Kinetics of high temperature deformation of lamellar Ti48Al–2Nb–2Cr

Based on microstructural observations, the deformation resistance of lamellar Ti48Al–2Nb–2Cr in the temperature range between 1000 and 1200 K is expressed by treating the material as a composite of regions with lamellar and globular structure deforming independently of each other. For stresses below 1000 MPa the dislocation velocity is lower in the lamellar regions compared to the globular ones due to a larger athermal hardening component caused by interfaces (lamella boundaries in the lamellar structure and (sub)grain boundaries in the globular structure). Combining deformation kinetics and structural evolution allows the modelling of the maximum deformation resistance and the subsequent softening resulting from the increase in globular volume fraction. The model is applied to predict the creep life.     

Microstructural evolution during high temperature deformation of lamellar Ti48Al–2Nb–2Cr

The microstructural evolution of lamellar Ti48Al–2Nb–2Cr during deformation at temperatures between 1000 and 1200 K was investigated by light optical and electron microscopy. The lamellar structure which initially covers more than 95% of the volume transforms during deformation into a globular structure consisting of equiaxed subgrains with a significant amount of large angle boundaries (<30%). The steady state subgrain size and the spacing of dislocations in the subgrain interior vary in inverse proportion to shear modulus normalized stress. Based on the steady state data the evolution of the volume fraction with globular (subgrain) structure and the characteristic spacings is quantified.     

Infrared joining strength and interfacial microstructures of Ti–48Al–2Nb–2Cr intermetallics using Ti–15Cu–15Ni foil

Infrared joining of Ti–48Al–2Nb–2Cr using Ti–15Cu–15Ni (wt%) foil as brazing filler metal was investigated at the temperature range of 1100∼1200°C for 30∼60 s in a flowing argon environment. The compressive tests show three types of fracture morphologies in which type I fails at the joint interface, but types II and III are fractured in the base-metal with the crack direction parallel to and perpendicular to the loading axis, respectively. Most of joined specimens were fractured through the base metal indicating that the infrared joined interface has relatively good joint strength. The compressive strength of type I specimen is about 319–322 MPa. Experimental results show that the shorter the real holding time or the higher the joining temperature, the larger the strength variation will be. The observed interfacial microstructures of Ti–48Al–2Nb–2Cr joint interfaces indicate that seven characteristic zones can be distinguished in the joint interfaces and each characteristic structure corresponds to one or more stable phases at T_w temperature. The observed microstructures and their evolutions of each zone are explained in detail in this study. The major difference between joint interfaces of Ti–48Al–2Nb–2Cr and Ti₅₀Al₅₀ alloys takes place on the base-metal interface zone and the columnar two-phase zone. The existence of Nb and Cr atoms in Ti–48Al–2Nb–2Cr alloy also has some influences on the microstructural evolution of the columnar two-phase zone and the continuous α₂-layer.     

Preparation of low-oxygen-containing Ti–48Al–2Cr–2Nb alloy powder by direct reduction of oxides

A high-purity Ti–48Al–2Nb–2Cr alloy powder with an oxygen content as low as 0.0572 wt.% and a particle size of <150 μm was produced from a mixture of TiO₂, Al₂O₃, Nb₂O₅, and Cr₂O₃ powders through reduction with magnesium and deoxidation with calcium. The phase and composition of the products were analyzed. The final product mainly included γ-TiAl and minor α₂-Ti₃Al phases, and Ti, Al, Cr, and Nb were homogenously distributed in the powder with a mole ratio of 49.73:43.51:2.05:1.98. The reduction and deoxidation mechanisms were investigated by thermodynamic modeling using the HSC Chemistry software and Pandat software based on the Ti alloy database.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

On the ω phase formation in Cr–Al and Ti–Al–Cr alloys

The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr₂Al phase (C11_b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.     

Evolution of lamellar structure in Ti–47Al–2Nb–2Cr–0.2W alloy sheet

The Ti–47Al–2Nb–2Cr–0.2W alloy sheets were obtained by hot pack rolling. The as-rolled sheet has an inhomogeneous duplex microstructure composed of elongated gamma grains and lamellar colonies. Heat treatments were conducted on the as-rolled sheets. The results show that the microstructures with different sizes and grain boundary morphologies were developed after different heat treatments. A coarse fully lamellar structure can be refined if the heating time, together with the cooling rate, is appropriately controlled. The grain growth exponent is found to be approximately 0.2, and the activation energy of grain boundary migration of the alloy is around 225 kJ/mol.     

Ti48Al2Cr2Nb合金高电位腐蚀的电流时间序列相空间重构分析

采用相空间重构法,研究Ti48Al2Cr2Nb合金在高电位电化学腐蚀过程中电流密度时间序列的动力学演化特征.结果表明:电化学系统均存在相似结构的吸引子,其轨迹线均分布在45°对角线上,随着时间以一定规律反复折叠和拉伸,且随着电位的提升越来越复杂、光滑,填充区域变大.结合腐蚀形貌可知,吸引子的具体形态与合金在相应电位下的...     

The crack and pore formation mechanism of Ti–47Al–2Cr–2Nb alloy fabricated by selective laser melting

In this study, a broad range of parameter combinations (laser power: 100–400 W; scanning speed: 10–90 mm/s) were used to fabricate Ti-47Al-2Cr-2Nb alloy at the layer thickness of 100 μm by selective laser melting (SLM). The preparation of the TiAl-single track by SLM was prone not only to balling and irregularity but also to cracking. Although the optimized process parameters were used to fabricate TiAl specimens, many pores and cracks still existed and a low density was achieved. To understand the mechanism for the crack and pore formation, the connections among the cracks, pores, and the process parameters were investigated in addition to the variation in the crack propagation with an increase in the number of deposition layers. The results indicated that the cracks originated in the third layer, because of the accumulation of residual stresses and the changes in the composition of Ti-47Al-2Cr-2Nb deposition layers. Additionally, the frequency of cracks constantly increased with an increase in the number of deposition layers. Preheating the substrate to 200 °C improved the degree of cracking to a certain extent, as the initiation layer for the cracks increased from the third layer to the fifth layer. Despite the achieved improvement, it was not possible to produce crack-free specimens on the SLM machine used for this study. Finally, there was a good metallurgical bond between the Ti-6Al-4 V substrate and the Ti-47Al-2Cr-2Nb deposition layers that was free of pore and crack defects. These findings suggest that using SLM to fabricate Ti-6Al-4 V/TiAl intermetallic laminate composites may potentially eliminate cracking and improve the properties of TiAl alloys.     

TiNb／Ti—48Al—2Cr—2Nb复合材料的力学性能和断裂特征

采用粉末冶金、热压成型工艺制备了TiNb/Ti-48Al-2Cr-2Nb(TACN)基复合材料。研究了复合材料的弯曲性能和断裂特征。实验结果表明：复合材料的性能较基体有较大的提高,用混合法则来进行预测复合材料的力学性能和实验中所得结果十分接近;这种工艺制备的复合材料界面由全片层组织构成,纤维和基体的结合状态良好,纤维损伤比例较小。     

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys have been studied. In order to optimize ductility and strength of the orthorhombic alloys with the nominal compositions of Ti–22Al–23Nb–3Ta and Ti–22Al–20Nb–7Ta, various thermomechanical processing steps were implemented as part of the processing route. With a special heat treatment before rolling to obtain a fine and homogeneous rolled microstructure, the rolled microstructure resulted in a good combination of high tensile yield strength and good ductility of the alloys through available solution and age treatments. The duplex microstructure with equiaxed α₂/O particles and fine O phase laths in a B2 matrix, deforming in α₂+B2+O phase field and treating in O+B2 phase field, possesses the highest tensile properties. The R.T. yield strength and ductility of the Ti–22Al–20Nb–7Ta alloy are 1200 MPa, and 9.8% respectively. The yield strength and ductility values of 970 MPa and 14% were also maintained at elevated temperature (650°C).     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Strength and fracture of two hot-rolled Ti–Al–Si–Nb dual phase alloys based on Ti3Al

Ti–Al–Si–Nb dual phase alloys are mainly composed of α₂-Ti₃Al matrix and Ti₅Si₃ silicide phases. In this paper, two alloys (402 and 405) whose Si contents are 2 and 5 at% respectively were arc melted and hot-rolled into sheets with different amounts of deformation. The silicide phase (Ti,Nb)₅(Si,Al)₃ was broken up into small pieces and redistributed in the α₂ matrix during the hot-rolling. Improved strength and ductility of the two alloys were observed after hot-rolling, which can be attributed to both the finely distributed reinforcement silicide phase and refinement of the matrix grain size. The mechanical properties of the two alloys are dependent on their volume fractions of the silicide phase: the strength of alloy 405 is higher than that of alloy 402, while alloy 402 is more ductile than alloy 405. The brittle–ductile transition temperature of the two dual phase alloys is between 600 and 800°C. The surface slip on the dual phase alloys was also observed. Obvious separation between the (Ti,Nb)₅(Si,Al)₃ particles and the α₂ matrix is found on the fracture surfaces obtained at high temperature, showing dimple-like morphology.     

Ti—48Al—2Cr—2Nb金属间的激光表面合金化组织与耐磨性

分别采用氮气及预涂ＴｉＮ粉、碳粉、ＴｉＣ粉等方法对Ｔｉ－４８Ａｌ－２Ｃｒ－２Ｎｂ金属间化合物进行激光表面合金化,制得了分别以ＴｉＮ,ＴｉＣ等硬质耐磨相为增强相的快速凝固“原位”金属基耐磨复合材料表面改性层,并分别在干滑动及磨料磨损条件下测试了所获激光表面合金化改性层的耐磨性。结果表明,对Ｔｉ－４８Ａｌ－２Ｃｒ－２Ｎｂ合金进行激光表面合金化改性处理后显微硬度和耐磨性大幅度提高。     

热处理对铸态Ti48Al2Cr2Nb1B合金组织和性能的影响

经真空自耗凝壳炉浇注得到Ti48Al2Cr2Nb1B合金试棒,通过显微组织观察和力学性能测试,研究了淬火及时效工艺对Ti48Al2Cr2Nb1B合金组织和性能的影响。结果表明,通过淬火+时效处理可以提高铸态Ti48Al2Cr2Nb1B合金的抗拉强度,但对其塑性的影响作用有限。经1 380℃×30 min/QC+1 240℃×6 h/AC处理后,Ti48Al2Cr2Nb1B合金可以获得均匀细小的双态组织,室温抗拉强度最高。     

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

NiTi-Al-based alloys are promising high-tem- perature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni--43Ti-4AI- 2Nb-2Hf （at%） was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope （SEM）, X-ray diffraction （XRD）, and tensile tests. Results show that the micro- structure of as-forged Ni-43Ti--4AI-2Nb-2Hf alloy con- sists of NiTi matrix, Ti2Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4A1-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4A1 alloy. At room temperature and 650℃, the yield stress of Ni--43Ti-4Al- 2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni--46Ti- 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.