Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

Abstract

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-m grain size and 1-m lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 ЊC and 815 ЊC. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the finegrained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.

Fracture in lamellar TiAl simulated with the cohesive model

“…The R-curve effect is correlated with the proposed accumulated shear ligament model. However, for finegrained fully lamellar microstructure of about 150 µm mean size of the colonies, the Rcurve effect is continuously reduced and the corresponding crack extension is detected within100 µm only [29]. The R-curve increases from 16 to 22 MPa√m.…”

“…From bend specimens, still in the global elastic regime, they found an R-curve effect, i.e. increasing fracture toughness from initiation (K Ic ≈ 16 MPa√m) up to saturation of about [25][26][27][28][29][30] MPa√m. This increase is confined within small crack extension, less than 200 µm, i.e.…”

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

“…performed by Kruzic et al [41], building on preliminary work by Chan and Shih [42]. This identified small crack growth by cleavage cracking along colony and interlamellar boundaries, as well as low-index crystallographic planes within lamellae and equiaxed grains.…”

Deformation Mechanisms in Niobium Silicide-Based Composites

“…Figure 16 shows fatigue crack growth data obtained for a range of RMICs including the cast/extruded Nb-10Si composite Bewlay et al, 1997c;Samant and Lewandowski, 1997), the DS MASC (Bewlay et al, 1997c), monolithic cast/extruded Nb-1 Si alloy, and TiAl based intermetallics (Chan and Shih, 1997;Larsen et al, 1995). Figure 16 indicates that the fatigue performance of monolithic Nb-1 Si is significantly better than that of the Nb-silicide based composite (Rao et al, 1994), and suggests that the good fatigue characteristics of the Nb in these systems dominates the composite creep performance due to (i) its intrinsic properties, (ii) its continuity, and (iii) its scale.…”