| Recent development and research work carried out using the fairly recently developedHCPEB technique have been reviewed by underlying its effects under the "melting","heating" and "evaporating" treatment modes.At first, different physical models for the HCPEB treatment of materials under the threedifferent treatment modes have been proposed and numerically solved.The results of calculations concerning the heating mode have revealed that (â…°) the quasistatic thermal stress is orientation dependent and has an amplitude of several hundred MPa,which is enough to induce significant plastic deformation; (â…±) the amplitude of the thermalstress wave is only about several tens of kPa, which in many cases can be omitted. Thereby,this suggests that the quasi-static thermal stress together with the fast thermal cycle shouldmake it possible to modify substantially the surface layers under the heating mode.Calculations concerning the melting mode have shown that (â…°) the melting starts from thesubsurface, (â…±) the depth of the melted layer-which was in good agreement with theexperimental observations - depends on the material properties and input electron beampower and (â…²) the melt solidifies at a speed of several m/s. The subsurface melting is one ofthe factors inducing the formation of craters and shock waves. The rapid solidificationauthorizes the homogenization of the melted layer and specific grain growth behaviors.Concerning the evaporating mode, the model can describe quantitatively the wholeevaporation process and the depth of evaporated layer. Therefore, these models have beendemonstrated to be useful tools to give new insight in the understanding of the phenomenainduced by the HCPEB treatment.Subsequently, the intriguing surface modifications associated with the HCPEB treatmentwere detailed. Techniques such as TEM, SEM, EBSD, X-ray diffraction and SNMS can beused, in a complementary manner, to analyze accurately the surface and subsurfacemodifications encountered in the materials. Some of the different features observed under thethree modes are in the following 3 parts.a) The major results corresponding to materials treated under the melting mode are:1. Homogenization of the melted layer.The high current pulsed electron beam is demonstrated to be an effective way to achievesurface homogenization of metallic materials containing precipitates/inclusions. The treatment induces the formation of craters by eruptions of the second phase inclusions thatlead to a selective purification of the surface melted layer.2. Ultra fine grain and nanostructure formations.The results gathered here show the very strong potential for surface nanocystallization ofmaterials with improved properties by HCPEB technique. The mechanisms were identified toproduce ultra fine microstructures from the melted liquid by taking advantage of the rapidsolidification from the melt as well as solid state martensitic transformation in steels.3. Metastable phase formations.The rapid thermal cycles generated by the HCPEB treatment has the potential ofgenerating metastable structures. This was particularly detailed in the case of different steels.For example, austenite was retained at room temperature in the D2 steel treated by HCPEBdue to the stabilization from high Cr and C content and the grain size effect.4. Formation of specific textures.The HCPEB irradiation has induced ultra fine grain structures with specific texture statesin the melted surface layers of a D2 steel. Rapid solidification of the supersaturated phase hasled to a mixed <200> (weak) + <220> (strong) parallel to ND fiber texture to which wasassociated a high portion of twin boundaries. The formation of twin boundaries is likely toplay a major role in the growth mechanisms of the highly undercooled melt that determinesthe presence of the final <220> texture.5. Surface alloying.Surface alloying by HCPEB under melting mode is an efficient way for improving thesurface properties of materials. The material pre-deposited on the surface can be rapidlymixed into the substrate during melting. This led to the modification of structure, phase andcompositions in the surface layer.6. Subsurface hardening.On the basis of a description of the transient thermo-mechanical processes associated withthe HCPEB bombardment, it is pointed out that the deep modification effect are caused by thestress wave originated from the quasi-static stress and sublayer melting. The intense stresswave interacts and modifies the structure and properties over a depth zone of a few hundredsmicrometers, far beyond the heat-affected zone.b) Under the combined action of the temperature and stress fields, significant modification ofthe microstructures and compositions were also demonstrated to take place at the surfacewithout melting. The main findings obtained under the so called heating mode are:1. Effectiveness of the thermomechanical cycles.It is clear that the HCPEB irradiation induces a dynamic temperature field in the surfaceof the material and that, concomitantly to the thermal effect, the pulse electron beam creates a dynamic stress field that causes intense deformation at the material surface and subsurface.The deformation is demonstrated to be orientation dependent and formed under the biaxialquasi-static thermal stress occurring during the HCPEB bombardment. For the 316L stainlesssteel, twinning was activated in grains having <111> close to ND while high misorientationdeformation gradients were created by intense crystallograplic slip in the other grains. For theFe (40 at%)Al intermetallic, local strain resulted in those grains having <100> close to ND tobe at a lower level on the surface than the other grains.2. Texture modification and grain refinement.The surface of an initially extruded Fe(40at%)Al sample characterised by a sharp <110>fiber texture parallel to the sample normal direction was modified towards a broader <321>fiber. Together with the texture evolution, a grain refinement effect was observed. Botheffects are due to the formation of new (low angle) grain boudaries induced by the repeatedcycles of deformation and dynamic recovery/recrystallization during the HCPEB treatment.3. Surface purification.Surface purification in particle reinforced alloys can also occur when the material istreated under the heating mode. As shown in the case of a Y2O3 oxide strengthened Fe (40at%)A1 alloy, the oxide particles were evaporated and removed by HCPEB treatment withoutany melting of the FeAl matrix.c) The effect of the HCPEB treatment for modifying the surfaces of materials underevaporating mode has also been clearly established. The main features generated by theevaporating mode are:1. Surface aspect modification.In addition to the formation of craters, the action of the electron beam under evaporatingmode is also to modify the surface morphology by forming a wavy aspect on the surface. Thiswavy aspect is the consequence of the combination of (â…°) melting and subsequent selective"local" evaporation and (â…±) deformation in the melted layer operating at the surface of thealloys.2. Selective evaporation.The evaporation mode can generate the selective evaporation of some species at thesurface of the treated alloys because they have lower vapor pressure.3. Evaporation/condensation phenomena.The effect of the evaporation mode is more pronounced after sufficient number of HCPEBpulses; a stage at which a part of the vapor often recondenses on the treated surface. This isclearly shown by (â…°) the presence of small protrusions and/or rounded droplets solidified onthe wavy surface and (â…±) the special composition distributions along depth after intensiveevaporation. 4. Special texture modification.A strong <110> fiber is created in the treated NiTi alloy after HCPEB treatment underevaporating mode due to the rapid solidification and evaporation/condensation mechanisms.Compared to the melting mode, the <110> oriented grains are often isolated island and, whengrouped together, are not twinned.Finally, the last section of this document has concentrated on potential applications of theHCPEB technique to improve the materials' properties. Under all the three treatment modes,HCPEB treatment is proved to be an efficient way to modify the surface of metallic materialsto taylor metallic materials' properties. The combination of super fast thermal cycles anddynamic stress fields makes it possible to modify substantially the surface characteristics and,in many cases, improve the mechanical properties faster and more efficiently than otherconventional surface treatment techniques. For examples, as a result of the ultra fine structureformations, metastable phase formations and strain hardening, surface hardness and wearresistance can be effectively improved. Selective surface purification effect can generatesurface layers having improved corrosion properties. Other possible applications of theHCPEB technique, for example rapid surface alloying, have been proposed in the literature.Overall, this thesis clearly demonstrate that the high potential of the HCPEB techniquecan be better achieved by a good control of the processing parameter in order to treat thesample surfaces under the most appropriate mode.
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