Formability of Ultrafine-Grained Interstitial-Free Steels

The stretch formability of ultrafine-grained (UFG) interstitial-free steel (IF-steel) produced by equal-channel angular extrusion/pressing (ECAE/P) via various strain paths was investigated with a miniaturized Erichsen test. A coarse-grained (CG) sample demonstrated high formability with an Erichsen index (EI) of 4.5 mm. Grain refinement by ECAE decreased the formability, but increased the required punch load (F _EI) depending on the applied strain paths. The EI values were 0.35, 2.90, and 3.91 mm for 8A-, 8Bc-, and 8C-processed samples, respectively. Decrease in the biaxial stretch formability was attributed to the limited strain-hardening capacity of the UFG microstructure. Also, the grain morphology of the UFG microstructure was found to be very influential on stretch formability. Heavily elongated grain morphology in the 8A-processed microstructure resulted in the lowest formability due to the increased cracking tendency through elongated grain boundaries. However, the UFG microstructures with equiaxed grains obtained after 8C and 8Bc ECAE resulted in better formability compared to 8A. The UFG microstructure reduced the roughness (orange peel effect) of the free surface of the biaxial stretched samples by decreasing the non-uniform grain flow leading to the so-called orange peel effect. It should be noted that the strength and ductility values gained from uniaxial tensile tests are not comparable directly to the EI and F _EI values determined from the Erichsen tests. Finally, it is important to emphasize that the UFG microstructure produced by a suitable strain path leading to equiaxed grains below 1 μm could be highly deformed even under multiaxial stress conditions.     

Equal-channel angular sheet extrusion of interstitial-free (IF) steel: Microstructural evolution and mechanical properties

Interstitial-free steel (IF-steel) sheets were processed at room temperature using a continuous severe plastic deformation (SPD) technique called equal-channel angular sheet extrusion (ECASE). After processing, the microstructural evolution and mechanical properties have been systematically investigated. To be able to directly compare the results with those from the same material processed using discontinuous equal channel angular extrusion, the sheets were ECASE processed up to eight passes. The microstructural investigations revealed that the processed sheets exhibited a dislocation cell and/or subgrain structures with mostly low angle grain boundaries. The grains after processing have relatively high dislocation density and intense micro-shear band formation. The electron backscattering diffraction (EBSD) examination showed that the processed microstructure is not fully homogeneous along the sheet thickness due probably to the corner angle of 120° in the ECASE die. It was also observed that the strengths of the processed sheets increase with the number of ECASE passes, and after eight passes following route-A and route-C, the yield strengths reach 463 MPa and 459 MPa, respectively, which is almost 2.5 times higher than that of the initial material. However, the tensile ductility considerably dropped after the ECASE. The limited ductility was attributed to the early plastic instability in the tensile samples due to the inhomogeneous microstructure. The specimen orientation with respect to the ECASE direction did not have a considerable effect on the stress-strain response. Appropriate low temperature annealing of ECASE-processed IF-steel resulted in a good strength-ductility balance.     

Microstructural evolution and mechanical properties of Al–40 wt.%Zn alloy processed by equal-channel angular extrusion

The Al-based Al–40 wt.%Zn alloy was subjected to multi-pass equal-channel angular extrusion (ECAE) via route-A and route-B_C. Before and after ECAE processing, microstructural evolution, the tensile properties, impact toughness and fracture behavior of the alloy were investigated.ECAE processing caused to elimination of the as-cast dendritic microstructure and formed a structure consisting of elongated, ribbon shaped α-phase via route-A and mostly equiaxed α-phase via route-B_C. ECAE processing also caused plastic instability as necking at early onset of deformation. As a result of more effective mechanical mixing via route-B_C, softening and necking occurred more apparently. The tensile and yield strength of the alloy increased just after first pass and then slightly decreased with increasing number of passes. On the other hand, its elongation to failure and impact toughness increased with increasing number of passes in both routes. The increase obtained via route-A is more pronounced in both properties. Fracture behavior of the as-cast alloy changed from brittle to ductile mode after multi-pass ECAE. Elimination of dendritic as-cast structure with reduction of porosities and deformation-induced homogenization by the effect of ECAE processing increased the ductility and impact toughness of the alloy and caused formation of a fracture surface consisting of micro-voids and dimples which indicates of ductile fracture. Attained experimental results indicate that multi-pass ECAE processing is very effective in improving the tensile elongation and impact toughness of binary Al–40 wt.%Zn alloy.     

Formability of friction stir processed low carbon steels used in shipbuilding

The stretch formability of a low carbon steel processed by friction stir processing (FSP) was studied under biaxial loading condition applied by a miniaturized Erichsen test. One-pass FSP decreased the ferritic grain size in the processed zone from 25 μm to about 3 μm, which also caused a remarkable increase in strength values without considerable decrease in formability under uniaxial loading. A coarse-grained (CG) sample before FSP reflected a moderate formability with an Erichsen index (EI) of 2.73 mm. FSP slightly decreased the stretch formability of the sample to 2.66 mm. However, FSP increased the required punch load (F_EI) due to the increased strength by grain refinement. FSP reduced considerably the roughness of the free surface of the biaxial stretched samples with reduced orange peel effect. The average roughness value (Ra) decreased from 2.90 in the CG sample down to about 0.65 μm in fine-grained (FG) sample after FSP. It can be concluded that the FG microstructure in low carbon steels sheets or plates used generally in shipbuilding provides a good balance between strength and formability.     

Microstructural evolution and mechanical properties of severely deformed Al-12Si casting alloy by equal-channel angular extrusion

Microstructural evolution and mechanical properties of Al-12Si alloy subjected to equal-channel angular extrusion (ECAE) were investigated. The high stresses imposed in ECAE led to the fragmentation of the needle-shaped eutectic silicon plates into smaller particles. The length and width of the Si particles decreased from 35.5±31.8 μm and 13.7±8.9 μm without ECAE to 2.7±1.1 μm and 1.3±0.4 μm after six ECAE passes, respectively. The average aspect ratio of 3.2±1.8 for the Si particles in the as-cast condition decreased to 1.18±0.8 after six ECAE passes with a corresponding increase in the average roundness of 0.26±0.19 to 0.61±0.19. ECAE increased the strength, ductility, and impact toughness of the alloy. The increase in the tensile and yield strengths after six passes was about 68 % and 100 %, respectively. The alloy after six ECAE passes exhibited 12 % elongation to failure, which was almost eight times higher than that of the as-cast alloy. The absorbed energy increased with an increase in the number of passes, finally reaching 11. 5 J/cm² after six passes, which is about four times higher than that of the as-cast alloy.     

Impact Toughness of Ultrafine-Grained Interstitial-Free Steel

Impact toughness of an ultrafine-grained (UFG) interstitial-free (IF) steel produced by equal-channel angular extrusion/pressing (ECAE/P) at room temperature was investigated using Charpy impact tests. The UFG IF steel shows an improved combination of strength and impact toughness compared with the corresponding coarse-grained (CG) one. The CG IF steel samples underwent a transition in fracture toughness values with decreasing temperature because of a sudden change in fracture mode from microvoid coalescence (ductile) to cleavage (brittle) fracture. Grain refinement down to the submicron (??320?nm) levels increased the impact energies in the upper shelf and lower shelf regions, and it considerably decreased the ductile-to-brittle transition temperature (DBTT) from 233?K (?40?°C) for the CG steel to approximately 183?K (?90?°C) for the UFG steel. Also, the sudden drop in DBTT with a small transition range for the CG sample changed to a more gradual decrease in energy for the UFG sample. The improvement in toughness after UFG formation was attributed to the combined effects of grain refinement and delamination and/or separation in the heavily deformed microstructure. Although an obvious change from the ductile fracture by dimples to the brittle fracture by cleavage was recognized at 233?K (?40?°C) for the CG steel, no fully brittle fracture occurred even at 103?K (?170?°C) in the UFG steel.     

High Strength and High Ductility of Ultrafine-Grained,Interstitial-Free Steel Produced by ECAE and Annealing

Interstitial-free steel (IF steel) underwent severe plastic deformation by equal-channel angular extrusion/pressing (ECAE/P) to improve its strength, and then it was annealed to achieve a good strength-ductility balance. The coarse-grained microstructure of IF steel was refined down to the submicron level after eight-pass ECAE. The ultrafine-grained (UFG) microstructure with high dislocation density brought about substantially improved strength but limited tensile ductility. The limited ductility was attributed to the small, uniform elongation caused by early plastic instability. The annealing at temperatures below 723 K (450 °C) for 1 hour did not lead to remarkable softening, whereas annealing at temperatures up to 923 K (650 °C) resulted in complete softening depending on the development of recrystallization. Therefore, the temperature of approximately 923 K (650 °C) can be considered as a critical recrystallization temperature for UFG IF steel. The annealing at 873 K (600 °C) for different time intervals resulted in different stress–strain response. Uniform tensile elongation increased at the expense of strength with annealing time intervals. After annealing at 873 K (600 °C) for 60 minutes, the yield strength, tensile strength, uniform elongation, and total elongation were found to be 320 MPa, 485 MPa, 15.1 pct, and 33.7 pct, respectively, showing the better combination of strength and ductility compared with cold-rolled samples.     

Enhancing the Damping Behavior of Dilute Zn-0.3Al Alloy by Equal Channel Angular Pressing

The effect of grain size on the damping capacity of a dilute Zn-0.3Al alloy was investigated. It was found that there was a critical strain value (≈1 × 10⁻⁴) below and above which damping of Zn-0.3Al showed dynamic and static/dynamic hysteresis behavior, respectively. In the dynamic hysteresis region, damping resulted from viscous sliding of phase/grain boundaries, and decreasing grain size increased the damping capacity. While the quenched sample with 100 to 250 µm grain size showed very limited damping capacity with a loss factor tanδ of less than 0.007, decreasing grain size down to 2 µm by equal channel angular pressing (ECAP) increased tanδ to 0.100 in this region. Dynamic recrystallization due to microplasticity at the sample surface was proposed as the damping mechanism for the first time in the region where the alloy showed the combined aspects of dynamic and static hysteresis damping. In this region, tanδ increased with increasing strain amplitude, and ECAPed sample showed a tanδ value of 0.256 at a strain amplitude of 2 × 10⁻³, the highest recorded so far in the damping capacity-related studies on ZA alloys.

     

Effect of severe plastic deformation on tensile properties and impact toughness of two-phase Zn–40Al alloy

Tensile properties and impact toughness of the severe plastically deformed Zn–40Al alloy were investigated. The material billets were subjected to equal-channel angular extrusion (ECAE). After processing, elongation to failure increased significantly with the increasing number of ECAE passes. ECAE also increased the strength levels after one pass, however, they were reduced with the higher number of passes. The observed softening of the alloy upon multiple ECAE passes was shown to be due to the deformation-induced homogenization and the continuous change in the composition of the constituting phases with the number of passes. In addition, the volume fraction of the hard phase decreased due to dissolution and/or breakage. The impact toughness of the alloy was improved by multi-pass ECAE due to the significant increase in ductility. These findings demonstrate that multi-pass ECAE effectively transforms brittle Zn–Al cast alloys into tougher materials with ductile fracture behavior.