Sankaran Shanmugam

The effect of strain rate and nitrogen content on cyclic deformation and substructural changes in 316LN stainless steel is investigated at temperatures 773, 823 and 873 K. Dynamic strain aging (DSA) and/or thermal-recovery processes are observed to control cyclic deformation, and the regimes of their predominance are mapped. An increase in nitrogen content and DSA enhanced cyclic stress and are found to offset thermal-recovery induced cyclic strength reduction. In addition, strain localization in the form of slip-bands impinging on grain boundary is observed. The predominance of thermal-recovery over DSA manifested as dislocation-poor channels, dislocation cells within and in-between planar slip-bands.

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.

Carbon nanotubes (CNT) have received huge attention from the scientific community in the last two decades due to their unique structure and properties. They have been considered for potential applications in various areas of science and technology. One of the major applications of CNT is as reinforcement for fabrication of light weight high strength composite materials for use in automobile and aerospace applications. Aluminium and its alloys are natural choices for such applications due to their low density, high specific strength and modulus. In the last decade, there have been significant advances in the processing of carbon nanotube reinforced aluminium matrix (Al-CNT) composites. New understanding has emerged due to research on several aspects such as damage to CNTs during processing, interfacial phenomena, novel methods of processing for improving CNT dispersion, tensile behaviour, numerical modelling and in situ tensile testing. This review summarizes the present status of the tensile properties of pure Al-CNT and Al alloy-CNT composites. The various processing routes for fabrication of Al-CNT composites have been compared in terms of the resulting microstructure, degree of CNT dispersion, extent of interfacial reaction and its effect on the tensile properties. Factors affecting strengthening efficiency and the strengthening mechanisms in Al-CNT composites are discussed.

Creep-fatigue interaction (CFI) behavior of 316LN stainless steel with varying nitrogen content (0.07, 0.14 and 0.22. wt.%) is investigated at temperature of 873. K under strain-controlled fatigue tests with a tensile-hold period of 1, 13.3 and 30 min. Dynamic strain aging, thermal-recovery and/or dislocation-precipitate interactions are found to strongly control CFI deformation. At all the nitrogen contents in 316LN SS, the amount of stress relaxation and the associated relaxation strain rate decreased with increase in hold duration. Irrespective of the hold period, wavy-slip dislocation structures (cells, tangles, walls) are noticed at 0.07. wt.% N, whereas dislocation structures such as planar slip bands impinging on grain boundary are noticed, in addition, at nitrogen contents above 0.07. wt.%. Nitrogen content played a dual-role in influencing the CFI life. At 1-min hold where the failure is by transgranular plus intergranular fracture, increase in nitrogen from 0.07 to 0.14. wt.% caused an improvement in CFI life. On the other hand, CFI life decreased with increasing nitrogen content at 13.3 and 30-min hold durations that induced significant intergranular damage. Intergranular damage is evidenced in the form of triple point cracks and interconnected networks of grain boundary decohesion.

Multiphase ferrite-bainite-martensite microalloyed steel produced through a two-step cooling followed by annealing route and a ferrite-pearlite steel obtained through air-cooling after forging were subjected to turning operation. The influence of process parameters such as cutting speed, feed and depth of cut on surface roughness on both materials was compared. The results show that the multiphase microalloyed steel exhibited high surface finish than air-cooled steel. The analysis of variance shows that the contribution of cutting speed and depth of cut on surface roughness are insignificant for both ferrite-bainite-martensite and ferrite-pearlite microstructures.

Electroless copper coatings were performed on purified carbon nanotubes (CNT), with varying deposition time and the optimum deposition time in terms of uniform deposition was determined to be 45. min. Different amounts of optimized Cu coated CNT (CNT (Cu)) and Al powders were ball milled. CNT (Cu) reinforced Al (Al-CNT (Cu)) composites were prepared by spark plasma sintering (SPS). Pure CNT reinforced Al (Al-CNT) composites were also prepared by SPS. The ball milled powders and composites were characterized using X-Ray diffraction, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy (TEM). Microhardness and compression properties of the composites were measured. TEM images of ball milled powders and composites revealed uniform distribution of CNT in matrix. Mechanical properties of Al-CNT (Cu) composites are superior to Al-CNT composites. The maximum enhancement in compressive strength of Al-CNT (Cu) composites is 154% for 2. wt% reinforcement; this enhancement is attributed to the copper coating on CNT surface.

Al-4.5 wt.% Cu alloy was spray atomized and deposited at varied spray heights ranging from 300 to 390 mm. The average grain sizes decreased from ∼ 29 to ∼ 18 μm and a concomitant increase in the hardness and the 0.2% yield strength (YS) with increase in the spray height. The respective hardness values of SF-300, SF-340, and SF-390 are 451 ± 59, 530 ± 39, and 726 ± 39 MPa and the YS are 108 ± 7, 115 ± 8, and 159 ± 10 MPa. The transmission electron micrographs revealed the morphological changes of the Al2Cu phase from irregular shaped to small plate-shaped and then subsequently to spheroidal shape due to high undercooling encountered during spray atomization with increase in spray height from 300 to 390 mm. The porosity of the spray formed deposits varied between 5 to 12%. Hot isostatic pressing of spray deposits reduced the porosity to less than 0.5% without any appreciable increase in grain size. A dislocation creep mechanism seems to be operative during the secondary processing. A comparison between as-spray formed and hot isostatically pressed deposits exemplifies improvement in mechanical properties as a result of elimination of porosity without affecting the fine grain sizes achieved during the spray-forming process. © 2014 ASM International.

Even anisotropic superplastic flow, which is a result of an elongated grain shape and texture, can lead to extreme elongations to fracture (superplasticity). Therefore, to identify the mechanisms of deformation present during superplastic flow alone, the effects of the microstructure should be eliminated first. Using an Al 5083 alloy, in which an equi-axed microstructure is present from the beginning, it is shown that grain boundary sliding, accompanied by grain rotations, is the rate controlling mechanism. © (2012) Trans Tech Publications, Switzerland.

In the present work, the hot deformation behaviour of as-cast medium Mn steels with nominal Mn content of 6, 8, and 10 wt% were studied using the thermomechanical simulator in the temperature and strain rate range of 1173 to 1373 K and 10−3 - 10 s−1 respectively. The flow curves of the three steels revealed significant work hardening followed by a plateau at strain rates ≥1s−1 in the entire temperature range. At lower strain rates (< 10−2 s−1) noticeable softening is observed after a strain of 0.2 in all three steels. The extent of softening is lower in 10 wt% Mn steel in comparison to the 6 and 8 wt% Mn steels. Analysis of processing maps reveals that in steels with 6 and 8 wt% Mn, regions of ‘high’ strain rate sensitivity (m) (0.2 - 0.25) are found to occur in similar temperature and strain rate regimes of 1273–1323 K and 10−2 s−1 respectively. These steels also showed similar apparent activation energies for hot deformation (380–390 kJmol−1), stress exponents (4.6) and dynamic recovery/recrystallization evolution with strain. The steel with 10 wt% Mn, has higher apparent activation energy for hot deformation (450 kJmol−1), higher stress exponent of 5.1, higher dynamic recovery and lower dynamic recrystallization. The observed activation energies correlate well with the activation energy for self-diffusion (QSD) of Fe in austenite containing Mn as an alloying element. In the ‘low’ m regimes, higher dynamic recovery (evaluated using kinetic analysis) was observed in the 10 % Mn steel as compared to the 6 and 8% Mn steels. In the ‘high’ m regimes, the dynamic recrystallization fraction (estimated from kinetic analysis and from parent grain reconstruction analysis) was lower in the 10 % Mn steel when compared to the 6 and 8 wt % Mn steels. This behaviour can be attributed to the higher dynamic recovery observed in the 10 % Mn steel. From the analysis of processing maps, kinetic analysis and microstructure, the optimum strain, strain rate and temperature for the thermomechanical processing of as-cast medium Mn steels with Mn in the range of 6 to 10 wt% are ≤ 0.6, ≤ 10−2 s−1 and 1273 to 1373 K respectively.