Enhancing Mechanical Properties Of Interstitial-free Steel And Pure Al Via Heterogeneous Lamellar Structures

Grain refinement is a promising strategy to produce bulk nanostructured metals with enhanced strength.However,the trade-off relationship between strength and ductility always limit the development and application of the nanostructured metals and alloys.Recently,researchers have reported that by adjusting the microstructure to form a multi-scale and heterogeneous lamellar structure can effectively overcome the shortcoming of poor ductility of the nanostructured metals.However,the research on heterogeneous lamellar single-phase materials are still limited.It is of great values to fabricate heterogeneous lamellar single-phase materials and invesigate their mechanical properties,the microstructural evolution and the strengthening mechanisms.In this thesis,the body centered cubic(BCC)structural interstitial free(IF)steel and face centered cubic(FCC)structural Al were selected as model materials and heterogeneous lamellar structural IF steel and pure Al were prepared by cold forge(CF)and accumulative roll-bonding(ARB)processing respectively to investigate the strengthening mechanisms.The heterogeneous lamellar IF steel was prepared by stacking cold rolled sheets(CR)and annealed sheets(AR)alternatively into multilayers,followed by by hot compression,cold forging and subsequent annealing treatment.The heterogeneous lamellar 2N/4N Al was prepared by stacking the commercial purity Al(2N Al)and high purity Al(4N Al)followed by ARB process and subsequent annealing treatment.The heterogeneous lamellar ARB6/2/6 2N Al were prepared by stacking the cold rolled 2N Al and annealed 2N Al followed by ARB process and subsequent annealing treatment.The microstructural evolution of deformed and annealed IF steel,2N/4N Al and ARB6/2/6 2N Al as well as the effect of microstructure on the deformation mechanisms were systematically studied.The strengthening mechanisms were discussed based on microstructure and quantitative analysis.The main conclusions are as following:(1)Heterogeneous lamellar IF steel has an average grain size of 216 nm in CR layers(97%reduction)and 323 nm in AR layers(85%reduction)respectively after cold forge.The ultimate tensile strength and uniform elongation is 604 MPa and 14%respectively after annealing at 600? for 1h.Heterogeneous lamellar IF steel has a better combination of strength and ductility than other IF-steel samples with a homogeneous layer structure.(2)Microstrucural evolution trend is different in CR layers and AR layers of cold forged IF steel when annealed at 600? for various time.At the early stage of annealing,grain recovery are dominatant in the CR layers due to high stored energy.Prolonging the annealing time resulted in a faster recrystallization process in the AR layers due to less exhausted stored energy by earlier recovery.After 1.5 h of annealing,the area fractions of the recrystallized microstructure(1)_Rex)is 72%in the AR layer,whereas the CR layers are only 50%even after annealing for 4h.(3)ARB6-2N/4N Al,ARB6-2N Al and ARB6-4N Al were fabricated by ARB process.The microstructure and mechanical properties were compared.The tensile results show that the ARB6-2N/4N Al has similar tensile elongation with ARB6-4N Al(7.5%and 8.5%,respectively),while ARB6-2N/4N Al has a strength 1.5 times higher than that of ARB6-4N Al(109 MPa and 68 MPa,respectively)after annealled at 225oC.Macroscopic DIC analyses show that the local strain distribution was more uniform in the ARB6-2N/4N Al than that in ARB6-4N Al at the same global tensile strains.This may be attributed to the heterogeneous lamellar structure delays the concentration of local strain in ARB6-2N/4N Al.Microscopic DIC analyses show that local deformation bands with higher plastic strains preferentially evolved in the softer 4N-Al layers and high shear strain areas extended in the 4N-Al layers.The deformation localized bands were terminated at interfaces with the harder 2N-Al layers.The termination of localized deformation bands suppressed macroscopic necking,resulting in larger tensile elongation and good balance of strength and ductility.(4)To investigate the effect of homogenous microstructure and heterogenous lamellar microstructure on the mechanical properties,lamellar 2N/4N Al was fabricated by 6 to 10 ARB cycles.The grain sizes tend to saturated after 8 cycles.Upon annealing at 250oC,the ARB6-2N/4N Al retained the heterogeneous lamellar microstructure,while ARB10-2N/4N Al were much homogeneous.ARB6-2N/4N Al showed continuous yield behavior(yield-drop phenomenon disappeared),but ARB10-2N/4N Al showed discontinuous yield behaviours(with yield-drop phenomenon and Lüders band).(5)ARB6/2/6 2N Al was fabricated by ARB process.The total reductions of the surface layer and core layer were 98.5%and 75.0%,respectively.The grain size of ARB2 layers(0.46?m)was larger than that of ARB6 layers(0.22?m),and ARB6layers have higher stored energy.The grains coarsed in ARB2 layers and ARB6 layers after annealing,but grain coarsening was more obvious in ARB6 layers.The ARB2layers underwent discontinuous recrystallization process while continuous recrystallization occurred in the ARB6 layers during annealing process.The normalized yield strengths and UTS of the ARB6/2/6 2N Al were close to those of ARB6-2N Al,while the uniform elongation and total elongation of the ARB6/2/6 2N Al recovered more rapidly than ARB6-2N Al after annealing.