| There were two pronounced drawbacks for synthesizing ultrafine grained (UFG) metals by accumulative roll bonding (ARB), the first was the limited range of applicable metals, and the other was the disappointingly low ductility for this kind of materials. Based on these two problems, this paper developed a modified route which included ARB at elevated temperature and cold rolling (CR) at room temperature (RT) to fabricate the1100/7075Al alloy multilayered sheets successfully. Low temperature annealing and thermomechanical treatment had been adopted to optimize the microstructures and mechanical properties of the representative samples. Tensile machine, optical microscopy, SEM-ECC/EBSD, TEM and XRD were used to investigate the microstructures and mechanical properties of the processed samples. The main conclusions included:For the multilayered sheets by ARB at460℃and subsequent CR at RT, as the number of layers increased, the yield strength (YS) revealed a stepwise enhancement, while the total elongation decreased initially and then improved:the YS first kept at280MPa-285MPa for samples with less than80layers, and then increased dramatically up to340MPa for the160and320layered samples, finally a maximum of370MPa was achieved for both the640and1280layered samples. All the samples exhibited poor ductility with total elongations less than5%. For the samples with comparably more layers, bulk ultrafine structure could be obtained without severe cold plastic deformation. The accelerated refinement of grains mainly derived from the CR process, and was related with four factors:the accelerated grains subdivisions caused by bending and necking of the layers, local stress concentration due to the multimodal distribution of grain size, additional shear strain resulting from the shear stress near layer interface, and the local grains distortions induced by frequently distributed oxide layer. Besides grain boundary strengthening and forest dislocations strengthening, layer interfaces strengthening was also one of the strengthening mechanisms.For the1280layered sheet by low temperature annealing, the optimum annealing process was150℃/30min, which gave an combination of optimized YS (331MPa), high ultimate strength (UTS,375MPa), a total elongation of8%and especially an uniform elongation of6.1%. The optimized mechanical properties with almost twice the elongation and only9%YS loss compared with that of the CR temper were achieved by remarkable recovery. The stepwise tensile strain hardening for the annealed sample was explained by the accelerated dynamic recovery due to layer interfaces and the observed shear bands during tensile straining.For the1280layered sheet by thermomechanical treatment, aging at120℃for12h exhibited a simultaneous improvement of strength and ductility comparing with that of solid solution and75%CR. The YS and UTS increased from328MPa and352MPa to342MPa and391MPa, respectively, while the total elongation and uniform elongation increased from3.4%and2%to10.5%and7.8%, respectively. The simultaneous enhancement of strength and ductility was attributed to the precipitate strengthening and recovery softening.For the intermediate1280layered plate fabricated by ARB at460℃, EBSD investigations indicated that some small grains in7075Al layers were surrounded by significantly large fraction of high angle boundary segments which had high mobility, and therefore could act as ready recrystallization nuclei and grow preferentially during annealing at400℃for1h, which resulted in the growth of grains across several alternant1100and7075Al layers, and this was supposed to be beneficial for improving the bonding. The present work highlighted an additional mechanism of metallurgical bonding that the grains spanned across layer interfaces by orientation operation. |
PDF Full Text Request