Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature

Abstract

Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520°C and 595°C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520°C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low , however, the FCGR is higher in the fine lamellar microstructure and is accompanied by-the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595°C, while the general level of the FCGR is higher than that at 520°C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520°C, however, the FCGR at low is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.

On the role of water vapor and oxygen on the fatigue crack propagation behavior at 550 °C of a Ti6242 alloy

“…At a ⌬K range higher than 35 MPa , the growth rates become similar to that in vacuum, which is consistent with the disappearance of the environmental influence. Faster growth rates in air at low ⌬K ranges are consistent with the general observation of an accentuated influence of environment on the fatigue crack growth behavior of metallic alloys [1][2][3][4] near the near-threshold region. Illustrations of the cracked surfaces given in Figures 3(a) and (b) show transgranular fracture features with ductile striations mixed with interlamellar decohesions in some grains (Figure 3(b)).…”

“…This model has been developed on the basis of experiments conducted mainly on a Ti6246 alloy, but also on the IMI834 and Ti6242 alloys. [24] As initially proposed by Wei et al [25] to account for the environmentally assisted fatigue crack propagation, a superposition model has been proposed as follows: [2] with [3] where E ϭ the Young modulus, ϭ stress parameter, and B ϭ a dimensionless parameter.…”

“…[3] is derived from the model initially proposed by McClintock [26] where the crack advance during one cycle corresponds to a fraction of the crack tip opening displacement range, and after having taking into account the existence of an effective threshold range ⌬K th for environmentally assisted propagation, as proposed by Suyitno et al [27] The intrinsic propagation (da/dN) 0 is given by Eq. [1].…”

Correlation Between Microstructure and Fatigue Property of Ti-6al-4v Alloy Used in Aerospace Applications

“…The way the first defects appear must be studied at the scale of microstructure in order to understand and to avoid any damage during service. It is well documented that fatigue crack initiation is strongly influenced by several characteristics of the microstructure like grain size, grain orientation and mis-orientation of the grain [9]. The present study aims to find a correlation between microstructure feature of Ti-6.3Al-4.17V and fatigue crack initiation sites as well as fatigue crack growth in the structure.…”

Crack growth behavior in dissimilar welded Ti based alloys under biaxial fatigue loading

“…Thus, a coarse microstructure including a large size of ↵ colonies would lead to many crack bifurcations. As a major result of these bifurcations, it has also been observed that for fatigue in mode I, an increase in crack bifurcation events enables to decrease the crack growth rate [7,8,9].…”