Fe Mn Al Ni Shape Memory Alloys Research Articles

The effect of precipitates on the superelastic response of [1 0 0] oriented FeMnAlNi single crystals under compression

FeMnAlNi shape memory alloys were recently discovered to have a small temperature dependence of the superelastic critical stress in a large superelastic temperature window from −196°C to 240°C. In this work, we investigated the effect of B2 nanoprecipitates on the superelastic characteristics of [100] oriented Fe43.5Mn34Al15Ni7.5 single crystals under compression, and found that the size, volume fraction and composition of precipitates strongly influence the transformation temperature, superelastic strain, critical stress for stress-induced martensitic transformation and stress hysteresis of the single crystals. The single crystals aged at 200°C for 3h show 7.2% superelastic strain with the precipitate size of about 6–10nm. Longer aging times or higher aging temperature reduces superelastic recovery due to the coarsening of the precipitates.

On the effect of gamma phase formation on the pseudoelastic performance of polycrystalline Fe–Mn–Al–Ni shape memory alloys

In recent years, iron-based shape-memory-alloys such as Fe–Mn–Al–Ni came into focus, due to superior pseudoelastic performance and concomitantly low costs of the alloying elements. The formation of the ductile γ-phase in polycrystalline Fe–Mn–Al–Ni and its impact on the pseudoelastic performance have not been addressed, yet. Based on modification of quenching conditions this study shows that controlled γ-phase precipitation at grain boundaries can suppress intergranular cracking without affecting pseudoelasticity. In situ incremental strain tests revealed good recoverability up to about 8% strain.

Superelastic response of a single crystalline FeMnAlNi shape memory alloy under tension and compression

Polycrystalline FeMnAlNi shape memory alloys were recently shown to possess a small temperature dependence of the stress for the onset of martensitic transformation (σSIM). In this work, the superelastic behavior of single crystalline Fe43.5Mn34Al15Ni7.5 samples oriented along the [100] direction was investigated under tension and compression after a precipitation heat treatment at 200°C. In constant strain, multi-temperature experiments, the single crystals showed a σSIM vs. temperature slope of 0.54MPa°C−1 in tension and 0.41MPa°C−1 in compression, and superelasticity over a wide temperature range from −80°C to 160°C. The irrecoverable strains in both tension and compression samples detected during the superelastic experiments were found to be due to retained martensite in detailed transmission electron microscopy investigations. The volume fraction of the retained martensite in the samples tested under tension was considerably larger than those for the samples tested in compression showing that the transformation is less recoverable in tension. The differences in the volume fraction of retained martensite and reversibility in both tension and compression are attributed to high density of dislocations in the tension samples as compared to the compression samples. The differences between the shape of the stress–strain curves under tension and compression are attributed to the lower number of martensite variants activated under tension as compared to compression, which were clearly verified with the transmission electron microscopy examinations.