Defects Of FCC Metal Materials Induced By High Current Pulsed Electron Beam

Recently, high-current pulsed electron beam (HCPEB) has been used as a toolfor material surface processing, and is becoming of incresing interest to materialprocessing. It is characterized by a high power density of 10⁸-10⁹ W/cm² at thetarget surface. During the interaction of incident pulsed electron beam with thesurface of materials, such a high energy is deposited only in a very thin layer withina short time and causes superfast processes such as heating, melting andevaporation. The improved physical, chemical and strength properties of thematerial unattainable with conventional surface treatment techniques may beobtained. Otherwise, we can observe the evolution of structure defects, especiallydefect clusters, induced by HCPEB irradiation due to a short pulsed time and revealthe natures and the formation sequences of the defect clusters. It is necessary tofurther understand the modification mechanism and irradiation damagecharacteristic caused by HCPEB irradiation. Therefore, we determine to investigatein detail the structures and microstructures of some typical metals induced byHCPEB.Based on a physical model, the temperature profile is simulated for the purealuminum The depth of heat-affected zone of the carbon steel is below 30μm. Themelting starts from 0.45μs and the site is at a sub-layer about 0.7μm from thesurface. The heating and quenching rate is approximately 10⁸K/s and 10⁶K/s,respectively.The pure aluminum with various pre-deformation extents was irradiated with4J/cm² energy density by using a high-current pulsed electron beam (HCPEB)source. The effect of the structure defects on the crater formation was investigatedusing optical microscopy and scanning electron microscopy (SEM). Thetemperature rises faster at a subsurface layer instead of at the outermost surface dueto the maximum energy deposition located at subsurface depth of the beam. Such asubsurface layer heating and melting mechanism causes eruptions of the subsurfacelayer liquid matters through the outermost surface and produces the typical surfacecrater morphology. The crater distribution density along with the grain boundaryand slipping band of dislocation were determined. The experimental results suggestthat the crater distribution density increases with increasing the pre-deformationextents of the aluminum sample, and the crater prefers to form at the structuredefects such as grain boundary and slipping band of dislocation, which seem to beembryos for craters. As a result of these experiments, the most probablemechanisms of crater formation on the surface of pure aluminum were established.After irradiated by HCPEB on different pre-deformed pure aluminum,structure defects and craters are analyzed systematically. The relationship betweenstructure defect and craters are studied. It is concluded that the main reason forforming craters is that sub-surface melts first and then spurt to the surface.Structure defects have great influence on the formation of craters. Craters willincrease with the increasing of structure defect and intend to be formed along grainboundaries and dense dislocations. Structure defects are the embryonic stage ofcraters. Moreover strong pulse electric beam can lead to a lot of voids caused byvacancies gathering at grain boundary and dislocations. Combining experimentalproof with theoretical analysis, the relationship between defect and craters arestudied.Surface treatment by HCPEB is carried out on different multi-crystal purealuminum, single-crystal Cu and nano-Cu. The influence of crystal defect oncraters is analyzed. It is shown in the experiment that craters of single-crystal Cu,multi-crystal pure aluminum and nano-Cu will dramatically increase in the order. Itis analyzed that HCPEB surface treatment increase the vacancies density of surfaceof material to 10-3, and vacancies gather lots of holes along grain boundary anddislocations.Analyzing surface microstructure of single-crystal Cu by TEM methods findsthat dislocations caused by HCPEB will increase as times of bombardmentsincrease and no longer increase when it gets to a maximum amount anddislocations density will slightly lower by multi bombardments. The quantity ofvacancy clusters (10²²-10²³) will reduce as times of bombardments increase anddimension (nm) increase. More dot defects exist in vacancy clusters.