#weekendusers How do some alloys remember their original shape after deformation?

12-04-2017

Shape Memory Alloys (SMAs) are metals that “remember” their original shape and go back to it when they are deformed. Their multiple applications go from medical devices, orthodontic material or automobile and aeronautics components. French scientists are studying what happens in their microstructure when they stretch.

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This weekend SMAs will keep busy a French team from the Centre Inter-universitatire de Recherche et d’Ingénierie des MATériaux (CIRIMAT), in Toulouse, and the Laboratory of Microstructure Studies and Mechaniscs of Materials (LEM3) in Metz, as they carry out experiments on ID11. Their goal is to gather information about the martensitic phase transformation in a copper-based shape memory alloy when exposed to stress.

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Younes Elhachi, PhD student, and Pavel Sedmak, post-doctoral researcher at the ESRF and local contact, in the experimental hutch. 

SMAs having become widely used in recent years has meant that scientists need to develop simulation tools to support the design of these alloys. X-ray diffraction can help to identify the behaviour of the different phases (austenite and martensite). All the data coming out of this experiment will set up a good database to validate mechanical models at the phase scale. Currently, micromechanical simulations are validated at the macroscopic scale and considered to provide  access to microscopic informations, due to a lack of experimental data at fine scales of the microstructures.

This is the second time that the team comes to ESRF’s ID11. “This beamline is the only one that allows to use both 3D X-ray diffraction method and Diffraction Contrast Tomography on the same sample”, explains Benoit Malard, main proposer of the experiment. Improvements in the tensile device since the first beamtime means more accuracy in their results. “We can see how the new crystal structure forms after applying a bit of stress and how the internal grains rotate”, explains Younes Elhachi, PhD student.

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Part of the team discusses the data. 

With the data, scientists will now analyze the behaviour of each individual grain considering its real grain-boundaries shape and the neighbouring grain. “This is of utmost importance as the martensitic transformation starts at the grain boundaries and it is influenced by neighbouring grains, which can induce local internal stress”, explains Sophie Berveiller, scientist at LEM3. “Then the polycrystal will be reproduced by finite elements and strained in tension to reproduce the tensile test; this will allow to improve the micro-mechanical models currently used”, she concludes. 

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Working hard on the beamline. 

Text by Montserrat Capellas Espuny

Top image: The microstructure of a Shape Memory Alloy (SMA). Credits: Sophie Berveiller.