Acoustic emission monitoring of orientation-dependent functional fatigue in superelastic CuAlMn single crystals

This study investigates the orientation dependence of functional fatigue in CuAlMn single crystals subjected to 100 superelastic compression cycles. Six crystallographic orientations, [ 4 3 4 ] , [ ̄ 4 ⁢ 4 1 ] , [ ̄ 2 ⁢ ̄ 3 ⁢ 4 ] , [ 1 3 2 ] , [ 4 ⁢ ̄ 2 ⁢ 1 ] , and [ 1 1 ⁢ ̄ 2 ] , were examined using a combination of in-situ acoustic emission (AE) monitoring, stereo digital image correlation (DIC) on two adjacent surfaces, and micromechanical calculations to determine the orientation-dependent activation of slip systems. Application of the HDBSCAN clustering algorithm to the AE data acquired during loading categorizes the signals into two clusters. Cluster 1, associated with the martensitic phase transformation, comprises high-energy AE signals in the 100–300 kHz frequency range. Cluster 2, attributed to dislocation plasticity and interface-boundary motion, consists of lower-energy AE hits spanning approximately 100–600 kHz. Orientations with poor superelastic stability— [ 1 3 2 ] , [ 4 ⁢ ̄ 2 ⁢ 1 ] , and [ 1 1 ⁢ ̄ 2 ] —exhibit more intense plasticity-related AE activity (cluster 2). Micromechanical calculations explain why the [ 1 3 2 ] , [ 4 ⁢ ̄ 2 ⁢ 1 ] , and [ 1 1 ⁢ ̄ 2 ] orientations show poorer fatigue resistance than the [ ̄ 4 ⁢ 4 1 ] , and [ ̄ 2 ⁢ ̄ 3 ⁢ 4 ] orientations, despite their much lower transformation stresses. It is revealed that the internal stresses generated by the formation of martensitic variant pairs substantially exceed the externally applied stresses, making internal stress the dominant factor governing promotion of plasticity and slip system activation. This internal stress is higher in the poorly performing orientations, regardless of their transformation stress level. The differences in the generated internal stresses are due to the formation of different lattice correspondence variant pairs that generate different levels of stress. The results also demonstrate that the superelastic cyclic stability of single crystals exhibits a non-monotonic dependence on transformation stress.