Role of crack nucleation versus propagation on the fatigue behavior of titanium-vanadium-iron-aluminum beta titanium alloy.

摘要

The current research is aimed at understanding the effect of microstructure on the fatigue behavior in a commercially used Ti-10V-2Fe-3Al beta titanium alloy. The effect of four beta-annealed microstructures (FC, o-aged, alpha-aged-LS and alpha-aged-HS) on fatigue crack nucleation, fatigue crack growth and the relative importance of the crack nucleation and crack propagation phases with respect to the total fatigue life was investigated.; Tensile deformation behavior revealed the formation of grain boundary cracking at yield point in the o-aged microstructure that was absent in the FC and alpha-aged-LS microstructures. A laser interferometric displacement test system was used to evaluate the elastic modulus (E) of individual grains as a function of the aging time, in the o-aged as well as in the alpha-aged-LS microstructure. It appears that the o-aged microstructure is elastically more anisotropic than the alpha-aged-LS microstructure that seems to reasonably explain the grain boundary cracking observed at yield.; The fatigue crack growth behavior was similar in FC, alpha-aged-LS and o-aged microstructures. The alpha-aged-HS microstructure had the worst near threshold response among the four microstructures.; Fatigue limit was the lowest in the o-aged microstructure and the highest in the alpha-aged-HS microstructure. The o-aged microstructure also exhibited the worst high cycle fatigue response. Fatigue crack nucleation was observed to occur by cleavage in all the four microstructures. Subsurface crack nucleation was observed at lower cyclic stress levels while cracks nucleated from surface at higher stress levels.; Fatigue crack nucleation site sizes were measured from the fracture surface and an equivalent crack nucleation site was calculated. A short crack analysis of the nucleation site sizes verified the long crack nature of these cracks, within the limits of experimental errors and crack closure restrictions. A generalized crack growth rate equation was developed for each microstructure that describes the entire regime of the fatigue crack growth curve. In the high cycle fatigue regime, residual life calculations indicate that in all the four microstructures, the fatigue crack propagation phase is only a small fraction of the total fatigue life. Thus, under high cycle fatigue conditions, the crack nucleation phase appears to dominate the fatigue process.