Studies On High-Temperature Deformation Behavior Of Commercially Pure Iron And Pure Aluminum Processed By Equal-Channel Angular Pressing

Higher internal stress is accumulated in the materials processed by equal channel angular pressing (ECAP) due to the severe plastic deformation and the obtained microstructures are normally in a metastable state. When the materials with the metastable structure are subjected to the action of applied load and temperature, the change in their structures will be very easy to occur, and thus leading to the decrease of mechanical properties. Currently, most researches on the materials prepared by ECAP are mainly focused on the room temperature mechanical behaviors and corresponding microstructural change, and so on. However, knowledge of their high temperature deformation behavior still remains less. Therefore, in the present work, the commercially pure Fe (CP Fe) with BCC structure and pure Al with FCC structure, prepared through1pass and4passes of ECAP technique with a Bc route, respectively, are selected as the target materials. The influences of annealing and strain rate on the high temperature deformation behavior, deformation characteristics and microstructures were investigated. The aim is to provide significant reference for the investigation of high temperature mechanical behaviors of ECAPed materials.The high temperature compressive yield stress of ECAP Fe is more than twice that of CP Fe. The slight softening occurs after rapid strain hardening for ECAP Fe, and then secondary strain hardening phenomenon appears again, and the higher temperature is, the more obvious softening phenomenon is. The yield stress first decreases fastly with increasing temperature, and then increases at300℃. Annealing at400℃effectively improves the softening phenomenon of plastic flow stage, here, yield stress is higher than that of ECAP Fe without annealing, but a certain degree of softening phenomenon still exists, which is associated with compressive temperature. The microstructures of original ECAP Fe are mainly composed of dislocation cells and lamellar structures, and there is a high dislocation density in lamellars. After annealing at400℃, the microstructures are mainly governed by the recrystallized grains and lamellars, but the dislocation density in lamellars becomes lower. The sub-structure consists primarily of dislocation cells and sub-grains after ECAP Fe annealed or unannealed are compressed at different temperatures, and with increasing temperature, the number of sub-grains rises and dislocation density decreases. At300℃, an amount of lamellar structure can still be observed. The shear deformation dominates, accompanied by slipping in grain interiors for ECAP Fe at less than200℃, and shear deformation along single direction at300℃is the governing deformation mode. The role of slipping in grain interiors becomes more and more notable with increasing temperature for ECAP Fe annealed at400℃.For ECAP A1compressed at two strain rates, the slight softening phenomenon occurs and subsequently strain hardening is observed at less than200℃. The yield stress decreases with increasing temperature, but rises at200℃and10-2s-1. With continuously raising temperature, the softening phenomenon in the plastic flow stage always maintains and the yield stress rapidly decreases, but the yield stress at10-2s-1is higher than that at10-3s-1.The deformation mode is dominated by the shear deformation, and temperature plays an important influencing role. For instance, the secondary shear bands can be found easily and obviously with increasing temperature at less than200℃, and disappear at more than200℃. The sub-structure is mainly sub-grains, and the size of sub-grains is uniform (around1.2μm) at room temperature and10-2s-1. With raising temperature, the distribution of sub-grain size becomes more and more non-uniform, and the phenomenon disappears and size increases to～2μm as temperature is300℃. However, at10-3s-1, the non-uniform distribution phenomenon of sub-grain size does not appear.In summary, annealing at400℃/lh could effectively improve the static compressive mechanical properties of the ECAP Fe. For ECAP Al, the decrease in strain rate at less than200℃enhances the uniformity of microstructures and thus leads to higher yield stress. When compressive temperature is more than200℃, the increasing of strain rate could obtain higher yield stress.