The maximum actuation frequency of magnetic shape-memory alloys ͑MSMAs͒ significantly increases with decreasing size of the transducer making MSMAs interesting candidates for small scale actuator applications. To study the mechanical properties of Ni-Mn-Ga single crystals on small length scales, two single-domain micropillars with dimensions of 10ϫ 15ϫ 30 m 3 were fabricated from a Ni-Mn-Ga monocrystal using dual beam focused ion beam machining. The pillars were oriented such that the crystallographic c direction was perpendicular to the loading direction. The pillars were compressed to maximum stresses of 350 and 50 MPa, respectively. Atomic force microscopy and magnetic force microscopy were performed prior to fabrication of the pillars and following the deformation experiments. Both micropillars were deformed by twinning as evidenced by the stress-strain curve. For one pillar, a permanent deformation of 3.6% was observed and ac twins ͑10M martensite͒ were identified after unloading. For the other pillar, only 0.7% remained upon unloading. No twins were found in this pillar after unloading. The recovery of deformation is discussed in the light of pseudoelastic twinning and twin-substrate interaction. The twinning stress was higher than in similar macroscopic material. However, further studies are needed to substantiate a size effect.
The magnetomechanical properties of ferromagnetic shape memory alloy Ni-Mn-Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy ͑AFM͒ and magnetic force microscopy ͑MFM͒ were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni-Mn-Ga single crystal. Experiments were performed in the martensite phase at 25°C and in the austenite phase at 55°C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature ͑55°C͒, the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25°C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys.
Is it possible for a rocky planet to have too much internal heating to maintain a habitable surface environment? In the Solar System, the best example of a world with high internal heating is Jupiter’s moon Io, which has a heat flux of approximately 2 W m-2 compared to the Earth’s 90 mW m-2. The ultimate upper limit to internal heating rates is the Tidal Venus Limit, where the geothermal heat flux exceeds the Runaway Greenhouse Limit of 300 W m-2 for an Earth-mass planet. Between Io and a Tidal Venus there is a wide range of internal heating rates whose effects on planetary habitability remain unexplored. We investigate the habitability of these worlds, referred to as Ignan Earth’s. We demonstrate how the mantle will remain largely solid despite high internal heating, allowing for the formation of a convectively buoyant and stable crust. In addition, we model the long-term climate of Ignan Earth’s by simulating the carbonate-silicate cycle in a vertical tectonic regime (known as heat-pipe tectonics, expected to dominate on such worlds) at varying amounts of internal heating. We find that Earth-mass planets with internal heating fluxes below 30 W m-2 produce average surface temperatures that Earth has experienced in its past (below 30 oC), and worlds with higher heat fluxes still result in surface temperatures far below that of 100 oC, indicating a wide range of internal heating rates may be conducive with habitability.
Ni-Mn-Ga is a ferromagnetic shape memory alloy that deforms by twin boundary motion. The magneto-mechanical properties depend strongly on the twin microstructure. A thermomechanical treatment was applied to a Ni-Mn-Ga single crystal with coexisting 10M and 14M martensite structures to create twin boundaries and align the short crystallographic c direction preferentially perpendicular to the surface. The resulting twin structure was characterized using atomic force microscopy (AFM) to obtain the surface relief caused by twinning. Magnetic force microscopy (MFM) was used to find the direction of easy magnetization (which coincides with the crystallographic c direction) of each twin. Among 18 possible twinning histories, ac twinning in 10M martensite was identified as the unique solution. The combination of AFM and MFM after thermomechanical treatment provides a nondestructive characterization of the twin microstructure including the identification of the crystallographic orientation of each twin and of each twin boundary plane.
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