Relationship Between Functional Fatigue and Structural Fatigue of Iron-Based Shape Memory Alloy FeMnNiAl

Abstract

Utilizing FeMnNiAl single crystals subjected to fatigue loading, we make the important observation that the crack nucleation from the notch and the ensuing crack trajectory follows the most favorable martensite variants. These variants form in an asymmetric pattern with respect to the crack plane because of the underlying elastic anisotropy which is accounted for in the driving force analysis of the fatigue cracks. When the recoverable strain on a particular variant is exhausted, new variants are activated; this in turn changes the crack path. The activation of new variants also results in transient deceleration of the fatigue crack growth rates and upon subsequent growth, fatigue crack growth trends merge with the steady state behavior. Two surface analysis using FIB/TEM facilitated the unambiguous identification of the specific martensite variants responsible for fatigue crack growth. Displacement fields obtained via digital image correlation allowed for the determination of the local stress intensity which is inevitably affected by the activation/arrest of martensite variants. Hence, we make a direct link between the complex functional fatigue behavior and the fatigue crack growth behavior. Overall, the results show the steps to be considered to develop a framework for understanding fatigue crack growth response of shape memory alloys.