| In recent decades, high current pulsed electron beam (HCPEB) has been developed as a new surface modification technique. HCPEB acting on a material can deposit abundant energy within a very short time and generate transient heating, melting and evaporation of the surface layer to which the energy has been delivered. The dynamic stress fields induced by the rapid heating process lead to intense modifications that can extend several hundreds of microns in depth in the material. Correspondingly, improvements of the wear and corrosion resistances can also be achieved.Titanium and titanium alloys, due to their good strength to density ratio, corrosion resistance and biocompatibility, are widely used in applications such as biomedical, aerospace and automotive fields. However, titanium and titanium alloys are all facing some serious surface-related disadvantages, such as soft surface, poor wear or corrosion resistance, that have strongly limited their further industrial applications. Therefore, surface modification techniques should be applied on titanium and titanium alloys in order to improve their global performance. On the other hand, titanium and titanium alloys exhibit allotropic phase transformation, and may undergo metastable phase transformation under rapid solidification leading to the formation of metastable martensitic phase a'or a" with respect to their content ofβstabilizers. Thus, we can not only get further understanding of the solid phase transformation bahaviors induced HCPEB treatment, but also can explore the potential industrial applications of HCPEB surface treatment on titanium and titanium alloys. Due to the simplicity of the single crystal structure and the elimination of large-angle grain boundaries, the HCPEB surface treatment on Ni-base single crystal alloys can help to understand the modification mechanisms both in the surface melted layer and in depth with respect to the initial crystal orientations. It is therefore of grate interest in achieving a better understanding of the HCPEB technique.To this end, titanium alloys (commercially pure Ti, a near a Ti alloy TA15 and aβmetastable Ti alloyβ-Cez) as well as Ni-base single crystal alloys with different orientations (AM1 (100), AM1 (111) and CMSX2 (100)) have been selected and investigated. The main conclusions are summarized as follows: 1) The transformation mechanism of the microstructure in the surface layer of the HCPEB treated near a titanium alloy TA15 has been examined and the results indicated that a single hexagonal a'martensite structure was formed in the surface melted layer.It was found that the top surface layer, that has been melted and partially evaporated during the HCPEB treatment, rapidly solidified asβand transformed into monolithicα' martensite on following rapid cooling. The transformation sequence in the melted layer was determined to beα+βresidual→β→L→β→α', while that in the unmelted HAZ layer was a +βresidual→β+βresidual→α'+βresidual. Due to the formation of uniform monolithic ultrafine phase in the treated surface layer, the corrosion rate decreased and the corrosion resistance increased in 5 wt% NaCl aqua solution.2) After HCPEB treatment, the orthorhombicα'' martensite phase can be examined in the surface layer of both the single-β-phase andα+βdouble-phaseβ-Cez Ti alloys. The stress level induced by HCPEB treatment is experimentally estimated using the triggering stress for stress-induced martensitic phase transformation. The estimation indicated that the stress level induced by HCPEB irradiation can reach GPa magnitude.3) Singleα' martensite phase was also achieved in the surface layer of the HCPEB treated pure Ti samples. The surface hardness and the wear resistance of treated samples were improved. The corrosion resistance in 5 wt.% NaCl aqua solution was also improved probably due to the removal of impurities under HCPEB.4) It was found that the surface melted layers of the HCPEB treated Ni-base single crystal superalloys solidified in an epitaxial manner. Many slip lines were present on the treated sample surfaces, which may be due to stresses generated by HCPEB treatment. TEM and EBSD cross section analysis revealed that, a linear increase of misorientation angles in the melted layer relative to the substrate were observed with the slope of for the treated AM1 (111) sample doubled that for the treated AM1 (100) sample. The faster increase of misorientation angle for the AM1(111) sample is due to that the preferred growth orientation for cubic metals is the<100> direction and more defects can be introduced when solidified along the <111> direction. Significant difference in cross section hardness between the AM1 (100) and AM1 (111) samples were also observed. For the AM1 (100) sample, a subsurface hardened zone was found, which is clearly due to the presence of high density dislocations induced by the stresses generated by the HCPEB treatment. For the AM1 (111) sample, however, a subsurface softened zone was observed. This subsurface softening can be attributed to the coarsening of the precipitates resulting from the HCPEB treatment.5) A morphological analysis of crater formation and evolution in the Ni-base single crystal alloy CMSX-2 induced by HCPEB treatment has been carried out. It was found that the crater density of the single-crystal superalloys decreased as the number of HPCEB pulse increased; moreover, the crater density of CMSX-2 single crystal alloy is one or two orders of magnitude than that of the previously studied polycrystalline metallic materials. The different stages of crater formation have been proposed according to the crater morphologies. Firstly, the melted liquid droplet formed beneath the surface; then the melted droplet expands, overflows and finally erupts towards the treated surface, causing the formation of craters with a hole in center.
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