Correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

This study is concerned with the correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation. The mixtures of TiC, SiC, Ti + SiC, or TiC+SiC powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures. The surface composite layers of 1.2 to 2.1 mm in thickness were homogeneously formed without defects and contained a large amount (30 to 66 vol pct) of hard precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates in the surface composite layer, improved the hardness and abrasive wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the abrasive wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate because of the precipitation of 66 vol pct of TiC and Ti₅Si₃ in the hardened martensitic matrix. During the sliding wear process, hard and coarse TiC and Ti₅Si₃ precipitates fell off from the matrix, and their wear debris worked as abrasive particles, thereby reducing the sliding wear resistance. On the other hand, needle-shaped Ti₅Si₃ particles, which did not play a significant role in enhancing abrasive wear resistance, lowered the friction coefficient and, accordingly, decelerated the sliding wear, because they played more of the role of solid lubricants than as abrasive particles after they fell off from the matrix. These findings indicated that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved abrasive and sliding wear resistance, although the abrasive and sliding-wear data should be interpreted by different wear mechanisms.     

Rapid synthesis of ternary carbide Ti3SiC2 through pulse-discharge sintering technique from Ti/Si/TiC powders

Ti/Si/TiC powders with molar ratios of 1:1:2 (M1) and 2:2:3 (M2) were prepared for the synthesis of a ternary carbide Ti₃SiC₂ by using the mixture method for 24 hours in an Ar atmosphere. The synthesis process was conducted at 1200 °C to 1400 °C under a pressure of 50 MPa, using the pulse-discharge sintering (PDS) technique. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction (XRD) technique and observed by optical microscopy and scanning electron microscopy. The results showed that the phases in all the samples consisted of Ti₃SiC₂ and small amounts of TiC, and the optimum sintering temperature was found to be in the relatively low range of 1250 °C to 1300 °C. By the standard additive method, the relative content of Ti₃SiC₂ was calculated. For the M1 samples, the lowest TiC content can be only decreased to about 3 to 4 wt pct, whereas the content of Ti₃SiC₂ in the M2 samples is always lower than that in the M1 samples. When the M2 powder was sintered at 1300 °C for 8 to 240 minutes, the TiC peaks were found to show a very low intensity, and the corresponding content of Ti₃SiC₂ was calculated to be higher than 99 wt pct. The grain size of Ti₃SiC₂ increased from 5 to 10 μm to 80 to 100 μm in the entire applied sintering temperature range. The relative density of the M2 samples was measured to be higher than 99 pct at sintering temperatures above 1275 °C. It indicates that the PDS technique can rapidly synthesize high-content Ti₃SiC₂ from the Ti/Si/TiC powders in a relatively low temperature range.     

Correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

The correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC + SiC powders and CaF₂ flux were placed on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2 to 2.1 mm in thickness were formed without defects and contained a large amount (up to 66 vol pct) of precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates and a hardened matrix in the surface composite layer, improved the hardness and wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate, because 66 vol pct of TiC and Ti₅Si₃ was precipitated homogeneously in the hardened martensitic matrix. These findings suggested that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved hardness and wear properties.     

Ti-6Al-4V-2Ni as a matrix material for a SiC-reinforced composite

Ti-6Al-4V-2Ni is being considered as a composite matrix material because of its potential for a lower consolidation temperature and reduced reaction product formation compared with conventional Ti-6A1-4V. Stress/strain-rate measurements of Ti-6Al-4V-2Ni in sheet form provided data for calculation of diffusion bonding parameters required for efficient consolidation. These data were used as consolidation parameters for fabrication of SiC (SCS-6) reinforced Ti-6Al-4V-2Ni. The composite with 10.5 vol pct SiC exhibits room temperature tensile strength approximately 80 pct of that observed for conventional Ti-6Al-4V/SiC having 35 to 40 vol pct SiC. Scanning and transmission electron microscopy revealed that the fiber-matrix reaction zone is roughly one-half the thickness of that found in SiC-reinforced Ti -6A1-4V, and that it consists of TiC and Ti₅Si₃. Nickel does not enter into the reaction zone products, but rather promotes the formation of Ti₂Ni in the matrix.     

Solid-state diffusion bonding of silicon nitride using titanium foils

This article presents an effective way to control the interfacial reaction during solid-state diffusion bonding of silicon nitride (Si₃N₄) using titanium foils. The interfacial structure and its growth kinetics were analyzed in detail with scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). The actual phase sequence of the joint interfaces bonded at temperatures between 1473 and 1673 K is concluded to be Si₃N₄/Ti₅Si₃(N)/α-Ti(N)+Ti₅Si₃(N), which is different from the phase sequence observed at room temperature after bonding. The joints are very weak due to the formation of a brittle Ti₅Si₃(N) layer at the interface. To suppress the growth of the Ti₅Si₃ layer, a nitrogen-solution treatment of titanium foils prior to each bonding experiment is implemented. Although a perfect prevention of the Ti₅Si₃(N) layer formation is not achieved with this treatment, it is shown that the growth of the layer is effectively suppressed enough to improve the joint strength to a level 3 times higher than the case in which pure titanium is employed.     

Improvement of the hardness and wear resistance of (TiC, TiN)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis and improvement of the hardness and wear resistance of Ti-6Al-4V surface-alloyed materials fabricated by a high-energy electron beam. The mixtures of TiC, TiN, or TiC + TiN powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then the electron beam was irradiated on these mixtures. In the specimens processed with a flux addition, the surface-alloyed layers of 1 mm in thickness were homogeneously formed without defects and contained a large amount (over 30 vol pct) of precipitates such as TiC, TiN, (Ti _x Al_1−x )N, and Ti(C _x N_1−x ) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification, including the formation of hard precipitates and hardened matrices in the surface-alloyed layers, improved the hardness and wear resistance. Particularly in the surface-alloyed material fabricated by the deposition of TiN powders, the wear resistance was greatly enhanced to a level 10 times higher than that of the Ti alloy substrate. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-based surface-alloyed materials with improved hardness and wear resistance.     

Microstructure and mechanical behavior of reaction hot-pressed titanium silicide and titanium silicide-based alloys and composites

Titanium silicide (Ti₅Si₃) and its composites show promise for applications at temperatures higher than 1000 °C. Dense Ti₅Si₃ was processed by reaction hot pressing of a TiH₂/Si powder mixture, which involved decomposition of TiH₂ into Ti and H₂ at around 800 °C, a chemical reaction between Ti and liquid Si at 1500 °C forming Ti₅Si₃ in situ, and densification under pressure. The use of fine TiH₂ particles led to the formation of a relatively fine-grained microstructure with fewer microcracks and higher hardness and fracture toughness values than those expected for a coarse-grained Ti₅Si₃. The addition of 8 wt pct Al as an alloying element led to the formation of Al_0.67Si_0.08Ti_0.25 and Al₂O₃ in situ and a solid solution of Al in Ti₅Si₃. Both alloying with Al and the addition of TiC as a reinforcement phase improved the room-temperature fracture toughness. Fracture toughness measurements were performed by three-point bend testing of single-edge notch bend (SENB) specimens, as well as by indentation techniques using different models, and the data have been compared. The role of different operating toughening mechanisms such as crack deflection, bridging, branching, and energy dissipation through microcracracking have been examined. The investigation has also shown that Ti₅Si₃ maintains a high yield strength value up to 1200 °C.     

Correlation of microstructure with hardness and wear resistance in (TiC,SiC)/stainless steel surface composites fabricated by high-energy electron-beam irradiation

Stainless-steel-based surface composites reinforced with TiC and SiC carbides were fabricated by high-energy electron beam irradiation. Four types of powder/flux mixtures, i.e., TiC, (Ti + C), SiC, and (Ti + SiC) powders with 40 wt. pct of CaF₂ flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with an electron beam. TiC agglomerates and pores were found in the surface composite layer fabricated with TiC powders because of insufficient melting of TiC powders. In the composite layer fabricated with Ti and C powders having lower melting points than TiC powders, a number of primary TiC carbides were precipitated while very few TiC agglomerates or pores were formed. This indicated that more effective TiC precipitation was obtained from the melting of Ti and C powders than of TiC powders. A large amount of precipitates such as TiC and Cr₇C₃ improved the hardness, high-temperature hardness, and wear resistance of the surface composite layer two to three times greater than that of the stainless steel substrate. In particular, the surface composite fabricated with SiC powders had the highest volume fraction of Cr₇C₃ distributed along solidification cell boundaries, and thus showed the best hardness, high-temperature hardness, and wear resistance.     

Correlation of microstructure with hardness and wear resistance in (TiC,SiC)/stainless steel surface composites fabricated by high-energy electron-beam irradiation

Stainless-steel-based surface composites reinforced with TiC and SiC carbides were fabricated by high-energy electron beam irradiation. Four types of powder/flux mixtures, i.e., TiC, (Ti+C), SiC, and (Ti+SiC) powders with 40 wt. pct of CaF₂ flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with an electron beam. TiC agglomerates and pores were found in the surface composite layer fabricated with TiC powders because of insufficient melting of TiC powders. In the composite layer fabricated with Ti and C powders having lower melting points than TiC powders, a number of primary TiC carbides were precipitated while very few TiC agglomerates or pores were formed. This indicated that more effective TiC precipitation was obtained from the melting of Ti and C powders than of TiC powders. A large amount of precipitates such as TiC and Cr₇C₃ improved the hardness, high-temperature hardness, and wear resistance of the surface composite layer two to three times greater than that of the stainless steel substrate. In particular, the surface composite fabricated with SiC powders had the highest volume fraction of Cr₇C₃ distributed along solidification cell boundaries, and thus showed the best hardness, high-temperature hardness, and wear resistance.     

Elevated-temperature oxidation behavior of titanium silicide and titanium silicide-based alloy and composite

The oxidation behavior of Ti₅Si₃ has been studied in air in the temperature range of 1200 °C to 1400 °C. The oxidation kinetics is slower than that predicted by the parabolic-rate law equation at 1200 °C, but is sharply enhanced beyond a temperature of 1300 °C. The oxidation kinetics of a Ti₅Si₃-8 wt pct Al alloy and a Ti₅Si₃-20 vol pct TiC composite at 1200 °C has also been investigated and compared to that of Ti₅Si₃. Alloying with Al does not alter the oxidation resistance much, but the presence of TiC reinforcements enhances the rate of oxidation significantly. The oxidation products have been identified and the mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Interface characterization of ceramic fiber-reinforced ti alloy composites manufactured by infrared processing

Interfacial reactions between several ceramic fibers (SCS-0, SCS-6, and carbon fibers) and a liquid titanium-nickel-copper alloy were investigated using electron microscopic analysis. Composite spec-imens were produced using a rapid infrared manufacturing (RIM) process. In SCS-O/Ti alloy com-posites, SiC dissolved in the alloy. The main reaction product was discontinuous agglomerates of titanium carbide which formed from the reaction between dissolved carbon and titanium. Polygonal precipitates of Ti₅Si₃, which are believed to have formed during cooling, were also noticed. Two distinct interface morphologies were observed in these composites: uniform fronts caused by iso-thermal dissolution and scalloped fronts formed as a result of an accelerated dissolution mechanism caused by localized heating. The presence of the accelerated dissolution mechanism suggests that SiC fibers cannot be infiltrated with liquid titanium alloys without applying a coating. In the C/Ti system, carbon fibers reacted with the liquid alloy to form a continuous layer of Ti_xC_1-x. Further growth of this layer occurred by the diffusion of carbon atoms across the reaction product. In SCS-6/Ti alloy composites, free carbon present in the coating formed a discontinuous layer of Ti^C,^, whereas SiC particles dissolved in the alloy. Due to channeled dissolution in the coating, the accel-erated dissolution mechanism was not observed in these composites. As a result, the presence of the carbon-rich coating prevented degradation of the fibers. Although the coating present on SCS-6 fibers moderately retarded reactions in the SiC/Ti alloy composite system during infrared liquid infiltration, it is recommended that the fibers be coated with pure carbon to effectively limit the attack of the fiber by molten titanium. Formaly Postdoctoral Fellow, Department of Materials Science and Engineering, University of Cincinnati     

Synthesis of nanocrystalline titanium carbide alloy powders by mechanical solid state reaction

A high-energy ball mill operated at room temperature has been used for preparing titanium carbide (TiC) alloy powders, starting from elemental titanium (Ti) and carbon (C) powders. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) have been used to follow the progress of the mechanical solid state reaction of Ti and C powders. A complete single phase of fcc-Ti₄₄C₅₆ alloy powders is obtained after a very short milling time (20 ks). The lattice parameter (a ₀ ) of the end product of Ti₄₄C₅₆ was calculated to be 0.4326 nm. The presence of excess starting reactant materials (Ti and/or C atoms) in the final product of the alloy powders could not be detected. The end product of Ti₄₄C₅₆ alloy powders possesses homogeneous, smooth spherical shapes with an average particle diameter of less than 0.5 μm. The internal structure of the particles is marked by fine cell-like features of about 3 nm. On the basis of the results of the present study, the mechanical alloying (MA) process appears to provide a powerful tool for the fabrication of Ti₄₄C{im56} alloy powders at room temperature. The mechanism of mechanical solid state reaction for formation of Ti₄₄C₅₆ alloy powders is discussed. Formerly Lecturer of Materials Science, Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City 11884, Cairo-Egypt.     

β-Ti Alloys with Low Young’s Moduli Interpreted by Cluster-Plus-Glue-Atom Model

β-Ti bio-alloys with low Young’s moduli (E) require multiple alloying using both BCC stabilizers and low-E elements. In this study, the cluster-plus-glue-atom model was introduced in the composition interpretation and design of Ti-based (Ti,Zr)-(Mo,Sn)-Nb alloys. Alloying elements are grouped according to their enthalpies of mixing with Ti, forming a general cluster formula [(Mo,Sn)(Ti,Zr)₁₄]Nb_{1 or 3}, where the square brackets enclose the nearest neighbor coordination polyhedron cluster CN14 of the BCC structure and Nb is the glue atom. The formulated compositions could reach simultaneously low E and high β-Ti structural stabilities, as exemplified by [(Mo_0.5Sn_0.5)(Ti₁₃Zr)]Nb₁ which had a Young’s modulus as low as 48 GPa in the suction-cast state.     

Discussion of “On the free energy of formation of TiC and Al4C3”

Several years ago, Banerji and Reif^[1] reported some very interesting studies on a process to react Ti and C in molten Al to form particles of TiC. The process was used to prepare a master alloy with a fine dispersion of TiC to inoculate Al for grain refinement. Approximately 2 wt pct of preheated graphite particles were stirred into the Al-5 to 10 pct Ti melts. The authors explained that the melts needed to be superheated above 1000 °C to avoid the undesirable formation of A1₄C₃ and Ti₃AlC at the TiC/melt interface. Their explanation for this phenomenon was based on thermodynamics. They observed that the standard free energy of formation curves for AI4C3 and TiC cross near 1175 °C, with A1₄C₃ having the lower free energy of formation below this temperature. There are several aspects of this work which merit further discussion.     

Electron Microscopic Study on the Suction Cast In Situ Ti-Fe-Sn Ultrafine Composites

The current investigation reports detailed study on the microstructural evolution in the suction cast hypereutectic Ti₇₁Fe_29?x Sn _x alloys during Sn addition with x = 0, 2, 2.5, 3, 3.85, 4.5, 6, and 10 at. pct and the solidification of these ternary alloys using SEM and TEM. These alloys have been prepared by melting high-purity elements using vacuum arc melting furnace under high-purity argon atmosphere. This was followed by suction casting these alloys in the water-cooled split Cu molds of diameters, ? = 1 and 3 mm, under argon atmosphere. The results indicate the formation of binary eutectic between bcc solid solution ??-Ti and B2 FeTi in all alloys. ??-Ti undergoes eutectoid transformation, ??-Ti ?? ??-Ti + FeTi, during subsequent solid-state cooling, leading to formation of hcp ??-Ti and FeTi. For alloys x < 2, the primary FeTi forms from the liquid before the formation of eutectic with minute scale Ti₃Sn phase. For alloys with 2 ?? x ?? 10, the liquid is found to undergo ternary quasi-peritectic reaction with primary Ti₃Sn, L+Ti₃Sn ?? ??-Ti+FeTi, leading to formation of another kind of FeTi. In all the other alloy compositions (3.85 ?? x ?? 10), Ti₃Sn and FeTi dendrites are observed in the suction cast alloys with profuse amount of Ti₃Sn being formed for alloys with x ?? 4.5. The current study conclusively proves that the liquid undergoes ternary quasi-peritectic reaction involving four phases, L + Ti₃Sn ?? ??-Ti + FeTi, which lies at the invariant point Ti_69.2±0.8Fe_27.4±0.7Sn_3.4±0.2 (denoted by P). Below P, there is one univariant reaction, i.e., L ?? ??-Ti + FeTi for all alloy compositions, whereas above P, liquid undergoes one of the univariant reactions, i.e., L + ??-Ti ?? Ti₃Sn (Sn = 2, 2.5, 3, and 4.5 at. pct) or L + FeTi ?? Ti₃Sn for alloys (Sn = 6, 10 at. pct). For alloy with Sn = 3.85 at. pct, the ternary quasi-peritectic reaction is co-operated by two monovariant eutectic reactions, i.e., L ?? ??-Ti + FeTi below P and L ?? FeTi + Ti₃Sn above P. Detailed microstructural information allows us to construct liquidus projection of the investigated alloys. The results are critically discussed in the light of available literature data.     

Electrochemical Corrosion and Impedance Studies of Porous Ti–xNb–Ag Alloy in Physiological Solution

Porous titanium (Ti) and its alloys are promising materials for orthopedic applications due to their low elastic modulus, high strength, excellent corrosion resistance, and biocompatibility. In this study, the porous Ti–xNb–5Ag (x = 25, 30 and 35 wt%) alloys were synthesized using the powder metallurgy approach. The effects of Nb content on the porosity, mechanical properties, and electrochemical corrosion behavior of the alloys were investigated. XRD analysis revealed that the porous alloys mainly consist of α-Ti, β-Ti, intermetallic compound (Ti₄Nb), and oxides of TiO₂ and NbO phases. Porous alloys possess the porosity ranging from 57 to 65%, due to the addition of NH₄HCO₃ (45 wt%). Increase in Nb content lead to a reduction in the elastic modulus and compression strengths of the sintered porous Ti–xNb–5Ag alloys. All three developed porous Ti–xNb–5Ag alloys show the optimum combination of elastic modulus and compression strength, which is suitable for orthopedic applications. These porous alloys exhibit excellent electrochemical corrosion resistance in the simulated body fluids, and the samples having low porosity exhibit higher corrosion resistance than high-porosity samples.

     

The effect of interfacial reaction layer thickness on fracture of titanium-SiC particulate composites

Composites of commercial-purity Ti reinforced with 10 vol pct of SiC particles have been produced by cospraying and by powder blending and extrusion. Interfacial reaction layers have been studied by electron and optical microscopy and by Auger electron spectroscopy (AES) of fracture surfaces. The work of fracture has been measured as a function of reaction layer thickness for extruded and heat-treated composites. Material with very thin reaction layers (<∼0.1 μn) can be produced by cospraying, but porosity levels are relatively high (∼5 to 10 pct). Extruded material has been produced with a thin reaction layer (∼0.2 μm) and low porosity (<1 pct). It appears that the rate of reaction conforms with published parabolic rate constant data over a wide range of time and temperature. The reaction layer always consists of TiC and Ti₅Si₃, but the TiC grains tend to be larger than those of Ti₅Si₃. As the reaction layer thickness becomes greater than about 1 μm, the work of fracture falls sharply and the cracking pattern changes from one involving fracture of SiC particles to one in which cracking between the particles and adjacent reaction zones becomes predominant. It is suggested that the volume contraction accompanying this reaction, calculated at about 4.6 pct from density data, has a significant effect in promoting crack formation in these locations by generating radial tensile stresses across the interface. Thus, for this particular composite system, the important effect of a thicker reaction layer may be that it promotes the formation of an interfacial crackvia an effect on the local stress state, rather than itself constituting a larger flaw in the form of a through-thickness crack assumed to be present.     

The Sintering, Sintered Microstructure and Mechanical Properties of Ti-Fe-Si Alloys

A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (??1?pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (Ti₅Si₃) being dispersed in both the ?? and ?? phases. Conversely, additions of ?>1?pct Si produce coarse and/or networked Ti₅Si₃ silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by Ti₅Si₃. Increasing the Fe content in the Ti-xFe-0.5/1.0Si alloys above 3?pct markedly increases the average grain size and changes the morphology of the ??-phase phase to much thinner and more acicular laths. Consequently, the ductility drops to <1?pct. Si reacts exothermically with Fe to form Fe-Si compounds prior to the complete diffusion of the Fe into the Ti matrix during heating. The heat thus released in conjunction with the continuous external heat input melts the silicides leading to transient liquid formation, which improves the densification during heating. No Ti-TiFe eutectoid was observed in the as-sintered Ti-Fe-Si alloys. The optimum PM Ti-Fe-Si compositions are determined to be Ti-3Fe-(0.5-1.0)Si.     

Diffusion Bonding of Microduplex Stainless Steel and Ti Alloy with and without Interlayer: Interface Microstructure and Strength Properties

The interface microstructure and strength properties of solid state diffusion bonding of microduplex stainless steel (MDSS) to Ti alloy (TiA) with and without a Ni alloy (NiA) intermediate material were investigated at 1173 K (900 °C) for 0.9 to 5.4 ks in steps of 0.9 ks in vacuum. The effects of bonding time on the microstructure of the bonded joint have been analyzed by light optical microscopy and scanning electron microscopy in the backscattered mode. In the direct bonded joints of MDSS and TiA, the layer-wise σ phase and the λ + FeTi phase mixture were observed at the bond interface when the joint was processed for 2.7 ks and above holding times. However, when NiA was used as an intermediate material, the results indicated that TiNi₃, TiNi, and Ti₂Ni are formed at the NiA-TiA interface, and the irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ have been observed within the TiNi₃ intermetallic layer. The stainless steel-NiA interface is free from intermetallics and the layer of austenitic phase was observed at the stainless steel side. A maximum tensile strength of ~520 MPa, shear strength of ~405 MPa, and impact toughness of ~18 J were obtained for the directly bonded joint when processed for 2.7 ks. However, when nickel base alloy was used as an intermediate material in the same materials, the bond tensile and shear strengths increase to ~640 and ~479 MPa, respectively, and the impact toughness to ~21 J when bonding was processed for 4.5 ks. Fracture surface observations in scanning electron microscopy using energy dispersive spectroscopy demonstrate that in MDSS-TiA joints, failure takes place through the FeTi + λ phase when bonding was processed for 2.7 ks; however, failure takes place through σ phase for the diffusion joints processed for 3.6 ks and above processing times. However, in MDSS-NiA-TiA joints, the fracture takes place through NiTi₂ layer at the NiA-TiA interface for all bonding times.     

Characterization of Ti carbosulfide precipitation in Ti microalloyed steels

The morphology and composition of the Ti carbosulfides observed in a family of steels containing 0.05 to 0.25 wt pct Ti were determined using optical and electron microscopy, electron microprobe analysis, and energy-dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS) techniques. It is demonstrated that the Ti carbosulfide phase has a Ti: S mole fraction ratio of 2∶1 and contains an appreciable level of carbon, its identity being Ti₄C₂S₂. The solubility product of Ti₄C₂S₂ in austenite is derived to be log [Ti] [C]^0.5[S]^0.5=−15,600/T+6.50 and that of TiS to be log [Ti] [S]=−17.640/T+8.20. The former lies between the values for TiN and TiC, whereas the latter is more soluble than TiC. Stringer inclusions consisting of globular Ti₄C₂S₂ surrounded by elongated MnS were observed in the steels with 0.05 to 0.18 wt pct Ti. The volume fraction of the stringers is shown to be related to the sulfur partition coefficient through an empirical power law function. W.J.Lju, formerly with the Department of Metallurgical Engineering, McGill University