673 K, respectively. Hardness test was also performed at room temperature up to 573 K with a step of 50 K. The mechanical properties of extruded Ti/CNTs composites at elevated temperature were remarkably improved by adding a small amount of CNTs, compared to extruded Ti matrix. These were due to the TiC dispersoids originated from CNTs effectively stabilized the microstructure of extruded Ti composites by their pinning effect. Moreover, the coarsening and growth of Ti grain never occurred even though they were annealed at 573, 673 K for 36 ks and 673 K for 360 ks, respectively.     

Synthesis of Fe/SiO2 composite particles and their superior electromagnetic properties in microwave band

Fe/SiO₂ composite particles were synthesized by hydrogen reduction of Fe₂O₃/SiO₂ precursor, which was prepared by sol-gel method. A reduction temperature higher than 600 °C is required for the complete conversion of Fe₂O₃ to Fe. Fe/SiO₂ composite particles exhibit superior complex permittivity and permeability in the microwave band. A reflection loss higher than − 70 dB as well as a broad absorption band can be simultaneously obtained for Fe/SiO₂-based coatings about 2 mm in thickness, suggesting that the Fe/SiO₂ composite particles are a promising candidate for high performance electromagnetic absorption materials.     

The relationship among microstructure evolution,mechanical property and in situ reaction mechanisms in preparing Cu–1.6wt%TiB2 alloys

The Cu–1.6wt%TiB₂ alloys have been prepared by combining mechanical alloying and heat treatment. The relationship among microstructure evolution, mechanical property and in situ reaction mechanisms in synthesizing Cu–1.6wt%TiB₂ alloys was investigated. It is shown that the temperature 750 °C measured by DSC can cause the occurrence of in situ reaction between B and Ti elements in the Cu–B–Ti alloy powders ball milled for 40 or 60 h; An upward trend of the hardness appears after heat treatment at 700 °C, and the highest value reaching 335HV, but a downward trend appears after heat treatment at either 500 °C or 900 °C; Many coarse B particles were formed in the copper matrix after heat treatment at 500 °C, the number and size of the formed B particles significantly increases with increasing time, but many reaction boundaries (or TiB₂ films) and finer TiB₂ particles can be formed after heat treatment at both 700 °C and 900 °C. The different in situ reaction models between Cu–B and Cu–Ti alloy micelles with different distribution levels were established and analyzed.     

Laser cladding composite coatings by Ni–Cr–Ti–B4C with different process parameters

To investigate the effect of laser process parameters on microstructure and properties of composite coating, the composite coatings were manufactured by laser cladding Ni–Cr–Ti–B₄C mixed powder on Q235 mild steel with different process parameters. The coatings are bonded with the substrate by remarkable metallurgical binding without cracks and pores. The composite coatings are consisted of in situ synthesized solid solution Ni–Cr–Fe, intermetallic compound (IMC) Ni₃Ti, Cr₂Ti, and ceramic reinforcements TiB₂, TiC. Results of scanning electron microscopy (SEM) revealed that the ceramic reinforcements became coarser with higher specific energy (E_s). There were independent ceramics TiB₂, TiC, eutectic ceramic TiB₂–TiC in coatings, and eutectic alloy–ceramic was detected. Compared with the substrate, the microhardness of coatings was increased significantly, and the maximum microhardness of coatings was approximately five times as high as the substrate. The wear resistance of coatings was improved dramatically than the substrate. Compared to the coatings with lower E_s, higher E_s led to lower microhardness and worse wear resistance ascribing to more Fe diffused into the coating from the substrate.     

Fabrication of in situ TiC locally reinforced manganese steel matrix composite via combustion synthesis during casting

In situ TiC particulates locally reinforced manganese (Mn) steel matrix composite was successfully fabricated via combustion synthesis of (Fe,Ti)–C system during casting. XRD results reveal that the phases of the composites consist of TiC, α-Fe and austenite. Microstructure of the locally reinforced manganese (Mn) steel matrix composite consists of three separate regions, i.e. a TiC particulate-reinforced region, a transition region, a steel matrix region. TiC particles in the reinforced region, having fine size of 2 μm, are distributed uniformly. The hardness and wear resistance of the TiC particulates locally reinforced composites are much higher than those of quenched Mn13 steel. Furthermore, the microstructure formation mechanism of the composite was discussed.     

Synthesis and phase evolution of Si-C-Ti powder derived from poly(methylsilaacetylene) and Ti

Si-C-Ti powder was synthesized by reactive pyrolysis of poly(methylsilaacetylene)(PSCC) precursor mixed with metal Ti powder. Pyrolysis of PSCC/Ti mixture with certain atomic ratio was carried out in argon atmosphere between 1300 °C and 1500 °C. The metal-precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy equipped with EDX. Ti₃SiC₂ phase was obtained from reaction of PSCC and Ti for the first time. Ti₃SiC₂ formation started at 1300 °C and its amount increased significantly at 1400 °C. In addition, liquid formed by additive CaF₂ could promote the formation of Ti₃SiC₂ phase.     

Solid-state synthesis of tungsten carbide from tungsten oxide and carbon, and its catalysis by nickel

Tungsten carbide has been produced by heating a mixture of tungsten oxide and carbon powder at 1300 °C for 2 h. Further batches were made with additional KCl, KCl + Ni, or KCl + Fe. The products were compared by XRD and SEM. A mixture of WC and W₂C was produced from the plain WO₃/carbon reaction, but adding 1 wt.% nickel assisted the formation of a pure WC phase. Both Ni and Fe assisted the growth of larger WC crystals.     

Effects of aging treatment on microstructure and tribological properties of nickel-based high-temperature self-lubrication wear resistant composite coatings by laser cladding

The NiCr/Cr₃C₂–WS₂ high-temperature self-lubrication wear resistant composite coatings were fabricated on substrate of a hot-rolled AISI304 austenitic stainless steel by laser cladding. The high-temperature phase stability of the composite coatings was evaluated by aging at 600 °C for 10 h, 30 h, 50 h, and the microstructures of the as-laser clad and aged coatings were examined by means of XRD, SEM, EDS, respectively. The sliding wear resistance of the as-laser clad and aged coatings was evaluated at 600 °C. The results show that NiCr/Cr₃C₂–WS₂ composite coating has excellent high-temperature phase stability, the γ-(Fe,Ni)/Cr₇C₃ eutectic phases, Cr₇C₃ and (Cr,W)C hard phases, CrS/WS₂ mixed solid lubricant phases all existed in the as-laser clad and aged coatings. The volume fraction of eutectic phases decreased gradually with the increasing of aged time due to their dissolution. The microhardness of the aged coating decreased slightly after aging the coating 50 h at 600 °C due to the dissolution of the eutectic phases and notable breaking or granulation of the Cr₇C₃ hard phase, but the tribological properties were not significantly affected by aging treatment.     

Epitaxial orientation and morphology of β-FeSi2 produced on a flat and a patterned Si(001) substrates

The epitaxial growth of β-FeSi₂ films produced on flat and patterned Si(001) substrates under various substrate temperatures (T_s) with deposition rates of Fe (V_Fe) was investigated by transmission electron microscopy (TEM). In the film deposited on the flat Si(001) substrate, precipitates of flat-bottom shaped β-FeSi₂ and those of round-bottom shaped α-FeSi₂ were formed at T_s = 500 °C and V_Fe = 0.02 nm/s. The β-FeSi₂ adopted the epitaxy to (001)_Si plane, while α-FeSi₂ selected the epitaxy to {111}_Si planes inside the Si matrix. At T_s = 350 °C and V_Fe = 0.01 nm/s, a continuous β-FeSi₂ layer were formed epitaxially on the Si(001) substrate without forming α-FeSi₂. It was found that the lower temperature and the higher Fe-concentration suppress the formation of α-FeSi₂ and promote the formation of β-FeSi₂. In addition, the morphology of β-FeSi₂ changed from fine isolated precipitates (islands) to a continuous layer with increasing the deposition rate and the substrate temperature. In the film deposited on the patterned Si(001) substrate at T_s = 500 °C and V_Fe = 0.02 nm/s, on the other hand, both β- and α-FeSi₂ precipitates were formed on the top-hills and the valleys of the patterned substrate, while only α-FeSi₂ precipitates were formed on the sidewalls. These results demonstrate that not only the growth conditions but also geometric situations affect strongly the epitaxial growth of FeSi₂ precipitates.