G.V.S. Sastry

On the dual slope Coffin-Manson relationship during low cycle fatigue of Ni-base alloy IN 718

Abstract A detailed TEM study has been conducted on specimens from LCF tests interrupted after different number of cycles at two strain amplitudes to find the reason for the observed dual slope C-M plot at room temperature in the alloy IN 718. The following conclusions emerge from this study: (a) In the low strain region the deformation mode is essentially twinning, (b) In the high strain region deformation principally occurs by slip band formation, (c) Change in mode of deformation has resulted in dual slope C-M plot.

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.

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.