Effect of prior cold work on age hardening of Cu–3Ti–1Cr alloy

The influence of 50%, 75% and 90% cold work on the age hardening behavior of Cu–3Ti–1Cr alloy has been investigated by hardness and tensile tests, and light optical and transmission electron microscopy. Hardness increased from 118 Hv in the solution-treated condition to 373 Hv after 90% cold work and peak aging. Cold deformation reduced the peak aging time and temperature of the alloy. The yield strength and ultimate tensile strength reached a maximum of 1090 and 1110 MPa, respectively, following 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation twins. The maximum strength on peak aging was obtained due to precipitation of the ordered, metastable and coherent β′-Cu₄Ti phase, in addition to high dislocation density and deformation twins. Over-aging resulted in decreases in hardness and strength due to the formation of incoherent and equilibrium β-Cu₃Ti phase in the form of a cellular structure. However, the morphology of the discontinuous precipitation changed to a globular form on high deformation. The mechanical properties of Cu–3Ti–1Cr alloy are superior to those of Cu–2.7Ti, Cu–3Ti–1Cd and the commercial Cu–0.5Be–2.5Co alloys in the cold-worked and peak-aged condition.     

Brazing of Mo and Nb using two active braze alloys

The brazing of molybdenum and niobium using two active braze alloys, 63Ag–35.25Cu–1.75Ti and 68.8Ag–26.7Cu–4.5Ti, has been extensively evaluated. Both infrared and traditional vacuum furnace brazing are included in the experiment. Two active braze alloys demonstrate excellent wettability on Mo and Nb substrates, especially upon increasing the test temperature from 900 to 950 °C. For 63Ag–35.25Cu–1.75Ti braze alloy, the brazed joint primarily consists of Ag-rich and Cu-rich phases. Because the Ti is completely miscible with Mo and Nb, there is no reaction product at the interface among the braze alloy and two substrates. Accordingly, dimple-dominated fracture is extensively observed in the brazed joint. The brazed Mo/68.8Ag–26.7Cu–4.5Ti/Nb joint primarily consists of Ag-rich, Cu-rich and Nb-rich phases. A few Cu–Ti intermetallics are found in the brazed joint due to higher Ti content in the braze alloy. However, the presence of Cu–Ti intermetallic phases demonstrates little effect on the shear strength of the brazed joint. Dimple-dominated fracture is observed for the 900 °C brazed joint. The Nb-rich phase is found in both brazed joints, and its amount is increased with increasing brazing temperature and/or time. Accordingly, the growth of the Nb-rich phase is greatly inhibited during rapid infrared brazing. The coarsening of the Nb-rich phase significantly deteriorates the shear strength of the brazed Mo/68.8Ag–26.7Cu–4.5Ti/Nb joint, and finally results in cleavage fracture of the brazed joint.     

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 10⁷ cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements.

An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ˜0.3 for the Cu–Ni coated pad against an as-received specimen and ˜0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.     

Surface hardening of biomedical Ti–29Nb–13Ta–4.6Zr and Ti–6Al–4V ELI by gas nitriding

The microstructure and hardness near the surface of a biomedical titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical Ti–6Al–4V ELI (Ti64). After gas nitriding, the microstructure near the specimen surface was observed by optical microscopy, X-ray diffraction (XRD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). In both alloys, two types of titanium nitrides (TiN and Ti₂N) are formed and the phase is precipitated by gas nitriding. Furthermore, the oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO₂ is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.     

Microstructure, mechanical properties and chemical degradation of brazed AISI 316 stainless steel/alumina systems

The main aims of the present study are simultaneously to relate the brazing parameters with: (i) the correspondent interfacial microstructure, (ii) the resultant mechanical properties and (iii) the electrochemical degradation behaviour of AISI 316 stainless steel/alumina brazed joints. Filler metals on such as Ag–26.5Cu–3Ti and Ag–34.5Cu–1.5Ti were used to produce the joints. Three different brazing temperatures (850, 900 and 950 °C), keeping a constant holding time of 20 min, were tested. The objective was to understand the influence of the brazing temperature on the final microstructure and properties of the joints. The mechanical properties of the metal/ceramic (M/C) joints were assessed from bond strength tests carried out using a shear solicitation loading scheme. The fracture surfaces were studied both morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The degradation behaviour of the M/C joints was assessed by means of electrochemical techniques.

It was found that using a Ag–26.5Cu–3Ti brazing alloy and a brazing temperature of 850 °C, produces the best results in terms of bond strength, 234 ± 18 MPa. The mechanical properties obtained could be explained on the basis of the different compounds identified on the fracture surfaces by XRD. On the other hand, the use of the Ag–34.5Cu–1.5Ti brazing alloy and a brazing temperature of 850 °C produces the best results in terms of corrosion rates (lower corrosion current density), 0.76 ± 0.21 μA cm⁻². Nevertheless, the joints produced at 850 °C using a Ag–26.5Cu–3Ti brazing alloy present the best compromise between mechanical properties and degradation behaviour, 234 ± 18 MPa and 1.26 ± 0.58 μA cm⁻², respectively. The role of Ti diffusion is fundamental in terms of the final value achieved for the M/C bond strength. On the contrary, the Ag and Cu distribution along the brazed interface seem to play the most relevant role in the metal/ceramic joints electrochemical performance.     

Characterization of prior cold worked and age hardened Cu–3Ti–1Cd alloy

The influence of cold deformation by 50%, 75% and 90% on the age-hardening behavior of a Cu–3Ti–1Cd alloy has been investigated by hardness, tensile tests and light optical as well as transmission electron microscopy. The hardness of Cu–3Ti–1Cd alloy increased from 111 Hv in the solution-treated condition to 355 Hv in 90% cold worked and peak aged condition. The yield and ultimate tensile strengths of Cu–3Ti–1Cd alloy reached maxima of 922 MPa and 1035 MPa, respectively, on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was brought about by the precipitation of ordered, metastable, coherent β′ Cu₄Ti phase, in addition to high dislocation density and deformation twins. Both the hardness and the strength of the alloy decreased on overaging due to the development of the incoherent equilibrium phase β Cu₃Ti in a cellular structure form. However, the morphology of the discontinuous precipitation was changed to globular form at high deformation levels.     

Joining of zirconia to metal with Cu–Ga–Ti and Cu–Sn–Pb–Ti fillers

The method of brazing by capillary impregnation of Cu–Ga melt through a titanium powder layer situated between brazed details is elaborated. Samples of ZrO₂ ceramic/metal brazed joints using Cu–Ga–Ti filler and Cu–Sn–Pb–Ti filler were fabricated. The joints’ shear strength was 277±37 MPa for the Cu–Ga–Ti and 156±25 MPa for the Cu–Sn–Pb–Ti.     

Hard amorphous Ti–Al–N coatings deposited by sputtering

Ti–Al–N coatings were deposited by direct current reactive magnetron sputtering using two titanium and two aluminum targets. Two series of films with Al/(Al + Ti) atomic ratios of ≈ 23.5 and ≈ 34.5% were studied. The amount of nitrogen in the films was varied from 0 to 44at.%. The incorporation of N atoms led to a change of the -Ti lattice preferential orientation from <100> to <001>, a decrease in the degree of crystallinity, and subsequently to the collapse of the crystalline structure. Annealing at 975K promotes the formation of the Ti₃Al compound. The hardness increases smoothly with the nitrogen content. The high hardness values (31 and 41GPa) measured for the films with the highest N contents may be explained by the deposition of a nanocomposite phase. For the Ti–Al–N film deposited with Al/(Al + Ti) atomic ratio of 34.5% the -Ti structure was completely transformed to TiO₂ upon oxidation. The high oxidation resistance of the film deposited with 44at.% N at 1075K is characteristic of Ti–Al–N films.     

Internal stress and adhesion of amorphous Ni–Cu–P alloy on aluminum

This study investigated the effect of saccharin on the internal stress and the adhesion of amorphous Ni–Cu–P deposited on aluminum. An amorphous Ni–Cu–P deposit with slight compressive stress can be produced when one adds 8–10 g/l saccharin into the Ni–Cu–P deposition solution. The stress relief mechanism was investigated. The addition of saccharin restrains the coalescence of the islands within Ni–Cu–P nodules and reverses the internal stress of the electroless Ni–Cu–P deposit from tensile to compressive. The adhesion strength of the Si/Ti/Al/Ni–Cu–P multilayer specimen obtained with 10 g/l saccharin is around 35 to 45 MPa, and the fracture occurs at the silicon substrate after the pull test. The shear strength of the Ti/Al/Ni–Cu–P bump (100×100 μm) on Si is 132.9±12.7 g, and the fracture occurs at the Ni–Cu–P deposit after the shear test. Moreover, the inhibition of coalescence of the fine islands within Ni–Cu–P nodules increases the brightness and the hardness of the deposit.     

Investigation into effects of re-shot-peening on fretting fatigue behavior of Ti–6Al–4V

The effects of re-shot-peening treatment on fretting fatigue life/strength and the recovery of residual stress of the initially shot-peened Ti–6Al–4V were investigated at room and elevated temperatures. After subjecting to fretting fatigue up to about 40% of the total expected life of the initially shot-peened Ti–6Al–4V or to thermal exposure to 370 °C only, residual stress relaxed in the range of 20–50% of its value before fretting fatigue. The magnitude of stress relaxation depended upon the applied load level and test temperature. Re-shot-peening successfully recovered the relaxed residual stress up to the same level as obtained after the initial shot-peening. Further, fretting fatigue life after re-shot-peening, excluding pre-re-shot-peening fatigue life, was very close to that of the initially shot-peened specimen at a given stress level and test temperature. It thus appears that re-shot-peening nullified the effect of fretting fatigue damage after the initial shot-peening.     

Electrochemical corrosion properties of Ti–6Al–4V implant alloy in the biological environment

The electrochemical corrosion behavior of Ti–6Al–4V implant alloy was investigated in three biological solutions, i.e. urine, serum and joint fluid. The corrosion properties of Ti–6Al–4V implant alloys were examined by using electrochemical techniques, such as the potentiodynamic method, cyclic voltammetry, electrochemical impedance spectroscopy (EIS). The electrochemical corrosion characteristics of Ti–6Al–4V implant alloys in three biological solutions were measured in terms of the corrosion potential (E_corr), the corrosion current density (i_corr), and ac polarization resistance (R_p). The corrosion kinetic parameters were calculated from both the Tafel plot analyses and EIS analyses. The dependence of impedance versus potentials was studied at 37 °C at various offset potentials in three biological solutions. The ac circuit model for Ti–6Al–4V implant alloy at corrosion interface in biological solution was proposed, which was based on a simple Randles equivalent circuit. It was found that the Ti–6Al–4V implant alloy in three biological solutions showed a characteristic of a capacitive behavior. The experimental results of Tafel plot analyses were found in good agreement with that of EIS analyses.     

CuTiNiZrV Amorphous Alloy Foils for Vacuum Brazing of TiAl Alloy to 40Cr Steel

Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer,filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25(at.%) foil was flat and thin,containing Ti19Al6 and Ti2Cu intermetallic compounds; however,the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges,consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However,fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases,but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.     

Infrared brazing Inconel 601 and 422 stainless steel using the 70Au-22Ni-8Pd braze alloy

Infrared brazing Inconel 601 and 422 stainless steel using the 70Au-22Ni-8Pd braze alloy is performed in the experiment. The brazed joint is primarily comprised of Au-rich and Ni-rich phases, and there is no interfacial intermetallic compound observed in the joint. The (Ni,Fe)-rich phase is observed at the interface between 422SS and the braze alloy, and the Ni-rich phase is found at the interface between the braze alloy and IN601. With increasing the brazing temperature and/or time, the microstructures of the brazed joint is coarsened. For the infrared brazed joint at 1050^∘C for 180 s shows the highest average shear strength of 362 MPa. In contrast, the shear strength of the infrared brazed joint is higher than that of the furnace brazed specimen due to coarsening of the microstructure in the furnace brazed joint.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Joining of Zirconia and Ti-6Al-4V Using a Ti-based Amorphous Filler

Polycrystalline ZrO2-3 mol.%Y2O3 was brazed to Ti-6Al-4V by using a Ti47Zr28Cu14Ni11(at.%) amorphous ribbon at 1123-1273 K in a high vacuum. The influences of brazing temperature on the microstructure and shear strength of the joints were investigated. The interfacial microstructures can be described as ZrO2/TiO+TiO2+Cu2Ti4O+Ni2Ti4O/α-Ti+(Ti,Zr)2(Cu,Ni) eutectic/acicular Widmanst¨aten structure/Ti-6Al-4V alloy. With the increase in the brazing temperature, the thickness of the TiO+TiO2+Cu2Ti4O+Ni2Ti4O layer reduced, the content of the α-Ti+(Ti,Zr)2(Cu,Ni) eutectic phase decreased, while that of the coarse α-Ti phase gradually increased. The shear strength of the joints did not show a close relationship with the thickness of the TiO+TiO2+Cu2Ti4O+Ni2Ti4O layer. However, when the coarse (Ti,Zr)2(Cu,Ni) phase was non-uniformly distributed in the α-Ti phase, or when α-Ti solely situated at the center of the joint, forming a coarse block or even connecting into a continuous strip, the shear strength greatly decreased.     

Long crack growth behavior of implant material Ti–5Al–2.5Fe in air and simulated body environment related to microstructure

The fatigue crack propagation behavior of Ti–5Al–2.5Fe with various microstructures for biomedical applications was investigated in air and in a simulated body environment, Ringer's solution, in comparison with that of Ti–6Al–4V ELI and that of SUS 316L stainless steel. The crack propagation rate, da/dN, of Ti–5Al–2.5Fe in the case of each microstructure is greater than that of the Widmanstätten structure in Ti–6Al–4V ELI in air whereas da/dN of Ti–5Al–2.5Fe is nearly equal to that of the equiaxed structure in Ti–6Al–4V ELI in air when da/dN is plotted versus the nominal cyclic stress intensity factor range, ΔK. da/dN of the equiaxed structure and that of the Widmanstätten structure in Ti–5Al–2.5Fe are nearly the same in air when da/dN is plotted versus ΔK. da/dN of Ti–5Al–2.5Fe is nearly equal to that of SUS 316L stainless steel in the Paris Law region, whereas da/dN of Ti–5Al–2.5Fe is greater than that of SUS 316L stainless steel in the threshold region in air, when da/dN is plotted versus ΔK. da/dN of Ti–5Al–2.5Fe or Ti–6Al–4V ELI is nearly the same in air and in Ringer's solution when da/dN is plotted versus the effective cyclic stress intensity factor range, ΔK_eff, whereas da/dN of Ti–5Al–2.5Fe or Ti–6Al–4V ELI is greater in Ringer's solution than in air when da/dN is plotted versus ΔK.     

Probabilistic modeling of fatigue crack growth in Ti–6Al–4V

This paper proposes an approximate approach to efficient estimation of some variabilities caused by the material microstructural inhomogeneities. The approach is based on the results of a combined experimental and analytical study of the probabilistic nature of fatigue crack growth in Ti–6Al–4V. A simplified experimental fracture mechanics framework is presented for the determination of statistical fatigue crack growth parameters from two fatigue tests. The experimental studies suggest that the variabilities in long fatigue crack growth rate data and the Paris coefficient are well described by the log-normal distributions. The variabilities in the Paris exponent are also shown to be well characterized by a normal distribution. The measured statistical distributions are incorporated into a probabilistic fracture mechanics framework for the estimation of material reliability. The implications of the results are discussed for the probabilistic analysis of fatigue crack growth.     

Cu-P-Sn-Ni钎料真空钎焊MGH956合金的研究

为扩展Cu-P基钎料在连接MGH956合金中的应用,采用新型Cu-P-Sn-Ni钎料对MGH956合金在800~890℃进行了真空钎焊,研究了不同钎焊温度和保温时间对焊缝组织及力学性能的影响.结果表明:在所研究的钎焊温度范围内保温5 min均可获得成形效果良好的钎焊接头,其主要由钎缝中心区和界面反应层组成,其中,钎缝中心区由α(Cu)固溶体基体和化合物Cu_3P+(Fe,Ni)_3P+FeCr组成,反应层由α(Fe)固溶体、Fe_3P和Cu_3P组成;随着钎焊温度的升高,反应层厚度逐渐增加,钎缝中心区中的化合物Cu_3P+(Fe,Ni)_3P+FeCr的形态也随之发生明显改变;各钎焊温度下获得的钎焊接头经室温拉伸,断裂均发生在钎缝中心区,断口形貌呈现韧性和脆性的混合断裂特征.830℃钎焊5 min的接头抗拉强度最大,为510.3 MPa,达到了母材抗拉强度的70.9%.     

Al–Si coating fused by Al + Si powders formed on Ti–6Al–4V alloy and its oxidation resistance

An Al–Si coating was successfully produced by means of the low oxygen pressure fusing technology for improving the oxidation resistance of Ti–6Al–4V alloy. The Al and Si concentration in coating and coating thickness could be controlled by adjusting powder mixing ratio and changing the technical parameters (fusing temperature and time), respectively. At 1273 K, the weight gain of the Al–20Si coating increased with prolonging fusing time and its equation could be described as Δm² = 3.62t. After 105 h oxidation, the oxidation rate of the Al–20Si coated specimen with fusing time 100 min was about two to four times than that of the Al–10Si coated specimen with fusing time 60 min.