Effect of thermomechanical treatment on the structural, phase transformations and properties of the Cu — Al — Ni shape memory alloys

The optical and electron microscopy, electron diffraction and X-ray diffraction have been used to study the effect of thermomechanical processing on grain structure, average grain size, structural and phase transformations in ternary shape memory Cu-Al-Ni alloys. In the studied alloys with a fixed content of Ni in an amount of 3 wt.%, the concentration of aluminum varied from 9 to 14 wt.%. It is shown that in the alloys after thermomechanical processing, including forging and homogenizing annealing using controlled recrystallization in austenite state and also subsequent quenching, grain-boundary decay and segregation are eliminated. It was found that the microstructure of alloys in a hot-tempered and quenched state with an aluminum content of 9–10 wt.%. is represented by grains with an average size of 60–80 $\mu$, with a content of 10–12% by weight of aluminum 100–350 $\mu$, whereas in alloys with an aluminum content of up to 14% by weight, the average grain size reaches 0.5–1 mm. At the same time, according to the mechanical tests at room temperature, as the aluminum content decreases, the tensile strength $\sigma_{u}$, the yield strength $\sigma_{m}$, the elongation $\delta$ increase. The increase in the mechanical properties of alloys is due to the refinement of the grain structure of the $\beta_{2}$-austenite and the package substructure of $\beta_{1}^{'}$- and $\gamma_{1}^{'}$ -martensites with decreasing aluminum content in alloys. Thus, for fine-grained alloys with 9.2 and 9.5% Al, the value of the relative elongation remains at a good level (grater than 10%), and for the remaining alloys with an aluminum content of 10–14 wt.% it does not exceed 5%. With an increase in the aluminum content in alloys, character of failure changes (from viscous to brittle) of samples under uniaxial tension.