Study Of Microstructure Evolution And Property Of Severe Plastic Deformed Cobalt-base And Titanium-base Materials

Severe plastic deformation(SPD) provides a technique involving a complex stress state or high shear into the materials, which provides an area of controlling the microstructure of the SPD materials. The material with quite different microstructure and excellent performance can be obtained by employing a subsequent heat treatment to release the energy introduced by SPD. However, different materials have different structural characteristics with different energy states. Thus, it is particularly important to systematically study the effeceive method on different materials to release the energy introduced by SPD. In the present study, we chose Ti Ni shape memory alloy, Sm Co/a-(Fe,Co) nanocomposite magnets and TiZrAlV alloy as model systems to study the microstructure evolution processed by severe plastic deformation and subsequent thermal treatments releasing the energy introduced by deformed samples by employing an X-ray diffraction(XRD), a transmission electron microscopy(TEM), a differential scanning calorimetry(DSC) and a micro material test machine(Instron 5948). And the subjects about deformation mechanism, crystallization kinetics and mechanical properties of these three advanced materials have been investigated as follows:Partially crystallized amorphous alloys Sm Co/Fe and Sm Co/FeCo were synthesized by mechanical alloying. The crystallization process and kinetics of these amorphous alloys upon annealing releasing the energy introduced by alloying have been investigated, as well as the nucleation, growth kinetics and atomic process of the nanocrystals in SmCo/FeCo alloy. For SmCo/Fe amorphous alloy, with the increase of the annealing temperature to 700 °C, a Sm Co5 phase and a Sm Co7 phase precipitate in sequence then the SmCo5 transforms to SmCo7. Differently, the SmCo/FeCo amorphous alloy processes only one phase transformation to SmCo7 after annealing to 700 °C. The preexisting α-(Fe,Co) phase possesses a growth activation energy smaller than the nucleation activation energy and growth activation energy for the newly formed SmCo7 phase. This demonstrates an easy growth of the α-(Fe,Co) nanocrystals and a relatively difficult formation of the SmCo7 phase, which leads to a large grain size for the α-(Fe,Co) soft phase in the α-(Fe,Co)/SmCo7 nanocomposite magnets. The activation volumes ΔVg*=0.67Ω and ΔVg*=0.99Ω(Ω=average atomic volume) for the growth of α-(Fe,Co) and SmCo7 nanocrystals were obtained, indicating that the atom diffusion during the growth of both the α-(Fe,Co) and the SmCo7 are assisted bythermal vacancies.The hierarchical structures were introduced into a quaternary TiZrAlV alloy by treating the room temperature rolling sample via recrystallization and two-step aging treatment to release the high storage of cold-work energy introduced by rolling. A combination of high strength(~1548 MPa) and high ductility(~6.6%) was achieved in one hierarchically structured sample. The effects of rolling and thermal treatment on the microstructure evolution and the tensile properties of the rolled sample have been investigated. The sample exhibits a higher wear resistance than the nanostructured counterpart, which can be attributed to the combination of high strength and high ductility provided by the multi-modal-laminated structure.The nanocrystalline TiNi with an average grain size below a critical size of 10–20 nm were prepared by annealing and room tensile process to release the energy introduced by electroplastic rolling. The effects of electroplastic rolling and thermal treatment on the microstructure evolution and the tensile properties of the rolled sample and the deformation mechanisms operating with stress have been investigated. We demonstrate a sequential variation of the deformation mechanism from grain boundary(GB) sliding and grain rotation to grain growth and dislocation activity with the increase of the deformation stress. The present study provides new insights into deformation mechanisms at small grain sizes, and has a direct implication for designing NC materials with high intrinsic ductility through employing mechanically driven grain growth and dislocation activity.