Mesoscopic Mechanical Behavior Of Polycrystalline Materials Subject To Large Deformation At Elevated Temperatures

Due to different combinations of temperature and grain size and strain rate, materials show quite novel mechanical behavior. In this dissertation, Investigations were performed aiming at metallic polycrystals with medium grain size. Numerical simulations and uniaxial tension tests were carried out to investigate the evolution of microstructure and mesoscopic viscoplastic behavior at elevated temperature.From an examination of the tensile behavior of CZ LY12 alloys under different temperature and strain rate, two kinds of deformation and intergranular fracture behaviors were observed that were opposite to the characteristic strain rate range in fine-grained superplasticity. The mechanisms are described as follows.The high ductility achieved at high strain rate is generally attributed to the dominant role of GBS accommodating mechanisms, which is considered as dislocation creep within grains controlled by subgrains. The presence of small amount of liquid at grain boundaries tends to form thin viscous layer which surrounds the finer grains. Intergranular fracture may occur at the grain boundaries between two fine grains not covered by viscous phases. The high ductility achieved at low strain rates is generally attributed to dislocation glide-creep accommodation mechanisms. The stress concentration produced by GBS is relaxed by this accommodation process. Large amount of liquid at grain boundaries assists the process of GBS. Thick viscous phases surround the coarser grains. Many filaments observed on the fracture surface may be the viscous phase at elevated temperature.It was observed, from experiments, that this alloy presented abnormal performance after undergoing low strain rate and high-temperature deformation. The flow stress reduced to only one percent of the yield point at room temperature and the elongation to fracture increase twice. This room-temperature high ductility strongly depends on tensile strain rate and temperature and the cooling rate. It is noted that the precipitation accounts for this high ductility at room temperature. So it can be concluded that ductility can be enhancedgreatly by controlling LY12 precipitation behavior.Based on the microscopic TEM and SEM observations, relationships between dislocation structure and rate-dependent/rate-independent behaviors under certain range of temperature and strain rate were presented. Interactions between grain boundaries and dislocations were analyzed. Moreover, the rate-dependent deformation could also be explained from the point of grain boundary energy.Based on test results, numerical simulations on uniaxial tension at 480 and different strain rate have been conducted. Simulated results showed that the position and the evolution process of necks are influenced by tensile strain rates. Uniform deformation covers only a small fraction of the total deformation. However, at the steady-like flow stage, restrictions and accommodations between scattered localizations promote large uniform deformation.On the basis of macroscopic simulated results, polycrystal model including grain boundary zones was established to model the mesoscopic processes. The reduced localizations can be understood by the effects that grain boundaries promote rotation of grains in shear bands, while the accommodating role of grain boundaries to deformation can be understood by restraining rotation of grains in non-shear bands. From the view of overcoming interlocking between neighboring grains, increasing grain boundaries relative thickness implied grains mutual motion more easily. Above analysis can provide instructions for grain boundary design and controlling in grain boundary engineering.Dynamics intergranular fracture process is modeled with considerations of grain boundaries damage. When intergranular damaged elements reached a certain numbers, the behavior of the corresponding representative elements will be softened. Intergranular failure path accelerates localized plastic deformation along some shear bands and alleviates others.