Interfacial microstructure and mechanical properties of diffusion-bonded joints of titanium TC4 (Ti-6Al-4V) and Kovar (Fe-29Ni-17Co) alloys

Diffusion bonding is a near net shape forming process that can join dissimilar materials through atomic diffusion under a high pressure at a high temperature.Titanium alloy TC4(Ti-6 Al-4 V)and 4 J29 Kovar alloy(Fe-29 Ni-17 Co)were diffusely bonded by a vacuum hot-press sintering process in the temperature range of 700-850°C and bonding time of 120 min,under a pressure of 34.66 MPa.Interfacial microstructures and intermetallic compounds of the diffusion-bonded joints were characterized by optical microscopy,scanning electron microscopy,X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS).The elemental diffusion across the interface was revealed by electron probe microanalysis.Mechanical properties of joints were investigated by micro Vickers hardness and tensile strength.Results of EDS and XRD indicated that(Fe,Co,Ni)-Ti,TiNi,Ti_2Ni,TiNi_2,Fe_2 Ti,Ti_(17) Mn_3 and Al_6 Ti_(19) were formed at the interface.When the bonding temperature was raised from 700 to 850°C,the voids of interface were reduced and intermetallic layers were widened.Maximum tensile strength of joints at 53.5 MPa was recorded by the sintering process at 850°C for 120 min.Fracture surface of the joint indicated brittle nature,and failure took place through interface of intermetallic compounds.Based on the mechanical properties and microstructure of the diffusion-bonded joints,diffusion mechanisms between Ti-6 Al-4 Vtitanium and Fe-29 Ni-17 Co Kovar alloys were analyzed in terms of elemental diffusion,nucleation and growth of grains,plastic deformation and formation of intermetallic compounds near the interface.     

The use of titanium aluminides to form electric-spark coatings

Titanium aluminides (TiAl₃, TiAl, Ti₃Al) fabricated by powder metallurgy were used as alloying electrodes for the formation of electric-spark coatings. Intermetallic coatings were deposited on steel substrates in argon or nitrogen. The microstructure and composition of fabricated coatings were investigated by scanning electron microscopy, X-ray structural analysis, and electron probe microanalysis. It is established that initial Ti–Al intermetallic phases are present in fabricated coatings; however, the ratio between Ti and Al concentrations is shifted to aluminum compared with the stoichiometric one. When depositing titanium aluminide in the nitrogen medium, titanium nitride is additionally formed in surface layers. Thermal and tribotechnical tests showed that the Ti₃Al coating deposited in nitrogen possesses high wear resistance and heat resistance.     

Phase Relations in CaO-SiO2-Al2O3-15 mass pct CaF2 System at 1523 K (1250 °C)

CaO-SiO₂-Al₂O₃-CaF₂ is a base system of mold flux for high Al steels. Phase equilibrium in CaO-SiO₂-Al₂O₃-15 mass pct CaF₂ system at 1523 K (1250 °C) was investigated using quenching method followed by X-ray diffraction and Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Isothermal section in this system at 1523 K (1250 °C) with Al₂O₃ being less than 25 mass pct and CaO/SiO₂ (mass pct) being between 0.43 and 1.25 was experimentally constructed. The liquidus composition and seven solid-liquid coexistence regions at 1523 K (1250 °C) were determined.     

Evolution of the Spectral Emissivity and Phase Transformations of the Al-Si Coating on Usibor<Superscript>®</Superscript> 1500P Steel During Austenitization

Usibor^® 1500P coupons are austenitized in a Gleeble 3500 thermomechanical simulator using a two-step heating procedure in an argon atmosphere. Variations in spectral emissivity are measured in-situ using a near infrared spectrometer and ex situ with a Fourier transform infrared reflectometer. Microstructural evolution and surface roughness are investigated using optical microscopy, FE-scanning electron microscopy, and a surface profilometer. A series of phase transformations of Al-Fe-Si intermetallic phases at the coating/steel substrate interface cause the surface phase and surface roughness to change, which in turn influences the spectral emissivity. At the beginning of the first heating step, the coupons have very low spectral emissivity, due to the molten Al-Si coating. Spectral emissivity increases significantly with increasing soak time from 5 to 12 minutes, associated with the surface phase transformation of the coating into Al₇Fe₂Si intermetallic phase and an increase in surface roughness. Through the second step heating at 1173 K (900 °C), the spectral emissivity shows a gradually decreasing trend with increasing soak time, caused by the surface phase transformation from Al₅Fe₂ into AlFe intermetallic phase with a decrease in surface roughness.     

Solubility of oxygen in Fe–Co melts containing titanium

Solutions of oxygen in Fe–Co melts containing titanium are subjected to thermodynamic analysis. The first step is to determine the equilibrium reaction constants of titanium and oxygen, the activity coefficients at infinite dilution, and the interaction parameters in melts of different composition at 1873 K. With increase in cobalt content, the equilibrium reaction constants of titanium and oxygen decline from iron (logK(FeO · TiO₂) =–7.194; logK(TiO₂) =–6.125; logK(Ti₃O₅) =–16.793; logK(Ti₂O₃) =–10.224) to cobalt (logK(CoO · TiO₂) =–8.580; logK(TiO₅) =–7.625; logK(Ti₃O₅) =–20.073; logK(Ti₂O₃) =–12.005). The titanium concentrations at the equilibrium points between the oxide phases (Fe, Co)O · TiO₂, TiO₂, Ti₃O₅, and Ti₂O₃ are determined. The titanium content at the equilibrium point (Fe, Co)O · TiO₂ ? TiO₂ decreases from 1.0 × 10^–4% Ti in iron to 1.9 × 10^–6% Ti in cobalt. The titanium content at the equilibrium point TiO₂?Ti₃O₅ increases from 0.0011% Ti in iron to 0.0095% Ti in cobalt. The titanium content at the equilibrium point Ti₃O₅ ? Ti₂O₃ decreases from 0.181%Ti in iron to 1.570% Ti in cobalt. The solubility of oxygen in the given melts is calculated as a function of the cobalt and titanium content. The deoxidizing ability of titanium decline with increase in Co content to 20% and then rise at higher Co content. In iron and its alloys with 20% and 40% Co, the deoxidizing ability of titanium are practically the same. The solubility curves of oxygen in iron-cobalt melts containing titanium pass through a minimum, whose position shifts to lower Ti content with increase in the Co content. Further addition of titanium increases the oxygen content in the melt. With higher Co content in the melt, the oxygen content in the melt increases more sharply beyond the minimum, as further titanium is added.     

Synthesis and structural characterization of nanocrystalline Ni<Subscript>50</Subscript>Al<Subscript>50 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compound prepared by mechanical alloying

Nanocrystalline Ni₅₀Al_{50 ? x} Mo _x (x = 0, 0.5, 1, 2.5, and 5) intermetallic powders were synthesized by mechanical alloying (MA). Microstructural characterization and structural changes of powder particles during mechanical alloying were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results confirmed that the synthesis behavior of NiAl intermetallic depends on the Mo content and milling time. The SEM micrograph outcomes confirmed the specimen with longer milling time includes finer and more homogenous particles with attention to the ones with lesser milling time. Mo enhance has a considerable effect on the lowering of crystallite size. The TEM image showed that the Ni₅₀Al₄₅Mo₅ nano-particles were less than 10 nm.     

Intermetallic diffusion coatings for enhanced hot-salt oxidation resistance of nitrogen-containing austenitic stainless steels

This article presents the preparation, characterization, and hot-salt oxidation behavior of nitrogen-containing type 316L stainless steel (SS), surface modified with intermetallic coatings. Three different types of intermetallic coating systems, containing aluminum, titanium, and titanium/aluminum multilayers, were formed by diffusion annealing of type 316L austenitic SS containing 0.015, 0.1, 0.2, and 0.56 pct nitrogen. Analysis by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and secondary ion mass spectroscopy (SIMS) confirmed the formation of various intermetallic phases such as AIN, Al₁₃Fe₄, FeAl₂, FeTi, Ti₂N, and Ti₃Al in the coatings. Hot salt oxidation behavior of the uncoated and surface-modified stainless steels was assessed by periodic monitoring of the weight changes of NaCl salt-applied alloys kept in an air furnace at 1023 K up to 250 hours. The oxide scales formed were examined by XRD and stereomicroscopy. Among the various surface modifications investigated in the present study, the results indicate that the titanium-modified alloys show the best hot-salt oxidation resistance with the formation of an adherent, protective, thin, and continuous oxide layer. Among the four N-containing alloys investigated, the titanium and Ti/Al multilayer modified 0.56 pct N alloy showed the best hot-salt oxidation resistance as compared to uncoated alloys. The slower corrosion kinetics and adherent scale morphology indicate that the surface-modified titanium intermetallic coatings could provide high-temperature service applications up to 1073 K, particularly in chloride containing atmospheres, for austenitic stainless steels.     

Influence of alloying the TiC<Subscript>0.5</Subscript>N<Subscript>0.5</Subscript> carbonitride with zirconium on an interaction mechanism with the Ni–Mo melt

The influence of alloying the TiC_0.5N_0.5 titanium carbonitride with zirconium on the mechanism and kinetic features of the contact interaction with the Ni–25%Mo melt (t = 1450°C, rarefaction 5 × 10^–2 Pa) is investigated for the first time by electron probe microanalysis and scanning electron microscopy. The main effects of the modifying influence of zirconium on the dissolution, phase formation, and structure formation processes which occur during the interaction of the Ti_1–n Zr _n C_0.5N_0.5 carbonitride (n = 0.05 and 0.20) with the Ni–Mo melt are revealed and the factors promoting their manifestation are analyzed. The practical absence of zirconium and nitrogen in the composition of the K-phase (the Ti_{1 – n} Mo _n C _x metastable solid solution, where n ≤ 0.65 and x = 0.7 ± 0.1) is confirmed experimentally. It is shown that the zirconiumenriched Ti_0.80Zr_0.20C_0.5N_0.5 carbonitride cannot be recommended as a refractory component of cermet because of the limitations of the chemical character.     

Grain growth behavior of cryomilled INCONEL 625 powder during isothermal heat treatment

Nanocrystalline INCONEL 625 powders were fabricated via cryomilling (mechanical alloying under a liquid nitrogen environment), and their grain growth behavior during isothermal heat treatment was investigated in detail. The grain size after milling for 8 hours was approximately 22 nm, measured by transmission electron microscopy (TEM) observations and X-ray diffraction (XRD). Along with this refined structure, the NiO and Cr₂O₃ oxide particles were distributed in the cryomilled material with average size of 3 nm. Following heat treatment at 800 °C, correspond to T/T _m = 0.65, for 4 hours, the grain size was approximately 240 nm, which represents an improved grain stability compared to that of conventional INCONEL 625 and cryomilled pure Ni. The improved grain stability of cryomilled INCONEL 625 is originated from a particle pinning effect by the oxide particles in addition to solute drag. The grain stability of the cryomilled powders at 900 °C was better than that at lower temperatures. This behavior was attributed to the formation of two types of secondary particles that precipitated at this temperature, which were identified as spherical NbC carbides and cylindrical-shaped Ni₃Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain-boundary pinning effect. Contribution of 30 pct Nb solute atoms in alloy on the forming precipitates on grain boundary, the grain growth will be restricted to approximately 200 nm, on the basis of a Zener mechanism. This calculation is in qualitative agreement with the experimental results. The observation that precipitation kinetics were accelerated over those of conventional INCONEL 625 was rationalized on the basis of the shortened diffusion paths and more nucleation sites available in the nanocrystalline materials.     

Effect of Bonding Time on Interfacial Reaction and Mechanical Properties of Diffusion-Bonded Joint Between Ti-6Al-4V and 304 Stainless Steel Using Nickel as an Intermediate Material

In the current study, solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200-μm thickness as an intermediate material was carried out in vacuum. Uniaxial compressive pressure and temperature were kept at 4 MPa and 1023 K (750 °C), respectively, and the bonding time was varied from 30 to 120 minutes in steps of 15 minutes. Scanning electron microscopy images, in backscattered electron mode, revealed the layerwise Ti-Ni-based intermetallics like either Ni₃Ti or both Ni₃Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) interface was free from intermetallic phases for all the joints. Chemical composition of the reaction layers was determined by energy dispersive spectroscopy (SEM–EDS) and confirmed by X-ray diffraction study. Maximum tensile strength of ~382 MPa along with ~3.7 pct ductility was observed for the joints processed for 60 minutes. It was found that the extent of diffusion zone at Ni/SS interface was greater than that of TiA/Ni interface. From the microhardness profile, fractured surfaces, and fracture path, it was demonstrated that the failure of the joints was initiated and propagated apparently at TiA/Ni interface near Ni₃Ti intermetallic for bonding time less than 90 minutes, and through Ni for bonding time 90 minutes and greater.     

The effect of thermal exposure on the mechanical properties of aluminum-graphite composites

The mechanical properties of aluminum-graphite composites were measured at room temperature in the as-received condition, after elevated temperature exposure and after thermal cycling. The composites were fabricated by solid-state diffusion bonding of liquid-phase Al-infiltrated Thornel 50 fibers. The results showed that the maximum longitudinal tensile strength of the as-received material was 80,000 psi (552 MN/m²), which corresponds well with the rule of mixture value. The composite strength was observed to vary widely, depending on the extent of wetting of the fibers by the aluminum. The strength of the composites in the transverse direction was generally very low, due to poor interfacial bonding. Aluminum carbide (A1₄C₃) formed at the surface of the fibers at temperatures greater than 500?C (773 K). Development of the carbide was shown to be diffusion-controlled and was dependent on the time and temperature used. It was shown that the tensile strength was virtually unaffected by heat-treatment up to 500?C (773 K); beyond that temperature a drastic degradation of tensile strength occurred. The degradation could be correlated with the extent of carbide development at the interface. Thermal cycling of the composites below 500?C (773 K) resulted in an observable degradation of the composite strength. Scanning electron microscopy of fractured surfaces indicated that the relatively weak interface governs the mode of failure in tension.     

Activity coefficient of titanium in iron-based melts under nitride formation/dissolution conditions

The initial parameters for calculating the activity coefficients of titanium in complex iron-based melts are determined. For this purpose, multiple regression analysis of 311 experimental results obtained by independent researchers is used. As a result, the initial parameters for calculating the activity coefficients of titanium (γ _Ti,1873 ^∞ = 0.059, its temperature dependence logγ _Ti ^∞ = ?14900/T + 6.73) and the temperature dependences of interaction parameters e _Ti ^j and r _Ti ^j are refined. The found value of γ _Ti,1873 ^∞ agrees with the results of a number of earlier investigations and is significantly higher than the values recommended in the most widely used handbooks. The application of the found parameters for calculating the nitrogen concentrations in equilibrium with TiN in the melts decreases the mean deviation of the calculated data from the experimental results to a level of at most ±13%, which is much better than in the case of using the data from the most widely used handbooks.     

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M₂₃C₆ was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti₄C₂S₂ which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M₂₃C₆ and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M₂₃C₆ and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.     

The Equilibrium Between Titanium Ions and Titanium Metal in NaCl-KCl Equimolar Molten Salt

The equilibrium between metallic titanium and titanium ions, 3Ti²⁺ ? 2Ti³⁺ + Ti, in NaCl-KCl equimolar molten salt was reevaluated. At a fixed temperature and an initial concentration of titanium chloride, the equilibrium was achieved by adding an excess amount of sponge titanium in assistant with bubbling of argon into the molten salt. The significance of this work is that the accurate concentrations of titanium ions have been obtained based on a reliable approach for taking samples. Furthermore, the equilibrium constant   $ {\text{K}}_{\text{C}} = (x_{{{\text{Ti}}^{{ 3 { + }}} }}^{\text{eql}} )^{3} /(x_{{{\text{Ti}}^{{ 2 { + }}} }}^{\text{eql}} )^{2} $ K C = ( x Ti 3 + eql ) 3 / ( x Ti 2 + eql ) 2 was calculated through the best-fitting method under the consideration of the TiOCl dissolution. Indeed, the final results have disclosed that the stable value of K_C could be achieved based on all modifications.     

Regularities of the contact interaction of titanium carbide with Ni and Ni–Mo melts

Regularities of the dissolution, the phase formation, and the structure formation implemented under the contact interaction conditions of titanium carbide of various compositions with Ni and Ni–(5–25%)Mo melts are investigated. It is originally established that the dissolution of carbide TiC_x in nickel-based melts is incongruent. Preferentially, carbon transfers into the melt at x ≥ 0.9 and titanium at x ≤ 0.8. The limiting stage of the dissolution is diffusion of metal atoms in the liquid phase. The formation regularities of carbide Ti_1–nMo_nC_x (K-phase)—the main product of the contact interaction in the TiC/Ni–Mo system—are revealed. It is established that the K-phase is formed under the relative excess conditions of the Ni–Mo melt preferentially according to the dissolution–isolation mechanism. The composition of autonomous isolations of the K-phase depending on the experimental conditions (1450°C, 0–25 h) varies in limits from Ti_0.4Mo_0.6C_0.7 (a = 4.27 Å) to Ti_0.7Mo_0.3C_0.6 (a = 4.29 Å). It is determined by the molybdenum concentration in the melt at the unsteady dissolution stage and by the concentration ratio between titanium and carbon in it at the steady-state dissolution stage.     

Effect of Bonding Temperature on Interfacial Reaction and Mechanical Properties of Diffusion-Bonded Joint Between Ti-6Al-4V and 304 Stainless Steel Using Nickel as an Intermediate Material

An investigation was carried out on the solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200-μm thickness as an intermediate material prepared in vacuum in the temperature range from 973 K to 1073 K (700 °C to 800 °C) in steps of 298 K (25 °C) using uniaxial compressive pressure of 3 MPa and 60 minutes as bonding time. Scanning electron microscopy images, in backscattered electron mode, had revealed existence of layerwise Ti-Ni-based intermetallics such as either Ni₃Ti or both Ni₃Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) diffusion zone was free from intermetallic phases for all joints processed. Chemical composition of the reaction layers was determined in atomic percentage by energy dispersive spectroscopy and confirmed by X-ray diffraction study. Room-temperature properties of the bonded joints were characterized using microhardness evaluation and tensile testing. The maximum hardness value of ~800 HV was observed at TiA/Ni interface for the bond processed at 1073 K (800 °C). The hardness value at Ni/SS interface for all the bonds was found to be ~330 HV. Maximum tensile strength of ~206 MPa along with ~2.9 pct ductility was obtained for the joint processed at 1023 K (750 °C). It was observed from the activation study that the diffusion rate at TiA/Ni interface is lesser than that at the Ni/SS interface. From microhardness profile, fractured surfaces and fracture path, it was demonstrated that failure of the joints was initiated and propagated apparently at the TiA/Ni interface near Ni₃Ti intermetallic phase.     

Cryogenic Mechanical Properties of Warm Multi-Pass Caliber-Rolled Fine-Grained Titanium Alloys: Ti-6Al-4V (Normal and ELI Grades) and VT14

The effect of microstructural refinement and the β phase fraction, V _β, on the mechanical properties at cryogenic temperatures (up to 20 K) of two commercially important aerospace titanium alloys: Ti-6Al-4V (normal as well as extra low interstitial grades) and VT14 was examined. Multi-pass caliber rolling in the temperature range of 973 K to 1223 K (700 °C to 950 °C) was employed to refine the microstructure, as V _β was found to increase nonlinearly with the rolling temperature. Detailed microstructural characterization of the alloys after caliber rolling was carried out using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). Complete spheroidization of the primary α laths along with formation of bimodal microstructure occurred when the alloys are rolled at temperatures above 1123 K (850 °C). For rolling temperatures less than 1123 K (850 °C), complete fragmentation of the β phase with limited spheroidization of α laths was observed. The microstructural refinement due to caliber rolling was found to significantly enhance the strength with no penalty on ductility both at room and cryogenic temperatures. This was attributed to a complex interplay between microstructural refinement and reduced transformed β phase fraction. TEM suggests that the serrated stress–strain responses observed in the post-yield deformation regime of specimens tested at 20 K were due to the activation of \( \left\{ {10\bar{1}2} \right\} \) tensile twins.     

Fabrication of vacuum-arc Ti-Al-N coatings using multicomponent SHS-compacted cathodes

The results of experiments on the deposition of vacuum-arc Ti-Al-N coatings using aluminum-containing pressed cathodes based on nonstoichiometric titanium carbide TC_0.5 are presented. The composition and morphology of coatings are investigated using electron probe microanalysis and scanning electron microscopy. Hardness and elasticity modulus were determined using the nanoindentation method. It is established that carbon is absent in the coating, while the fraction of aluminum in it decreases compared with its content in the cathode material. A superhard coating with nanohardness H = 59 GPa and the Young modulus E = 475 GPa is obtained from the cathode with the calculated composition TiC_0.5-30%Al. By the ratio of hardness to the elasticity modulus, which equals H/E = 0.124, this coating is an amorphous-crystalline material. When using cathodes based on refractory nonstoichiometric titanium carbide, the content of the drop phase in coatings substantially lowers with no separation of the plasma flow. In general, the application of multicomponent SHS-compacted cathodes based on nonstoichiometric titanium carbide TiC_0.5 makes it possible to fabricate superhard nanocrystalline coatings according to the standard technology using a serial vacuum-arc installation.     

The undercooling of aluminum

An important parameter affecting microstructure development during solidification is the amount of undercooling prior to nucleation. The undercooling potential of aluminum has been assessed by thermal analysis measurements on powder dispersions of the liquid metal. A number of variables have been identified which influence the undercooling of powder Al samples including powder coating, powder size, melt cooling rate, and melt superheat. Surface analysis by Auger electron spectroscopy indicates that changing the medium in which the powders are produced is an effective method of altering the coating chemistry. Factorial design analysis has been employed to quantify the potential of processing variables to increase the undercooling level obtainable in aluminum. The factorial analysis indicates that control of the powder coating through changing the medium in which the powders are produced is most effective in decreasing the nucleation temperature. Additionally, the finest powders produced in the medium which induces the least catalytic coating, when cooled at high rates,T = 500 °C/s, from low superheats,T _s = 710 °C, are found to achieve the deepest undercooling, ΔT = 175 °C. These studies provide the basis for further increases in undercooling and for future investigations into the solidification reactions which produce both stable and metastable structures in aluminum alloys.