High Temperature Corrosion and Protection of Titanium Alloys and TiAl Intermetallics

ＨｉｇｈＴｅｍｐｅｒａｔｕｒｅＣｏｒｒｏｓｉｏｎａｎｄＰｒｏｔｅｃｔｉｏｎｏｆＴｉｔａｎｉｕｍＡｌｌｏｙｓａｎｄＴｉＡｌＩｎｔｅｒｍｅｔａｌｌｉｃｓＴａｎｇＺｈａｏｌｉｎ；ＷａｎｇＦｕｈｕｉａｎｄＷｕＷｅｉｔａｏ（唐兆麟）（王福会）（吴维）（Ｓｔａｔ...     

Microstructure and interface reaction of investment casting TiAl alloys

In order to research the microstructure of TiAI alloy and TiAl-mould reaction between TiAI and ceramic mould shells prepared with the low cost binder in investment casting, the ceramic mould shells were prepared with low cost binder and refractory materials. Using two kinds of casting methods （gravity casting and centrifugal casting）, the titanium aluminum alloys with rare earth element （Ti-47.5Al-2Cr-2Nb-0.3Y and Ti-45Al-5Nb-0.3Y） were cast into the mould shells. The microstructures of investment casting titanium aluminum alloys were observed by optical microscope （OM）. The distributions of elements of topping investment on the surfaces of titanium aluminum alloys castings were analyzed by the means of electron probe micro-analysis （EPMA）, and the mechanical properties were studied. The results show that the microstructures of two kinds of titanium aluminum alloys are both lamella shape, and lamella is thin. The thickness of reaction and diffusing layer of Ti-47.5Al-2Cr-2Nb-0.3Y alloy is about 80 μm, and that of Ti-45Al-5Nb-0.3Y is less than 30 μm.     

Production and characterization of TiAl ribbons with Y additions

Rapid solidification method was employed to produce TiAl based alloy ribbons and finally to produce TiAl based alloys with novel microstructure and properties. TiAl based alloys with nominal compositions of Ti-46Al-2Cr-2Nb （atomic ratio） and various yttrium additions （0, 0.5%, 1.0%, 1.5%, 2.0%, atom fraction） were produced using the melt spinning method. For comparison, cast alloys with the same compositions were prepared by argon arc melting as well. It is shown that the as cast microstructure of Y free alloy is lamellar. With addition of Y up to 1.5%, the microstructure of the alloy changes from lamellae to dendrites, while more addition of Y leads to a finer dendrite structure. For the rapidly solidified TiAl based alloy ribbon, refined microstructure is produced. The increase of Y addition modifies the phase constituents from pure y-TiAl phase of Y flee alloy gradually to γ-TiAl＋α2-Ti3Al and γ-TiAl＋α2-Ti3Al＋small amount of Al2Y phase. Though the phase constituents on the two sides of ribbons, surface contacting roll and free surface, are essentially the same, the microstructure of the former one is much finer than that of the latter one.     

Filling and solidification of TiAl melt in centrifugal field

A model was established based on the combination of the equation of continuity, the equation of conservation of momentum and the equation of general energy to describe the filling and solidification of TiAl melt by permanent mold centrifugal casting. The model was solved numerically and the filling and solidification processes in the centrifugal field were discussed. The results indicate that the centrifugal field essentially influences the filling and solidification processes of TiAl melt. The melt will first fill the cavity along the back boundary until it reaches the end. After the end is fully filled, the whole cavity will be filled gradually by the way that free surface of the melt moves towards the entrance, hence the entrance is the last part to be filled. Furthermore, the mechanism by which internal defects can be formed in centrifugally cast TiAl components were interpreted.     

Wear and isothermal oxidation kinetics of nitrided TiAl based alloys

Gas nitridation of TiAl based alloys in an ammonia atmosphere was carried out.The evaluation of the surface wear resistance was performed to compare with those of the non-nitridd alloys.It is concluded that high temperature nitridation raised wear resistance of TiAl based alloys markedly.The tribological behaviors of the nitrided alloys were also discussed.The oxidation kinetics of the nitrided TiAl based alloys were investigated at 800-1000℃ in hot air.It is concluded that nitridation is detrimental to the oxidation resistance of TiAl based alloys under the present conditions.The nitrided alloys exhibit increased oxidizing rate with the prolongation of nitridation time at 800℃.However,alloys nitrided at 940℃ for 50h display a sign of better oxidation resistance than the other nitrided alloys at more severe oxidizing conditions.The parabolic rate law is considered as the basis of the data processing and interpretation of the mass gain vs time data.As a comparison with it ,attempts were made to fit the data with the power law.The oxidation kinetic parameter kn,kp and nwere measured and the trends were discussed.     

Improving hot deformability of TiAl alloys by minor additions of Ni and Mg

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｉＡｌａｌｌｏｙｓａｒｅｐｏｔｅｎｔｉａｌａｅｒｏｓｐａｃｅｅｎｇｉｎｅｍａｔｅｒｉ ａｌｓｂｅｃａｕｓｅｏｆｔｈｅｉｒｌｏｗｄｅｎｓｉｔｙａｎｄｈｉｇｈｐｅｒｆｏｒｍａｎｃｅａｔｅｌｅｖａｔｅｄｔｅｍｐｅｒａｔｕｒｅ[1] .Ｓｅ     

Microstructure characterization and tensile properties of β phase containing TiAl pancake

The microstructure and tensile properties of Ti-44Al-6V-3Nb-0.3Y (at.%) alloy after canned forging were investigated. SEM results showed that the TiAl pancake exhibits inhomogeneous microstructure, which can be ascribed to the temperature drop and friction between billet and outer pack during forging, as well as the intrinsic anisotropy of lamellar colony. By means of TEM observation and EBSD analysis, the microstructure in the dominant area of the pancake was further characterized. This deformation area consists of 87.7% content of γ grains plus some refined lamellar colonies and the rest of B2 grains. The grain size ranges between 1 μm and 8.5 μm. High-angle boundaries dominate the deformation microstructure, several substructures and twins are observed as well. Additionally, current forged alloy exhibits excellent high temperature tensile strength and noteworthy yield stress anomaly (YSA), with ultimate tensile strength 680 MPa and yield strength 620 MPa at room temperature, increasing to 850 MPa and 750 MPa at 700 °C, respectively. The anomalous strengthening of current TiAl alloy is temperature dependent and can be interpreted by the dislocation cross-slip pinning mechanism.     

Effect of trace tungsten on the microstructure of TiAl alloys containing Nb and B

New TiAl alloys, containing 45 at.% A1, 7 at.% Nb, x at.% W, and 0.15 at.% B （x = 0, 0.2, 0.4, and 0.7） were prepared by arc melting and drop casting consequently. Using optical microscopy, scanning electron microscopy （SEM）, and electron superprobe technologies, the effects of tungsten on the microstructural evolution of the TiA1 alloys, including the colony size and lamellar spacing, were analyzed. It was found that cellular structures and dendrites were formed in the as-cast TiA1 alloys, and heavy metals, such as niobium and tungsten, tend to segregate strongly at the interface of the cellular structures and dendrites. Trace tungsten can effectively impede the grain growth and narrow the interlamellar spacing. 0.4 at.% tungsten is more effective in refuting the microstructure of the TiAI alloys.     

Interface microstructure and formation mechanism of diffusion-bonded joints of TiAl to steel 40Cr

1　ＩＮＴＲＯＤＵＣＴＩＯＮＩｎｒｅｃｅｎｔｙｅａｒｓ,ｃｏｎｓｉｄｅｒａｂｌｅｉｎｔｅｒｅｓｔｈａｓｂｅｅｎｇｉｖ ｅｎｔｏＴｉＡｌｉｎｔｅｒｍｅｔａｌｌｉｃｓｂｅｃａｕｓｅｏｆｉｔｓｕｎｉｑｕｅｐｒｏｐｅｒｔｉｅｓｓｕｃｈａｓｌｏｗｄｅｎｓｉｔｙ ,ｇｏｏ     

Microstrucmre and mechanical properties of Al2O3/TiAl composite

Al2O3/TiAl composites were fabricated by PAXD （pressure-assisted exothermic dispersion） method. The effects of Nb205 content on the microstructure and mechanical properties of the composites were investigated. The results show that the ultimate phases of the composite consist of TiAl, Ti3Al, Al2O3 and a small amount of NbA13. SEM reveals that a submicron γ＋（α2/γ） dual phases structure can be presented after sintered at 1 200 ℃, Furthermore, with the increase of Nb205 content, the ratio of TiAl to Ti3Al phase decreases correspondingly, the grains of the corflposites are remarkably refined, and the produced Al2O3 particles are uniformly dispersed. When 6% Nb205 is added, the composite has the best comprehensive properties. It exhibits a Vickers hardness of 4.77 GPa and a bending strength of 642 MPa. Grain-refinement and dispersion-strengthening are the main strengthening mechanisms.     

Investigations of microstructures and properties in electron beam welded joints of TiAl to TC4

The normally centered electron beam and non-centered electron beam welding of TiAl to TC4 was investigated in order to analyze the electron beam weldability between TiAl/TC4 dissimilar materials. Macroscopic cold crack easily occurred near TiAl substrate in the joints. The optimal tensile strength was related to the welding heat input. The weld structures were composed of bulky columnar grains and equiaxed grains. The isolated phases consisted of large quantities of α2 -Ti3Al phase, small quantity of B2 phase, γ-TiAl phase and YAl2 phase. Insufficient melting of the base metal occurred in the weld when the beam position leaned to the TC4 side. The tensile strength could be improved when the deflection was limited in the optimum range. Otherwise, non-fusion zone was easily generated in the weld, which led to the low tensile strength.     

Effect of Heat Treatment on Microstructure and Properties of as-Forged TiAl Alloy with β Phase

采用包套锻造技术成功制备Ti-45Al-5.4V-3.6Nb-0.3Y合金锻饼,并研究热处理对该合金锻饼组织和性能的影响。通过热处理得到3种不同的组织形态,分别为双态、近层片、和全层片组织,分析热处理过程中合金组织的演变规律,并对不同组织形态的合金进行力学性能测试。结果发现,双态组织的合金具有最好的室温塑性,其值可达1.35%,近层片组织的合金具有较高的室温强度,屈服强度为605.31MPa,断裂强度为665.75MPa。     

Improvement of microstructure and mechanical properties of TiAl−Nb alloy by adding Fe element

In order to improve mechanical properties and optimize composition of TiAl−Nb alloys, Ti46Al5Nb0.1B alloys with different contents of Fe (0, 0.3, 0.5, 0.7, 0.9, and 1.1 at.%) were prepared by melting. Macro/microstructure and compression properties of the alloys were systematically investigated. Results show that Fe element can decrease the grain size, aggravate the Al-segregation and also form the Fe-rich B2 phase in the interdendritic area. Compressive testing results indicate that the Ti46Al5Nb0.1B0.3Fe alloy shows the highest ultimate compressive strength and fracture strain, which are 1869.5 MPa and 33.53%, respectively. The improved ultimate compression strength is ascribed to the grain refinement and solid solution strengthening of Fe, and the improved fracture strain is due to the reduced lattice tetragonality of γ phase and grain refinement of the alloys. However, excessive Fe addition decreases compressive strength and fracture strain, which is caused by the severe Al-segregation.     

Recent advances of wrought TiAl alloys

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｗｏ ｐｈａｓｅ ｇａｍｍａＴｉＡｌａｌｌｏｙｓｈａｖｅｇｏｏｄｓｔｒｅｎｇｔｈｒｅｔｅｎｔｉｏｎａｂｉｌｉｔｙａｔｈｉｇｈｔｅｍｐｅｒａｔｕｒｅｓｗｈｉｃｈ ,ｉｎｃｏｍｂｉｎａｔｉｏｎｗｉｔｈｌｏｗｄｅｎｓｉｔｙ ,ｍａｋｅｓｔ     

Evaluation of TiAl and 40Cr diffusion bonding quality using ultrasonic reflection spectrum

Combined with ultrasonic pulse-echo technique, reflection spectrum analysis was introduced to evaluate TiAl and 40Cr diffusion bonding quality. Frequency dependence of reflection coefficient was used to distinguish perfect bonding from imperfect bonding. It is found that the reflection coefficient from perfect bonding interface does not vary with frequency. When the size of imperfections is much smaller than the wavelength of ultrasound, the reflection coefficient depends on frequency. When the size of imperfections is the same order of or even larger than the wavelength of ultrasound, the reflection coefficient does not exhibit frequency dependence. However the amplitude of imperfect interface is higher than the amplitude of perfect bonding interface. A combination of reflection spectrum analysis and ultrasonic pulse-echo technique provides more accurate information about the bonding quality of dissimilar materials.     

Mcrostructure of TiAl alloy prepared by intense plastic deformation and subsequent reaction singering

TiAl alloy was prepared by intense plastic deformation and subsequent reaction sintering.The effect of plasetic deformation on the microstructure of sintered TiAl alloy was investigated using energy dispersive X-ray spectroscopy(EDS),optical microscopy and transmission electron microscopy(TEM).The results show that the intense plastic deformation of reacting Ti and Al phases caued by high energy ball milling refines the as-sintered microstructure.The longer the milling time,the finer the grain size of γ and lamellar(α2 γ）phases.The finer grain size improves the properties of the TiAl alloy.It is also found that th volume fraction of lamellar(α2 γ)phases increases first,then decreases with increasing milling time.Based on the experimental results theoretical discussion was presented.     

Microstructure of TiAl alloy prepared by intense plastic deformation and subsequent reaction sintering

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｉＡｌａｌｌｏｙｓａｒｅｓｕｉｔａｂｌｅｆｏｒｈｉｇｈ ｔｅｍｐｅｒａｔｕｒｅａｐ ｐｌｉｃａｔｉｏｎｓｉｎａｉｒｃｒａｆｔｇａｓｔｕｒｂｉｎｅｅｎｇｉｎｅｓａｎｄｏｔｈｅｒｔｒａｎｓｐｏｒｔａｔｉｏｎｖｅｈｉｃｌｅ ,ｏｗｉｎｇ     

The effect of different crystal conditions of filler metal on vacuum brazing of TiAl alloy and 42CrMo

Ti-based filler metals made by transient solidification and normal crystallization were selected for the vacuum brazing of the TiAl alloy and 42CrMo under different processing parameters. The results show that the tensile strength of the joint of transient solidified filler metal is higher than that of normal crystallized filler metal under the same processing parameters. By the analysis of scanning electron microscope（SEM） and X-ray diffracting （XRD） , it is found that the higher strength maybe caused by the generating of TiAl , TiNi and TiCu at the interface of joint made by transient solidified filler metal.     

Research on investment casting of TiAl alloy agitator treated by HIP and HT

Using TiAl alloy to substitute superalloy is a hot topic in aeroengine industry because of its low density, high elevated temperature strength, and anti-oxidization ability. In this research, Ti-47.5AL-2Cr-2Nb-0.2B alloy was used as the test material. By applying a combination process of ceramic shell mold and core making, vacuum arc melting and centrifugal pouring, and heat isostatic pressing (HIP) and heat treatment (HT) etc., the TiAl vortex agitator casting for aeroengine was successfully made. This paper introduced key techniques in making the TiAl vortex agitator with investment casting process, provided some experimental results including mechanical properties and machinability, and explained some concerns that could affect applications of TiAl castings.