Study on notch fracture of TiAl alloys at room temperature

Abstract

In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of twophase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study. [1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.

Fracture behaviour of precracked specimens of a TiAl alloy

“…The inverse ductility/K 1C relationship is also observed in this paper, showing a lower tensile ductility 15 higher fracture toughness of the coarser NFL microstructure (Table 3).…”

“…In a previous work, 15 observing of surface fracture process of very thin specimens in situ, the present authors indicated that cracks prefer to initiate and propagate along lamellar interfaces, which are the weakest link in the near fully lamellar microstructure. The interlamellar strength was calculated to be less than 100 MPa and is significantly lower than the translamellar strength.…”

“…15 The tensile conditional yield strength and the tensile fracture stress and strain of the specimens with the finer NFL microstructure are inferior to those of specimens with the coarse DP microstructure. These phenomena are attributed to the much higher sampling volume in tensile tests, which involves the entire volume within the gauge.…”

“…10b. 15 Figure 11 shows the competition between two fatigue cracks initiating at the same notch root. One is composed of two interlamellar sections (marked 1) and a short translamellar section (marked 2) and propagates further, and the other (marked 3) stops at the boundary of a large grain with unfavourable orientation.…”

The Effects of Cr and Mo on the Microstructure and Mechanical Properties of As-Cast TiAl Alloys

“…A diamond-coated blade cutting tool was used to cut the as-cast TiAl alloys into metallographic and compressive samples. The metallographic samples were polished and then etched in a solution of 10 vol.% HNO 3 , 5 vol.% HF, and 85 vol.% water [20]. X-ray diffraction (XRD) using a diffractometer (Siemens D5000) with Cu Kα anode (λ = 1.54056 Å) was used to identify the phases present in the samples.…”