Research On Surface Morphology And Microstructure Of 7075 Aluminum Alloy Irradiated By Helium Ion

In this paper,aerospace 7075 aluminum alloy was used as the experimental material,and the ion implantation method was used to simulate the damage in space caused by He⁺irradiation.The research mainly focused on the SRIM calculation of the He⁺irradiation process,the change of the surface morphology,the irradiated defects and the evolution of the microstructure in the irradiated layer.And then,the causes of these defect damages were discussed.In this experiment,fluences of 0,10¹⁵,10¹⁶ and 10¹⁷ions/cm² were implanted into the 7075 aluminum alloy samples,and the samples were examined by using optical microscope?OM?,scanning electron microscopy?SEM?,energy dispersive spectrometer?EDS?,atomic force microscopy?AFM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?.The results of the study are as follows:The energy loss,range and concentration distribution of He⁺in 7075 Al alloys were calculated by SRIM-2008 software.The distribution of recoil atoms?Al,Zn,Mg,Cu?and their displacement energy,lattice binding energy and surface binding energy were also calculated.Besides,SRIM simulated the generation of point defects and sputtering phenomenon during He⁺irradiation process.SRIM further analysis showed that 50keV helium ion irradiation will cause atomic displacement damage.When the fluence was 10¹⁷ions/cm²,the peak value of the displacement damage was about 6dpa.By analogy,10¹⁶ions/cm² corresponded to 0.6dpa and the damage of 10¹⁵ ions/cm²was 0.06dpa.For the transmission of a single helium ion in the 7075 aluminum alloys,97 lattice atoms will be displacemented,including 4 permutation collisions and 93vacancies.It is concluded that the point defects caused by the cascade collision and the implanted helium atoms are the source of irradiated damage.The experimental results showed that the irradiation pits and the micro-crack were the main damage morphologies observed on the surface of the irradiated samples.The irradiation pits were almostly distributed on constituent particles S?Al₂CuMg?and the micro-cracks were mainly distributed on constituent particles Al₇Cu₂Fe.With the increase of dose,the number of pits was more and more,and the size also continued to grow even grow into holes,which resulted in formation of micro-cracks around them.Therefore,helium ion irradiation will lead to the increasing of surface roughness.In addition,helium ion irradiation resulted in partially missing of S?Al₂CuMg?and Al₇Cu₂Fe particles and the loss became more serious with the increased dose.In addition,He⁺irradiation has a great influence on microstructure of the irradiated layer.The point defects and clusters caused by irradiation led to the lattice distortion and produced compressive stress in the crystal.Moreover,He⁺irradiation did not generate a new phase.Instead,the density of MgZn₂ phase decreased.Besides,high-density point defects and implanted He⁺can form supersaturated solid solutions and high stress zones,causing the occurrence of radiation defects,such as dislocations,dislocation loops and helium bubbles.The density of dislocations increased with the increase of irradiation dose,and movement and aggregation of dislocations will produce dislocation structures in different forms.Dislocation walls and subgrain structure were found in the 0.6dpa samples.In the 6dpa samples,dense dislocation entanglement appeared in subgrain.In addition,a large amount helium bubbles were found at 0.6dpa,and the helium bubbles became larger at dose of 6dpa.Except for bubbles,dislocation loops were observed at 6dpa.The presence of dislocation loops and helium bubbles prevented the movement of dislocations.