SURFACE LAYER HIGH-ENTROPY STRUCTURE AND ANTI-CORROSION PERFORMANCE OF AERO-ALUMINUM ALLOY INDUCED BY LASER SHOCK PROCESSING

7075 aluminum alloy is an ultra-high strength alloy containing Al, Zn, Mg, Cu and Cr elements,and is widely used in the aviation industry, but it has severe intergranular corrosion characteristics. The high-entropy alloys are composed of more than five major metallic elements and possess excellent corrosion resistance.When laser shock, featuring ultra high energy as well as the thermodynamic and kinetic loading characteristics farfrom-equilibrium states, acts on the surface of alloys with multiple elements, high-entropy alloy surface layer with specific properties may be obtained. In this work, surface modification of 7075-T76 aluminum alloy by laser shock was investigated. The microstructure, formation cause of the amorphous/nano- crystalline composite high- entropy alloy surface layer obtained by laser shock, hardness and corrosion resistance of the laser were analyzed by means of SEM and TEM. The results show that the adiabatic shear thermal effect induced by super high energy, ultra-fast process of laser shock causes surface alloy system to occur entropy increase effect and partitioning. The high mixing entropy contributes to the randomization increase of the alloy system. Thus, the elements in the system spontaneously self- organize in accordance with the law of Boltzmann. The dynamical formation of the nano- crystalline grains coordinates the thermodynamic equilibrium during the process. The strain- hardened layer is composed of amorphous microstructure and nanocrystalline grains, and the total depth of it reaches up to about 100 μm. After 1time laser shock,the depth of the surface high entropy layer is about 20 μm, of which the diameter of the nanocrystalline grains is 6~8 nm. After 3 times laser shock, the thickness of the layer can increase to more than 40 μm, and the diameter of the nanocrystalline grains is 2~3 nm. Meanwhile, the intense ultra high strain-rate induced by the laser shock makes precipitates deform, producing parallelly distribution of deformation twins in order to balance the laser energy. After repeated laser shocks, the hardness of the amorphous/nanocrystalline layer gradually closes to that of the matrix of the alloy because of the disappearing of the support of grain boundaries to the strength, the dislocation strengthening effect in nano-crystalline grains, and the coherent relationship between precipitates and matrix. Due to that the amorphous microstructure can prevent galvanic effect around precipitates, and nano-crystalline has good chemical stability, the nano-crystalline/amorphous composite high-entropy layer on surface of 7075-T76 aluminum alloy induced by laser shock can significantly improve the corrosion resistance, and effectively block the intergranular corrosion of the alloy.