Diffusive and diffusionless phase transitions in severely deformed Cu-Al-Ni shape memory alloys

Gabriel A. López 1 Iñaki López-Ferreño 2 Boris B. Straumal 3,4,5 Askar Kilmametov 4 Horst Hahn 4 Brigitte Baretzky 4 María L. Nó 1 Jose San Juan 2
1Applied Physics II, University of the Basque Country, Spain
2Condensed Matter Physics, University of the Basque Country, Spain
3Laboratory of Interfaces in Metals (LIM), Institute of Solid State Physics, Russian Academy of Sciencies, Russia
4Insitute of Nanotechnology, Karlsruher Insitute of Technology, Germany
5Laboratory of Hybrid Nanomaterials, National University of Science and Technology MISIS, Russia

Severe-plastic-deformation (SPD) drives the accelerated mass transfer and diffusive phase transitions like dissolution of precipitates, decompositions of a supersaturated solid solution, amorphization and/or nanocrystallization. However, such diffusive transformations taking place simultaneously with diffusionless (displacive or martensitic) phase transitions have not been thoroughly investigated in heavily deformed materials. The purpose of the current work was to investigate the combination of displacive (austenite ↔ martensite) and diffusive (decomposition of supersaturated solid solution) phase transitions in two Cu-Al-Ni shape memory alloys (SMA) during high pressure torsion (HPT). The idea of applying an SPD processing is to achieve nanostructured Cu-based SMAs to overcome the typical brittleness of this family of alloys. After homogenization in the one-phase (austenitic) β-area of the Cu-Al-Ni phase diagram and quenching, the first alloy was in martensitic state, and the second one remained austenitic (β3 phase). Afterwards, HPT was performed for both alloys at room temperature. A detailed microstructural characterization of the final material was carried out by electron microscopy and X-ray diffraction together with some functionality tests. It was observed that the applied processing led to the precipitation of stable phases, α1-phase in the first case and γ1-phase in the second one, as if they were annealed at a certain elevated effective temperature Teff. After HPT, both alloys contained mainly β’3 martensite with a certain amount of γ’3 martensite. Thus, the HPT-driven diffusive transformations (precipitation of α1 and γ1 phases) influence the subsequent displacive (martensitic) transformation. Simultaneously, a dramatic grain refinement was achieved, what opens new possibilities to investigate functional properties in nanostructured Cu-Al-Ni alloys. [1]

[1] B.B. Straumal, A.R. Kilmametov, G.A. López, I. López-Ferreño et al. Acta Mater. 125 (2017) 274.









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