The effect of austenite aging at 823 K on the microstructures and martensitic transformation behavior of Co 46 Ni 27 Ga 27 alloy has been investigated using optical microscopy （OM）, transmission electron microscopy （TEM）, X-ray diffraction analysis （XRD）, and differential scanning calorimeter （DSC）. The microstructure observation results show that the unaged Co 46 Ni 27 Ga 27 alloy is composed of the tetragonal nonmodulated martensite phase and face-centered cubic γ phase. It is found that a new nanosized fcc phase precipitates in the process of austenite aging, leading to the formation of metastable age-affected martensite around the precipitates with composition inhomogeneity. Two-stage reverse martensitic transformation occurs in the samples aged for 2 and 24 h due to the composition difference between the age-affected martensite and the original martensite. For the Co 46 Ni 27 Ga 27 alloy aged for 120 h, no reverse transformation can be detected due to the disappearance of the metastable age-affected martensite and the small latent heat of the original martensite. The martensitic transformation temperatures of the Co 46 Ni 27 Ga 27 alloy decrease with an increase in aging time.
Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Nis6.xMn25FexGa19 （x = 0, 2, 4, 6, 8） shape memory alloys were investigated using optical microscopy （OM）, X-ray diffraction analysis （XRD）, differential scanning calorimeter （DSC）, and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56Mn25Ga19 alloy has no effect on phase structure. These alloys all exhibit a ther- moelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature （Mp） decreases from 356℃ for x = 0 to 20℃ for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni-Mn-Ga alloys. Substituting Fe for Ni in Ni56Mn25Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.
The microstructure, martensitic transformation behavior, mechanical properties and shape memory effect of Ni54Mn25Ga15Al6 high-temperature shape memory alloy were investigated. By comparing with the Ni54Mn25Ga21 alloy, the effect of Al addition on the properties of Ni-Mn-Ga alloys was analyzed. The results show that the Ni54Mn25Ga15Al6 alloy has a single-phase tetragonal non-modulated martensite structure with lamellar twins. The martensitic transformation start temperature of this alloy is up to 190 ℃, displaying the promising application as a high-temperature shape memory alloy. Al addition in Ni-Mn-Ga alloy can decrease the martensitic transformation temperatures due to the effect of size factor and improve the strength and plasticity. However, the shape memory effect is reduced remarkably with the Al addition.
