Thermal activation of fatigue crack growth: Analysing the mechanisms of fatigue crack propagation in superalloys

Abstract

A method for deriving apparent activation energies, E app , for fatigue crack growth (FCG) is presented. It is shown how these E app values can be used to analyse the main underlying mechanism causing the progression of the crack. The E app values for fatigue crack growth in air and vacuum, and with and without a dwell are obtained for a range of Ni based superalloys for turbine disc applications. The apparent activation energies range from about zero to about 300kJ/mol. The analysis allows identification of alloys that are prone to FCG through diffusion controlled mechanisms, notably grain boundary oxidation or grain boundary creep, as well as alloys that are nearly immune to FCG by grain boundary oxidation or creep.

Environmental and Dwell Effects on the Damage Tolerance Properties of ATI 718Plus® Alloy

“…the darker-colo behind the crack des showed no observed in c d of the crack t e in agreement w ontaining supera oundaries has be atigue crack prop air using SE ima dominately trans δ phase crack obular particles fatigue crack p ht-coloured carb with the SEM BS 11, illustrates t The difference b oured carbides w k front. In comp evidence of ox crack-free grain tip, as shown in with other studie alloys that oxidat een linked to en pagation [13]. aging.…”

“…However, upon closer inspection using the SEM BSE detector, the observed fracture morphology of HT2, as shown in Figure 9, is actually predominately intergranular just to a lesser degree than HT1, as mentioned earlier. Dwell fatigue cracking in HT2 occurred For Ni-based superalloys such as 718Plus and Waspaloy, fatigue crack propagation may be either time or cycle dependent or some combination thereof, largely governed by such variables as temperature and environment [12,13]. The main mechanism for crack propagation in a cycle-dependent case is the cyclic plastic deformation immediately ahead of the crack tip, usually resulting in a transgranular type of fracture [4].…”

“…In the case of timedependent fatigue crack propagation, environmental effects (i.e. oxidation) and creep are the dominant damage mechanisms and these effects are diffusion-controlled [12,13]. As such, timedependent crack propagation is typically intergranular because the grain boundaries are preferential locations for higher diffusivity [12].…”

“…It has been reported that the main environmental effects of timedependent fatigue crack propagation in Ni-base superalloys is dynamic embrittlement and oxidation of grain boundaries and other microconstituents such as γ′ and grain boundary carbides, the latter two of which are stress-assisted mechanisms [12,13,14]. The dynamic embrittlement process, which is believed to be the governing mechanism for intergranular failure of Ni-base superalloys, involves short-range atomic oxygen diffusion (or other gaseous species) into the grain boundaries, which lead to grain boundary decohesion [14].…”

Grain Boundary Deformation and Fracture Mechanisms in Dwell Fatigue Crack Growth in Turbine Disk Superalloy ME3

“…In vacuum, since Q is idealized as a linear function of K max , K max ath is taken to be equal to that at Q = 0 which in turn identifies a single fracture process operating at the crack tip. The nature of the operating damage process is determined by comparing the current results of Q for ME3 to those identified in relation to known mechanisms by Starink and Reed [9]. The activation energy in vacuum in the range of 130-140 kJ/mol, corresponds to a single damage mechanism involving grain boundary (GB) creep.…”