TiAl金属间化合物中（Al,Ag）3Ti,Ti3AlC的析出形态

用透射电镜观察了在不同温度，不同时间时效时Ｌｌ０－ＴｉＡｌ金属间化合物中形成的Ｌｌ２－（Ａｌ，Ａｇ０３Ｔｉ，Ｔｉ３ＡｌＣ析出相的形态以及与基体之间的位向关系，讨论了析出相的形态与ＴｉＡｌ基体间的错配度的关系。     

Morphology of L10-TiAl(Ag) precipitation in Ag-modified L12–Al3Ti

Transmission electron microscopy (TEM) examinations of Ag-modified Al₃Ti with an L1₂-ordered structure have revealed the precipitation of L1₀–TiAl upon aging after quenching from higher temperatures. TEM observations revealed that plate-like L1₀–TiAl precipitates lie on {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. A quantitative X-ray microanalysis for determining the chemical compositions of precipitate and matrix has been done by using an analytical electron microscope. The L1₂ phase field in the Ti–Al–Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.     

Microstructure of carbide precipitates in L12−Ni3Al and L10−TiAl

The crystallographic structures of carbide formed in Ni₃Al- and TiAl-based intermetallics containing carbon are investigated in this study using transmission electron microscopy. In an L1₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2 mol.% to 3.0 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide were formed in the matrix by aging at temperatures around 973 K after solution annealing at 1423 K. TEM examination revealed that the M₂₃C₆ phase and the matrix lattices have a cube-cube orientation relationship and maintain partial atomic matching at the {111} interface. After prolonged aging or by aging at higher temperatures, the M₂₃C₆ precipitates adopt a rod-like morphology elongated parallel to the <100> directions. In L1₀-ordered TiAl containing from 0.1 mol.% to 2.0 mol.% carbon, TEM observations reveal that needle-like precipitates, which lie only in one direction parallel to the [001] axis of the L1₀ matrix appear in the matrix mainly at dislocations. Selected-area electron diffraction (SAED) patterns analyses showed that the needle-shaped precipitate is perovskite-type Ti₃AlC. The orientation relationship between the Ti₃AlC and the L1₀ matrix was found to be (001)_Ti3AlC//(001)_{L10 matrix} and [010]_Ti3AlC//[010]_{L10 matrix}. By aging at higher temperatures or for a longer period at 1073 K, plate-like precipitates of Ti₂AlC with a hexagonal structure form on the {111} planes of the L1₀ matrix. The orientation relationship between the Ti₂AlC and the L1₀ matrix is (0001)_Ti2AlC//(111)_{L10 matrix} and Ti2AlC//L10 matrix.     

PRECIPITATION AND MICROSTRUCTURE OF Cr MODIFIED Al3Ti

1. IntroductionThe tetragonal D022 compound A13Ti has recently received attention as a possible structural material because of its low density, 3.4g/cm', and its good oxidation resistanced'2].However, D0zz-AlsTi has the disadvalltages, that it is a line compound and is extremelybrittle at room temperature, presumably due to the lack of slip systems associated withthe crystal symmetry[2]. It is known that partially replacing some of the aluminum atomsin AlsTi with Cr[31, Ni[4], or Ag['] etc…     

Precipitation behavior of Al-Ti-Ag alloy system

Transmission electron microscopy (TEM) examinations of Al₃Ti with an L1₂-ordered structure have revealed the precipitation of D0₂₃-Al₁₁Ti₅ and L1₀-TiAl upon aging after quenching from higher temperatures. TEM observations revealed that fine uniform precipitation of Al₂Ti occurs when the supersaturation is sufficiently high, and, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L1₂-Al₃Ti is supersaturated with DO₂₂-Al₃Ti, DO₂₃-Al₁₁Ti₅ with a multidomain structure is formed during aging. On the other hand, plate-like L1₀-TiAl precipitates lie on the {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. The Ll₂ phase field in the Al-Ti-Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

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.     

Ti3AlC2掺杂SPS法制备Ti2AlC/TiAl复合材料的研究

通过2TiC-Ti-1.2Al体系的原位热压反应制备了Ti₃AlC₂陶瓷,然后以59.2Ti-30.8Al-10Ti₃AlC₂(wt%)为反应体系,采用放电等离子烧结技术制备出Ti₂AlC/TiAl基复合材料。借助XRD、SEM分析了产物的相组成和微观结构,并测量了其室温力学性能。结果表明：原位热压烧结产物由Ti₃AlC₂和TiC相组成,Ti₃AlC₂呈典型的层状结构,TiC颗粒分布在其间。SPS法制备的Ti₂AlC/TiAl基复合材料主要由TiAl、Ti₃Al和Ti₂AlC相组成,Ti₂AlC增强相主要分布于基体晶界处,表现为晶界/晶内强化作用。力学性能测试表明：Ti₂AlC/TiAl基复合材料的密度、维氏硬度、断裂韧性和抗弯强度分别为3.85 g/cm₃、5.37 GPa、7.17 MPa?m_1/2和494.85 MPa。     

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.     

Microstructure evolution and nitrides precipitation in in-situ Ti2AlN/TiAl composites during isothermal aging at 900 °C

Microstructures of Ti₂AlN/TiAl composites prepared by in-situ method were characterized in in-situ and aging treatment conditions and the nitride precipitation was investigated in Ti₂AlN/TiAl composites aged at 900 °C for 24 h after being heat treated at 1400 °C for 0.5 h. The in-situ composites consist of γ+α₂ lamellar colonies, equiaxed γ grains and Ti₂AlN reinforcements. Matrix with nearly fully lamellar structure formed after solution and subsequently aging treatment. With the increase of Ti₂AlN content, the nearly fully lamellar structure becomes instable for the aged composites. According to TEM study, fine Ti₂AlN precipitates are found to distribute at the grain boundaries of lamellar colony. Needle-like Ti₃AlN precipitates arrange in line with growing axis parallel to [001] direction of the γ-TiAl matrix and another needle-like Ti₃AlN precipitates with lager size distribute at the dislocations.     

Interfacial reaction studies of B4C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites

The interfacial reactions of B₄C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites were investigated by scanning electron microscope and transmission electron microscope. The detailed microstructures as well as the chemical composition throughout the reaction zone were identified. For SiC_f/B₄C/TiAl composite, the reaction zone from B₄C coating to TiAl matrix is composed of 4 layers, namely, a carbon-rich layer, a mixed layer of TiB₂ + amorphous carbon, a TiC layer and a mixed layer of TiB + Ti₂AlC. For SiC_f/C/TiAl composite, the reaction zone from C coating to TiAl matrix is composed of 3 layers, namely, a fine-grained TiC layer, a coarse-grained TiC layer and a thick Ti₂AlC layer. For both kinds of composites, the reaction mechanisms of the interfacial reactions were analyzed, and the corresponding reaction kinetics were calculated. The activation energies of interfacial reaction in SiC_f/B₄C/TiAl composite and SiC_f/B₄C/TiAl composite are 308.1 kJ/mol and 230.7 kJ/mol, respectively.     

TiAl3对燃烧合成Ti3AlC的影响

以单质粉末Ti，Al和碳黑为原料，研究了添加金属间化合物TiAl3对燃烧合成Ti3AlC的影响。实验结果表明，仅以单质粉末Ti，Al和碳黑为原料，按Ti3AlC化学计量比配料，燃烧产物主要物相是Ti2AlC和TiC，无Ti3AlC。但在保持原料配比不变的情况下，在反应物原料中添加金属间化合物TiAl3（0～23．5％，质量分数）后，可得到Ti3AlC相物质，其含量随TiAl3的增加而显著增多，成为燃烧产物的主要物相之一。从动力学和热力学角度探讨了TiAl3对燃烧合成Ti3AlC的影响机理。     

Fabrication and mechanical properties of Ti2AlC/TiAl composites with co-continuous network structure

Ti₂AlC/TiAl composites with different volume fractions were prepared by hot pressing technology, and their reinforced structural characteristics and mechanical properties were evaluated. The results showed that when the reinforced phase volume fraction of Ti₂AlC was 20%, three-dimensional interpenetrating network structures were formed in the composites. Above 20%, Ti₂AlC phase in the composites accumulated and grew to form thick skeletal networks. The microplastic deformation behavior of Ti₂AlC phase, such as kink band and delamination, improved the fracture toughness of the composites. Comparative analysis indicated that the uniform and small interconnecting network structures could further reinforce the composites. The bending strengths of composites prepared with 20 vol.% Ti₂AlC reached (900.9±45.0) MPa, which was 25.5% higher than that of TiAl matrix. In general, the co-continuous Ti₂AlC/TiAl composite with excellent mechanical properties can be prepared by powder metallurgy method.     

Formation of TiC/Ti2AlC and α2+γ in in-situ TiAl composites with different solidification paths

In order to improve the mechanical properties of TiAl alloys, TiAl composites with different solidification paths were synthesized by metallurgical method. Results show the TiC disappears and Ti₂AlC increases when the Al content is more than 42% (at.%, similarly hereinafter). Small TiC particles are located in Ti₂AlC grains with irregular shapes when the Al content is 40%, and they translate into clubbed Ti₂AlC with increasing of Al. This metallurgy method can solve the defects of the Al lacking and the residual TiC. The γ phase increases between lamellar colonies with the increasing of Al. When the Al content is 48%, the fully lamellar structure transforms into a duplex microstructure and there are small Ti₂AlC phases in γ phases, because the forming of Ti₂AlC phase must consume Al. The compressive strength increases up to 1678.68 MPa as Al content is 46 at.%, and then decrease to 1460.22 MPa, the compressive strain increases and then keeps stabilization with the increasing Al. The maximum strength improves 38.82% and the maximum strain improves 121.37%. The Ti₂AlC/TiAl composites fracture behaviors are load transferring behavior, crack deflection, trans-lamellar cracking and extraction of carbide reinforcements. The Ti₂AlC phase and the fully lamellar structure improve the mechanical properties.     

The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1–5 at.%) in TiAl alloys with different compositions, and in Ti₃Al with the compositions of Ti–26 at.%Al–(1–2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti–Al–V–Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti₃Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti₃Al. The experimental results are interpreted in terms of a Bragg–Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached.     

Mechanical milling of Ti–Ni–Si filler metal for brazing TiAl intermetallics

The Ti–Ni–Si filler metal was manufactured by mechanical milling of TiH₂, Ni and Si powder mixture. The microstructure of the filler metal and TiAl brazed joint was analyzed by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The effect of milling time on the brazing powder was investigated. It was found that NiSi phase formed when the milling time exceeded 120 min. The typical microstructure of the TiAl brazed joint using Ti–Ni–Si filler metal was TiAl/Ti₃Al/TiAlNi₂/Ti₃Al + Ti₅Si₃/TiAlNi₂/Ti₃Al/TiAl. The effect of Si on the microstructure was investigated and the result suggested that Si addition resulted in the aggregation of Ti and formation of Ti₃Al phase in the middle of joint. The optimal parameters were brazing temperature of 1140 °C and holding time of 30 min. The fracture was brittle and propagated between the TiAlNi₂ layer and Ti₃Al + Ti₅Si₃ layer.     

添加TiAl对燃烧合成Ti3AlC2粉体的影响

采用Ti，Al和C粉末为反应物原料，研究了添加金属间化合物TiAl对燃烧合成Ti3AlC2的影响。从动力学和热力学的角度探讨了TiAl对燃烧合成Ti3AlC2的影响机理。实验结果表明，仅以单质粉末Ti，Al和碳黑为原料，按Ti3AlC2化学计量比配料，燃烧产物的主要物相是TiC，只能得到少量Ti3AlC2相，但在保持原料配比不变的情况下，在反应物原料中添加金属间化合物TiAl(20％-35％)(质量百分数)后，燃烧合成产物中Ti，AlC2的含量显著增加，成为燃烧产物的主要物相，而TiC的含量则显著减少。燃烧产物中Ti3AlC2的含量随添加TiAl量的增加而显著增多。     

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. TiC, TiN and TiC+TiN powder mixtures containing 50% CaF₂ flux were deposited on the surface of a Ti–6Al–4V alloy substrate, and irradiated by high-energy electron beam to form 1-mm-thick, defect-free surface composite layers. The surface composite layers contained a large amount (over 30 vol.%) of precipitates such as TiC, TiN, (Ti_xAl_1−x)N and Ti(C_xN_1−x) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification including the formation of hard precipitates and hardened matrices in the surface composite layers improved hardness and high-temperature hardness two to four times greater than that of the substrate. In particular, the surface composite fabricated with TiN powders had the highest hardness because of the highest volume fraction of TiN and (Ti_xAl_1−x)N distributed in the hardened N-rich acicular α-Ti matrix. These findings suggest that the (TiC,TiN)/Ti–6Al–4V surface composites can be used for structural materials requiring excellent thermal resistance.     

Ti-Al-C体系中添加TiAl3对燃烧合成 Ti3AlC2粉体的影响

以单质粉末Ti，Al和碳黑为原料，按Ti3AlC2化学计量比配料，燃烧产物主要物相是TiC1，只能得到少量Ti3AlC2相，但在保持原料配比不变的情况下，在反应物原料中添加金属间化合物TiAl3(质量分数为0-23．5％)，燃烧产物中Ti3AlC2的含量随添加TiAl3量的增加而显著增多，成为燃烧产物的主要物相。从热力学和动力学的角度探讨了TiAl3对燃烧合成Ti3AlC2的影响机理。     

Phase equilibria in Ti3Al-Nb alloys at 1000 °C

Titanium aluminides are considered to be the future high-temperature structural materials for turbine applications. Major focus is on α₂Ti₃Al based and γTiAl based alloys. Niobium additions to Ti₃Al alloys is found to improve the room-temperature ductility. Thus phase equilibria in Ti-Al-Nb system is of practical significance with regard to their processing and high-temperature phase stability characteristics. In the present research, four alloys with compositions Ti-22Al-12Nb, Ti-21A1-16Nb, Ti-20Al-20Nb, and Ti-25Al-25Nb (all in atom percent) were equilibrated at 1000 °C for 225 hours and then quenched in water. The quenched alloys were characterized for phase relations by optical microscopy, X-ray diffraction (XRD), and electron probe microanalysis (EPMA). Based on the phase analysis, the ternary isotherm of the Ti-Al-Nb system at 1000 °C was constructed on the Ti₃Al-rich side. The orthorhombic Ti₂AlNb phase was observed in the sample with Ti-25Al-25Nb composition signifying the presence of this phase at 1000 °C.