A B S T R A C T γ -TiAl-based materials have the ability to provide superior creep strength, high yield strength and Young's modulus at temperatures as high as 700 • C. This has led to the consideration of γ -TiAl-based materials for use in microelectromechanical systems (MEMS) intended for application at elevated temperatures. One γ -TiAl-based material under consideration for these applications is the composition known as Alloy 7/Ti-46Al-5Nb-1W (at%). In this case, this material is tested in the as-wrought form with a fully lamellar structure and a colony size ≈75 µm. The effect of loading volumes of this material smaller than conventional samples is investigated. This is carried out in order to improve understanding of the fracture toughness behaviour, which ultimately will be required of components of this material with a cross-section ≈18 × 6 µm. Here, we focus upon a specially developed machine, which provides accurate loading of microsized cantilever beams through a diamond tip. The directional nature of the fracture toughness properties of this material is considered in relation to the local orientation of the lamellar microstructure and the resulting failure modes occurring.Microelectromechanical systems (MEMS) are intended for applications including microsensors, instrumentation and fluid dosing in environments that require the ability to withstand high temperatures. These environments could include aeronautics and space travel, where significant advantages can be expected to be gained from miniaturization, leading to cascading benefits such as reduced fuel requirements and cost. The implementation of devices such as these requires the development and understanding of materials that are capable of operating at these temperatures; that can be effectively used to produce MEMS devices and provide the necessary mechanical properties. One material class currently being considered for such applications is the fully lamellar (FL) γ -TiAl-based alloys. These materials have been developed by the aerospace industry with a view to using them to replace the heavier Ni-based superalloys in gas turbine engine compressors. 1,2 γ -TiAl materials offer a high yield stress, creep limit and Young's modulus combined with a low density Correspondence: T. P. Halford. E-mail: t halford@ames.pit.titech.ac.jp at temperatures up to 700 • C. 3 Application of these materials on the micro-and nano-scale, however, requires an improved understanding of the effect of a reduction in size upon their mechanical properties. The variation in these properties occurring due to the orientation effect of the lamellar microstructure must also be considered. This is particularly significant as previous, bulk, testing of these materials has demonstrated a distinct variation in the properties of the three failure modes occurring in the lamellar microstructure. 4 These modes, in the order of increasing toughness, are described as interlamellar, intralamellar and translamellar. The interlamellar failure mode requires comparatively small driving fo...
We have developed a new type of mechanical testing machine for micro-sized specimens, which can apply a small static or cyclic load, and have investigated fracture and fatigue crack growth behavior of micro-sized specimens. Cantilever beam type specimens (10 μm × 10 μm × 50 μm), with notches were prepared from thin films of a Ni-P amorphous alloy by focused ion beam machining. Fatigue and fracture toughness tests were carried out in air at room temperature using the mechanical testing machine. Fatigue and fracture testing was completed successfully for micro-sized cantilever specimens. Once fatigue crack growth occurs, rapid sample failure was observed in these micro-sized specimens. This indicates that the fatigue life of micro-sized specimens is mainly dominated by crack initiation. This also suggests that even a micro-sized surface flaw can be a fatigue crack initiation site which will shorten the fatigue life of micro-sized specimens. As a result of fracture toughness tests, plane strain criteria for small scale yielding were not achieved for this amorphous alloy. Plane stress and plane strain dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This suggests that fracture mechanics is still valid for such micro-sized specimens.
The inertia friction welding process is being extensively investigated for the joining of high strength titanium alloys for aerospace applications. Although it offers solid state joining, the thermal cycle and deformation involved results in microstructural inhomogeneity across the weld interface. In this paper, the fatigue crack propagation behavior in an inertia welded microstructure in a high strength, high temperature α/β titanium alloy is considered. The fatigue crack propagation behavior in corner notched weld specimens at varying stress ratios is studied at room and elevated temperatures and compared with that of the parent material. Fatigue crack growth rates at lower stress intensity ranges are comparable with those in the parent material. However, in weld specimens tested at room temperature, unstable crack growth occurs at lower stress intensity range values compared to that at high temperature. Fracture surface observations show that this difference is related to a change in fracture mode from transgranular to intergranular/mixed mode during room temperature tests. This change in fatigue crack growth mechanism is deduced to be due to low ductility intergranular failure of grain boundary α in the refined transformed beta microstructure across the weld interface.
A micro-sized testing technique has been applied to investigate the fracture properties of lamellar colonies in a fully lamellar Ti-46Al-5Nb-1W alloy. Micro-sized cantilever specimens with a size ≈ 10 × 10 × 50 μm3 were prepared by focused ion beam machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the micro-sized specimens by focused ion beam machining. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness (KQ) values obtained were in the range 1.4–7 MPam1/2. Fracture surface observations indicate that these variations are attributable to differences in local lamellar orientations ahead of the notch. These fracture toughness values are also lower than those having been previously reported in conventional samples. This may be due the absence of significant extrinsic toughening mechanisms in these micro-sized specimens. Fracture mechanisms of these alloys are also considered on the micrometer scale. The results obtained in this investigation give important and fundamental information on the development of TiAl based alloys with high fracture toughness.
High strength γ-TiAl based alloys, such as Ti-46Al-5Nb-1W (Alloy 7), which were originally developed for gas turbine and automotive applications are now being considered for application in Micro Electro Mechanical Systems (MEMS). This requires the evaluation of these materials upon the microscale. As international standards do not currently exist for the evaluation of the mechanical properties of samples with dimensions equivalent to those required by MEMS devices, the development of new methods was required. The method developed here is intended for the fatigue testing of samples measuring ≈ 10µm (B) x 20µm (W) x 40µm (L). This is completed using a machine recently developed at Tokyo Institute of Technology to load samples of lamellar γ-TiAl based material to failure in compressive bending. This method is intended to work alongside methods previously developed for the fracture toughness testing of similar microsized cantilever bend specimens.In this work sample cantilevers of Alloy 7 are Focussed Ion Beam (FIB) machined from foil ≈ 20µm thick and their stress -life (S-N) fatigue behaviour evaluated. The dependence of fatigue life upon lamellar orientation for a given peak stress / stress range is considered. The effect of the reduced scale of these samples upon the mean and scatter of these sample lifetimes is also considered through comparison with previous data obtained from the S-N testing of macrosized samples of the same material.
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