Surface Modification Of Pure Aluminum By High Current Pulsed Electron Beam

In recent years, high current pulsed electron beam (HCPEB) has been intensely developed for surface modification of materials. This paper first describes the mechanism of the generation of I-ICPEB on the 揘adezhda-2?system and the testing results of its characteristic parameters. Then, experimental results are presented on samples of pure aluminum. Based on the analyses by metallography, SEM, TEM and microhardness distribution, the mechanism for the 1-fCPEB treatment are investigated. After a discussion on the interaction of accelerated electrons with solid materials, a theoretical model for the study of energy deposition of electrons in materials is proposed on the basis of some empirical formula. The temperature and stress fields during the treating process are simulated according to the model using numerical calculation. At last, examples of HCPEB treatments on some practical alloys such as stainless steel lCrl8Ni9Ti and bearing steel GCrI5 are presented. A duplex treatment using successively ion implantation and HCPEB post-treatment is carried out to reveal the effects of electron beam aided enhanced diffusion.The HCPEB source consists mainly of anode spark plasma, cathode explosive electron emission and magnetism restricted conveyance. High current pulsed electron beam with round section diameter of c1?Omm, energy up to 4OkeV, current density of lOkA, and adjustable pulse duration between 3 and 6 jis can be produced in this system. By selecting cathode accelerate voltage, pulse duration and specimen-anode distance, different HCPEB can be attained.For aluminum irradiated by HCPEB of 28keV and 4.5 p.s, surface melting of about I jim in thickness is observed. Craters, characteristic for high-energy beam bombardment, are observed on the melted surface. A large amount of point defects are formed in the near surface zone. They tend to accumulate themselves near dislocations and grain boundaries. Due to these point defects, preferentially perforated holes with {220} planes as the hole walls are produced after electrochemical thinning for TEM sample preparation. Melting layer is separated from the unmelted matrix by a clear boundary. Stacking faults and mosaic domains are formed beneath the melted layer. Special wave front morphology is found both near the surface layer and in the interior about 0.5mm beneath the surface. Thisis the direct evidence for supporting the presence of plastic stress wave. The microhardness varies as a function of the depth from the surface. Its peak value appears at about 40 ~tm after single electron pulse. After multi-pulse treatment, the distribution of microhardness becomes more complicated, with influenced depth up to hundreds of micrometers.A three-dimension model was proposed based on the beam-target interaction process and experimental parameters. The alternate finite differential and finite element methods are adopted in the calculation to solve the equations of temperature and stress fields. Heat compensation and enthalpy are considered separately. The computational results show that rapid heating and cooling at rate of 108K/s occur in the surface layer of lOj.tm thick during the treating process of aluminum. The sharp temperature gradient induces a thermal stress up to l0~ Pa, affecting a depth zone up to 301.tm, far beyond the heat-affected zone.For stainless steel lCrl8Ni9Ti treatment, an increase in surface hardness is accompanied by an enhancement of the corrosion resistance. Under proper treating conditions, the surface of steel GCrI5 can be hardened without loosing its original bulk ductility. Post-treatment by HCPEB after ion implantation changes the ion distribution profile to the depth of micrometer level due to enhanced diffusion induced by electron beam bombardment.