R. Manna

Strengthening Behavior of Bulk Ultra Fine Grained Aluminum Alloys

Billets of aluminum and aluminum alloys have been deformed at room temperature using a die having equal channels of 10 mm diameter intersecting at an inner angle of 120° and outer arc of 60° by equal channel angular pressing (ECAP) to ultra fine grain (UFG) size level, adopting route Bc. Mechanical properties were evaluated by tensile testing and microhardness measurement. Effects of alloying elements on strengthening were explored. The strengths increase rapidly at first few passes and then reach to a saturation level. The improvement in strength at initial passes of ECAP is due to work hardening and subgrain or dislocation cell formation. However, strengthening at large number of passes is due to the grain refinement alone. The rate of strengthening as a function equivalent strain decreases to a minimum. The strengthening level of bulk UFG alloys is about 3.5 to 4.5 times to that of starting materials. The major cause of strengthening is grain refinement apart from solute strengthening. Among Mg, Zn and Ag alloying elements, the strengthening effect is highest for Mg and lowest for Ag. Ductility is regained without affecting the strength after sufficient number of passes when microstructure becomes equiaxed and ultra-fine in size. However, ductility of UFG Al alloys is lower than that of their coarse grained counterpart.

Effect of Flash Annealing on Ultra-Fine Grained Low-Carbon Steel Processed Through Equal-Channel Angular Pressing Followed by Cryorolling

Low-carbon steel (LCS) work pieces are deformed through equal-channel angular pressing (ECAP) up to an equivalent strain of 16.8 (28 passes) at room temperature. Microstructures are characterized by optical microscopy and transmission electron microscopy. Mechanical properties are evaluated by tensile testing and hardness measurements. The material gets refined to ultra-fine level of average grain size of 0.2 μm and the ultimate tensile strength improves from 368 MPa to ultra-high strength of 1008 MPa. ECAP followed by cryorolling of low-carbon steel at −50 °C for 75% of reduction in the area produces bulk nanostructured grains of size 87 nm. The strength of the material further enhanced to 1238 MPa. The strengthening is due to reduction in grain size and high defect density. But the material loses its ductility due to high defect density and nonequilibrium nature of grain boundaries. Flash annealing at 600 °C of bulk nanostructured low-carbon steel produces the bimodal grain size distribution of UFG and micron-sized grains in the microstructure which partially recovers uniform elongation of 20% with improved strength. The material still maintains hardness which is twice as that of as-received material but it decreases with increase in flash annealing temperature.

Development of High-Strength Bulk Ultrafine-Grained Low Carbon Steel Produced by Equal-Channel Angular Pressing

Low carbon steel (LCS) workpieces have been deformed by equal-channel angular pressing (ECAP) at a large equivalent strain of 16.8 at room temperature. The mechanisms of microstructural refinement, strengthening, hardening, and fracture behavior are investigated. LCS becomes refined by a sequence of mechanisms of elongation of grains, splitting of elongated grains to bands at low strain, subdivision of bands to cells at intermediate strain, elongation of bands to ribbon grains, and breaking of ribbons to near-equiaxed grains at a high strain level. ECAP of LCS at εvm = 16.8 refines the material to near-equiaxed grains of size 0.2 µm having a high-angle grain boundary fraction of 82.4 pct and average misorientation angle of 40.8 deg. The ultrafine-grained (UFG) LCS contains a dislocation density of 1.7 × 1015 m2. In the initial passes of ECAP, the yield and tensile strengths increase rapidly due to rapid grain refinement, reduction in domain size, and increase in dislocation density. At high strain levels, strengthening can be attributed to a combination of grain refinement, dissolution of cementite in the ferrite matrix, and increase in misorientation angle. At εvm = 16.8, the ultimate tensile strength (UTS) reaches >1000 MPa with a consequent drop in ductility to ≈10.6 pct. Reduction in ductility is found to be due to high dislocation density, high stored energy in the matrix, and occurrence of nonequilibrium grain boundaries. The LCS at low equivalent strain fails by ductile fracture. The dimple size and its volume fraction decrease, but their number density and stored energy increase with increasing equivalent strain. Beyond a critical equivalent strain of 9, the material fails by ductile-brittle fracture. At εvm = 16.8, equal-channel angular pressed UFG LCS fails mainly by cleavage fracture.

Grain Refinement Mechanisms Oterative during Equal Channel Angular Pressing of Aluminium

Grain refinement of aluminum deformed by equal channel angular pressing is strongly dependent on the amount of strain. The refinement process at low to high strain level involves elongation of the existing grains by shear deformation, their subdivision into bands and subgrain formation within bands, intersection of the bands during subsequent passes and finally conversion of the subgrains to grains by continuous dynamic recrystallization process. At room temperature the conversion of subgrains to grains takes place by progressive lattice rotation.