Electron Tomography Of Dislocation Structures In An Al-Cu-Mg Alloy

Dislocation is an important defect in the microstructure of Al-Cu-Mg alloy,which has a profound influence on the plastic deformation behavior of alloys and the precipitation behavior of strengthening phase.Systematic and quantitative characterization of the geometrical and crystallographic characteristics of dislocations is of great significance for a deep understanding of the microscopic mechanism and macroscopic behavior of many dynamic processes within materials.Generally,the results obtained by conventional transmission electron microscopy(TEM)are usually two-dimensional projection of the three-dimensional(3D)structure in the direction of the electron beam,thus are unable to fully reveal the exact 3D morphology and spatial relationship among dislocations.In this research,the quenching induced dislocation structures in an Al-Cu-Mg alloy and the deformation induced dislocation structure in a10% cold rolled pure Al were detailed studied by means of dislocation tomography and correlative crystallographic analysis.Besides,combining in-situ heating TEM with dislocation tomography,the quenching induced Frank partial dislocation loops and their evolution mechanism during annealing were further studied.The main conclusions are as follows:(1)The quenching induced dislocation loop usually shows elliptical shape,with the ellipticity increase from 0.54 to 0.86 with the increase of the major axis size of dislocation loops.The dislocation loops with the Burgers vectors of a/2<110> can be classified into the edge type loop on {110} planes and the mixed type loop on {111}planes.Specifically,the majority of the location planes of the {110} type dislocation loops deviates from their {110} planes,leading to nearly-edge dislocation loop.The deviation angle is less than 18.2°,and the average deviation angle is 5.0°.By means of slip or climb driven by thermal stress,dislocation loops can approach each other and form dislocation locks through reactions.(2)The Burgers vector of helical dislocation is a/2< 110 >,and the deviation angle between helical axis and Burgers vector is usually less than 15 °.This deviation probably results from climb of local dislocation segment driven by vacancy migration under thermal stress after high temperature quenching.(3)The quenching induced a/3<111> type Frank partial dislocation loops show both elliptical and polygonal shapes.There are two evolution processes for Frank partial dislocation loops during room temperature and in situ annealing,i.e.climb on {111}planes through vacancy diffusion which makes Frank partial dislocation loops vanished,and react with Shockley partial dislocation to form a/2<110> prismatic dislocation loop with its location planes unchanged.(4)After 10% cold rolling,the typical dislocation boundaries in pure aluminum are characterized by hexagonal and quadrilateral dislocation networks.The hexagonal dislocation network is composed of dislocations with three Burgers vectors,two of which come from the slip systems with the relatively large Schmid factors,while the third is a reaction product.These three dislocation components form dislocation locks,leading to fairly stable dislocation boundaries.The quadrilateral dislocation network is composed of dislocations with two Burgers vectors.The dislocation components are also from the slip systems with the relatively large Schmid factors.However,these two dislocations do not react with each other,resulting in a metastable dislocation boundary.