ECCI Characterization of Dislocation Structures at a Non-propagating Fatigue Crack Tip: Toward Understanding the Effects of Mn-C and Cr-N Couples on Crack Growth Resistance

We attempted to clarify the underlying mechanisms of the enhanced small-fatigue-crack resistance of Fe-Mn-C twinning-induced plasticity (TWIP) steel and high-nitrogen austenitic steel. To this end, we performed electron channeling contrast imaging near the tips of non-propagating fatigue cracks in Fe-18Cr-14Ni steel without a significant amount of interstitials, Fe-23Mn-0.5C TWIP steel, and Fe-25Cr-1N austenitic steel. The fatigue crack non-propagation limits of the TWIP steel and high-nitrogen steel were higher than that of the steel without a significant amount of interstitials; the higher limits of the TWIP steel and high-nitrogen steel are attributed to the Mn-C and Cr-N interactions, respectively. The enhanced small-fatigue-crack resistance of the Fe-23Mn-0.5C steel is attributed to local hardening at the crack tip caused by an increase in the dislocation density via dynamic strain aging. The enhanced dislocation planarity of the Fe-25Cr-1N steel, which is a result of the Cr-N interaction, is a significant factor that influences (i.e., increases) the crack resistance. The enhanced dislocation planarity results in dislocation pile-up stress at the crack tip, thereby preventing dislocation emission from the crack tip.