High Pressure Nano Mechanics: An In Situ Synchron X-ray Study

The thesis focuses on nano mechanics on metal Fe, Co, Ni and ceramics TiC and diamond using the in-situ high temperature-high pressure synchrotron x ray diffraction. Though the detail anylasis of position, broadening and intensity of the diffraction peaks, we derive the model for the models for the nano mechanics and especially the different behaviors of the dislocations. The main results include:(1) We present a comparative study of mechanical properties of bcc nano-crystalline iron (nano-Fe) and microncrystalline iron (micron-Fe), nano-crystalline cobalt (nano-Co) and microncrystalline cobalt (micron-Co) by in-situ high-pressure synchrotron x-ray diffraction under tri-axial compression. For nano-Fe with a starting high dislocation density of 1016 m-2, the peak broadening is almost reversible upon unloading from 8.6 GPa to atmospheric pressure, indicating that no additional dislocations are built up during compressive deformation inside grains, at grain boundaries or twin boundaries. Furthermore, an orientation dependent surface strain is found to be stored in the surface layer of the bcc nano Fe, which is in agreement with the core-shell model of the nano crystals. For micron-Fe, a significant and continuous peak sharpening and the associated work softening were observed after the sample is yielded at pressures above 2.0 GPa, which can be presumably attributed to a pressure-induced dislocation annihilation. This finding/interpretation supports the hypothesis that the annihilation of dislocations is one of the dominant mechanisms underlying the plastic energy dissipation. The determined yield strength of 2.0 GPa for nano-Fe is more than 15 times higher than that for micron-Fe (0.13 GPa), indicating that the nano scale grain-size reduction is a substantially more effective strengthening mechanism than the conventional carbon infusion in iron. Relative to micron Co, nano Co exhibits extra degrees of strain induced peak broadening during loading yet a better strain recoverability after unloading. These observations suggest different deformation mechanisms with intragranular strains dominated in nano Co and intergranular strains in micron Ni. The determined flow stresses are 2.0 and 2.9 GPa, respectively, for micron and nano Co, indicating that Co is the strongest metal among 3d transition metals. It is observed that the bulk modulus of nano Co (216 GPa) is 17% higher than that of micron Co (184 GPa). This finding supports a generalized model of nanocrystals with pre-compressed surface layers.(2) Compression-tensile cyclic deformation experiments are conducted on the nano Ni (8-12 nm, 1016 m-2 dislocation density) at three different confining pressure (16 ton, 32 ton and 50 ton) and temperature ( up to 900°C) by in situ synchrotron x-ray deformation dia apparatus. From the strain-stress analysis, the yield strengths at compression and tensile mode are almost the same, the confining pressure shows no effect on the yield strength on nano Ni, indicating that the dislocations, although in very high density, are frozen inside the grains and no longer effect the elastic/plastic deformation. The nano Ni can endure substantial mechanical fatigue during the cyclic stress loading. From the (111)/(200) peak intensity ratio study, we find that the grain rotation is one of the main factor for the yielding of nano Ni. Furthermore, the nano Ni could maintain its strength until 190°C, the stress relaxation begin at 220°C.(3) Thermoelastic properties and equation of states are studied on titanium carbide at pressures and temperatures up to 8.1 GPa and 1273 K using synchrotron x-ray diffraction. The nano TiC and the micron TiC almost have the same bulk modulus which means the core-shell model in nano metals is not suitable for ceramics, like TiC. The strain-stress relations of nano TiC at 2 GPa is determened by d-dia press with in-situ synchrotron diffraction. Different from nano Ni, the nano TiC have very strong history effect during the deformation. During the first deformation cycle, the deformation behavior of nano TiC is though the grain crack, while, in the second deformation cycle, the plastic deformation is mediated by dislocations, a yield strength of 8 GPa of nano TiC is derived based on the strain-stress curve.(4) The constitute properties are obtained at NSLS using deformation-DIA on polycrystalline and nano diamond at different P-T conditions. As expected, even at total strains up to 15%, we did not observe the yield point of diamond at room temperature and a confining pressure of 4 GPa. However, for deformation at 1000 and 1200...