Phase Transformation And Formation Mechanism Of Nano-twins In Pulse Electrodeposited Nanocrystalline Co-Ni Alloys

Nanocrystalline material is one of the hot topics in the research field of materials. Materials with nano-sized grains always show certain unique properties, such as electronic, optical and catalytic characteristics. The phase transformations and grain growth always take place when the temperature changes. So it is important to investigate grain growth and phase stability in nanocrystalline materials.Meanwhile, the formation of some substructures, such as nano-twins, can improve the performance of nano-materials. In the present years, many research works have been done on physical properties and mechanical properties of metallic materials with nano-scale twins. However, the research of strengthening mechanism of twin boundaries and the development of nano-twinned metals still face many challenges, among which to develop the preparation of nano-twinned metals is one of the biggest challenges.In the present work, a series of nanocrystalline Co-Ni alloys with different components were prepared by pulse electrodeposition. The content of this paper includes: 1. the preparation of nanocrystalline Co-Ni alloys by pulse electro-deposition; 2. grain growth and phase transformation in nanocrystalline Co-Ni alloys; 3. formation mechanism and properties of nano-twins.The grain growth and phase transformation in nanocrystalline Co-Ni alloys during annealing treatments or tensile tests were characterized by DSC(differential scanning calorimetry), XRD(X-ray diffraction) and TEM(transmission electron microscopy). The phase transformation is significantly inhibited by the nano-grains, while the phase transformation from HCP(Hexagonal-Close-Packed) to FCC(Face-Centered-Cubic) and grain growth are the mutual promotion processes, which can be triggered by a sufficiently high temperature or a sufficient applied strain.A nano-twinned structure had been observed in the Co-Ni deposits after annealing treatments or tensile tests, accompanying phase transformation coupled with grain growth. This way has provided a new method to generate nano-twinned structure via phase transformation coupled with grain growth. This FCC phase nano-twinned structure demonstrates an extraordinary stable within the temperature range from 4.2K to 1000 K, which may facilitate its applications in a wide temperature range. The abnormal stability of the nano-twinned structure is attributed thus to the structure itself.