Possible mechanisms for degradation of photo induced micro actuation effect in a ferromagnetic shape memory alloy at high temperatures

摘要

Ferromagnetic shape memory alloys (FSMAs) have been in active research nowadays for their unique properties of responses to both temperature changes and external magnetic fields. Recently, one such material, a CoNiAl based system, showed a photoinduced microactuation (PIMA) effect in which an incident spot of a tiny laser beam could induce actuation in these systems. This is unparalleled in ordinary materials, and its real cause is yet to be found. However, we tried to still use it in real world engineering applications, and for that, the limitations on its usability should be found. With this idea, we studied here in detail the effect of continuous degradation of the PIMA effect on a ribbon sample through TEM, as it was exposed to higher temperatures from room temperature. While the as-spun alloy had microstructured B2 ordered (Ni, Co)-Al micrometer sized grains embedded in a continuous L1(2) ordered cobalt-rich matrix channel, at 400 degrees C where the PIMA effect was fully destroyed, Co-rich nanosized precipitates were found to have formed inside the B2 grains, the actual FSMA component in the sample, and these are also found to have been oxidized. Concurrent oxidation studies showed the formation of an irregular shaped composite oxide layer with a thickness of about several hundred nanometers at the top surface. These are possibly the reasons behind the hindrance in the actuating motion. This was corroborated by magnetic studies also. Published under license by AIP Publishing.