Papers by Author: Peter Au

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Authors: Han Liang Zhu, Dong Yi Seo, Kouichi Maruyama, Peter Au
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.
Authors: Dong Yi Seo, H. Saari, Peter Au, J. Beddoes
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.
Authors: Dong Yi Seo, S. Bulmer, H. Saari, Peter Au
Abstract: The microstructures and mechanical properties of three powder metallurgy Ti-48Al-2Cr- 2Nb-xW alloys (where x=0, 0.5, and 1 atomic percent (at.%)) are presented. The results indicate that a solution heat treatment combined with controlled cooling generate a fully lamellar (FL) microstructure without the formation of detrimental Widmanstätten or massively transformed γ phases. Aging causes coarsening of the FL microstructure in the alloys containing 0%W and 0.5%W, while almost no coarsening occurs in the 1%W sample. The addition of W to the base composition results in the formation of precipitates at the lamellar interfaces and grain boundaries during aging which helps stabilize the FL microstructure. The amount of W and the aging time affect the room temperature hardness values and tensile properties.
Authors: Dong Yi Seo, S. Bulmer, H. Saari, Peter Au
Abstract: The microstructures and tensile properties of a fully lamellar Ti-48Al-2Cr-2Nb, and two tungsten-modified versions, Ti-48Al-2Cr-2Nb-0.5W and Ti-48Al-2Cr-2Nb-1.0W (atomic percent) are investigated. Gas atomized powders are consolidated by hot isostatic pressing followed by solution treatment and aging. The microstructures are characterized by optical, scanning electron, and transmission electron microscopy and mechanical properties are characterized by room temperature tensile testing. The solution heat treatment, combined with controlled cooling, generates relatively fine, fully lamellar grains. Tungsten reduces the propensity for martensitic gamma formation during cooling, and slows down lamellar coarsening as well as the formation of equiaxed gamma phase during aging. The aging treatment stabilizes the microstructure and, in the tungsten-modified alloys, causes beta phase precipitation at lamellar interfaces and grain boundaries. Both aging and tungsten additions increase the alloy strength and reduce ductility. The fracture morphologies of the alloys are similar and exhibit mixed-mode fracture consisting of inter- and intra-granular cracking, as well as inter-lamellar cracking.
Authors: Dong Yi Seo, Scott Bulmer, H. Saari, Han Liang Zhu, Peter Au
Abstract: Pre-alloyed powders with a nominal composition of Ti-48Al-2Cr-2Nb+1W were consolidated by hot isostatic pressing (HIP). After the HIP process, a step cooled heat treatment (SCHT) with a carefully controlled cooling rate was applied to homogenize the HIP’ed microstructure and produce a fully lamellar microstructure. Following the SCHT, various isothermal aging at 950 °C and step aging processes form interfacial precipitates at the lamellar interfaces. The morphology, size, and distribution of the precipitates are dependent on the aging condition. Creep tests were carried out in air at 760 °C and 276 MPa to investigate the effect of interfacial precipitates. Primary creep resistance and creep life of the 8 and 144 hr aged conditions are improved substantially compared to the unaged condition due to the existence of the interfacial precipitates. However, the step aging process improves the creep resistance only slightly, probably because of the size and distribution differences of the interfacial precipitates compared to the 144 hr aged condition. Microstructure control is important since it has a substantial influence on creep behavior, especially primary creep resistance.
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