Influences Of External Field On The Microstructures And Properties During Annealing In 1050 Alloy Prepared By CryoECAP Technology

The microstructures and mechanical properties of 1050 alloy fabricated by cryogenic equal channel angular pressing (cryoECAP) after annealing at different temperatures without or with electric current and high magnetic field are investigated by optical microscopy, electron back-scattered diffraction (EBSD), and hardness, tensile and positron annihilation tests.The annealing of 8 cryoECAPed sample without and with electric current are divided into:(a) annealing at 50-200℃ with 250A electric current for 4h; (b) annealing at 150-400℃ with 500A electric current for Ih. The results show that hardness and strength of samples annealed at 50-200℃ without and with electric current both increase. The phenomenon of annealing induced hardening occurs at 150℃ without electric current, and happens at 100-120℃ with electric current. It can be seen that the application of electric current makes the temperature of annealing induced hardening move to low temperture.According to the grain growth ratio (compared to the grain size in as the cryoECAPed samples), the annealing temperature at 150-400℃ with 500A electric current can be divided into:low temperature region at 150-250℃ and high temperature region at 300-400℃. At low temperature region, grain grows slowly, the grain growth ratios are 150℃/0.14,200℃/0.56, 250℃/1.33; the fraction of low angle boundaries changes little, is 0.48 when annealed at 250℃. At high temperature region, grain grows obviously, the grain growth ratios are 300℃/7.67,350℃/11.22,400℃/17.89; the fraction of low angle boundaries reduces rapidly to 0.386, and then increases with increasing temperature. Comparing with the sample annealed at same temperature without electric current, the grain size is slightly higher at low temperature region, and is much smaller at high temperature region in sample annealed with 500A electric current; the grain size distribution is more homogeneous, no abvious grain growth happens. It can be seen that the application of electric current promotes the recovery process at low temperature, but suppresses the recrystallization process at high temperature.The results of hardness and tensile tests show that the hardnesses and strengths of samples annealed at 150℃ are slightly higher than those of initial 8 cryoECAPed sample, and then decrease gradually with increasing temperatures. The hardness and strength decrease significantly when annealed at 300℃, and then reach saturated values at ≥350℃. The elongations in samples annealed with 500A electric current are higher than those in sample annealed without electric current.The phenomenon of annealing induced hardening of cryoECAP 8 passed samples occurs at 150℃ without high magnetic field, and is at 80℃ with high magnetic field.The results of positron annihilation test indicate that the fraction of monovacancies increases and the fraction of divacancies decreases with increasing annealing temperature. With the application of magnetic field, the concentration of monovacancies is higher than that in samples annealed without magnetic field. Vacancies associated with dislocations appear in samples annealed at 200℃ with high magnetic field. In addition, high magnetic field increases the concentration of vacancies increases in annealed samples.