Acoustic emission screening of thermo-mechanically processed shape memory alloys in tension and compression – The role of irreversible processes in Fe-Ni-Co-Al-Ti-B SMA

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Iron-based shape memory alloys (SMA) have been in the focus of interest for some time due to their low material costs and significantly simplified manufacturing conditions compared to Ni-Ti-FGL. Nano-scaled precipitates in the alloys Fe-Ni-Co-Al-X (X = Ti, Ta) and Fe-Mn-Al-Ni are indispensable for the realization of shape memory effects (SME). At the same time, they also have a significant impact on the resulting mechanical properties. For single crystals of these alloy systems, the relationship between precipitates, transformation behavior, mechanical properties and microstructure is already well understood. Through the application of thermo-mechanical process chains, promising material states for SME have already been achieved for polycrystals. However, in both cases a comprehensive database for understanding functional and structural fatigue mechanisms is still missing. Irreversible processes such as dislocation motion, interaction of different martensite variants and interactions with nano-scaled precipitates, which are associated with phase transformation, play a significant role in this context. An excellent method to characterize the kinetics of phase transformation and the underlying microstructural processes is the measurement and evaluation of acoustic signals during mechanical testing in these materials. The aim of the proposed project is to elucidate the role of irreversible processes for both pseudoelasticity and thermally-induced phase transformation by applying acoustic emission analysis. The alloy system Fe-Ni-Co-Al-Ti-B is used for the investigations. For the first time, acoustic emission is used to investigate the processes of thermoelastic martensitic phase transformation under mechanical loading in order to understand the kinetics of the ongoing deformation processes. Acoustic emission measurements are accompanied by further complementary in situ characterization techniques such as digital image correlation, thermography and X-ray diffraction but also by comprehensive microstructural investigations using scanning and transmission electron microscopy. Starting point of the investigations are differently oriented single crystals with a high degree of reversibility. The results of the acoustic emission analysis at these states serve as a benchmark for the understanding of the processes in polycrystalline material states. The polycrystalline states are adjusted by thermomechanical processes (rolling, heat treatment) in such a way that either a strong <001> texture for expected good reversibility or states with a high degree of irreversibility are present. The long-term goal of the project is (i) the prediction of functional degradation using real-time screening by means of acoustic emission during the application of SMAs and (ii) the prediction of suitable microstructures for different application scenarios of SMAs.