Materials Science Forum Vols. 747-748

Abstract: The corrosion behavior of porous Nb-containing TiAl based alloys in HCl, NaOH and NH₃H₂O aqueous solutions with various concentrations has been studied simultaneously at room temperature. The results indicate that the porous alloys have good acidic resistance which is probably based on the combined work of Ti-Ti metallic bond and covalent bond. And pitting corrosion is the main corrosion behavior of Nb-free TiAl alloys during the first 40 hours, while intergranular corrosion is the main corrosion behavior later. Nb-containing alloys have better acid corrosion resistance due to inhibiting intergranular corrosion by Nb element. In addition, the porous TiAl based alloys are much more resistant to corrosion in alkaline solutions than in acidic solutions, owing to the different oxide films formed in alkalis solutions.

Abstract: First-principles method has been used to study the intrinsic brittlement of Mo₃Si. The crystal constants, formation energy, cohesive energy, electronic structure, elastic constants of Mo₃Si were calculated. The results were in good agreement with experiment data. Electronic structures showed that the strong covalent bonding between the nearest neighbour Mo atoms, which arrange perpendicularly each other, leads to embrittlement of Mo₃Si.

Abstract: The isothermal static corrosion of Ti-45Al-8Nb and Ti-53Al-20Nb (at.%) in liquid Zn-55Al-1.6Si (wt.%) at 630°C was studied. Incubation periods without any corrosion occurred are exhibited in both alloys. After the incubation time, the corrosion rate increases noticeably which is controlled by the reaction-diffusion and dissolution mechanism. Thick transition layers were formed. The EDS results show that Si and Al diffused into the TiAl-Nb alloys preferentially, while Zn was hindered from diffusing and segregated in spots. All the transition layers are porous and composed mainly of Al₃Ti (Nb), SiTi and AlNbZn₂. The fracture of the transition layers of Ti-45Al-8Nb should be attributed to the defects of the lamellar colony boundaries. nanoIndenter was used to check the mechanical properties of the materials. The results show that the transition layers have higher micro-hardness and elastic modulus than the matrix.

Abstract: The influence of laser remelting on the mechanical property and fracture behavior of the superplastic diffusion bonding between TiAl intermetallics and TC4 alloy was investigated. The joint bonded by the pre-remelted samples displayed well diffusion, but the mechanical properties of the joint should be further enhanced. The mechanical properties of the joint pre-remelted under the diffusion bonding conditions of 915/80MPa/1h are lower than that of the joint without remelting. After the heat treatment on pre-remelted joint sample at 860, the mechanical properties have been enhanced greatly.

Abstract: nterdiffusion coefficients in α2-Ti3Al and γ-TiAl of conventional TiAl and TiAl-8Nb alloy were measured at the temperature ranging from 1273K to 1523K. Single-phase diffusion couples were employed, and the concentration profiles of Al after annealing were measured by an electron probe microanalyzer (EPMA), and the interdiffusion coefficients were calculated according to the Boltzmann-Matano method. The results showed that there was no significant concentration dependence of interdiffusion coefficients for all the alloys with various phases, and the values of interdiffusion coefficients covered three orders of magnitude (E-17-E-14) with the increase of temperature according to Arrhenius law. In α2-Ti3Al and γ-TiAl phase of conventional TiAl alloys, the pre-exponential factor and activation enthalpy were D0=3.95×10-5m2s-1,Q=276KJmol-1 ;D0=7.26×10-5m2s-1,Q=275KJmol-1 respectively. The pre-exponential factor and activation enthalpy were D0=4.54×10-6m2s-1, Q=244KJmol-1 in γ-TiAl phase of TiAl-8Nb alloys. However, the temperature dependence of interdiffusion coefficients in α2-Ti3Al of TiAl-8Nb alloys did not follow Arrhenius laws very well. With the addition of Nb, the interdiffusion coefficients increased significantly in α2-Ti3Al, but changed slightly in γ-TiAl at high temperature.

Abstract: Cellular Al-0.24wt.%Sc and Al-0.41wt.%Sc foams with a porosity ranging from 37 to 86% were successfully fabricated via the melt-foaming method with the calcium particles as the thickening agent and titanium hydride as blowing agent. The corrosion rate increased with the increasing of porosity for the cellular Al-0.24wt.%Sc, Al-0.41wt.%Sc and Al foams. Large size Al₃Sc precipitates can accelerate the corrosion of the Al foams. However, T6-treatments could improve the corrosion resistance significantly. With T6-treatments, 0.24 wt.%Sc addition to Al foams exhibited unconspicuous decrease in the corrosion resistance but much improved strength.

Abstract: TiAl intermetallic alloy was prepared by gelcasting. The alloy powders were synthesized through reaction diffusion. The microstructure was investigated by XRD and SEM. The results showed that titanium aluminide powders were synthesized from elemental powders under the condition of 600°C for 3h. The solid loading of metal slurry was 45%. After gelcasting, drying and sintering at 1480°C for 2h, TiAl with the relative density of 93% was successfully prepared.

Abstract: Low pressure turbine blades (LPT) made by investment casting from intermetallic titanium aluminide alloys for aero-engine applications have about 50% weight saving compared with that from nickel-based counterparts. Investment casting process of the low pressure turbine blades for high Nb containing TiAl alloy was simulated by Procast. The height of the blade is about 125mm and the thinnest part of it is about 6mm. Compositions of the cast and mould are Ti-45.5Al-8Nb (at %) and Zircon sand, respectively. The simulation result showed that there were porosities appearing in the centre of blades, which may be due to the formation of isolated liquid. In this work, the simulation, analysis and comparison of different casting ways were carried out. The result showed that compared with top and bottom casting, blades made by side casting have less porosity defects. And then the casting temperature, casting velocity, mould preheating temperature and interface heat transfer coefficient were optimized based on orthogonal design. The result also indicated that the influence of process parameters to porosity defects of blades can be ranked from strong to weak as follow: casting temperature>shell mould preheating temperature>casting velocity>interface heat transfer coefficient. When the casting temperature was 1700, the mould preheating temperature was 500, the casting velocity was 0.5 m·s^-1, and the interface heat transfer coefficient was 500 W·m^-2·K^-1, the volume of porosity defects was the smallest.

Abstract: Effects of annealing treatment on microstructure and the compressive properties of hot-worked Ti-45Al-8Nb-(W, B, Y) alloy were investigated. Microstructure of the extrusion plus multi-step forging pancake before and after heat treatment was analyzed by SEM and TEM, respectively. The annealing was conducted by holding samples at 1100°C for 2hrs, and followed by air cooling and furnace cooling. The mechanical properties were measured by Instron test machine. The microstructure evolution during compressive deformation was analyzed by TEM. The results showed that after the annealing the microstructure change could not be observed under SEM but can be observed under TEM. Many dislocation clusters were removed by heat treatment. The heat treated samples had similar compression behaviors with the pancake. TEM investigation showed that the numerous twin intersections occured in γ matrix during compression. The twin spaces tended to decrease as the deformation and the intersection increasing.

Abstract: In this study, based on cast Ti-47.5Al-2.5V-1.0Cr (at. %) alloy with the directional lamellar microstructure, the influence of exposure at 750 in air on room temperature tensile ductility was investigated as a function of time, ranging from1hr to 300hr, in order to determine the onset exposure time for a significant reduction in RT ductility and the fracture mechanism for the exposed specimens. The results indicated that this alloy could still retain more than 2.0% of its ductility after exposure at 750 for 150hr, and its ductility decreased to 1.58% and ~1.0% even after exposure for 200hr and 300hr, respectively. High ductility for the directional lamellar microstructure could be retained after exposure for 150hr, which was attributed to its high crack origination and propagation resistance at specific lamellar orientation. And the fracture mechanism of the exposed specimen with this special structure would also be discussed.