Subramanya Sarma Vadlamani

The present study discusses the grain boundary microstructural control in a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (commonly known as alloy D9) through a one-step thermomechanical treatment. The experimental methodology adopted in this investigation was based on the strain annealing approach in which a small amount of strain (5 to 15 pct) was imparted on the solution-annealed (SA) sample. The cold-deformed samples were subsequently annealed at various temperatures (1173 to 1273 K) for different time periods (0.5 to 2 hours). It was observed that annealing after 5 pct deformation induces anomalous grain growth with a moderate increase in number fraction of coincidence site lattice (CSL) boundaries. However, a prestrain of 10 to 15 pct followed by annealing at 1273 K for 0.5 to 2 hours was found to be a suitable thermomechanical processing schedule to increase the number fraction of CSL boundaries (particularly Σ3 and its variants) significantly. Further, the well-connected network of random grain boundaries present in the SA specimen was substantially disrupted in these processing conditions due to the incorporation of Σ3 and its variants. The preceding results were discussed with reference to strain-induced grain growth vis-à-vis strain-induced boundary migration (SIBM) following deformation and annealing. © The Minerals, Metals & Materials Society and ASM International 2008.

The effect of low-temperature on the active deformation mechanism is studied in pure copper. For this purpose, cryogenic wire drawing at liquid nitrogen temperature (77 K) was performed using molybdenum disulfide lubrication. Microstructural investigation and texture analysis revealed severe twin formation in the cryogenically drawn copper, with a broad twin size distribution. The spacing of the observed deformation twins ranges from below 100 nm, as reported in previous investigations, up to several micrometers. The extent of twin formation, which is significantly higher when compared to other cryo-deformation techniques, is discussed with respect to the state of stress and the texture evolution during wire drawing. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This paper presents a study of microstructure, mechanical properties and corrosion behaviour of a near alpha Ti-5Ta-1.8Nb alloy subjected to cryo-rolling. Grain size decreased with increase in cryo-rolling strain (εcr), from about 5μm in the annealed sample down to ~20nm at a εcr=2.3. Dislocation density initially increased steeply up to εcr=0.69 and then decreased with increasing strain up to 2.3. The decrease in the dislocation density was attributed to the formation and interaction of pile-ups resulting in annihilation of dislocations. Hall-Petch analysis showed two distinct slopes, region I, with a rapid increase in stress and region II with a negligible increase in stress with decrease in grain size. This was explained in terms of changes in dislocation density and grain size. Fractography studies revealed dimples and microvoids in the coarse grained alloy but the nanocrystalline alloy revealed a mixture of shear bands at the surface and smaller ductile dimples at the centre. Evaluation of the corrosion behaviour in boiling and concentrated nitric acid in three phases (liquid, condensate and vapour) of the nanocrystalline samples revealed almost no deterioration of the corrosion resistance of the Ti alloy when compared to the solution annealed alloy. © 2014 Elsevier B.V.

An attempt has been made to predict the segregation of carbon in continuously annealed hot-rolled low-carbon aluminum-killed steel (LCAK) through thermodynamic calculations. It is found that the surface carbon content calculated by the Guttmann model is comparable with the experimental results for inert atmosphere condition, whereas a large discrepancy is observed for the conventional continuous annealing furnace atmosphere. The main reason for this difference is the water-gas reaction that takes place on the surface of the steel sheet. It is proposed that this reaction, along with furnace atmosphere, determines the extent of carbon content on the sheet's surface. Based on this, the surface carbon content for various furnace conditions could be predicted. © 2010 Elsevier B.V.

Supersaturated Cu-3 at.% Ag alloy was processed by rolling at liquid nitrogen temperature and subsequent annealing at 623 K up to 20 min. It was found that after annealing, an inhomogeneous solute atom distribution developed, since the Ag particles with small size and/or large specific interfacial energy were dissolved due to the Gibbs-Thomson effect. In the region where the solute concentration increased, a high dislocation density was retained in the Cu matrix even after annealing, while in the region where the Ag solute content did not increase, the dislocation density decreased by more than one order of magnitude. Therefore, in the cryorolled and annealed samples, heterogeneous microstructures were developed where both the dislocation density and the solute concentration varied considerably. © Published under licence by IOP Publishing Ltd.

Ultrafine grained (ufg) and nanocrystalline (nc) materials are widely researched due to significant improvements in yield and fracture strength. However, achieving a reasonable ductility in these materials is still a challenge. Recent results have shown that the combination of high strength and ductility could be achieved in precipitation hardening alloys through severe plastic deformation followed by annealing/ageing treatments. In the present work, the solutionised plates of an Al-Mg-Si alloy (modified AA6061 alloy) were subjected to severe cold rolling at room and liquid nitrogen temperatures to a true strain ∼1.6. The rolled sheets were aged to induce precipitation. The equilibrium second phase distribution for the above alloy was calculated using CALPHAD. The rolled and aged samples were analysed using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), hardness and tensile tests. The stored energy obtained from DSC measurements was found to be independent of the rolling temperature. The volume fraction of S {1 2 3} 〈6 3 4〉 orientation is predominant (∼40%) in both the rolling conditions. The strength and ductility were simultaneously improved following ageing of the cryorolled (CR) and room temperature rolled (RT) samples. Transmission electron microscopy analysis revealed dislocation cell structures in the CR and RT conditions. Analysis of second phases revealed fine spherical Mn rich precipitates (most likely Al6Mn) following ageing. © 2009 Elsevier B.V. All rights reserved.

The influence of Mo alloying on annealing-induced hardening in ultrafine-grained (UFG) Ni is studied. The hardening observed after low temperature annealing is explained by the annihilation of mobile dislocations and a concomitant clustering of the remaining dislocations into low energy configurations. This study reveals that, with increasing Mo concentration, the hardening effect decreases as the Mo solute atoms hinder the annihilation and rearrangement of dislocations. This trend is the opposite to that observed in electrodeposited Ni–Mo alloys where the larger alloying element concentration yields a higher annealing-induced strengthening effect. The difference is attributed to the different deformation mechanisms in UFG and nanocrystalline Ni–Mo alloys.

Severe cold rolling and short intercritcal annealing is often used to produce ultra-fine grained ferrite and martensite dual phase steels. In this paper, microstructure and texture of Nbmicroalloyed steel following cold rolling and short intercritical annealing is investigated. The results show that cold rolling and annealing resulted in ultra-fine grained dual phase steel consisted of polygonal ferrite in the range of ∼1-2 μm in size. In cold rolled material, the texture components are γ fiber (<111>//normal direction) and α fiber (<110>//rolling direction). Partial recrystallization texture was observed following intercritical annealing. © (2012) Trans Tech Publications, Switzerland.

Ultrafine-grained (UFG) dual-phase (DP) steel was produced by severe cold rolling (true strain of 2.4) and intercritical annealing of a low carbon V-Nb microalloyed steel in a temperature range of 1003 K to 1033 K (730 °C to 760 °C) for 2 minutes, and water quenching. The microstructure of UFG DP steels consisted of polygonal ferrite matrix with homogeneously distributed martensite islands (both of size <1 µm) and a small fraction of the inter lath films of retained austenite. The UFG DP steel produced through intercritical annealing at 1013 K (740 °C) has good combination of strength (1295 MPa) and ductility (uniform elongation, 13 pct). The nanoscale V- and Nb-based carbides/carbonitrides and spheroidized cementite particles have played a crucial role in achieving UFG DP microstructure and in improving the strength and work hardening. Analysis of work hardening behavior of the UFG DP steels through modified Crussard–Jaoul analysis showed a continuously varying work hardening rate response which could be approximated by 2 or 3 linear regimes. The transmission electron microscopy analysis on post tensile-tested samples indicated that these regimes are possibly related to the work hardening of ferrite, lath, and twin martensite, respectively.

Solid particle erosion wear behaviour of A356 and A356/TiB2 in situ composites has been studied. A356 alloy reinforced with in situ TiB2 particles was fabricated by the reaction of halide salts with aluminium melt and the formation of Al3Ti brittle phase is completely suppressed. The composites show good grain refinement of α-Al and modification of eutectic Si. These in situ composites show high hardness and better erosion resistance than the base alloy. Though the sizes of in situ formed TiB2 reinforcement particles are smaller than the erodent SiC particles, TiB2 particles are able to effectively resist the erodent particles. Design of experiment has been used to run the solid particle erosion experiment. An attempt has also been made to develop a mathematical model by using regression analysis. Analysis of variance (ANOVA) technique is applied to check the validity of the developed model. Student's t-test is utilized to find out the significance of factors. The wear mechanism has been studied by analyzing the surface of the worn specimen using scanning electron microscopy and energy dispersive X-ray microanalysis. © 2007 Elsevier B.V. All rights reserved.