In-situ synthesis of Ti-Si-C fine grained composite with different amount of TiC: Microstructure and mechanical properties

The present work reports the in-situ synthesis of the Ti-Si-C composite so that TiC phase is increased in the composite by suitable adjustment of starting reactants during self-propagating high temperature synthesis. The detailed microstructure and mechanical behaviour of the composite has been studied. The microstructure, density, phase present, amount of melt phase, hardness and modulus were directly affected by adiabatic temperature of the reaction. The highest value of microhardness was found to be around 2200 HV_0.05. The nanoindentation revealed the hardness range from 22 to 12 GPa and modulus varied from 445 to 281 GPa for different percentages of TiC into matrix.     

In situ fabrication of ZrC powder obtained by self-propagating high-temperature synthesis from Al-Zr-C elemental powders

The in situ synthesis of ZrC powder utilizing self-propagating high-temperature synthesis (SHS) reaction that occurred in the compact consisting of Al, Zr and C powders was investigated. The result shows when Al contents were 0-40 wt.% SHS reaction proceeded favorably. The as-products display a uniform distribution of ZrC particles with the sizes ranging from 8 μm at Al free to 50 nm at 40 wt.% Al. The temperature curve, coupled with the quenched treatment, indicates that the Al-Zr reaction to form ZrAl₃ initiated and then C reacted with ZrAl₃ to form the more stable ZrC phase. It also proves that the mechanism of reaction-precipitation should be responsible for the formation of ZrC in this system. Al has been playing an important role in determining the formation of ZrC, not only as a diluent to inhibit the ZrC grains from coarsening, but also as an intermediate reactant to participate in the total reaction.     

Processing titanium foams using tapioca starch as a space holder

In this paper, the fabrication of open cellular titanium foams by using an innovative spacer through powder metallurgy was investigated. Current space holders used to fabricate titanium foam have brought attention to issues such as solubility, removal time, expensiveness and lack of environmental friendliness. Starch, as a new spacer, has been utilized in this research to overcome these issues. Starch is an environmentally friendly organic polymer that can be burnt off easily by heating. Starch is chemically stable and is not soluble in titanium.Titanium foams were fabricated in this study using starch. The mechanical properties and pore structures were investigated by SEM. The porosities of the fabricated titanium foams were 64-79%, and the sizes of open cellular pores were 100-300 μm. The observed yield strength of the foams was in the range of 23-41 MPa, and the Young's moduli were 1.6-3.7 GPa. Finally, the XRD results of titanium before and after sintering were studied to ensure that the foams produced featured no contamination.     

Surface engineering techniques used for improving the mechanical and tribological properties of the Ti6A14V alloy

The microstructure, mechanical and tribological properties of composite intermetallic layers of the Ti-Al system were investigated. The layers on the two-phase (α + β) Ti6Al4V titanium alloy were obtained using the hybrid method, which consist of two stages. First, the titanium alloy was covered with aluminum layer by magnetron sputtering. In the second stage, coated specimens were treated under glow discharge conditions. The surface treatment makes it possible to obtain the diffusional composite layers which considerably improve the mechanical and wear resistance properties of the material. These properties cannot be achieved using simple methods of surface engineering. The chemical and phase composition of the composite intermetallic layers was investigated. It has been shown that the hybrid method significantly increases the hardness and wear resistance of the titanium alloy and can widen the range of application of the material. Because of the diffusional character of the intermetallic layers they possess good adhesion to the titanium substrate.     

Ion beam assisted deposition of TiN-Ni nanocomposite coatings

Hard and tough nanocomposite coatings consisting of hard TiN nanograins embedded in a soft metallic intergranular phase of Ni have been produced using ion beam assisted deposition. The chemical composition has been obtained by Rutherford Backscattering and the microstructural properties: phases, grain size, and texture of the coatings have been investigated by X-Ray Diffraction. In the composition range 0-22.5 at.% Ni, δ-TiN is the only crystalline phase and Ni appears as an X Ray amorphous phase. The hardness increases up to a maximum of 41 GPa at ~ 7 at.% Ni which corresponds to a TiN crystallite size of ~ 8 nm and a Ni intergranular phase thickness of roughly 1 monolayer. It is shown that the hardness enhancement in TiN-Ni nanocomposite coatings is not correlated with residual stresses, but rather with the intrinsic properties of the nanostructure. An important improvement in wear resistance is obtained for the coatings exhibiting the highest toughness and not the highest hardness. These results show that ion assisted processing is an effective tool for producing dense TiN-Ni nanocomposite coatings and tailoring their structure and mechanical properties.     

Crystallization of anodic titania on titanium and its alloys

Crystallization of amorphous anodic films grown at constant current density on sputtering-deposited titanium, and Ti-Si and Ti-Al alloys, in ammonium pentaborate electrolyte, has been examined directly by transmission electron microscopy. In the case of titanium, anatase develops at relatively low voltage in the inner film region, formed by inward migration of oxygen species. In contrast, the outer film region, formed at the film/electrolyte interface, is composed of amorphous oxide only. Oxide crystals are particularly found near the plane, separating the two regions, which is located at a depth of 35-38% of the film thickness. Oxide zones, of size ∼ 1 nm, with a relatively ordered structure, developed at the metal/film interface, are considered to lead to transformation of the inner region structure. The incorporation into the film of either aluminium or silicon species suppresses the formation of crystalline oxide to much increased voltages. However, eventually nanocrystals form at ∼40% of the film thickness, probably originating from pre-cursor nuclei in the air-formed on the as-deposited alloy.     

Mechanical, tribological and erosion behaviour of super-elastic hard Ti-Si-C coatings prepared by PECVD

Titaniun carbide (TiC) based coatings prepared by low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) are investigated as attractive candidates for wear resistance, and particularly for protection against solid particle erosion. In the present work, we incorporated silicon (Si) as an alloying element to TiC, to obtain ternary nanostructured Ti-Si-C films. The incorporation of Si in TiC resulted in significant microstructural, mechanical and tribological modifications. By controlling the Si content in the films, we observed a transition between films consisting of fine nano-sized TiC crystallites (nc-TiC) embedded in an amorphous C:H matrix (a-C:H) to a microstructure formed by nc-TiC encapsulated in a-SiC/a-C:H matrix. This allowed one to selectively control the main mechanical characteristics, namely the hardness (H), the Young's modulus (E), and the friction coefficient (μ), in the range of 14-32 GPa, 140-240 GPa, and 0.16-0.6, respectively. For films prepared under optimized conditions, high elastic strain to failure and high resistance to plastic deformation of the Ti-Si-C films, expressed by H/E and H³/E² ratios, resulted in an 8 fold increase of the erosion resistance at an impact angle of 90° compared to a bare steel substrate. Erosion resistance at 30° increased by a factor of 22 compared to bare substrate due to a simultaneous combination of high H and low μ. Taking into consideration the severe erosion test conditions and the Ti-Si-C film thickness of less than 5 μm in this work, further improvement is expected for thicker films.     

Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium

Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.     

Fabrication of In Situ TiBCSi Composite by Hot-Press Sintering

An attempt has been made to synthesize TiBSiC composite using titanium, silicon carbide, and boron carbide as the raw materials by hot-press sintering. The TiBSiC bulk composite with a high hardness (20.5 GPa) and high compressive strength (2010 MPa), with more than 99.1% density of theoretical value, has been successfully synthesized by the hot-press sintering. The composite was found to consist of fine TiB₂, TiC, and Ti₃SiC₂ using XRD analysis. SEM and EDX analyses showed uniform distribution of phases and an average grain size of around 2-3 μm. Nanoindentation studies showed the modulus of the composite is about 448 GPa.     

A new TiB2 + CrSi2 composite – Densification,characterization and oxidation studies

A new composite of TiB₂ with CrSi₂ has been prepared with excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM with EDAX. Mechanical and physical properties were evaluated. Extensive oxidation studies were also carried out. A near theoretical density (99.9% TD) was obtained with a small addition of 2.5 wt.% CrSi₂ by hot pressing at 1700 °C under a pressure of 28 MPa for 1 h. The microstructure of the composite revealed three distinct phases, (a) dark grey matrix of TiB₂, (b) black phase – rich in Si and (c) white phase – Cr laden TiB₂. Hardness and fracture toughness were measured as 29 ± 2 GPa and 5.97 ± 0.61 MPa m^1/2, respectively. Crack branching, deflection and bridging mechanisms were responsible for the higher fracture toughness. With increase in CrSi₂ content, density, hardness and fracture toughness values of the composite decreased. Thermo gravimetric studies revealed the start of oxidation of the composite at 600 °C in O₂ atmosphere. Isothermal oxidation of these composites showed better oxidation resistance by formation of a protective oxide layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Effects of CrSi₂ content, temperature and duration of oxidation on the oxide layer formation are reported. Activation energy of the composite was calculated as ∼110 kJ/mol using Arrhenius equation. Diffusion controlled mechanism of oxidation was observed in all the composites.     

Low temperature sintering of ZrC-SiC composite

High-energy ball milling and spark plasma sintering were adopted to prepare ZrC-SiC composite. Zirconium carbide, silicon, and graphite powders were used as raw materials. ZrC-30 vol.%SiC was sintered to a relative density of >96.1% at 1800 °C. The composite showed a fine microstructure. The fracture strength reached up to 523.4 MPa, Vickers’ hardness 18.8 GPa, fracture toughness 4.0 MPa m^1/2, and elastic modulus 390.5 GPa.     

Structure and microhardness of nanocrystalline composite alloys on the basis of Al and Ti

A nanotechnology for production of layered composite nanocrystalline Al-Si, Al(1Hf + 0.2Nb + 0.2Sn, wt %)-Si, Al(0.5Ce + 0.5Re + 0.1Zr, wt %)-Si, and Ti-Si alloys has been suggested. The structure of the nanocrystalline composites obtained has been studied by the methods of optical microscopy and scanning and transmission electron microscopy. Experimental data on the microhardness of layered composite nanocrystalline alloys on the basis of aluminum and titanium are given. The microhardness of the nanocrystalline two-layered Al-Si composite in comparison with submicrocrystalline aluminum increased by a factor of three. In the nanocrystalline two-layered Ti-Si composite (compacted from powders), the microhardness also increased by a factor of almost six in comparison with nanocrystalline titanium. In the nanocrystalline two-layered composite alloys of aluminum, the increase in microhardness in the case of the Al(Hf,Nb,Sn)-Si alloy was up to 45% as compared to the composite on the basis of the metallic foil and by a factor of two as compared to the powder-compact composite; and in the case of the Al(Ce, Re,Zr)-Si alloy, by a factor of 1.6 and 2.7, respectively. An increase in the number of layers in the composites had no significant effect on the level of microhardness.     

Microstructure, mechanical and wear properties of laser processed SiC particle reinforced coatings on titanium

Laser processing of Ti-SiC composite coating on titanium was carried out to improve wear resistance using Laser Engineered Net Shaping (LENS™) — a commercial rapid prototyping technology. During the coating process a Nd:YAG laser was used to create small liquid metal pool on the surface of Ti substrate in to which SiC powder was injected to create Ti-SiC metal matrix composite layer. The composite layers were characterized using X-ray diffraction, scanning and transmission electron microscopy equipped with fine probe chemical analysis. Laser parameters were found to have strong influence on the dissolution of SiC, leading to the formation of TiSi₂, Ti₅Si₃ and TiC with a large amount of SiC on the surface. Detailed matrix microstructural analysis showed the formation of non-stoichiometric compounds and TiSi₂ in the matrix due to non-equilibrium rapid solidification during laser processing. The average Young's modulus of the composite coatings was found to be in the range of 602 and 757 GPa. Under dry sliding conditions, a considerable increase in wear resistance was observed, i.e., 5.91 × 10^− 4 mm³/Nm for the SiC reinforced coatings and 1.3 × 10⁻³ mm³/Nm for the Ti substrate at identical test conditions.     

The Ti-Si-C system (Titanium-Silicon-Carbon)

In the present article different isothermals of Ti-Si-C system at temperatures ranging from 1250 to 2877 °C, previously reported by [1966Bru], [1989Tou], [1991Wak], and [1993Sei1], were assessed and redrawn on the basis of the recently reported binary alloy phase diagram of Ti-Si, Ti-C, and Si-C.     

Adhesion and composite micro-hardness of DLC and Si-DLC films deposited on nitrile rubber

Thin films of hydrogenated diamond-like carbon (DLC) and silicon (Si) doped diamond-like carbon (Si-DLC) have been deposited on acrylonitrile butadiene rubber (NBR) using a closed field unbalanced magnetron sputtering ion plating system. A sputter cleaning process was integrated into the deposition process so as to reduce the likelihood of re-contamination between the cleaning and deposition stages. The deposited coatings showed excellent adherence with an adhesion rating of 4 A for films with a Si-C interlayer. The composite micro-hardness was highest for DLC films at 15.5 GPa for indentation load of 147.1 mN using a Vickers micro-hardness tester. Tribological tests undertaken under normal load of 5 N using a pin-on-disc tribometer for all of the samples of DLC and Si-DLC films, with and without Si-C interlayer, show a friction increase between 0.25 and 0.4 to between 0.45 and 0.6. This friction increase has been related to the micro-hardness of the films.     

Microstructure and mechanical behavior of a Ti-24Nb-4Zr-8Sn alloy processed by warm swaging and warm rolling

A combination processing technique of warm swaging and warm rolling is proposed to refine grains and improve the mechanical properties of a multifunctional β-type Ti-24Nb-4Zr-8Sn (wt.%) alloy. The results show that a highly swirled marble-like microstructure can be easily produced by warm swaging at an initial temperature of 573 K, whereas it has little effect on the nonlinear elastic deformation compared with the hot forged alloy with an equiaxed microstructure. Although the swirled microstructure has the limitation of an inhomogeneous distribution, swaging has the great advantage of refining the initial subgrains produced by hot forging with little loss of ductility. The following warm rolling at an initial temperature of 673 K results in a uniform microstructure comprising β phase with a size less than ∼200 nm and the precipitation of nanosized α phase. Therefore, significant grain refinement was achieved through the formation and refinement of the subgrains. The ultrafine grained alloy exhibits large scale nonlinear deformation behavior with a recoverable strain of up to ∼3.4% in combination with a high strength of ∼1150 MPa, a low elastic modulus of ∼56 GPa and good ductility of ∼8%. Such an improvement in mechanical properties indicate great potential for biomedical applications.     

Bulk texture evolution of Cu-Nb nanolamellar composites during accumulative roll bonding

A combination of accumulative roll bonding (ARB) and rolling is used to fabricate nanolamellar Cu-Nb multilayers with individual layer thicknesses (h) of 600 μm ? h ? 10 nm with a total strain imposed between 0.5 and 11.6. Neutron diffraction, scanning electron microscopy and transmission electron microscopy are used to characterize the microstructures and measure orientation distribution functions of both phases as a function of layer thickness. Fiber plots are calculated from the orientation distribution functions in order to understand the texture evolution in the Cu and Nb layers with increasing strain. Results are compared with rolling studies of single phase Cu, single phase Nb, and cast Cu-20 wt.% Nb composite. Results indicate that textures develop in the Cu and Nb layers during ARB that are distinct from classical rolling textures frequently observed both in their single-phase counterparts and in rolled composites. The atypical texture that develops shows a preferential strengthening of specific β fiber components at the expense of others in Cu and a strengthening of the α fiber at the expense of the γ fiber in Nb. No dynamic recrystallization is observed in Cu, even at strains above 99.99%, further delineating the behavior from single phase and composite behavior previously observed. Viscoplastic self-consistent (VPSC) polycrystal simulations were carried out to provide an understanding of the texture evolution in accumulative roll bonding. Enforcing planar slip in Cu leads to texture evolution for VPSC consistent with observations. A reasonable fit for Nb could be produced via the selection of specific {1 1 0} and {1 1 2} slip systems.     

Fabrication of ZrC particles and its formation mechanism by self-propagating high-temperature synthesis from Fe-Zr-C elemental powders

ZrC particles were prepared via self-propagating high-temperature synthesis (SHS) reaction from 0 to 30 wt.% Fe-Zr-C elemental powder mixtures. The ZrC particles of the SHS products greatly decreased from about 10 μm with an irregular shape in free Fe addition to nano-meter order with a nearly spherical shape in 30 wt.% Fe addition. The reaction mechanism of ZrC during the SHS processing was explored through the microstructural observation and phase constituents analysis on the combustion-wave quenched sample in combination with differential thermal analysis (DTA). For the low Fe contents, the solid-state reaction between C and Zr powders should be responsible for the formation of ZrC. While for the high Fe contents, the formation mechanism of ZrC could be ascribed to the dissolution of C into the Fe-Zr melt and the precipitation of ZrC. The addition of Fe to Zr-C reactants not only inhibits the growth of ZrC particles but also promotes the occurrence of ZrC-forming reaction.     

Electroless ternary Ni-W-P alloys containing micron size Al2O3 particles

In the present investigation electroless ternary NiWP-Al₂O₃ composite coatings were prepared using an electroless nickel bath. Second phase alumina particles (1 µm) were used to codeposit in the NiWP matrix. Nanocrystalline ternary NiWP alloys and composite coatings were obtained using an alkaline citrate based bath which was operated at pH 9 and temperature at 88 ± 2 °C. Mild steel was used as a substrate material and deposition was carried out for about 4 h to get a coating thickness of 25 ± 3 µm. Metallographic cross-sections were prepared to find out the coating thickness and also the uniform distribution of the aluminum oxide particles in NiWP matrix. Surface analysis carried out on both the coatings using scanning electron microscope (SEM) showed that particle incorporation in ternary NiWP matrix has increased the nodularity of composite coatings compared to fine nodular NiWP deposits. Elemental analysis of energy dispersive X-ray (EDX) results showed that codeposited P and W elements in plain NiWP deposit were 13 and 1.2 wt.%, respectively. There was a decrease in P content from 13 to 10 wt.% with a marginal variation in the incorporated W (1.01 wt.%) due to the codeposition of aluminum oxide particles in NiWP matrix. X-ray diffraction (XRD) studies carried out on as-plated deposits showed that both the deposits are X-ray amorphous with a grain size of around 3 nm. Phase transformation studies carried out on both the coatings showed that composite coatings exhibited better thermal stability compared to plain NiWP deposits. From the XRD studies it was found that metastable phases such as NiP and Ni₅P₂ present in the composite coatings heat treated at major exothermic peak temperature. Annealed composite coatings at various temperatures revealed higher microhardness values compared to plain NiWP deposits.     

The ω-phase formation in titanium upon deformation under pressure

The ω-phase formation in titanium of the VT1-00 grade upon deformation under pressure has been investigated by X-ray diffraction analysis and diffraction electron microscopy. The deformation was effected by two methods: shear under a pressure of 6 GPa in Bridgman anvils and high-strain-rate equal-channel angular pressing at a pressure of about 2 GPa. Upon deformation under a pressure of 6 GPa, the ω phase is formed as grains that are isolated or clustered into groups. Upon deformation at a pressure of 2 GPa, this phase arises in the form of nanosized particles that are orientationally related to the α phase. After deformation by shear under pressure using one revolution of the anvils, new grains of the α phase up to 2–3 μm in size have been detected. The grains are nearly free of dislocations and have wavy boundaries. The origin of these grains is tracealle the reverse ω → α phase transformation that takes place upon pressure release and occurs via the “normal” rather than martensitic mechanism, at the expense of migration of the inter-phase boundaries. Upon heating, the reverse ω → α transformation at 100°C does not yet begin, whereas at 220°C the transformation proceeds almost completely. A temperature distribution in the titanium sample upon shear under pressure at a rate of 0.3 rpm has been calculated; according to this distribution, the maximum temperature rise is 12 K.