Mechanism And Numerical Simulation Of Materials Modification By High Current Pulsed Electron Beam

This dissertation focuses on mechanism of material surface modification in high current pulsed electron beam (HCPEB) processing. Based on our physical model for temperature and stress fields, we resolve the temperature field and coupled dynamic thermal stress field in the process of material surface modification by the HCPEB bombardment for the first time by an alternate implicit differential method. These numerical simulation results.explain satisfactorily certain important experimental phenomena associated with the HCPEB processing such as formation of surface craters, deep modification effects, and plastic deformations.Non-linear and non-equilibrium thermodymanic-mechanical equations are used to fully describe dynamic processes induced by the HCPEB bombardment, such as the transient temperature field and the coupled temperature-dynamic thermal stress field. Specifically, quasi-static stress, thermoelastic stress, shock stress, quenching stress, and residual stress are analyzed.In the temperature field simulation, following assumptions and approximations are taken into account: temporal functions of pulsed voltage and power are fitted with experimentally measured data, phase transition related physical parameters such as the specific heat capacity and heat conductivity etc. are treated temperature-dependentally, and the latent heat is also compensated with temperature. These numerical simulations are carried out for Aluminum and a Cr-steel to reveal the temperature distributions in the samples, which provide quantitative information of the melting layer thickness (1-5 urn), the heating/cooling rate (108~109K/s), the temperature gradients (107-108K/m), the initial subsurface layer melting spots (about 1μm), and the maximum crater depths(1-3μm), etc.. The crater formation is, for the first time, related to the subsurface layer heating and melting, which cause eruptions of the subsurface layer liquid through the outer solid surface.The formation and evolution of stresses are also resolved in these numerical simulations. If the target is not melted (either before melting or due to insufficient beam energy density), there exist two kinds of stress fields induced by the bombardment, a thermoelastic stress wave and a quasi-static thermal stress. After the target starts melting, a thermal stress shock is generated due to the liquid state metalerupting force. The shock is also a stress wave propagating along the direction of the electron beam. Due to high magnitudes of the quasi-static stress and the shock stress wave, plastic deformation often occurs. During the cooling process the quenching stress is produced, and after the process a residual stress is also remained in the target. The magnitudes of quasi-static stress, quenching stress and residual stress in the near surface area are in a range of several hundreds of MPa to several GPa. Whereas, the thermoelastic stress wave has small amplitudes, typically less than 0.1 MPa, too weak to exert any influence to the materials. The shock stress is hwoever similar to the quasi-static stress in amplitudes.In the heat-affected zone, the quasi-static stress, generally higher than the dynamic yield strength of common materials, is responsible for the plastic deformation, in both twinning and in dislocation modes. The shock stress wave and the stresses associated with cooling can also lead to plastic deformations, and their affected depth extends far beyond the heat-affected zone. Thus the presence of defect structures such as twinnings, dislocations, slipping bands, and surface cracks in Al and steels are all in results of those stresses.The cross-section microhardness measurement for aluminum and steels processed by HCPEB shows typical oscillatory distribution profiles over hundreds of micrometers in depth. This special microhardness distribution can be explained by shock wave transmission, reflection and overlapping. Especially in multi-pulse HCPEB treatments, the effects of the stress wave are accumulated and show complicated stress patterns. The observation of wave front con...