Shape Memory Alloy Synthesis

Cu-Al-Ni alloy sample. (Photo: Matthew Beaudouin-Lafon)

Shape memory alloys (SMAs) have a fascinating property: after being deformed, heat returns them to their original shape. Our team took on the challenge of synthesizing our own SMAs using simple manufacturing techniques and reproducing the "shape-memory" effect.
We cast various Cu-Al-Ni (copper, aluminum, nickel) alloys and used different heat treatments until we achieved the characteristic microstructure of our target material. We were not able to reproduce the mechanical properties of SMAs, but we got some pretty amazing pictures!
For more details, refer to our technical report.

Optical micrograph of non-heat-treated sample.
Etchant: 2 parts H2SO4 : 1 part 3% H2O2 : 2 parts H2O.
While the striped sections, with a variety of orientations, are most likely the martensitic β′ phase that we were loking for, the presence of orange and silver sections, copper-rich α and aluminum-rich γ1 phases (respectively), indicates that the sample was not quenched rapidly enough to prevent them from forming. An alloy with good shape-memory properties requires a uniform transformation into pure martensite.

Optical micrograph of annealed sample.
Etchant: 2 parts H2SO4 : 1 part 3% H2O2 : 2 parts H2O.
The annealed sample clearly depicts the needle-like, twinned structure of β′ martensite. The different orientations of the grains minimize the strain in the sample; upon deformation, the variants align themselves in one direction and allow the transformation from martensite back to austenite without breaking any bonds.