Abstract
The near threshold fatigue crack growth in ultrafine-grained (UFG) copper at room temperature was studied in comparison to conventional coarse-grained (CG) copper. The fatigue crack growth rates da/dN in UFG copper were enhanced at ΔK ≤ 7 MPa√m compared to the CG material. The crack closure shielding, as evaluated using the compliance variation technique, was shown to explain these differences. The effective stress intensity factor amplitude AKeff appears to be the same driving force in both materials. Tests performed in high vacuum on UFG copper demonstrate the existence of a huge effect of environment with growth rates higher of about two orders of magnitude in air compared to high vacuum. This environmental effect on the crack path and the related microstructure is discussed on the basis of fractography observations performed using scanning electron microscope and completed with field emission scanning electron microscope combined with the focused ion beam technique.
Highlights
Severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP), have been widely used to obtain ultrafine grain (UFG) metals with high tensile strength and fairly large plasticity at low temperatures
It can be seen that material subjected to 5 passes exhibits a considerable increase in the crack growth rates in the range of stress intensity factor below 7 MPa√m; the curves merge together at higher ∆K values, namely above 7 MPa√m (Fig. 1a)
It comes out from this figure that the differences observed in the nominal curves between UFG and CG copper are rationalized in terms of the effective stress intensity factor range ∆Keff after crack closure correction
Summary
Severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP), have been widely used to obtain ultrafine grain (UFG) metals with high tensile strength and fairly large plasticity at low temperatures. Reduction in the crack growth resistance with decreasing grain size was reported in Cu, Ni, Ti and Al-alloys [3,4,5,6,7]. This detrimental effect is attributed to the greater availability of grain boundaries in orientation favorable for crack propagation in a material with an UFG microstructure. The current study aims to investigate whether or not the intrinsic propagation mechanism is altered by UFG microstructure For this purpose, propagation of long fatigue cracks was studied in the mid stress intensity factor amplitudes and in the near threshold range in UFG copper obtained by ECAP. The crack growth mechanisms are discussed according to the testing environment based on local fracture surface observations and focused ion beam (FIB) cuts
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
