Solid state phase transformations in Ti–Al–C and Ti–Al–Nb–C alloys

Abstract

Results are reported of an investigation of solid state transformations in a series of α₂ based alloys having an aluminium content of 26 at.-% with carbon up to 3 at.-%; two α₂ basedquaternary Ti–Al–Nb–C alloys with 5 and 12 at.-%Nb and 3 at.-%C were also studied. Ordering occurs in the ternary Ti–Al–C alloys and also in the 23Al–5Nb–3C alloy on quenchingfrom 1250°C. Additional carbide precipitation was not observed in the ternary Ti–Al–C alloys on reheating to 750°C. Additions of niobium resulted in the presence of the β phase at 1050°C in the 5%Nb alloy and at 1050 and 750°C in the 12%Nb alloy. In the quaternary Ti–Al–Nb–C alloys, (Ti, Nb)₃AlC was found to be the primary phase and was present in the microstructure over the temperature range studied. In the 21Al–12Nb–3C alloy, the ordered β phase transformed to α″₂ martensite on quenching from 1250;amp;#x00B0;C.

MST/1306     

Mechanical, physical, and chemical characterization of Ti–35Nb–5Zr and Ti–35Nb–10Zr casting alloys

Titanium is used due its excellent properties in medical and dentistry areas. With the objective of exploiting better mechanical properties, not altering its biocompatibility, it was intended to add niobium and zirconium to the titanium, being formulated two alloys Ti–35%Nb–5%Zr (alloy 1) and Ti–35%Nb–10%Zr (alloy 2) wt% produced by an arc melting method. The chemical analysis of the samples was accomplished by X-ray fluorescence, and the microstrutural evaluation by scanning electron microscopy and X-ray diffraction. The mechanical tests were: Vickers hardness, tensile strength, mechanical cycling, and fracture analysis. The results allowed characterizing the alloy 1 as α + β type and the alloy 2 as β type. It is found that the alloy 1 presented larger hardness and smaller tensile strength than the alloy 2. The fractures, after the tensile test, were of the ductile type and, after the mechanical cycling, they were of the mixed type for both alloys.     

Mechanical properties and defective effects of bcc V–4Cr–4Ti and V–5Cr–5Ti alloys by first-principles simulations

V–(4–5) wt.% Cr–(4–5) wt.% Ti alloys are important candidate structural materials for the first-wall and blanket in future fusion reactor. Thus it is necessary to study the fundamental mechanical properties and the irradiation effects of the V-based alloys. Within a random solid solution model, the elastic constants and ideal strength of the V–4Cr–4Ti and the V–5Cr–5Ti alloys were calculated and compared with those of pure V solid. According to the theoretical Cauchy pressure and the ratio of bulk modulus and shear modulus, both alloys exhibit good ductility. Within the 250-atom supercell, inclusion of one vacancy defect or one interstitial H (He) atom will further enhance the ductility of these alloys.     

Microstructure and grindability of as-cast Ti–Sn alloys

In this study, the structure, microhardness, and grindability of a series of binary Ti–Sn alloys with tin contents ranging from 1 to 30 wt% were investigated. Commercially pure titanium (c.p. Ti) was used as a control. The experimental results indicated that all the Ti–Sn alloys showed hcp α structure, and the hardness values of the Ti–Sn alloys increased with greater Sn contents, ranging from 246 HV (Ti–1Sn) to 357 HV (Ti–30Sn). Among these Ti–Sn alloys, the alloy with 30 wt% Sn content showed the highest hardness value. The grindability of each metal was found to be largely dependent on the grinding conditions. The addition of Sn to c.p. Ti did contribute to improving the grindability of c.p. Ti. The Ti–Sn alloys with a higher Sn concentration could be ground more readily. The grinding rate of the Ti–20Sn alloy at 1200 m/min was about 2.8 times higher than that of c.p. Ti. Additionally, the grinding ratios of the Ti–10Sn, Ti–20Sn, and Ti–30Sn alloys at 1200 m/min were about 2.8, 2.7, and 3.4 times that of c.p. Ti, respectively. Our research suggests that the Ti–Sn alloys with Sn contents of 10 wt% and greater developed here are good candidates for machining by the CAD/CAM method.     

Composition and lattice parameters of silicide and matrix in cast Ti–Si–Al–Zr alloys

Abstract

The silicide chemistry, i.e. the type, composition, and lattice parameters of the silicide in as cast titanium based Ti–Si–Al–Zr alloys, has been studied. It has been shown that the stoichiometry of the silicide in the alloys can be expressed as (Ti_1?x , Zr_x)₅(Si_l?y, Al_y) _2·76?3·04(0≤X< 0·2, 0≤y<0·1). The presence of Al and Zr in the silicide increases its lattice parameters. Addition of Al coarsens the eutectic silicide and slows the formation of secondary silicide precipitates by solid state reaction. Addition of zirconium refines the eutectic silicide and promotes secondary silicide precipitation. The silirides are low in Al and rich in Zr, whereas the Ti matrix is rich in Al and low in Zr. The lattice parameters of the Ti matrix are decreased by Al and increased by Zr.

MST/1427     

Investigation of joining Al–C–Ti cermets and Ti6Al4V by combustion synthesis

The paper has addressed a route for the welding of titanium alloy (Ti6Al4V) and Al–C–Ti powders by the combustion synthesis (CS) method. Al–C–Ti powders were compressed in the titanium alloy pipes with relative densities of 65%, and then the powder compact was sintered by two reaction mode at the same time as the annulus of titanium alloy and the synthesized product were joined. The paper has studied the effects of reaction mode and Al content in starting powders on the structure and property of the welded joints. And it has also discussed the microstructure of welded joints by laser-induced combustion synthesis (LCS). The mechanical properties of the welding seam have been also tested. The results show that LCS welding has realized fusion welding and the welding seam has good mechanical properties. Furthermore, SEM analysis has indicated that nano-size grains of TiC were formed in the joint layer.     

Corrosion and corrosion-fatigue behavior of cp-Ti and Ti–6Al–4V laser-marked biomaterials

The aim of this work was to determine the influence of laser surface modification treatments on mechanical and electrochemical behavior in Ti and Ti–6Al–4V implants. For each metal, different samples were laser modified simulating the markings according to the international requirements. (It is necessary in each metallic biomaterial to mark the serial, batch and company numbers.) Microstructural changes produced by this treatment were observed: (a) the melting zone with small grain sizes and martensitic structures in above-mentioned metals and (b) the heat-affected zone (HAZ) with alpha phase in cp-Titanium with bigger grain sizes and Widmanstatten structure in Ti–6Al–4V. Positive tensile residual stress was determined by means X-ray analysis in the zones marked by laser. Furthermore, corrosion behavior was studied in a simulated body fluid at 37°C. Pitting was observed in different zones near the HAZ and the results showed a decrease of the corrosion resistance in the laser treated samples. Residual stresses and the martensitic microstructures favoured the decrease of the corrosion-fatigue life around 20% of both metals under physiological conditions.     

Microstructure and properties of Ti–Nb–V–Mo-alloyed high chromium cast iron

The correlations of microstructure, hardness and fracture toughness of high chromium cast iron with the addition of alloys (titanium, vanadium, niobium and molybdenum) were investigated. The results indicated that the as-cast microstructure changed from hypereutectic, eutectic to hypoeutectic with the increase of alloy contents. Mo dissolved in austenite and increased the hardness by solid solution strengthening. TiC and NbC mainly existed in austenite and impeded the austenite dendrite development. V existed in multicomponent systems in forms of V alloy compounds (VCrFe₈ and VCr₂C₂). With the increase of alloy additions, carbides size changed gradually from refinement to coarseness, hardness and impact toughness were increased and then decreased. Compared with the fracture toughness (6 J/cm²) and hardness (50·8HRC) without any alloy addition, the toughness and hardness at 0·60 V–0·60Ti–0·60Nb–0·35Mo (wt%) additions were improved and achieved to 11 J/cm² and 58·9HRC, respectively. The synergistic roles of Ti, Nb, V and Mo influenced the solidification behaviour of alloy. The refinement of microstructure and improvement of carbides morphologies, size and distribution improved the impact toughness.     

Phase relationships involving TiC and Ti3AlC (P phase) in Ti–Al–C system

Abstract

Titanium rich alloys of the Ti–Al–C system have been investigated to determine the constitution in the range 1250–750°C with particular reference to phase equilibria and transformations involving TiC and Ti₃AlC (P phase). Alloys with varying aluminium and carbon concentrations up to 15 at.-%Al and 15 at.-%C have been studied using scanning and transmission electron microscopy and X-ray diffraction. The isothermal sections at 1250, 1050, 1000, and 750°C reported by previous workers are reviewed. Equilibria involving the phases β-Ti, α-Ti, Ti₃Al (α ₂), TiC, and Ti₃AlC (P phase) have been investigated and partial isothermal sections at 1050 and 750°C are presented. A revision of the previously established Ti–Al–C isothermal section at 750°C is shown involving equilibria of α+α ₂+P and α+P+TiC. The orientation relationship between the P phase and Ti–Al (α/α ₂) matrix has been found to be (0001)_{α/α 2} ∥{111}_P and 〈12¯10〉_{α/α 2} ∥〈110〉_P.     

Active metal brazing of SiO_2–BN ceramic and Ti plate with Ag–Cu–Ti+BN composite filler

SiO_2–BN ceramic and Ti plate were joined by active brazing in vacuum with Ag–Cu–Ti+BN composite filler.The effect of BN content,brazing temperature and time on the microstructure and mechanical properties of the brazed joints was investigated.The results showed that a continuous Ti N–Ti B_2reaction layer formed adjacent to the SiO_2–BN ceramic,whose thickness played a key role in the bonding properties.Four Ti–Cu compound layers,Ti_2Cu,Ti_3Cu_4,Ti Cu_2and Ti Cu_4,were observed to border Ti substrate due to the strong affinity of Ti and Cu compared with Ag.The central part of the joint was composed of Ag matrix,over which some fine-grains distributed.The added BN particles reacted with Ti in the liquid filler to form fine Ti B whiskers and Ti N particles with low coefficients of thermal expansion(CTE),leading to the reduction of detrimental residual stress in the joint,and thus improving the joint strength.The maximum shear strength of 31 MPa was obtained when 3 wt%BN was added in the composite filler,which was 158%higher than that brazed with single Ag–Cu–Ti filler metal.The morphology and thickness of the reaction layer adjacent to the parent materials changed correspondingly with the increase of BN content,brazing temperature and holding time.Based on the correlation between the microstructural evolution and brazing parameters,the bonding mechanism of SiO_2–BN and Ti was discussed.     

Microstructure and notched tensile fracture of Ti–6Al–4V to Ti–4.5Al–3V–2Fe–2Mo dissimilar welds

Dissimilar welding of Ti–6Al–4V (Ti-6-4) to Ti–4.5A1–3V–2Fe–2Mo (SP-700) alloys was performed using a CO₂ laser. The microstructure and notched tensile strength (NTS) of the dissimilar welds were investigated in the as-welded and post-weld heat treatment (PWHT) conditions. Moreover, the results were compared with homogeneous laser welds with the same PWHT. The dilution of SP-700 with the Ti-6-4 alloy caused the formation of fine needle-like α + β structures, resulting in the exhibition of a moderately high fusion zone (FZ) hardness of HV 398. The high FZ hardness (HV 438) for the weld with the PWHT at 482 °C was associated with low NTS or high notch brittleness. The fracture appearance of the notched tensile specimen was related to its inherent microstructure. With increasing the PWHT temperature, the thickness of grain boundary α increased, which promoted an intergranular dimple fracture. By contrast, fine shallow dimples were present in the peak-aged weld, which was induced by the refined α + β microstructures in the basket-weave form.     

Formation of hydroxyapatite layer on bioactive Ti and Ti–6Al–4V by simple chemical technique

Bioactive coatings on cp-Ti and Ti–6Al–4V were prepared by a simple chemical technique. Specimens of cp-Ti and Ti–6Al–4V were initially immersed in a 5 M NaOH solution at 60 °C for 24 h which resulted in the formation of a porous network structure composed of Na₂Ti₅O₁₁ and TiO₂. The specimens were then immersed in a Ca-rich solution either at 60 °C or at 36.5 °C for 24 h. During this treatment Na⁺ was released and Ti–OH groups were formed. Subsequently, TiO₂ dissociated from the Ti–OH group and combined with calcium ions to form calcium titanate (CaTiO₃), which was embedded in a titania gel layer during the immersion period. The specimens were then immersed in r-SBF at 36.5 °C for 1–30 days. After immersion in r-SBF for 3 days, HAp (hydroxyapatite) spheroids began to deposit on the substrates, and within a week the surfaces were covered. The HAp spheroids were 5 μm in size with a Ca/P ratio of 1.68 which was close to bone-like apatite (1.67). The average thicknesses of HAp layer after immersion in r-SBF for 3 days, 1 week, and 2 weeks were 3.8, 5.6, and 6.4 μm, respectively. A scratch test, used to evaluate the adhesive strength of the HAp layer, showed that the HAp layer was not scraped off until the applied load reached 26 N.     

Interface evolution of SiO2 glass ceramic and Ti–6Al–4V alloy joint brazed with Ag–Cu–Ti alloy

Abstract

In the present paper, SiO₂ glass ceramic and Ti–6Al–4V alloy were successfully brazed with Ag–21Cu–4·5Ti active braze alloy. The interfacial microstructure and evolution course of SiO₂ glass ceramic/Ti–6Al–4V joint were studied in detail. According to the experimental results, active element Ti plays a quite important role in the formation of reaction layers on the joint interface. The reaction products of the joint are TiSi₂, Ti₄O₇, TiCu, Cu₂Ti₄O and Ti₂Cu respectively. The interface evolution can be generally described by four stages, which are solution and diffusion of atoms, reaction among atoms, formation of reaction layers and precipitation of solid solution layers respectively.     

Comparison of residual stresses in Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo linear friction welds

Abstract

In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α–β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d ₀ variation across the weld has been evaluated using the biaxial sin²Ψ technique with laboratory X-rays. It was found that failure to account for the d ₀ variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (～750 MPa) than for Ti-64 (～650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+β titanium alloy.     

Surface modification of Ti–Nb–Zr–Sn alloy by thermal and hydrothermal treatments

Surface modifications by thermal and hydrothermal treatments in solution with calcium ions were investigated with the aim of improving bioactivity and wear resistance of a Ti–Nb–Zr–Sn alloy. The results showed that the first step of thermal treatment at 600 °C significantly increases the surface hardness and energy by forming oxides of Ti and Nb. The second step of hydrothermal treatment in a boiled supersaturated Ca(OH)₂ solution induces a bioactive layer containing CaTiO₃, CaCO₃, Ca(OH)₂ and TiO₂. Using this treatment, a complete Ca–P layer can be formed within 3 days of soaking in simulated body fluid (SBF). The origin of such fast apatite formation was analyzed by comparison with single step thermal or hydrothermal treatment and with thermal plus hydrothermal treatment without calcium ions. The results suggest that the increase of surface energy by thermal treatment and the incorporation of calcium ions by the hydrothermal treatment in calcium ion solution play important roles in the formation of bioactive apatite.     

Microstructure and corrosion behavior of Ti nanoparticles reinforced Ni–Ti composite coatings by electrodeposition

A survey on electrochemical codeposition of Ti nanoparticles in Ni matrix coating is given. The influences of Ti nanoparticle loadings in electrolyte on the microstructure, microhardness and corrosion behavior of Ni–Ti coatings were investigated. The results showed that a pyramidal surface structure evolved into a spherical surface structure of the coatings with increasing Ti nanoparticle loading. The content of Ti in the Ni–Ti coatings first increased and reached the maximum value of 7.1 vol.% at the loading of 16 g/L, then decreased due to agglomeration of nanoparticles. The [2 0 0] preferred orientation gradually evolved to [1 1 1] orientation with increasing Ti nanoparticle loading. At Ti nanoparticle loading of 16 g/L, the minimum crystallite size (44 nm) and maximum microstrain (0.25%) were obtained. The microhardness of the Ni–Ti coatings was improved and obtained the maximum value at the loading of 16 g/L. The anti-corrosion behavior of the Ni–Ti coatings had increased trend with increasing Ti nanoparticle loading. The pitting corrosion and the selective dissolution of Ti nanoparticles happened in corrosion of Ni–Ti coating electrodeposited at the loading of 16 g/L in a 3.5 wt.% NaCl solution.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Characterization and corrosion behavior of NiTi–Ti2Ni–Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi–Ti₂Ni–Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni–Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni–Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Interface microstructure analysis of SiO2 glass ceramic and Ti–6Al–4V alloy joint brazed with Ti–Zr–Ni–Cu alloy

Abstract

In the present paper, SiO₂ glass ceramic was joined to Ti–6Al–4V alloy with 35Ti–35Zr–10Ni–15Cu (wt-%) filler foil. The whole brazing process was performed under vacuum circumstances at different temperatures (850–1000°C) for several holding times (1–30 min). According to results of scanning electron microscopy, energy dispersive spectrometry, electron probe X-ray microanalysis and X-ray diffraction analysis, the reaction products of the interface are Ti₂O, Zr₃Si₂, Ti₅Si₃, Ti based solid solution and Ti₂(Cu,Ni). There is residual TiZrNiCu braze alloy on the SiO₂ glass ceramic/Ti–6Al–4V alloy interface after brazing. Besides, the interface evolution model of the joint was described by four stages: diffusion and solution among atoms, formation of reaction products, precipitation and growth of reaction layers respectively.     

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.