钛/铝/钛层状复合材料的爆炸复合制备及后续热压处理研究

本文以纯钛板与纯铝板为原料,通过爆炸复合法制备钛/铝/钛层状复合材料,之后采用热处理以及热压工艺对钛/铝/钛层状复合材料进行进一步处理。研究结果表明：复合板界面主要由波状界面和平直状界面构成,铝元素与钛元素在界面上发生了互扩散,界面结合性能优良,可以承受后续较大的二次塑性变形;热处理后的复合板界面发生明显扩散,在热处理25 h后热压2.5 h后铝层完全反应,扩散反应层主要由TiAl₃相以及Ti₂Al₅相构成。     

TEM and DTA study on the stability of Al5Ti3- and h-Al2Ti-superstructures in aluminium-rich TiAl alloys

The phases Al₅Ti₃ and h-Al₂Ti, which are superstructures of the L1₀ TiAl structure, are frequently observed in as-cast and low-temperature-annealed aluminium-rich TiAl alloys. The strong decrease of the solubility of aluminium in TiAl with decreasing temperature leads to a supersaturation of the solid solution with aluminium during cooling. The decomposition of the supersaturated TiAl results in the precipitation of the superstructure phases at low temperatures. The evolution of the Al₅Ti₃ and h-Al₂Ti phases and the resulting microstructures were studied as a function of time, temperature, and composition by TEM and DTA investigations on Ti–Al alloys with 55 to 64 at.% Al. Both superstructures were found not to be equilibrium phases. Al₅Ti₃ is metastable below a composition-dependent critical temperature in the range of about 750–900°C with a maximum value reached near the stoichiometric composition. Above this temperature, Al₅Ti₃ rapidly dissolves. Extended lamellar Al₅Ti₃+TiAl microstructures have been found in a Ti–60 at.% Al alloy after low-temperature annealing, whereas in Ti–62 at.% Al large single-phase domains of Al₅Ti₃ have grown. h-Al₂Ti is a metastable phase at least up to 1200°C. It slowly transforms into the equilibrium phase r-Al₂Ti during annealing.     

Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium

Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.     

冷轧热处理钛-铝箔制备微叠层Ti-Al系金属间化合物板材

通过钛箔、铝箔叠加烧结制备出微叠层Ti-Al系金属间化合物合金板材。对不同烧结条件下获得的板材组织和相组成进行分析。结果表明,当烧结温度到达到Al的熔点以上时,高温自蔓延反应（SHS）在Ti箔和Al箔之间发生,生成α-Ti、Ti3Al、TiAl、TiAl2和TiAl3等相;随着烧结时间的延长,α-Ti、TiAl2和TiAl3逐渐消失,最终获得包含Ti3Al和TiAl的叠层结构板材。由于铝的熔化,柯肯达尔效应和反应前、后摩尔体积的变化在烧结过程中产生大量的孔洞,随后的热压处理将孔洞消除并获得致密的合金板材。     

Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.     

铝/钛异种金属旁路分流MIG电弧熔钎焊特性

以1 mm厚6061铝合金与TC4钛合金板为试验材料,进行旁路分流MIG电弧熔钎焊工艺试验,得到均匀美观的焊缝成形.分别采用金相显微镜、扫描电镜、万能拉伸试验机进行研究.结果表明,界面层上方金属间化合物以柱状晶形式存在,呈现短而密集的状态.界面层靠近钛侧易形成AlTi,而界面层靠近铝侧易形成Al₃Ti,TiSi₂等金属间化合物,由于焊接过程中熔池内部温度的变化不均,会使界面层中出现Al₁₁Ti₅过渡相.拉伸试验表明,接头最高抗剪强度达182.6 MPa,约为铝母材的97.6%,断裂发生在铝母材热影响区,断口出现一定量的颈缩.     

Catalytically active cobalt-copper-oxide layers on aluminum and titanium

Oxide coatings modified with cobalt and copper oxides are obtained on titanium and aluminum by means of combining plasma electrolytic oxidation (PEO) in silicate and zirconate electrolytes and impregnation in nitrate solutions followed by annealing. The effect of PEO coatings that were preliminarily formed on aluminum and titanium in different electrolytes on the composition and surface morphology of cobaltcopper oxide composites and their activity with respect to CO oxidation is studied. The maximum total concentration of cobalt and copper is found to be typical of composite layers based on SiO₂ + Al₂O₃/Al, while the minimum content is observed in the case of layers based on ZrO₂ + TiO₂/Ti, the PEO bases being characterized by the highest and lowest water-absorbing capacity, respectively. The effect of PEO coatings on the catalytic activity of cobalt-copper oxide catalysts decreases in the series SiO₂ + TiO₂/Ti > SiO₂ + Al₂O₃/Al > Ce₂O₃ + ZrO₂ + TiO₂/Ti > ZrO₂ + TiO₂/Ti.     

Ti/Al异种合金电弧熔钎焊接头界面断裂行为分析

采用TIG电弧的方法实现了钛合金与铝合金熔钎焊连接,分析了不同焊丝形成的熔钎焊接头的界面组织和断裂特征.结果表明,纯铝接头界面为单一的TiAl3相,裂纹主要沿着TiAl3反应层与焊缝之间的界面扩展.拉伸时首先从坡口拐角启裂,当裂纹扩展至接头反面时,断裂扩展形式转变为从焊缝金属撕裂,接头抗拉强度为139MPa.添加Al-Cu-La焊丝的接头界面结构为TiAl3+ Ti2Al20La双化合物层,拉伸时沿TiAl3反应层与钛合金界面开裂,以界面内的微裂纹为裂纹源并向反应层内扩展,属于准解理断裂,接头抗拉强度达270 MPa.稀土La元素作用下形成的双化合物层是提高接头强度的关键.     

Ordering process of Al5Ti3, h-Al2Ti and r-Al2Ti with f.c.c.-based long-period superstructures in rapidly solidified Al-rich TiAl alloys

Change in microstructure and stability of superstructural phases in Al-rich TiAl alloys containing 58.0–62.5 at.% Al were investigated using melt-spun ribbons. Ordering processes of long-period ordered phases such as Al₅Ti₃, h-Al₂Ti and r-Al₂Ti in the L1₀ matrix during annealing were examined. The presence of Al₅Ti₃ and h-Al₂Ti phases in the L1₀ matrix was confirmed in melt-spun Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons by electron diffraction patterns, while diffuse scattering corresponding to the Al₅Ti₃ superstructure appeared in Ti–58.0 at.% Al ribbon. In Ti–58.0 at.% Al ribbon, the Al₅Ti₃ phase developed as an island in the L1₀ matrix having an obscure coherent boundary at and below 800°C, while it dissolved during annealing above 800°C. Although the r-Al₂Ti phase was finally formed as an equilibrium phase, the ordering of Al₅Ti₃ and metastable h-Al₂Ti phases in Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons occurred prior to the precipitation of the r-Al₂Ti during annealing below 800°C. The priority for the ordering process is discussed on the basis of crystal symmetry and periodicity of Al layers parallel to the (002) plane. The anti-phase boundaries (APBs) based on the Al₅Ti₃-type ordering were observed along {110) planes in Ti–62.5 at.% Al ribbon annealed at 700°C and their energies were calculated using the interaction energy between neighbouring atoms.     

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.     

The improvement of high temperature oxidation of Ti–50Al by sputtering Al film and subsequent interdiffusion treatment

The high temperature oxidation resistance of Ti–50Al can be improved by sputtering an Al film and subsequent interdiffusion treatment at 600 °C for 24 h in high vacuum. In these conditions, a TiAl₃ layer is formed on the surface, which exhibits good adhesion with Ti–50Al substrate and provides high oxidation resistance. Cyclic and isothermal oxidation tests show that the Ti–50Al with 3–5 μm Al film can dramatically reduce the oxidation at 900 °C in air, at which the parabolic oxidation rate constant K_p of specimen with 5 μm Al film is only about 1/15,000 of that of bare Ti–50Al. XRD and SEM results indicate that the TiAl₃ layer can promote the formation of a protective Al₂O₃ scale on the surface as well as react with γ-TiAl to form TiAl₂ during the oxidation. Simultaneously, layers of Al₂O₃/TiAl₂/Al-enriched γ-TiAl/Ti–50Al are also formed on specimens. The TiAl₂ layer thickness will decrease gradually with increasing the oxidation time. After oxidation at 900 °C for 300 h, there is a clearly discontinuous thin layer of Ti₃₇Al₅₃O₁₀ compound observed in between Al₂O₃ and TiAl₂.     

Fabrication of TiAl intermetallic phases by heat treatment of warm sprayed metal precursors

Warm Spraying is an atmospheric coating process based on high-velocity impact bonding of powder particles. By decreasing the temperature of combustion gas via mixing with nitrogen the oxidation of feedstock powder can be effectively controlled. This is particularly important for Ti-based coating materials, which rapidly oxidize at elevated temperatures.In this study, Ti–Al composite coatings were fabricated by the Warm Spray process using a mixture of titanium and aluminum powders as a feedstock and applying a two-stage heat treatment at 600 and 1000 °C to obtain intermetallic phases. The microstructure, chemical and phase composition of the deposited and heat-treated coatings were investigated using SEM, EDS and XRD. The experimental results show that TiAl₃ was the first intermetallic phase formed during the first-stage heat treatment. The growth of TiAl₃ layer occurred mainly by diffusion of Al into Ti particles. Significant porosity that developed during the heat treatment was caused mainly by Kirkendall effect. After the second-stage heat treatment, a coating layer with TiAl as the dominant phase was obtained with about 20 vol % porosity.     

Preparation and oxidation resistance of a crack-free Al diffusion coating on Ti22Al26Nb

A crack-free Al diffusion coating has been developed to improve the oxidation resistance of Ti22Al26Nb. It was produced by a two-step method; an Al film was deposited on the substrate alloy by arc ion plating followed by a diffusion process conducted at 873 K in pure Ar to form the Al diffusion coating. The two-step method lowers the temperature required to form the diffusion coating, which dramatically decreases the thermal stress developed in the coating and results in it being crack-free. The oxidation resistance of the non-coated Ti22Al26Nb alloy in isothermal and cyclic tests in air at 1073 K was poor, but the coated specimens possessed excellent oxidation resistance because a protective α-Al₂O₃ scale formed. The life of the Al diffusion coating greatly depends upon the rapid initial formation of a protective Al₂O₃ scale and interdiffusion between coating and substrate. Once the stable Al₂O₃ scale has formed and the composition changes from (Ti, Nb)Al₃ into (Ti, Nb)Al₂, the coating has a long life.     

Formation of Brittle Phases During Pulsed Current Gas Tungsten Arc Welding of Titanium to Aluminum Alloys

Welding of titanium alloy TA15 to aluminum alloy Al 2024 was conducted by pulsed current gas tungsten arc welding using AlSi12 filler metal. Formation process of phases near the Ti/Al interface was discussed. Titanium and aluminum were partially fusion welded in the upper part while brazed together in the middle and bottom parts of the joint. In the upper part of the joint, intermetallics Ti₃Al + Ti₅Si₃, TiAl + Ti₅Si_3, and TiAl₃ were formed as three layers orderly from the titanium side to the weld metal. In the middle and bottom parts of the joint, intermetallics Ti₅Si₃ and TiAl₃ were formed as two layers near the Ti/Al interface.     

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.     

Nucleation and growth of titanium aluminide in an explosion-welded laminate composite

Processes of nucleation and growth of titanium aluminide in a 23-layer aluminum-titanium composite produced by explosion welding have been studied. In the vortex zones of seven upper welds, microvolumes of melted metal whose microhardness is ??5500 MPa have been revealed, which corresponds to the microhardness of the intermetallic compound Al₃Ti. No formation of titanium aluminide in welded junctions that were not subjected to additional heat treatment has been revealed by X-ray diffraction. The holding of the composites at 630°C is accompanied by the formation of interlayers of intermetallic compounds of the Al₃Ti type. Intermetallic compounds of two morphological types are formed in the welds. In the regions of vortex zones, compact precipitates of Al₃Ti are formed; in the other regions of the welds, intermetallic compounds in the form of a film are precipitated. The intermetallic compounds of the first type grow more rapidly and in final account absorb the precipitates of the film type. The activation of diffusion in the upper junctions that occurs upon heating of the welded composites is favored by the nonequilibrium state of the material caused by the strain hardening of the initial samples. In the welds located deeper than the 13th layer, no signs of the formation of compact intermetallic compounds have been revealed upon the annealing for 5 h and less.     

A structural and compositional analysis of a TiO<Subscript>x</Subscript> diffusion barrier for indium tin oxide/Si contacts

We investigated the structural and compositional changes of titanium oxides as a diffusion barrier between indium tin oxide (ITO) and Si under two different Ti oxidation conditions: (1) annealing of the Ti layer deposited on Si in air followed by ITO deposition (Type 1) and (2) annealing in nitrogen after the deposition of ITO/Ti on Si (Type II). The diffusion barrier layer in both samples namely the Ti layer oxidized under different conditions, consisted of two regions: a region composed of a mixture of silicide and titanium oxide near the Si substrate and a titanium oxide region near the ITO layer. However, the titanium oxide in the Type 1 samples was composed of TiO₂ and Ti₂O₃ phases, whereas Ti₂O₃ was dominant in the Type II samples. In addition, the Type I and II samples showed the formation of voids in the middle of the barrier layer and in the region near the ITO layer, respectively. Therefore, the electrical and optical properties of ITO/TiO_x/Si are dependent on the structural and compositional changes of the diffusion barrier layer.     

Synthesis and high-temperature stability of titanium aluminide matrixin situ composites

A premixture of elemental powders of titanium and aluminum was supplied as a spray material for the direct fabrication of titanium aluminide matrixin situ composites by means of reactive low-pressure plasma spraying with a nitrogen and hydrogen mixed plasma gas. The aluminum content varied from 10 to 63 wt.% in the premixtures. The matrix of sprayed layers consisted of three kinds of titanium aluminides—Ti₃A1, TiAl, and TiAl₃—which begin to form on a low-carbon steel substrate immediately after deposition. The formation of nitrides, which act as a reinforcement, occurs both during the flight of liquid droplets and on the substrate. The nitrogen content is approximately 4 to 5 wt.% in the sprayed intermetallic matrix composites, regardless of the aluminum content of the premixtures. The kinds of titanium aluminides andin situ nitrides developed depend on the aluminum content of the premixtures. The homogeneity of the distribution of aluminum and titanium in sprayed intermetallic matrix composites has been improved by vacuum annealing. The predominant TiAl phase that formed in the sprayed intermetallic matrix composites with a Ti-36 wt.% AI premixture increases in quantity through annealing. Although some minor nitrides disappear through annealing, the principal reinforcement, Ti₂AlN, does not decompose, but increases in quantity. The hardness of sprayed intermetallic matrix composites varies with aluminum content of the premixtures, but is always greater than that of sprayed titanium aluminides containing no nitrides. Annealing does not reduce the hardness of sprayed intermetallic matrix composites. Sprayed and annealed intermetallic matrix composites with a Ti-36 wt.% Al premixture maintain their hardness of approximately 500 HV up to 800 K. Hence, reactive low-pressure plasma spraying offers a promising fabrication method for titanium aluminide matrixin situ composites, which are expected to excel in wear resistance applications at elevated temperatures.     

Characterisation of anodic oxide films formed on titanium and two ternary titanium alloys in hydrochloric acid solutions

The purpose of this paper is to characterise the anodic oxide films formed on titanium and two ternary titanium alloys, Ti‐15Mo‐5Al and Ti‐10Mo‐10Al in 1%, 10% and 20% hydrochloric acid solutions at 25, 50, and 75°C. The anodic film on titanium in hydrochloric acid is stable between + 0.6 V (SCE) and + 2.0 V (SCE). For our new ternary titanium alloys, the passive film is formed at about + 0.6 V (SCE) and is stable to + 2.0 V (SCE). The anodic polarization curves for alloys differ from the base metal curve, presenting two peaks for the critical passivation current density in the active‐passive potential range. At the first current peak (the first critical passivation potential E_cr1) a porous titanium pentaoxide (Ti₃O₅) is formed. When the potential reaches the second current peak (the second critical passivation potential E_cr2) the compact and protective titanium dioxide (TiO₂) is formed. The impedance spectra exhibit the typical behaviour for a passive film i.e. a near capacitive response illustrated by a phase angle close to − 90 ° over a wide frequency range. The oxide film on titanium and its alloys in hydrochloric acid solutions exhibits a high resistance and a low capacitance (with the increase of the potential) attributed to the surface roughness decrease as the oxide layer thickens.