Fracture tests of microsized TiAl specimens

Abstract

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...

Fracture toughness testing of nanocrystalline alumina and fused quartz using chevron-notched microbeams

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“…The radii of the resulting notch roots range from a few tens to several hundreds of nanometres (e.g. [11,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]). Beyond the need to produce a notch of sufficient sharpness, another difficulty with ion milling lies in producing a uniform notch depth and/or width: for this reason, in Refs.…”

Plane stress fracture toughness of freestanding nanoscale thin films

“…Available online at www.sciencedirect.com The challenges in mechanical testing of very thin specimens with sub-micron flaws have constrained prior studies to thicker specimens [13,14], or specimens attached to a substrate [15,16]. Specimens thicker than a few microns fall in the plane strain category, which is established in the fracture mechanics literature.…”

Metals and Alloys

“…The investigations of intermetallics for MEMS materials are, however, very limited. Halford et al (2005) performed fracture tests on microsized, lamellar-structured TiAl and revealed that the lamellar orientation at the crack tip affects their fracture behavior. For the microsized, lamellar-structured TiAl, fracture toughness values (K Q ) are 1.5-7 MPa m 1/2 and are significantly smaller than that obtained for bulk material (18 MPa m 1/2 ) (Halford 2003).…”