Microstructures Of Metal Materials Induced By High Current Pulsed Electron Beam

Recently, high-current pulsed electron beam (HCPEB) has been used as a tool for material surface processing, and is becoming of incresing interest to material processing. It is characterized by a high power density of 108-109 W/cm2 at the target surface. During the interaction of incident pulsed electron beam with the surface of materials, such a high energy is deposited only in a very thin layer within a short time and causes superfast processes such as heating, melting and evaporation. The improved physical, chemical and strength properties of the material unattainable with conventional surface treatment techniques may be obtained. Otherwise, we can observe the evolution of structure defects, especially defect clusters, induced by HCPEB irradiation due to a short pulsed time and reveal the natures and the formation sequences of the defect clusters. It is necessary to further understand the modification mechanism and irradiation damage characteristic caused by HCPEB irradiation. Therefore, we determine to investigate in detail the structures and microstructures of some typical metals induced by HCPEB. The pure aluminum with various pre-deformation extents was irradiated with 4J/cm2 energy density by using a high-current pulsed electron beam (HCPEB) source. The effect of the structure defects on the crater formation was investigated using optical microscopy and scanning electron microscopy (SEM). The crater distribution density along with the grain boundary and slipping band of dislocation were determined. The experimental results suggest that the crater distribution density increases with increasing the pre-deformation extents of the aluminum sample, and the crater prefers to form at the structure defects such as grain boundary and slipping band of dislocation. As a result of these experiments, the most probable mechanisms of crater formation on the surface of pure aluminum were established. We study the microstructure and phase transformation on the near-surface of 0.20%C and 0.45% C carbon steel during the HCPEB processing by using electron microscopy. Based on a physical model, the temperature profile is simulated for carbon steel. The depth of heat-affected zone of the carbon steel is below 10μm. The melting starts from 0.6 μs and the site is at a sublayer about 0.25 μm from the surface. The heating and quenching rate is approximately 108K/s and 106K/s, respectively. Obvious changes in microstructure and significant hardening occurred in the depth of 200-250μm from the surface after HCPEB irradiation. The laminated structure of pearlite were substituted by dispersive rounded-like cementites in the near-surface. The effect of HCPEB treatment can reach more than 500μm depth from the surface after HCPEB post treatments. The intense stress wave induced by HCPEB treatment is the origin of material modification in deeper regions. Rapid heating and solidification induced heavy plastic deformation, which caused the formation of the dislocation cells by 1-pulse bombardment. After multi-pulse bombardments, both austenite and carbide types of nanostructure particles were formed from the supersaturated Fe (C) solid solution phase. After 10-pulses bombardments, besides austenite and carbide types of nanostructure particles, additional amorphous structure was formed in the local zones. The supersaturated carbon solution in the parent ferrite phase due to dissolution of cementite is the origin of the nanostructured formation. The 304L austenite stainless steel was irradiated by HCPEB in different processing. The deformation microstructures were observed by using transmission electron microscopy (TEM). The relationship between stress characteristic and the microstructures has been investigated. The experimental results show that very high stress value was induced and significant hardening and reduction of ductility occurred in the irradiated materials during HCPEB irradiation. Dislocation tangles and small, isolated size stacking faults smaller than about 20nm were dominant at one pulse HCPEB irradiation. Large stacking faults (about a few hundreds of nanometers) and twin bands became dominant at multi-pulses HCPEB irradiation. Higher strain is located near the twin bands. More broaden and intersect twin bands were observed in the sample with ten pulses irradiation. Based on the experimental results, the deformation mechanism of 304L austenite stainless steel, formationmechanism of twins and the physical mechanism of the stress induced by HCPEB irradiation were discussed. In order to investigate high speed deformation mechanism of aluminum, we have irradiated the specimens of single-crystals aluminum by using a Nadezhda-2 HCPEB device with 1Jcm-2 and 4Jcm-2 energy density, respectively. It was found that deformation twinning occur in the specimens treated by 1Jcm-2 energy density and both of 1 and 5 pulses on irradiated surface. No melting evidence is associated to the twins producing. Intersecting twin lamellae with approximately 200-400 μm long were formed in the surface of 1 pulse specimen, whereas parallel twins with approximately 200 μm long were observed in the surface of 5 pulses specimen. In the case of 4J cm-2 energy density, surface melting occurred; leaving crater-like surface and no deformation twinning took place. In the case of 1J cm-2 energy density, tremendous numbers of point defects are formed in the samples of aluminum single crystal due to {111} atomic plane displacement simultaneous along <111> directions. After 1 and 5 pulses, vacancies agglomerate to form various vacancy clusters, and no dislocation reaction happen during vacancies agglomeration. Interstitial atoms are spent in order to increase the number atomic planes, which produce a change in sample shape, and (or) escape from surface. They mainly agglomerate at irradiated near-surface. There is close relation between dislocation loop and SFT and between SFT and void. The present results seem to indicate that the formation sequence is firstly the dislocation loop, follow by SFT and finally the void. The vacancy clusters are formed whthin a time scale which is shorter than a HCPEB pulse duration (1.5μs). The evolution of interstitial atomic clusters is important to form vacancy clusters and deformation behavior. The numbers of interstitial atom increase with increasing of the HCPBE pulses. They agglomerate to evolve to complicated dislocation structures. Voids can also form by aggregation of vacancy clusters along dislocation lines. After 1 and 5 pulses, no interstitial atomic clusters are observed. In the case of10 pulses, large numbers of atomic clusters and dislocation structures are present at the near-surface of the sample while deformation twins are absent.