I. Karaman, Texas A&M University, College Station, TX; H. E. Karaca, B. Basaran, University of Kentucky, Lexington, KY; Y. I. Chumlyakov, Siberian Physical Technical Institute, Tomsk, Russia; H. J. Maier, University of Paderborn, Paderborn, Germany
Ferromagnetic SMAs have attracted increasing interest because of the ability to obtain one order of magnitude higher recoverable magnetic field induced strain than other active materials. A recently discovered ferromagnetic shape memory CoNiAl and CoNiGa alloys have promising shape memory characteristics for medium to high temperature applications. We have conducted an extensive work to capture several aspects of the shape memory behavior of both alloys in single and polycrystalline forms. It has been demonstrated that these alloys have perfect pseudoelasticity with low stress hysteresis even at temperatures higher than 200 °C, high strength for dislocation slip, large recoverable strain levels (10%), large pseudoelastic temperature window (>250 °C), low stress for martensite reorientation, and stable response to cyclic deformation at high temperatures. It was observed that the stress hysteresis decreases with increasing temperature above austenite finish temperature and temperature hysteresis decreases with increasing constant stress level. Strong orientation dependence and tension/compression asymmetry in shape memory characteristics has also been investigated. An unexpected phenomena, transformation induced twinning in the parent phase, was observed. This mechanism led to recovery of more strain than the applied one under tension due to back transformation into the twinned structure instead of into the initial single crystal. Selected experimental findings on single and polycrystals that summarize these findings will be presented and the challenges will be addressed.
Ferromagnetic SMAs have attracted increasing interest because of the ability to obtain one order of magnitude higher recoverable magnetic field induced strain than other active materials. A recently discovered ferromagnetic shape memory CoNiAl and CoNiGa alloys have promising shape memory characteristics for medium to high temperature applications. We have conducted an extensive work to capture several aspects of the shape memory behavior of both alloys in single and polycrystalline forms. It has been demonstrated that these alloys have perfect pseudoelasticity with low stress hysteresis even at temperatures higher than 200 °C, high strength for dislocation slip, large recoverable strain levels (10%), large pseudoelastic temperature window (>250 °C), low stress for martensite reorientation, and stable response to cyclic deformation at high temperatures. It was observed that the stress hysteresis decreases with increasing temperature above austenite finish temperature and temperature hysteresis decreases with increasing constant stress level. Strong orientation dependence and tension/compression asymmetry in shape memory characteristics has also been investigated. An unexpected phenomena, transformation induced twinning in the parent phase, was observed. This mechanism led to recovery of more strain than the applied one under tension due to back transformation into the twinned structure instead of into the initial single crystal. Selected experimental findings on single and polycrystals that summarize these findings will be presented and the challenges will be addressed.
