Abstract:
Polycrystalline FeMnAlNi shape memory alloys were recently shown to possess a small temperature dependence of the stress for the onset of martensitic transformation (sigma(SIM)). 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 degrees C. In constant strain, multi-temperature experiments, the single crystals showed a sigma(SIM) vs. temperature slope of 0.54 MPa degrees C-1 in tension and 0.41 MPa degrees C-1 in compression, and superelasticity over a wide temperature range from -80 degrees C to 160 degrees 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.
