Cu And Cuzn Alloys Such As Extrusion Angle Channel Grain Refining Efficiency Of Fault Correlation Studies

Equal channel angular extrusion (ECAE) is a processing method of severe plastic deformation (SPD). It draws growing attention due to its ability to fabricate ultra-fine grained materials in bulk form and thus to improve their mechanical and physical properties. To date, researches on ECAE mainly focus on revealing the effect of various processing parameters (processing route, die parameter, pass number, etc.) on grain refinement efficiency. The effect of stacking fault energy (SFE) on the grain refinement efficiency in different materials under the same processing conditions, however, requires further studies.Three as-cast materials with different SFEs, namely, T2pure copper (78mJ/m2), H90brass (or Cu-10wt.%Zn,35mJ/m2) and H65brass (or Cu-35wt.%Zn,14mJ/m2) were considered. Detailed microstructure analyses were conducted on these materials after being processed by ECAE in a die with Φ=120°via optical microscope (OM), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) techniques. Vickers microhardness tests were also performed to discuss the effect of SFEs on the mechanical properties during ECAE.(1) By analyzing the feature of OM microstructures, we evaluated the features and densities of slip bands in T2、H90and H65after ECAE via different processing routes and up to different passes, and preliminarily discussed the effect of SFEs on the grain refinement efficiency under the different processing condition. The results show that, for route A, the inclination angle of developed slip bands with respect to the extrusion direction (ED) decreases with the increase of pass number, until about15°after eight passes; the inclination angle stays almost unchanged with the increase of pass number for routes Bc and C, and it is about60°.We can get the preliminary conclusion that for a material with the same number of passes but three different processing routes, grain refinement efficiency decreases from routes A to Bc and then C. The difference in refinement between routes A and BC is relatively small. Route A leads to the most efficient refinement while route C leads to the least; for materials with different SFEs processed under the same condition, more uniform microstructure with a higher density of slip bands are obtained with the decrease of SFE.(2) Quantitative comparisons of the mesoscopic microstructure features and misoritation angle distributions were conducted between T2and H90after ECAE to examine the effect of processing conditions and SFE on the grain refinement with the aid of EBSD analyses.The results suggest that with the increase of pass number from1to8, the microstructure becomes more uniform with a smaller grain size and a greater proportion of high angle grain boundaries (HAGBs). The average grain size decreases and the proportion of HAGBs increases with a decrease of SFE under the same processing conditions. There are distinct differences among microstructures of the same material after8passes via different routes. Route A leads to a microstructure consisting of mainly elongated grains together with some fine equiaxed grains. Route Bc leads to a microstructure with mainly fine equiaxed grains and some coarser grains. Route C leads to a microstructure of nearly equiaxed grains which are larger than that of route A or Bc. Grains refine in the following order of routes:A>BC>C.(3) The effect of SFE on the microstructure features and underlying deformation mechanisms was investigated with the aid of TEM analysis on the dislocation structures and selected area electron diffraction (SAED) pattern. For materials with different SFEs, TEM observation shows that there are no apparent twins in the deformed T2pure copper while there are apparent deformation twins in the lower-SFE H90and H65. Since the motion of dislocations is obstructed by the occurrence of twins, the development of twins has the effect of increasing the number of sub-grain (or grain) boundaries, minimizing average grain size, and improving the proportion of HAGBs. Dislocation slip is the only mechanism during deformation in T2pure copper while both slip and twinning operate in H90and H65. Furthermore, the grain size becomes smaller and the grain refinement becomes more significant with the decrease of SFE.(4) Discussion of the effect of material’s SFE on its mechanical properties is done by analyzing the microhardness in the ECAE-processed T2、H90and H65via different processing routes and passes.The results suggest that for a material processed up to the same number of passes but via three different routes, the hardness decreases in the following order of routes:A>Bc>C. Under the same processing conditions, the hardness increases with the decrease of SFE in the material and the differences between different materials become more apparent at a higher strain level. These tendencies are consistent with the effect of SFE and processing conditions on the grain refinement.