Uniform Elongation and Yield-Drop Phenomenon in Magnetically Annealed 1050 Aluminum Alloy Prepared by CryoECAP

The ultrafine grained(UFG)1050 aluminum alloy was prepared by equal channel angular pressing at cryogenic temperature,namely cryoECAP process.The tensile behavior and microstructures of UFG 1050 aluminum alloy after annealing at 90-210°C for 4 h without and with high magnetic field of 12 T were investigated by tensile tests,transmission electron microscope,and electron backscattered diffraction analyses.After cryoECAP and annealing treatments,the 1050 aluminum alloy has ultrafine grains with 0.7-1.28μm in size,the ratio of ultimate tensile strength to yield strength is less than 1.24,and the uniform elongation is less than 2.3%.With increasing the annealing temperature from 90°C to 210°C,the yield-drop phenomenon becomes more obvious due to the decrease in mobile dislocations to maintain the applied strain rate during tensile deformation.The uniform elongation decreases from 1.55%to 0.55%,the dislocation density reduces from 5.6×10^(14)m^(−2)to 4.2×10^(13)m^(−2),and the fraction of high-angle grain boundaries(HABs)increases from 63.8%to 70.8%.These phenomena cause the higher annihilation rate of dislocations,thereby leading to the degradation of strain hardening effect.During annealing under high magnetic field at 90-210°C,the low fraction of HABs(61.7%-66.2%)can provide a slower annihilation rate of dislocations,therefore resulting in the higher uniform elongation(0.64%-1.60%)and slower decrease in the flow stress after the yield peak.

Bake hardening behavior of TRIP and DP steels

The bake hardening （BH） behavior of transformation-induced plasticity （TRIP） and dual-phase （DP） steels after different prestrains was studied. The experimental results indicate that TRIP steel exhibits good BH ability while DP steel does not, and prestrain displays a strong effect on the BH values of both steels. The comparison of microstructures of the two steels showed that the hard second phase in the matrix might be harmful to the BH ability. For deformed specimens, baking resulted in a loss of uniform elongation, but there was no obvious decrease in uniform elongation for unprestrained specimens.

Stress-strain behavior of multi-phase high performance structural steel

A series of ferrite/bainite(F/B) multi-phase steels containing different volume fractions of ferrite were obtained.The effect of soft phase(ferrite) content on the work-hardening behavior of the steel was studied by the finite element simulation with V-BCC model and the modified Crussard-Jaoul(C-J) analysis.It is shown that the multi-phase steels have an excellent anti-deformation ability,such as higher stress ratio(R t1.5 /R t0.5),higher uniform elongation and lower yield to tensile strength ratio.For the F/B multi-phase steels,increasing the proportion of ferrite would help to increase the uniform elongation.However,introducing much more fraction of ferrite would not be helpful to improve the stress ratio of multi-phase steel.The ferrite plastic strain constrained by bainite would be beneficial to increasing the work hardening rate.The optimum proportion of ferrite will result both higher stress ratio and uniform elongation in multi-phase steel.