中文摘要：

用7075-T76航空铝合金进行激光冲击表层改性实验,借助SEM和TEM分析冲击层的微观结构、表层获得的非晶/纳米晶复合材料高熵合金层的演变过程和成因及力学性能与抗腐蚀性能.结果表明,激光冲击的超高能量、超快过程导致的绝热剪切热效应诱导材料表层合金体系发生熵增效应及重新配分.合金体系混合熵的增大促进组元间的混乱度增高,弱化了杂质原子的不良作用.激光冲击提供的外场能量促进熵的增量转化为合金中形成非晶态组织所需Gibbs自由能ΔGconf的降低.多组元铝合金在多次激光冲击强大的外场能量作用过程中,各组元间按照Boltzmann定律自发重组,动态析出的纳米晶组织则发挥过程中体系不平衡度的协调作用,使所获高熵非晶组织更符合Boltzmann关系的热力学要求.通过热力学自调整和微结构重组,激光冲击层最终由非晶/纳米晶颗粒复合组成.同时,激光冲击的超高应变率诱导的强烈微观应力使时效析出相发生整体塑性形变,产生平行分布的形变孪晶,协同吸收激光冲击能量.由于晶界强化消失和位错密度降低,激光冲击主要体现为结构重组效应.激光冲击表层的硬度在单次激光冲击后有所提高,随冲击次数增加,硬度逐步与基体硬度持平.激光冲击造成的强烈形变可使铝合金表层内纳米晶尺寸减小至2~3 nm.非晶态消除了在第二相周围的原电池腐蚀,从而使航空铝合金7075-T76表面激光冲击所获非晶/纳米晶复合材料表层的抗腐蚀性能明显改善.

英文摘要：

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 en