Behavior Of Super-undercooling And Directional Solidification Of Co-Ni-Ga(Al) Alloys

As new smart materials, ferromagnetic shape memory alloys (FSMAs) not only have the properties of traditional shape memory alloys, such as the temperature controlled shape memory effect and superelasticity, but exhibit the unique magnetic field controlled shape memory effect as well as the magnetic field induced strain. Co-based FSMAs, including Co-Ni-Ga and Co-Ni-Al, with high magnetocrystalline anisotropy energy and good hot-working ability have recently attracted much interest from material researchers.The dissertation systematically investigated the oriented-growth pattern of matrix, the competitive growth relationship of multiphase, the characters of nucleation and growth of precipitation and the changes of martensitic transformation temperatures of Co-Ni-Ga and Co-Ni-Al FSMAs. The microstructural evolution, oriented growth and transformation temperatures of the alloys were studied with various processing methods, such as undercooling rapid solidification, undercooling directional solidification, zone melting liquid metal cooling directional solidification and related heat treatment. The relationship of oriented-polycrystal growth with solidification condition was concluded, and a nevol approach of preparing single-crystal was proposed, which drafted a foundation for the appliance of Co-based FSMA. The main findings are the follows:By glass fluxing combined with superheating cycling, Co-Ni-Ga melt was denucleated by 70% Na₂B₄O₇ + 30% NaSiCa glass,and obtained bulk undercooling of above 200 K which guarantees the success of undercooling rapid solidification of Co-Ni-Ga. The effect of undercooling on microstructure in two-phase (?+?) alloys (Co₅₀Ni₂₅Ga₂₅, Co₅₀ Ni₂₂Ga₂₈) and single-phase Co₄₅Ni₂₅Ga₃₀ alloy was investigated. Results show that the grain size and the volume fraction of the?phase in as-solidified Co₅₀Ni₂₅Ga₂₅ alloys can be controlled by melt undercooling. With the increasing of undercooling, the?-phase size obviously decreases, its distribution becomes homogeneous and its volume fraction grows. However, the?phase completely disappears in the undercooled Co₅₀ Ni₂₂Ga₂₈ alloy, which hence becomes a single martensitic alloy. The different results of the two two-phase alloys treated by undercooling are attributed to the segregation-free solidification due to bulk undercooling. The martensite lath is obviously refined and a mass of sub-grains are produced with the increase of undercooling in Co₅₀ Ni₂₂Ga₂₈ and Co₄₅Ni₂₅Ga₃₀ alloys. Undercooling rapid solidification introduces a number of dislocations and large internal stress, which give rise to the production of sub-grains in recovery, while the refinement of microstructure is due to the increase of nucleation rate in undercooled melt. For the annealed alloys, the grain size and volume fraction of?phase obviously increase and the sub-grains completely disappear.The effect of undercooling on martensitic transformation temperatures of Co-Ni-Ga alloys is also remarkable. For martensitic Co₄₅Ni₂₅Ga₃₀ alloys, the martensitic transformation temperatures totally increases by 50 K after being treated at undercooling of 220 K; that of Co₅₀ Ni₂₂Ga₂₈ alloys is also observed to rise by about 26 K. The increase of martensitic transformation temperature results from the large internal stress caused by undercooling solidification. Thus, it can be concluded that martensitic transformation may be directly induced by high undercooling for austenitic alloys. Accordingly, the austenitic Co₅₀ Ni₂₀Ga₃₀ alloy was denucleated and a maxium undercooling of 298 K was achieved. The microstructure tranformates from single austenite to single martensite after undercooling treated. The martensitic transformation temperatures are also elevated by 40 K and reach above room temperature. Moreover, pre-martensitic transformation is observed in the Co₅₀ Ni₂₀Ga₃₀ alloy undercooled by 179 K. Therefore, martensitic transformation can be directly induced by undercooling.Bulk textured martensitic Co-Ni-Ga rods were successfully prepared by the novel technique of undercooling directional solidification. The as-grown rods have very homogenous composition along the axis, and the microstructure exhibits coarse columnar crystal with strong [111]M texture, which confirms that the undercooling directional solidification is feasible and effective in preparing Co-Ni-Ga oriented alloys. The effect of triggering undercooling on the microstructure and texture of directional solidified Co₅₀ Ni₂₀Ga₃₀ was also discussed. Undercooled melt was triggered to directionally solidify at the undercooling of 70 K and 186 K respectively. The microstructure of the Co₅₀ Ni₂₀Ga₃₀ rod triggered by 70 K exhibits an evolution process of fine equiaxed grain to coarse columnar crystal and to coarse equiaxed grain, while that of 186 K shows martensitic single crystal with sharp cracks, which are approximatively perpendicular to each other and form quite regular parallelograms. The two rods with different triggering undercooling are respectively with (110)A and (111)M preferred orientation.In order to develop the appliance of Co-based alloys, the low-cost alloy of Co-Ni-Al was studied to find out the effects of solidification rate and temperature gradient on the polycrystal microstructure and crystal orientation. By means of zone melting liquid metal cooling directional solidification, a serial of Co₃₂Ni₄₀Al₂₈ rods were prepared at large scopes of temperature gradient and solidification rate. Results show that the rod exhibits coarse columnar crystal with strong orientation at the condition of high temperature gradient and low solidification rate. On the contrary, the microstructure exhibits cellular crystal or equiaxed grain with non-preferred orientation. Based on the results, austenitic Co₃₇Ni₃₄Al₂₉ oriented rods were grown under super-high temperature gradient. At the temperature gradient of 800 K/cm, the rod along the <110> direction grows up at the solidification rate of 15?m/s; when the solidification rate increases to 150?m/s, the preferred orientation changes to <100> direction. The phenomenon is more remarkable at the temperature gradient of 1200 K/cm. The change of preferred orientation is attributed to the different kinetics of crystal growth under different conditions. Thus, the selective oriented growth of Co-Ni-Al crystals can be obtained by controlling solidification condition.