Irradiation Damage And Mechanisms On Al And Al Alloys Under Ga Ion Milling

Over the last a few decades,Focused Ion Beam（FIB）technology has played an important role in micro-nano processing.It became an essential tool in making specimens for in-situ Transmission Electron Microscopy（in-situ TEM）,in-situ Scan Electron Microscopy（in-situ SEM）,Atom Probe Tomography（APT）and other advanced characterization experiments.Such investigations have significantly expanded our knowledge about the microstructure and properties of materials.Meanwhile,FIB milling can produce some surface damage and modification on the material.This not only affects the properties of the material itself,but also causes confusion and misunderstanding between the artificial defects and the intrinsic structure of the material.Therefore,it is very important to understand the damage mechanism of focused ion beam.In this investigation,APT tips were milled/irradiated by using FIB with Ga ions,and characterized by using TEM and APT.The data was analyzed by using HDBSCAN-De Ba CL cluster algorithm,and Monte Carlo simulation was implemented to estimate irradiation effect.Al light metal,Al-Cu age-hardening alloy and Al-Mg unage-hardening alloys were studied to understand the mechanism of surface damage due to ion-beam processing,irradiation-induced phase transition mechanism,microstructure change during aging and oxidation behavior under different environments.This research is to develop deep understanding about the damage behavior mechanism and microstructure stability of micro-nano samples propared by using FIB,and provides information to help properly using FIB by selection of processing parameters,and to correctly interprete experimental resuts.The main outcomes of this investigation are as follows:The removal effects of damage layer on different alloy samples processed by ion beam with low accelerating voltage were systematically investigated.It was found that although most research suggests that beam with low accelerating voltage can reduce the surface damage layer induced by the beam with high accelerating voltage,but for pure Al single crystal samples due to the strong channeling effect^[1],the beam with low accelerating voltage not only couldn’t reduce the Ga implantation in the pure Al needle-shape sample,even could aggravate the ion implantation and cause clusters to grow up.The addition of alloying elements could effectively reduce the channel effect,so the implanted Ga concentration of alloys were smaller than that of Al single crystals,and the alloys had better damage removal effects for the ion beam process with low accelerating voltage.Therefore,it is suggested that the ion beam process with low accelerating voltage should be avoided for single crystal pure Al to further aggravate the damage.It was found that Ga ion irradiation can induce accelerated phase transition（θphase）on the surface of Al-Cu alloy tips with the redistribution of Ga element.For the first time,we found that the formation and growth of surfaceθphase is a combination reaction process,rather than the generally thought that phase transformation induced by ion irradation was a precipitation behavior in the block aging hardening alloys.Cu-rich clusters in the matrix formed during pre-aging process could provide nucleation forθphase,and Cu-rich layers on the surface of Al-Cu layer induced by electropolishing provided Cu source forθphase growth.It was the first time that the mechanism of the rapid formation and growth ofθphase on the surface of Al-Cu aging hardening alloy induced by Ga ion irradiation was discovered and explained from the kinetics theroy,and several key factors affecting the formation ofθphase were revealed.By studying the subsequent aging behavior of Al,Al-Cu and Al-Mg alloy tip samples irradiated by Ga ion beam at room temperature,the diffusion behavior of alloy elements and the evolution of defects of these samples during the aging process at room temperature were discovered.Preventing damage effects of different alloys was different,which cause different diffusion behaviors of solute atoms（uphill or downhill diffusion）during the aging process.It was revealed that surface depletion zone of solute atoms（Mg and Cu）in Al alloy irradiated by Ga ion is dominated by diffuse behavior rather than differential sputtering,which fills up the gap in related research field.The oxidation mechanism and Ga diffusion behavior in different environments of Al-Ga amorphous layer induced by FIB process were revealed.The Al-Ga amorphous layer induced by Ga ion irradiation in pure Al can grow slowly in the air at room temperature,and in this process,Ga diffused from the oxide layer to the matrix,resulting in that Ga cannot be detected in the oxide layer.The oxidation rate of the amorphous layer could be accelerated under electron beam,and the oxidation layer with a thickness more than 315 nm could rapidly grow within half an hour.Ga atoms have no time to diffuse during this process and part of them were retained in the oxidation layer.The results provided important experimental evidence for the accelerated oxidation behavior of FIB irradiated samples induced by electron beam.