Materials Science Forum Vols. 539-543

Abstract: This paper reviews two kinds of well-aligned fibrous microstructures produced by directional solidification of NiAl-Mo and Fe-Fe2Ta eutectics. In both these composites, fiber size and spacing decrese inversely as the square root of the growth rate. Tensile tests as a function of temperature showed that the NiAl-Mo composite has a higher yield strength and lower ductile-to-brittle transition temperature than the NiAl matrix. For the Fe-Fe2Ta composite, yield strengths in excess of 700 MPa were obtained at temperatures to 600°C, with elongations to fracture of ~3% which remained roughly constant at temperatures to 950°C.

Abstract: The paper presented the texture evolution during primary recrystallization and following grain growth in the heavily cold-rolled Ni3Al single crystals. It turned out that the texture evolution occurred in the two stages. First, primary recrystallization caused the drastic change of the as-rolled texture. Then, as grain growth proceeded, the texture returned to the same one as the as-rolled textures. This texture return can be designated as Texture memory effect. The mechanism of the texture memory effect was discussed based on the analysis of the orientation relationship between the as-rolled and the primary recrystallization textures.

Abstract: High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials. In this study, it is applied for the investigation of an intermetallic γ-TiAl based alloy. Not only the diffraction angles, but also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to approach lattice parameters and grain sizes as well as crystallographic relationships. An in-situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl which exhibits high internal stresses. With increasing temperature the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. The data obtained by high-energy synchrotron X-ray diffraction, extremely rich in information, are interpreted step by step.

Abstract: Fine-grained fully lamellar (FGFL) structures of XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8vol.%TiB2) (at.%) were stabilized to varying degrees by different aging treatments. Specimens with and without aging were creep tested at 760°C and 207 MPa. It was found that during creep deformation, degradation of the lamellar structure involving coarsening within the colonies and spheroidization at colony boundaries occurred, forming fine globular structures at the colony boundaries and increasing the creep rate. Aging treatments stabilized the lamellar structure and retarded the coarsening and spheroidization processes during creep deformation. As a result, the aged specimens exhibited lower minimum creep rates and longer creep lives than the unaged specimens. A multiple step aging stabilized the lamellar structure to the greatest extent and suppressed other degradation processes during aging, resulting in the best creep resistance. These results demonstrate that the multiple step aging is the optimal aging condition for stabilizing FGFL XD TiAl alloys.

Abstract: In this study, high-temperature deformation behavior of newly developed beta-gamma TiAl alloys was investigated in the context of the dynamic-materials model (DMM). Processing maps representing the efficiency of power consumption for microstructure evolution were constructed utilizing the results of compression test at temperatures ranging from 1000oC to 1200oC and strain rates ranging from 10-4/s to 102/s and Artificial Neural Network simulation method. With the help of processing map and microstructural analysis, the optimum processing condition for the betagamma TiAl alloy was investigated. The role of β phase was also discussed in this study.

Abstract: Fully lamellar structures of powder metallurgy (PM), investment cast, and directionally solidified (DS) TiAl alloys containing β stabilizer were produced after stepped cool heat treatment, and interface β precipitates were formed after aging at 950°C. In addition, a columnar grain structure combined with a fully lamellar structure aligned with the load direction and interface β precipitates were formed by directional solidification and subsequent heat treatments. Creep test results of PM TiAl indicate that controlling the initial microstructures is also critical for balancing the primary and steady-state creep resistance during short and long-term tests. DS TiAl alloy exhibits a significant reduction of the primary strain and creep rate compared to polycrystalline TiAl due to the unique DS microstructure. Therefore, a DS microstructure with proper lamellar orientation and controlled interface β precipitation is the ideal if maximum time to a relatively small (<0.5%) strain is the design criterion of merit.

Abstract: Several beta stabilizing elements such as Mo, Cr, W, V and Fe have been added to Ti- 22Al-27Nb alloy for substituting a portion of Nb in order to further improve the tensile properties and creep resistances of orthorhombic Ti2AlNb-based alloys. Six compositional modified alloys Ti- 22Al-19.2Nb-2Cr, Ti-22Al-12.5Nb-2W-2Cr, Ti-22Al-10.8Nb-2Mo-2Cr, Ti-22Al-16Nb-2Cr-2V, Ti-22Al-11Nb-2Mo-1Fe, Ti-22Al-16.3Nb-2V-1Fe were prepared by plasma arc melting. The phase constitutions of these alloys were found to be B2+O or B2+O+α2 phases. The tensile properties were investigated at room temperature, and the creep behaviors were investigated under 650oC/310MPa and 650 to 750oC/200MPa. The results showed that Mo+Fe and W+Cr addition improved effectively the 0.2% yield strength and creep resistance. Ti-22Al-11Nb-2Mo-1Fe alloy exhibited the lower transient creep strain and steady-state creep rate, and longer 1% creep-strain lifetime than Ti-22Al-27Nb alloy at 650 to 700oC creep. The dislocation-controlled creep deformation mechanism was suggested to the creep behaviors of the Mo+Fe-modified alloy.