Infrared brazing of Ti50Ni50 shape memory alloy using two Ag–Cu–Ti active braze alloys

Microstructural evolution, shape memory effect and shear strength of infrared brazed Ti₅₀Ni₅₀ shape memory alloy using Cusil-ABA^® and Ticusil^® active braze alloys are investigated. The Ag–Cu eutectic braze alloy can readily wet Ti₅₀Ni₅₀ substrate by minor titanium additions. The brazed Ti₅₀Ni₅₀/Cusil-ABA^®/Ti₅₀Ni₅₀ joint is mainly comprised of Cu-rich, Ag-rich and CuNiTi phases. On the other hand, the brazed Ti₅₀Ni₅₀/Ticusil^®/Ti₅₀Ni₅₀ joint consists of Ag-rich, Cu-rich and TiCu₂ phases. Because the chemical composition of Ticusil braze alloy is located inside the huge miscibility gap, the molten braze tends to be separated into two liquids during brazing. One is rich in Ag, and the other is rich in both Cu and Ti. The Ag-rich liquid does not react with Ti₅₀Ni₅₀ substrate. In contrast, the copper content is depleted from the matrix of brazed joint due to the formation of interfacial TiCu₂ phase. The TiCu₂ phase is less detrimental to the shape memory effect than CuNiTi phase during the shape recovery bending test. Shear strength of brazed joints exceeds 200 MPa for both braze alloys if the brazing time exceeds 180 s. However, thick interfacial CuNiTi and TiCu₂ layers can deteriorate the shear strength.     

Ag-Cu+WC复合钎料钎焊ZrO2陶瓷和TC4合金

采用新型Ag-Cu+WC复合钎料进行ZrO₂陶瓷和TC4合金钎焊连接,探究了接头界面组织及形成机制,分析了钎焊温度对接头界面结构和力学性能的影响. 结果表明,接头界面典型结构为ZrO₂/TiO+Cu₃Ti₃O/TiCu+TiC+W+Ag_(s,s)+Cu_(s,s)/TiCu₂/TiCu/Ti₂Cu/TC4. 钎焊过程中,WC颗粒与Ti发生反应,原位生成TiC和W增强相,为Ti-Cu金属间化合物、Ag基和Cu基固溶体提供了形核质点,同时抑制了脆性Ti-Cu金属间化合物的生长,优化了接头的微观组织和力学性能. 随钎焊温度的升高,接头反应层的厚度逐渐增加,WC颗粒与Ti的反应程度增强. 当钎焊温度890 ℃、保温10 min时,复合钎料所得接头抗剪强度达到最高值82.1 MPa,对比Ag-Cu钎料所得接头抗剪强度提高了57.3%.     

Details on the Formation of Ti<Subscript>2</Subscript>Cu<Subscript>3</Subscript> in the Ag-Cu-Ti System in the Temperature Range 790 to 860 °C

Silver-copper-titanium (Ag-Cu-Ti) ternary alloys are often used as active braze alloys for joining ceramics to metals at temperatures ranging from 780 °C (the melting point of the Ag-Cu eutectic) up to 900 °C. When Ti/Ag-Cu joints are brazed at low temperature (near 800 °C), the intermetallic compound Ti₂Cu₃ (tetragonal, P4/nmm, a = 0.313 nm, c = 1.395 nm) is systematically missing from the interface reaction layer sequence. An experimental investigation based on isothermal diffusion experiments in the Ag-Cu-Ti ternary system has then been undertaken to clarify the issues of thermal stability and formation kinetics of this compound. Evidence has been found for the stability of Ti₂Cu₃ at temperatures ranging from 790 to at least 860 °C. By heat treating Ag-Cu-Ti powder mixtures at 790 °C for increasing times, it has moreover been shown that Ti₂Cu₃ forms at a much slower rate than the two adjacent Ti-Cu compounds: TiCu₄, the first phase to form, and Ti₃Cu₄. This explains why although thermodynamically stable, Ti₂Cu₃ is not obtained when temperature is too low or reaction time too short.     

Vacuum brazing of TZM alloy to ZrC particle reinforced W composite using Ti-28Ni eutectic brazing alloy

Reliable brazing of TZM alloy and ZrC particle reinforced (ZrC_p) W composite was achieved in this study by using Ti-28Ni eutectic brazing alloy. The typical interfacial microstructure of TZM/Ti-28Ni/ZrC_p-W brazed joint consisted of a Ti solid solution (Ti(s, s)) layer, a continuous Ti₂Ni layer and a diffusion layer mainly composed of W particles and (Ti, Zr)C particles. With an increase of brazing temperature, more ZrC particles and W particles entered the molten brazing alloy, which broadened the brazing seam and diminished the Ti₂Ni layer, resulting in the disappearance of the Ti₂Ni layer eventually. Meanwhile, more Ti(s, s) stripes were observed on the TZM side. The presence of continuous Ti₂Ni intermetallic phase and Ti(s, s) stripes structure in joints deteriorated the joining properties, which resulted in the formation of brittle fracture under shear test. In addition, the fracture path was related to the brazing temperature, and cracks initiate and propagate in the continuous Ti₂Ni layer at lower temperatures. However, the fracture path tended to be located at the TZM substrate close to the interface between TZM and the brazing seam when the brazing temperature exceeded 1040 °C. The optimal room temperature shear strength reached 120.5 MPa when brazed at 1040 °C for 10 min and the fracture surface exhibited cleavage fracture characteristics, and the shear strength at high temperature of 800 °C for the specimens with highest shear strength at room temperature reached 77.5 MPa.     

Phase diagram of the Sn–Zn–Ti system

Equilibrated Sn–Zn–Ti alloys and (Sn + Zn)/α-Ti diffusion couples have been studied by scanning electron microscopy, metallography, and differential scanning calorimetry. For the first time an isothermal section, at 600 °C, of the ternary Sn–Zn–Ti system has been constructed. A previously unknown ternary phase with approximate formula Ti₃Sn₂Zn₆ (probable homogeneity interval in the range Ti₅Sn₄Zn₁₁ to Ti₅Sn₃Zn₁₂) has been found.The solubility ranges of the titanium based solid solutions and the intermetallic phases have been looked for. As far as we could detect and in agreement with theoretical considerations, zinc dissolves more in Ti–Sn phases than tin into Ti–Zn compounds. Titanium additions of 3 and 4 at.% Ti do not influence significantly the Sn–Zn eutectic temperature. The experimentally determined melting enthalpies of the nearly eutectic alloys have values around 100 J g⁻¹.     

Phase equilibria in the Ti-rich corner of the Ti-Si-Ga system

Phase relations in the ternary Ti-Si-Ga system have been established experimentally by means of a study of alloy samples in the as-cast condition and annealed at 1350 °C. The alloys were prepared by arc melting. The investigation was carried out using physico chemical methods of analyses (metallography, X-ray powder diffraction, differential thermal analysis, and electron probe microanalysis over a limited composition range with samples containing less than 38 at.% Ga and more than 62 at.% Ti. Liquidus and solidus surface projections, the isothermal section at 1350 °C, and the isopleth at 68 at.% Ti are presented. Three surfaces of primary crystallization of phases have been established: extended ones for Ti₅(Si,Ga)₃ and β (Ti-base solid solution) and a narrow one of Ti₂Ga. The monovariant curves separating these are due to the eutectic reactions L↔β+Ti₅(Si,Ga)₃ and L↔β+Ti₂Ga and to the L+Ti₅(Si,Ga)₃↔Ti₂Ga peritectic reaction. The three-phase region (β+Ti₅(Si,Ga)₃+Ti₂Ga) results from the four-phase eutectic reaction L↔β+Ti₅(Si,Ga)₃+Ti₂Ga. The composition of the ternary eutectic point E and the compositions of the coexisting solid phases have been determined. The solubilities of Si in the gallides, and of Ga in Ti₅Si₃ and of both the elements in Ti are given.     

Phase equilibria in the Ti-rich corner of the Ti-Si-Ga system

Phase relations in the ternary Ti-Si-Ga system have been established experimentally by means of a study of alloy samples in the as-cast condition and annealed at 1350 °C. The alloys were prepared by arc melting. The investigation was carried out using physico chemical methods of analyses (metallography, X-ray powder diffraction, differential thermal analysis, and electron probe microanalysis over a limited composition range with samples containing less than 38 at.% Ga and more than 62 at.% Ti. Liquidus and solidus surface projections, the isothermal section at 1350 °C, and the isopleth at 68 at.% Ti are presented. Three surfaces of primary crystallization of phases have been established: extended ones for Ti₅(Si,Ga)₃ and β (Ti-base solid solution) and a narrow one of Ti₂Ga. The monovariant curves separating these are due to the eutectic reactions L↔β+Ti₅(Si,Ga)₃ and L↔β+Ti₂Ga and to the L+Ti₅(Si,Ga)₃↔Ti₂Ga peritectic reaction. The three-phase region (β+Ti₅(Si,Ga)₃+Ti₂Ga) results from the four-phase eutectic reaction L↔β+Ti₅(Si,Ga)₃+Ti₂Ga. The composition of the ternary eutectic point E and the compositions of the coexisting solid phases have been determined. The solubilities of Si in the gallides, and of Ga in Ti₅Si₃ and of both the elements in Ti are given.     

High hydrogen permeability in the Nb-rich Nb–Ti–Ni alloy

The microstructure and the hydrogen permeability of the Nb-rich Nb–Ti–Ni alloy, i.e., the Nb₅₆Ti₂₃Ni₂₁ alloy were investigated and compared with those of the Nb₄₀Ti₃₀Ni₃₀ alloy. The Nb₅₆Ti₂₃Ni₂₁ alloy consisted of a combination of the primary phase bcc- (Nb, Ti) solid solution with the eutectic phase {bcc- (Nb, Ti) + B2-TiNi}. The volume fraction of the former and the latter phases were 62 and 38 vol.%, respectively. The Nb₅₆Ti₂₃Ni₂₁ alloy showed the higher Φ value of 3.47 × 10⁻⁸ (mol H₂ m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 1.8 times higher than that of the Nb₄₀Ti₃₀Ni₃₀ alloy, which has been reported to be highest in the Nb–Ti–Ni system. The present work demonstrated that the Nb-rich Nb–Ti–Ni alloys consisting of only the primary phase bcc- (Nb, Ti) and the eutectic phase {bcc- (Nb, Ti) + B2-TiNi} are promising for the hydrogen permeation membrane.     

Formation process of liquid in interface of Ti／Cu contact reaction couple

By using the Ti/Cu contact reaction couples, the dissolution behavior of Ti and Cu in the eutectic reaction process was investigated under different conditions. The results show that the formation of eutectic liquid phase has a directional property, i.e. the eutectic liquid phase forms first at the Cu side and then spreads along the depth direction of Cu. The width of the eutectic liquid zone when Ti is placed on Cu is wider than that when Ti is placed under Cu. The shape of the upside liquid zone is wave-like. This phenomenon indicates that the formation process and spreading behavior in the upside are different from those in the underside, and there exists void effect in the Cu side of underside liquid zone, this will result in the delaying phenomenon of the contact reaction between Ti and Cu,and distinctly different shapes of the both liquid zones. The formation process of Ti/Cu eutectic liquid zone is similar to that of the traditional solid-state diffusion layer, and the relationship between the width of liquid zone and holding time obeys a square root law.     

Microstructure and properties of Ti64.51Fe26.40Zr5.86Sn2.93Y0.30 biomedical alloy fabricated by laser additive manufacturing

From the perspective of biomechanics and forming technology, Ti−Fe−Zr−Sn−Y eutectic alloy was designed using a “cluster-plus-glue-atom” model, and then the alloy was prepared by laser additive manufacturing (LAM) on pure titanium substrate. The mechanical properties of the alloy were evaluated using micro-hardness and compression tester, and the elastic modulus was measured by nanoindenter. The results show that the alloy exhibits a high hardness of HV (788±10), a high strength of 2229 MPa, a failure strain of 14%, and a low elastic modulus of 87.5 GPa. The alloy also has good tribological, chemical, forming, and biological properties. The comprehensive performances of the Ti_64.51Fe_26.40Zr_5.86Sn_2.93Y_0.30 alloy are superior to those of the Ti_70.5Fe_29.5 eutectic alloy and commercial Ti−6Al−4V alloy. All the above-mentioned qualities make the alloy a promising candidate as LAM biomaterial.     

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

The common Ti₄₄Ni₄₇Nb₉ and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti₄₄Ni₄₇Nb₉ alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti₅₀Ni₄₀Cu₁₀ alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.     

Hydrogen permeable Ta–Ti–Ni duplex phase alloys with high resistance to hydrogen embrittlement

Crystal structures, microstructures and hydrogen permeability Φ of as-cast Ta–TiNi alloys on the line connecting the compositions of the primary (Ta, Ti) and the ternary eutectic phases have been investigated to find out highly hydrogen permeable duplex phases alloys with high resistance to the hydrogen embrittlement. The alloys on this line show microstructures of (1) the eutectic {(Ta, Ti) + TiNi} phase, (2) the primary (Ta, Ti) phase + the eutectic {(Ta, Ti) + TiNi} phase, and (3) the (Ta, Ti) solid solution, although a little amount of unidentified (impurity) phases are included in these samples. The value of Φ increases with increasing Ta content and the volume fraction of the primary (Ta, Ti) phase, which indicates that the primary phase contributes mainly to the hydrogen permeation. The Ta₅₆Ti₂₃Ni₂₁ alloy, containing the 61 vol.% primary phase, shows the highest Φ of 2.18 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−0.5 at 673 K, which is 1.3 times higher than that of the previous most high Φ alloy (Ta₅₃Ti₂₈Ni₁₉). The more Ta-rich alloys on this line, i.e., containing a small amount of the eutectic phase, are broken down by the hydrogen embrittlement, suggesting that the eutectic phase suppresses the hydrogen embrittlement.     

PTLPB of Si3N4 to FA-129 using nickel as a core interlayer

Partial transient liquid phase bonding was applied to a silicon nitride/iron aluminide alloy (FA-129) joint. This joint was composed of two independent joining systems. The joint configuration was Si₃N₄/75Cu–25Ti/Cu/Ni/Al/FA-129. The results demonstrate that homogenization has no effect on the kinetic of the reaction layer formation between the ceramic and the nickel core due to a previous dissolution reaction. The reaction layer composition evolves from a TiN–Ti₅Si₃ layer system after reaction during the first soaking time, to a single TiN layer after the complete cycle. The dissolution of the Ti₅Si₃ layer in the Ni core becomes the driving factor influencing the properties of the joint. The composition of the copper–nickel alloy interlayer changes with homogenization treatment, passing from a maximum of 70–20 wt% Cu, depending on the conditions. NiAl and Ni₃Al intermetallics were observed at the interface between the nickel and the FA-129 and their thicknesses varies with the homogenization conditions. The strength values obtained were within the range of 80 MPa, with the ceramic reaction layer being the region where all failures occurred.     

Microstructure and mechanical properties of diffusion bonded joints between titanium and stainless steel with copper interlayer

Abstract

The solid state joining of titanium to stainless steel with copper interlayer was carried out in the temperature range of 850–950°C for 7·2 ks in vacuum. The interface microstructures and reaction products of the transition joints were investigated with an optical microscope and a scanning electron microscope. The elemental concentration of reaction products at the diffusion interfaces was evaluated by electron probe microanalysis. The occurrence of difference in intermetallics at both interfaces (SS/Cu and Cu/Ti) such as CuTi₂, CuTi, Cu₄Ti₃, χ, FeTi, Fe₂Ti, Cr₂Ti, α-Fe, α-Ti, β-Ti, T₂(Ti₄₀Cu_60?xFe_x; 5<x<17), T₄(Ti₃₇Cu_63?xFe_x; 5<x<7) and T₅(Ti₄₅Cu_55?xFe_x; 4<x<5) has been predicted from the ternary phase diagrams of Fe–Cu–Ti and Fe–Cr–Ti. These reaction products were detected by X-ray diffraction technique. The maximum tensile strength of ～91% of Ti strength and shear strength of ～74% of Ti strength along with ～ 7·2% ductility were obtained for the joint bonded at 900°C due to better coalescence of mating surfaces. At a lower joining temperature of 850° C, bond strength is poor due to incomplete coalescence of the mating surfaces. With an increase in the joining temperature to 950°C, a decrease in bond strength occurred due to an increase in the volume fraction of brittle Fe–Ti base intermetallics.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

Wetting and interfacial phenomena in Ni–HfB2 systems

The wettability and reactivity between polycrystalline hot-pressed HfB₂ and liquid Ni, Ni–Ti and Ni–B alloys have been investigated by the sessile drop method up to 1520 °C. Independently of the sintering aids (B₄C, HfSi₂), high-temperature interactions led to the formation of a bimodal interface profile, due to the competition between the strong dissolution of HfB₂ in the liquid phase and drop spreading along the substrate surface. Ni demonstrates good wetting with the HfB₂ substrates (θ = 20°). Compared to Ni, high Ti-containing alloys reveal much faster wettability kinetics with a final contact angle below 10°, accompanied by the formation of an interfacial Ti-rich reaction product. The eutectic NiB alloy shows complete wetting and fast spreading on HfB₂ and minor dissolution of the ceramic into the liquid, a clear indication that higher B contents could optimize the interfacial structure. These results are of practical interest for liquid-assisted composite synthesis and joining of HfB₂-based ceramics.     

The oxidation behavior of Ti–46.5Al–5Nb at 900 °C

The early stages of Ti–46.5Al–5Nb (at%) oxidation at 900 °C have been investigated combined with TEM and STEM. The results reveal that a layer composed of polycrystalline TiO₂ and amorphous Al₂O₃ phase formed firstly after 5-min oxidation. The base alloy connected with the oxide scale has some deformation compared with the inner full lamellar TiAl structure. After 30-min oxidation, the phases of γ-Al₂O₃, κ-Al₂O₃, titanium nitrides and Ti₅Al₃O₂ were formed in the area from the nitride layer to base alloy. After 50 h of oxidation, Ti₅Al₃O₂ vanishes at the interface of oxide scale/base alloy in Ti–46.5Al–5Nb, contrary to the continuous formation of Ti₅Al₃O₂ in γ-TiAl at the interface of oxide scale/base alloy, Al₃Nb phase formed in this zone, which hinders the continuous formation of Ti₅Al₃O₂.     

Formation, properties and microstructure of amorphous/crystalline composite Ag20Cu30Ti50 alloy using miscibility gap

The aim of the work was to produce the amorphous/crystalline composite with uniform distribution of fine crystalline soft phase. Silver–copper–titanium Ag₂₀Cu₃₀Ti₅₀ alloy was prepared using 99.95 wt% Ag, 99.95 wt% Cu, 99.95 wt% Ti that were arc-melted in argon atmosphere. Then the alloy was melt spun on a copper wheel with linear velocity of 33 m/s. Investigation of the microstructure for both arc-melt massive sample and melt-spun ribbons was performed with use of scanning electron microscope (SEM) with EDS, light microscope (LM) and X-ray diffraction. The thermal stability was evaluated by differential scanning calorimetry (DSC). The properties such as Young modulus and Vickers hardness number before and after crystallization of the amorphous matrix were measured with use of nanoindenter. The microstructure was investigated by transmission electron microscope (TEM). It was found, that the alloy has a tendency for separation within the liquid state due to the miscibility gap which resulted in segregation into Ti–Cu–Ag matrix and Ag-base spherical particles after arc-melting. During rapid cooling through the melt spinning the Ag₂₀Cu₃₀Ti₅₀ alloy formed an amorphous/crystalline composite of fcc silver-rich spherical particles within the amorphous Ti–Cu–Ag matrix.     

Tribological properties of titanium aluminides coatings produced on pure Ti by laser surface alloying

Titanium aluminides coatings were in-situ synthesized on a pure Ti substrate with a preplaced Al powder layer by laser surface alloying. The friction and wear properties of the titanium aluminides coatings at different normal loads and sliding speeds were investigated. It was found that the hardness of the titanium aluminides coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating. Friction and wear tests revealed that, at a given sliding speed of 0.10 m/s, the wear volume of pure Ti and the titanium aluminum coatings all increased with increasing normal load. At a given normal load of 2 N, for pure Ti, its wear volume increased with increasing sliding speed; for the titanium aluminides coatings, the wear volume of Ti₃Al coating and TiAl coating first increased and then decreased, while the wear volume of TiAl₃ coating first decreased and then increased with increasing sliding speed. In addition, the friction coefficients of pure Ti and the titanium aluminides coating decreased drastically with increasing sliding speed. Under the same dry sliding test conditions, the wear resistance of the titanium aluminium coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating.     

Effects of alloying on the microstructures and mechanical property of Nb-Mo-Si based in situ composites

Nb-Mo-Si based alloys have been prepared by arc melting in a water-cooled copper crucible under an argon atmosphere. The effects of Al, Cr, Hf and Ti additions on the phase components and stability, microstructures and mechanical property of Nb-Mo-Si based alloys have been studied. The results indicated that the additions of Al, Cr and Ti elements do not change the phase components of Nb-20Si-6Mo alloys, which are composed of Nb solid solution (Nb_ss) and β-Nb₅Si₃. The phase component is α-Nb₅Si₃ and Nb_ss in Nb-20Si-6Mo-3Hf alloy. The additions of Cr and Ti element make the Nb_ss/Nb₅Si₃ eutectic morphology anomalous and coarsening. The element segregation is obvious found with the additions of Hf and Ti. The enrichments of Hf and Ti change the compositions of retained melt and promote the formation of fine eutectic structure. After heating treatment at 1873K for 10 h, β-Nb₅Si₃ phase translates into α-Nb₅Si₃ phase and γ-Nb₅Si₃ phase. The eutectic structures tend to anomalous and coarsening. The Ti solid solution (Ti_ss) phase was found in Nb-20Si-6Mo-20Ti alloy and the formation mechanism of Ti_ss phase was discussed. The high temperature (1523 K) compression strength of as cast Nb-Mo-Si based alloys increased with the additions of Al, Cr, Hf, and decreased with Ti addition.