Production of TiC reinforced-aluminum composites with the addition of elemental carbon

TiC reinforced Al matrix composites were produced by the additions of elemental carbon to both Al + 4%Ti and Al + 5%Ti alloys. It is shown that the microstructure, phase composition as well as fracture behavior of the composites produced are controlled by the processing parameters, such as temperature, amount of excess carbon and duration. Composite microstructure subjected to 1300 °C for 15 min includes only TiC particles where the fracture occurs in a ductile manner whilst composites subjected to 1200 °C for 30 min contain Al₃Ti and TiC particles which show mixed mode of fracture behavior where Al₃Ti particles resulted in brittle fracture due to their coarser size.     

Brazing continuous carbon fiber reinforced Li2O–Al2O3–SiO2 ceramic matrix composites to Ti–6Al–4V alloy using Ag–Cu–Ti active filler metal

C_f/LAS composites and TC4 alloy were brazed successfully by vacuum brazing using Ag–Cu–Ti active filler metal. The interfacial microstructure was characterized by a scanning electron microscope, energy dispersive spectrometer and X-ray diffraction. The effects of brazing temperature on the interfacial microstructure and joint properties were investigated in details. Various phases including TiC, TiSi₂, Ti₃Cu₄, Cu (s,s), Ag (s,s), TiCu and Ti₂Cu were formed in the brazed joints. Interfacial microstructure varies greatly with the increase of brazing temperature, while the amount of Ti₂Cu reduced, but no new phase is generated. The optimal shear strength of the joint is 26.4 MPa when brazed at 890 °C for 10 min. Shear test indicated that the fracture of the brazed joints went through the TiSi₂ + TiC layer close to the C_f/LAS composites interface.     

Microstructure and mechanical properties of hot extruded 6016 aluminum alloy/graphite composites

The incorporation of graphite particles into AA6016 aluminum alloy matrix to fabricate metal/ceramic composites is still a great challenge and various parameters should be considered. In this study, dense AA6016 aluminum alloy/(0-20 wt%) graphite composites have successfully been fabricated by powder metallurgy process. At first, the mixed aluminum and graphite powders were cold compacted at 200 MPa and then sintered at 500 ℃ for 1 h followed by hot extrusion at 450 ℃. The influence of ceramic phases(free graphite and in-situ formed carbides) on microstructure, physical and mechanical properties of the produced composites were finally investigated. The results show that the fabricated composites have a relative density of over 98%. SEM observations indicate that the graphite has a good dispersion in the alloy matrix even at high graphite content. Hardness of all the produced composites was higher than that of aluminum alloy matrix. No cracks were observed at strain less than 23% for all hot extruded materials.Compressive strength, reduction in height, ultimate tensile stress, fracture stress, yield stress, and fracture strain of all Al/graphite composites were determined by high precision second order equations. Both compressive and ultimate tensile strengths have been correlated to microstructure constituents with focusing on the in-situ formed ceramic phases, silicon carbide(SiC) and aluminum carbide(Al_4 C_3). The ductile fracture mode of the produced composites became less dominant with increasing free graphite content and in-situ formed carbides. Wear resistance of Al/graphite composites was increased with increasing graphite content. Aluminum/20 wt% graphite composite exhibited superior wear resistance over that of AA6016 aluminum alloy.     

Microstructural characteristics and evolution of Ti2AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process

The microstructural evolution of TiAl matrix composites with a novel network distribution of Ti₂AlN particle reinforcement was studied. The composites were synthesized by reaction hot pressing method using pure Al and nitrided Ti powders as initial materials. Pure Ti powders nitrided at 600 °C for a certain time in an atmosphere of flowing nitrogen turned into new compound Ti(N) powders, which have a shell of titanium nitrides (such as TiN, Ti₂N and TiN_0.3) and a core of Ti–N solid solution. Within the composites synthesized, Ti₂AlN particles, produced by in situ reaction, exhibit a network distribution. The special shell/core structure of the compound Ti(N) powders contributes to this architecture. Nitriding time of the Ti powders greatly affects the microstructure of the composites. Increasing the nitriding time is beneficial to the distribution of Ti₂AlN particles in a continuous network form. However, too long nitriding time can result in the aggregation of Ti₂AlN particles and thus destroy the uniformity of the network structure. The in-situ synthesized Ti₂AlN/TiAl composites with uniform network structure have a superior mechanical property, and their compressive strengths at 800 °C and 1000 °C are 1112 MPa and 687 MPa, respectively.     

Microstructure evolution and mechanical properties of GH984G alloy with different Ti/Al ratios during long-term thermal exposure

GH984G alloy is a low cost Ni–Fe based wrought superalloy designed for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the microstructure evolution and tensile properties of GH984G alloy with different Ti/Al ratios during thermal exposure at different high temperatures are investigated. Detailed microstructure analysis reveals that the Microstructure of alloys with different Ti/Al ratios are similar after standard heat treatment, and the primary precipitates are γ′, MC, M₂₃C₆ and M₂B. However, η phase precipitates at grain boundary in the alloy with high Ti/Al ratio after thermal exposure at 750 °C for 570 h. By contrast, the microstructure stability of the alloy with lower Ti/Al ratio is excellent. There is no detrimental phase even if after thermal exposure at 750 °C for 5000 h in the alloy with lower Ti/Al ratio. γ′ coarsening plays a great role on the tensile strength, and the critical size range of γ′ could be defined as approximately 27–40 nm. The influence of η phase on tensile strength has close relationship with its volume fraction, the high volume fraction results in the decrease of tensile strength. The tensile strength of the alloy with lower Ti/Al ratio is obviously higher than the alloy with higher Ti/Al ratio and the yield strength has no obvious decrease during long-term thermal exposure at 700 °C. It is demonstrated that the thermal stability of microstructure and mechanical properties of GH984G alloy can be improved by moderately decreasing Ti/Al ratio to satisfy the requirement of A-USC plants.     

Structural and chemical characterization of precipitates in Al-2024/TiC composites

Structural characterizations based on transmission electron microscopy observations were carried out on as-fabricated and heat-treated Al-2024/TiC composites. These composites types reinforced with TiC particles were produced with a pressureless melt infiltration route at 1200 °C for 2 h under argon atmosphere. The composites were heat-treated at 530 °C during 150 min, cold-water quenched and subsequently artificial and natural aged at 190 °C for 12 h in an argon environment and at room temperature for 96 h, respectively. Different precipitate types were obtained and they were identified as CuAl₂, Al₃Ti, Ti₃AlC and Ti₃Al. Most of the precipitates were found to be uniformly distributed in the matrix and some regions show precipitates which have a cubic morphology (Ti₃Cu). High-resolution electron microscopy images were partially used for the characterization of the precipitates in these composites.     

Microstructure and mechanical properties of in situ TiC and Nd2O3 particles reinforced Ti–4.5 wt.%Si alloy composites

(TiC + Nd₂O₃)/Ti–4.5 wt.%Si composites were in situ synthesized by a non-consumable arc-melting technology. The phases in the composites were identified by X-ray diffraction. Microstructures of the composites were observed by optical microscope and scanning electron microscope. The composite contains four phases: TiC, Nd₂O₃, Ti₅Si₃ and Ti. The TiC and Nd₂O₃ particles with dendritic and near-equiaxed shapes are well distributed in Ti–4.5 wt.%Si alloy matrix, and the fine Nd₂O₃ particles exist in the network Ti + Ti₅Si₃ eutectic cells and Ti matrix of the composites. The hardness and compressive strength of the composites are markedly higher than that of Ti–4.5 wt.%Si alloy. When the TiC content is fixed as 10 wt.% in the composites, the hardness is enhanced as the Nd₂O₃ content increases from 8 wt.% to 13 wt.%, but the compressive strength peaks at the Nd₂O₃ content of 8 wt.%.     

Development and characterization of laser clad high temperature self-lubricating wear resistant composite coatings on Ti–6Al–4V alloy

To enhance the wear resistance and friction-reducing capability of titanium alloy, a process of laser cladding γ-NiCrAlTi/TiC + TiWC₂/CrS + Ti₂CS coatings on Ti–6Al–4V alloy substrate with preplaced NiCr/Cr₃C₂–WS₂ mixed powders was studied. A novel coating without cracks and few pores was obtained in a proper laser processing. The composition and microstructure of the fabricated coating were examined by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) techniques, and tribological properties were evaluated using a ball-on-disc tribometer under dry sliding wear test conditions at 20 °C (room-temperature), 300 °C, 600 °C, respectively. The results show that the coating has unique microstructure consisting of α-Ti, TiC, TiWC₂, γ-NiCrAlTi, Ti₂CS and CrS phases. Average microhardness of the composite coating is 1005 HV_0.2, which is about 3-factor higher than that of Ti–6Al–4V substrate (360 HV_0.2). The friction coefficient and wear rate of the coating are greatly decreased due to the combined effects of the dominating anti-wear capabilities of reinforced TiC and TiWC₂ carbides and the CrS and Ti₂CS sulfides which have excellent self-lubricating property.     

Effects of Zr,Ti and Sc additions on the microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe alloys

The evolution of microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe (wt.%) alloys, micro-alloyed with Zr, Ti and Sc, were investigated. The addition of 0.2%Zr to base alloy accelerates the precipitation of Si-rich nano-phase in α-Al matrix, which plays an important role in improving the mechanical properties of an alloy. The tensile strength increases from 102 MPa for the base alloy to 113 MPa for the Zr-modified alloy. Adding 0.2%Zr + 0.2%Ti to base alloy effectively refines α-Al grain size and accelerates the precipitation of Si and Cu elements, leading to heavy segregation at grain boundary. By further adding 0.2%Sc to Zr + Ti modified alloy, the segregation of Si and Cu elements is suppressed and more Si and Cu precipitates appeared in α-Al matrix. Accompanied with the formation of coherent Al₃Sc phase, the tensile strength increases from 108 MPa for the Zr + Ti modified alloy to 152 MPa for the Sc-modified alloy. Due to excellent thermal stability of Al₃Sc phase, the Sc-modified alloy exhibits obvious precipitation hardening behavior at 350 °C, and the tensile strength increases to 203 MPa after holding at 350 °C for 200 h.     

Nanocrystalline matrix TiC–Al3Ti and TiC–Al3Ti–Al composites produced by reactive hot-pressing of milled powders

Mechanically alloyed nanocrystalline TiC powder was short-term milled with 40 vol.% of Al powder. The powders mixture was consolidated at 1200 °C under the pressure of 4.8 GPa for 15 s and at 1000 °C under the pressure of 7.7 GPa for 180 s. The bulk materials were characterised by X-ray diffraction, light and scanning electron microscopy, energy dispersive spectroscopy, hardness, density and open porosity measurements. During the consolidation a reaction between TiC and Al occurred, yielding an Al₃Ti intermetallic. The microstructure of the produced composites consists of TiC areas surrounded by lamellae-like regions of Al₃Ti intermetallic (after consolidation at 1200 °C) or Al₃Ti and Al (after consolidation at 1000 °C). The mean crystallite size of TiC is 38 nm. The hardness of the TiC–Al₃Ti and TiC–Al₃Ti–Al composites is 13.28 GPa (1354 HV1) and 10.22 GPa (1041 HV1) respectively. The produced composites possess relatively high hardness and low density. The results obtained confirmed satisfactory quality of the consolidation with keeping a nanocrystalline structure of TiC.     

The influences of trace TiB and TiC on microstructure refinement and mechanical properties of in situ synthesized Ti matrix composite

Ti–5Al–5Mo–5V–1Fe–1Cr Ti alloy and Ti–5Al–5Mo–5V–1Fe–1Cr Ti matrix composites containing different weight fractions of trace TiB and TiC are fabricated via in situ synthesis method. The as-cast ingots are subjected to thermo-mechanical processing and heat treatment. The Widmannstatten structure is obtained after the heat treatment. The microstructure length scales of the materials are identified. The identification indicates that 0.4 wt.% TiB and 0.1 wt.% TiC can reduce the average size of the β grains by more than 50%. Whereas the extent of the microstructure refinement gradually decreases while increasing the weight fraction of the trace reinforcements. The influences of weight fraction and morphology of the trace TiB and TiC on microstructure refinement are researched in this work. Moreover, the tensile properties of the heat-treated materials are examined. It is revealed that Hall–Petch mechanism plays an identically important role in improving the mechanical properties of the composites comparing with the load bearing and dispersion strengthening of the trace reinforcements.     

The influences of trace TiB and TiC on microstructure refinement and mechanical properties of in situ synthesized Ti matrix composite

Ti–5Al–5Mo–5V–1Fe–1Cr Ti alloy and Ti–5Al–5Mo–5V–1Fe–1Cr Ti matrix composites containing different weight fractions of trace TiB and TiC are fabricated via in situ synthesis method. The as-cast ingots are subjected to thermo-mechanical processing and heat treatment. The Widmannstatten structure is obtained after the heat treatment. The microstructure length scales of the materials are identified. The identification indicates that 0.4 wt.% TiB and 0.1 wt.% TiC can reduce the average size of the β grains by more than 50%. Whereas the extent of the microstructure refinement gradually decreases while increasing the weight fraction of the trace reinforcements. The influences of weight fraction and morphology of the trace TiB and TiC on microstructure refinement are researched in this work. Moreover, the tensile properties of the heat-treated materials are examined. It is revealed that Hall–Petch mechanism plays an identically important role in improving the mechanical properties of the composites comparing with the load bearing and dispersion strengthening of the trace reinforcements.     

Effect of (α + β) heat treatment on microstructure and mechanical properties of (TiB + TiC)/Ti–B20 matrix composite

For the first stage, a metastable β titanium alloy, Ti–3.5Al–5Mo–4V–2Cr–2Sn–2Zr–1Fe reinforced with trace amounts of TiB whiskers and TiC particles was fabricated by vacuum arc melting process and hot forging followed by heat treatment at 780 °C/740 °C, then by aging at 500 °C, 550 °C, 570 °C and 600 °C. For the second stage, the unreinforced titanium alloy was also fabricated by the same process. The microstructural characteristics were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Traces of TiB whiskers and TiC particles (2.2 vol.%) with a volume ratio of 2:3 synthesized in situ exerted a hybrid reinforcing effect on the β titanium alloy. The reinforcements were uniformly distributed in the matrix and the elastic modulus was improved about 25 GPa. Ultimate tensile strength and yield strength achieves about 1625 MPa and 1500MPa respectively, with ductility at 7% when the aging temperature is 500 °C. The ductility of (TiB + TiC)/(Ti–3.5Al–5Mo–4 V–2Cr–2Sn–2Zr–1Fe) matrix composite could be enhanced by increasing the aging temperatures. After 780 °C followed by aging at 570 °C, excellent strength and plasticity properties were obtained (ultimate tensile strength of matrix alloy is 1350 MPa with elongation of 18% and ultimate tensile strength of composite is 1500 MPa with elongation of 13%).     

Microstructure and brazing mechanism of porous Si3N4/Invar joint brazed with Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler

Porous Si_3N_4 was brazed to Invar alloy in this study, and Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was designed to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallic compounds. The effects of the brazing temperature and the thickness of Cu interlayer on the microstructure and mechanical properties of brazed joints were investigated. The typical microstructure of the joint brazed with multi-layered filler was porous Si_3N_4/TiN + Ti_5Si_3/Ag-Cu eutectic/Cu/Ag-Cu eutectic/Cu-rich layer + diffusion layer/Invar. When the brazing temperature increased, the reaction layer at the ceramic/filler interface grew thicker and the Cu interlayer turned thinner. As the thickness of Cu interlayer increased from 50 to 150μm, the joint strength first increased and then decreased. In this research, the maximum shear strength(73 MPa) was obtained when being brazed at 1173 K with a 100μm Cu interlayer applied in the filler, which was 55% higher than that brazed with single Ag-Cu-Ti brazing alloy and had reached 86% of the ceramic. The release of residual stress and the barrier effect of Cu interlayer to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallics played the major role in the improvement of joint strength.     

Structural and biocompatible characterization of TiC/a:C nanocomposite thin films

In this work, sputtered TiC/amorphous C thin films have been developed in order to be applied as potential barrier coating for interfering of Ti ions from pure Ti or Ti alloy implants. Our experiments were based on magnetron sputtering method, because the vacuum deposition provides great flexibility for manipulating material chemistry and structure, leading to films and coatings with special properties. The films have been deposited on silicon (001) substrates with 300 nm thick oxidized silicon sublayer at 200 °C deposition temperature as model substrate. Transmission electron microscopy has been used for structural investigations. Thin films consisted of ~ 20 nm TiC columnar crystals embedded by 5 nm thin amorphous carbon matrix. MG63 osteoblast cells have been applied for in vitro study of TiC nanocomposites. The cell culture tests give strong evidence of thin films biocompatibility.     

A developed method for producing in situ TiC/Al composites by using quick preheating treatment and ultrasonic vibration

Quick preheating treatment of Al–Ti–C was introduced in the fabrication of in situ TiC/Al metal matrix composites in our research. Al–Ti–C pellets were preheated in the furnace at 750 °C, in which the pure aluminum was melted. After adding the preheated pellets into the molten aluminum, the thermal explosion reaction of Al–Ti–C took place in a short time. In situ TiC particles synthesized in the pure molten aluminum were spherical in morphology and most of which were smaller than 1 μm in size. The synthesizing temperature of in situ TiC/Al composites was decreased significantly by using the quick preheating treatment, at least 150 °C lower than those used in the conventional methods. In addition, high-intensity ultrasonic vibration was applied into the melt to disperse TiC particle-reinforcement into the matrix and degas the melt as well. In situ TiC particles were distributed uniformly in the matrix, and the porosity in the composites was below 1% due to the effect of ultrasonic vibration. Furthermore, the microhardness test indicated that a homogeneous microstructure of in situ TiC/Al composite was obtained.     

Evaluation of wear characteristics of Al3Tip/Mg composite

The dry sliding wear tests were performed for a novel developed Al₃Ti_p/Mg composite under the ambient temperatures at 25–200 °C and the loads of 25–150 N. The wear rate of the composite increased with increasing the load, but reduced with increasing the ambient temperature. The Al₃Ti_p/Mg composite had relatively lower wear rates than AZ91D alloy under the loads of less than 100 N at 25 °C. At 200 °C, the Al₃Ti_p/Mg composite presented an absolutely higher wear resistance than AZ91D alloy, and the mild-severe wear transition was delayed. These were attributed to Al₃Ti particulates and the mechanical mixing layer formed on the worn surfaces, which hindered the plastic deformation and thermal softening of the matrix. The mechanical mixing layer contained MgO, Fe–Ti–O, Al₃Ti, Mg₁₇Al₁₂ and Mg and thickened with increasing the ambient temperature. The predominant wear mechanisms of the composite were oxidation wear and delamination wear.     

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.     

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%.     

In situ joining of titanium to SiC/Al composites by low pressure infiltration

A method of in situ joining of titanium to SiC/Al composites by low pressure infiltration was proposed. The effect of infiltration temperature on microstructure and bending strength of in situ joining composites was investigated and the best infiltration temperature was confirmed to be 710 °C. The interfacial region of SiC/Al/Ti composites was consisted of Ti substrate, Al–Ti interfacial layer, Al layer and SiC/Al composite. The bending strength of SiC/Al composites kept nearly constant as the infiltration temperature changed while that of SiC/Al/Ti composites was influenced significantly by the infiltration temperature. The fracture occurred at the Al–Ti and Al–SiC/Al interfaces alternately as infiltrated at 670 °C. But as the infiltration temperature was increased to 710 °C, the fracture occurred only at the Al–SiC/Al interface which shows a great interfacial bonding at the Al–Ti interface. The formation of Al–Ti brittle intermetallics and the effect of crystallization and grain coarsening are two possible reasons which lead to the decrease of bending strength when the infiltration temperatures were increased from 710 °C to 730 °C.