Hot-Workability of Gamma-Based TiAl Alloys during Severe Torsional Deformation
- PDF / 1,616,393 Bytes
- 16 Pages / 593.972 x 792 pts Page_size
- 113 Downloads / 291 Views
NTRODUCTION
GAMMA-BASED titanium aluminide alloys suffer from brittle fracture, which persists to relatively high temperatures. Quality issues associated with ingot metallurgy are therefore of major concern for the fabrication and reliability of components. These involve shrinkage porosity, segregation of alloying elements, coarse microstructure, and texture. Attaining chemical homogeneity and refinement of the microstructure is therefore the most important prerequisite for engineering applications. Thermomechanical processing probably offers the most effective way of controlling the microstructure in gamma-based titanium aluminide alloys. To date, the traditional metal-forming operations, such as extrusion, forging, and rolling, have been used. The current status that has been achieved in these processing routes was described in recent review articles.[1–5] Although the feasibility of these methods has been demonstrated on an industrial scale, hot-worked products often suffer from insufficient consolidation, which is manifested by incomplete recrystallization and significant chemical inhomogeneity.[6,7] These quality issues are a major limitation for the reliability of components manufactured from semifinished products and also impede secondary processing. Thus, further improvement of hot-working procedures with regard to structural and chemical homogeneity needs to be addressed. A possible way could be to enhance the imparted mechanical energy, which triggers dynamic recrystallization. This can be achieved by torsional deformation, which imparts much higher strains than compression, extrusion, and rolling. Its feasibility has been shown in a previous study on extruded materials.[8] For efficiency reasons, ingot breakdown by torsion U. FRO¨BEL, Scientist, and F. APPEL, Scientist, are with the GKSS Research Centre, Institute for Materials Research, MaxPlanck-Straße 1, D-21502, Geesthacht, Germany. Contact e-mail: [email protected] Manuscript submitted August 10, 2006 Article published online July 18, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
alone is a main objective. In the present study, the investigations were therefore performed on alloys in the as-cast and hot-isostatically pressed (‘‘hipped’’) state. It became apparent that the cast and hipped morphology favors particular hot-working mechanisms, which will be discussed in the following.
II.
EXPERIMENTAL PROCEDURE
The hot-working experiments were conducted on two c-based TiAl alloys with the compositions Ti-46.5Al (alloy 1) and Ti-46.5Al-5Nb (alloy 2). The Nb-bearing alloy 2 belongs to a group of TiAl alloys, which is currently investigated for engineering applications.[5,9] Because Nb additions may significantly affect hot-working behavior, alloy 1 was investigated as a reference. The two materials were supplied by the Flow Serve Corp. (Dayton, USA) as induction skull melted and hipped ingots of 70-mm diameter and 1000-mm length. The torsion specimens were prepared by spark erosion, turning, and drilling so that the sample and ingot axes were parallel. The
Data Loading...
