Role of nanotwins on fatigue crack growth resistance – Experiments and theory

• Experimental fatigue crack growth behaviors of Ni–2.89%Co are investigated.
• DIC and TEM characterizations are used to study the microstructure.
• Atomistic simulations are employed to study the role of cross-lip at a twin boundary.
• Lattice friction stress is obtained from Peierls–Nabarro model.
• ΔKth,eff levels are predicted and the sensitivity of input variables is studied.

The study of near-threshold fatigue crack growth has long remained an empirical field due principally to the highly microstructure-sensitive nature thereof. The primary challenges have been to forward physical model(s) informed by the governing micromechanism(s), which would be able to predict the experimental behaviors devoid of empiricism. Today, we have sophisticated experimental techniques (e.g. digital image correlation, electron microscopy) as well as atomistic simulation tools (e.g. molecular dynamics) at our disposal to finally revisit the century old fatigue problem in the light of physical phenomena therein. This paper is geared towards achieving such a feat with a very special type of materials, nano-twinned alloys, as the candidate materials, which are of great recent interest due to their reportedly superior damage properties. Specifically, we investigate how the microstructural features (e.g. slip transfer mechanism at coherent twin boundaries, twin thickness/spacing, frictional stress, pre-existent near-tip slip density) can be modulated to improve the damage resistance. The results suggest that these parameters considerably affect the crack propagation impedance (as quantified in terms of ΔKeffth). A thorough discussion of the current findings and the most recent literature developments in this regard are provided.