| In recent years, there has been an increasing interest in ferromagnetic shape memory alloys (FSMAs) duo to their unique ability to produce very large output strains and rapid response frequency. However, the output stress in some of these FSMAs, such as Ni-Mn-Ga, only about 2MPa was achieved. This dramatically limits the extensive applications of this kind of alloys. To enhance the field-induced output strain and stress, a reversible magnetic-field-induced phase transformation was observed in Ni-Co-Mn-In alloys. The output stress can be up to 100MPa, about 50 times than that of Ni-Mn-Ga. This can be explained from the different mechanism between these two alloys. The shape memory effect of Ni-Mn-Ga is caused by the matensite variant reorientation as the result of magnetic induced twin boundary motion. And the shape memory effect of Ni-Co-Mn-In alloys relies on the magnetic-field-induced phase transformation. Therefore, Ni-Co-Mn-In alloys is paid more attention recently. This paper used thermal-simulation facilities to compress the Ni45Co5Mn36.7In13.3 alloy under different condition. The evolution of the microstructure was studied. By optimizing the experimental parameters, we found fine recrystallizing structure. In addition, precipitate was observed in some compressed samples. By analyzing the precipitate with optical microscopes, X-ray diffractometer, scanning electron microscopy, we draw some conclusions as followed:1. By optimizing the temperature, strain rate, we get the dynamic recrystallization structure in the Ni45Co5Mn36.7In13.3 alloy without microscopy or microscopic crack.2. As the temperature is higher than 800℃, meanwhile the stain rate is lower than 4x10-3s-1, we can observe the precipitate in metallographic investigation. The precipitates have two kinds of size. The smaller ones distribute along the grain boundary, and the lager ones present in the severely-deformed area.3. we analyze the content of the main elements in the precipitates using EDX, the results show that the composition change between these two precipitates is small, but they are different from the matrix. Especially in Co and In content. The amount of In is about 1/5 of the matrix meanwhile the Co is three times than the matrix. Ni and Mn have little changes. This is mainly because In has much bigger atomic radius, so it is hard to precipitate from the matrix, as well as the Co has the same occupation with In. The line scanning results agree well with the above analysis.4. XRD experiment of the deformation samples is carried out. Though the precipitate is found in these samples, but there are no other peaks except the peaks of the austenite. This shows that the precipitate have the same structure with the matrix.5. The Vibration Sample Magnetometer (VSM) results of these deformation samples shows that as the amount of precipitate increasing, the saturation intensity decrease dramatically. In sample C, we got the VSM curve of the matensite, there was no saturation intensity found within 2T. But after annealed at 900℃for 15h, the VSM curve changed to the same form as the original alloy. We can confirm that the matensite is caused by the deformation as the stain rate is much faster (6xl0-3s-1).6. This paper also tried to study the effect of Boron on the deformability of this system of alloys. But as the content of B was more than 0.1%, the B precipitate was formed along the grain boundary. This kind of precipitate caused the crack during the deformation process. The suitable parameter should be optimized in the future experiment. |
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