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Acta Metall Sin  2015, Vol. 51 Issue (1): 57-66    DOI: 10.11900/0412.1961.2014.00279
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SURFACE LAYER HIGH-ENTROPY STRUCTURE AND ANTI-CORROSION PERFORMANCE OF AERO-ALUMINUM ALLOY INDUCED BY LASER SHOCK PROCESSING
LUO Xinmin1(), WANG Xiang1, CHEN Kangmin1,2, LU Jinzhong3, WANG Lan1, ZHANG Yongkang4
1 School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013
2 Analysis and Test Center, Jiangsu University, Zhenjiang 212013
3 School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013
4 School of Mechanical Engineering, Southeast University, Nanjing 210089
Cite this article: 

LUO Xinmin, WANG Xiang, CHEN Kangmin, LU Jinzhong, WANG Lan, ZHANG Yongkang. SURFACE LAYER HIGH-ENTROPY STRUCTURE AND ANTI-CORROSION PERFORMANCE OF AERO-ALUMINUM ALLOY INDUCED BY LASER SHOCK PROCESSING. Acta Metall Sin, 2015, 51(1): 57-66.

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Abstract  

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 far-from-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 1 time 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.

Key words:  laser shock      aero-aluminum alloy      surface modification      high-entropy alloy (HEA)      amorphous      nano-crystalline grain      anti-corrosion     
ZTFLH:  TG115  
Fund: Supported by National Natural Science Foundation of China (Nos.51275220 and 51105179)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2014.00279     OR     https://www.ams.org.cn/EN/Y2015/V51/I1/57

Fig.1  Schematic of laser shock processing
Fig.2  TEM images of matrix (a) and precipitate (b) after aging in 7075-T76 aluminum alloy sheet
Fig.3  SEM images of laser shocked layers of 7075-T76 aluminum alloy after 1 time (a) and 3 times (b) laser shock
Fig.4  Hardness of 7075-T76 aluminum alloy after 1 and 3 times laser shock
Fig.5  TEM (a) and HRTEM (b) images of aluminum alloy 7075-T76 after 1 time laser shock
Fig.6  TEM image of 7075-T76 aluminum alloy at micro-area in the laser shocked layer (The inset shows the corresponding SAED pattern)
Fig.7  TEM images of matrix (a) and precipitate morphology (b) in 7075-T76 aluminum alloy after 2 times laser shock
Fig.8  HRTEM images of the matrix of 7075-T76 aluminum alloy after 3 times laser shock

(a) amorphous structure

(b) composite microstructure of amorphous and nano-crystalline grains

Fig.9  TEM (a) and HRTEM (b) images of deformation micro-twins of precipitate in 7075-T76 aluminum alloy after 3 times laser shock
Fig.10  Surface morphologies of 7075-T76 aluminum alloy after corrosion test

(a) raw material

(b) after 1 time laser shock

(c) after 3 times laser shock

Fig.11  Cross section morphologies of 7075-T76 aluminum alloy after corrosion test

(a) raw material (b) after 3 times laser shock

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