R. Jayaganthan

A Comparative study on Mechanical Properties of Al 7075 alloy processed by rolling at cryogenic temperature and room temperature

The mechanical properties and microstructural characteristics of a precipitation hardenable Al 7075 alloy subjected to rolling at liquid nitrogen temperature and room temperature are reported in this present work. The Al 7075 alloy was severely rolled at cryogenic temperature and room temperature and its mechanical properties were studied by using tensile tests and hardness. The microstructural characterization of Al 7075 alloy were made using SEM/EBSD, TEM and DSC. The cryorolled Al 7075 alloys have shown improved mechanical properties as compared to the room temperature rolled Al alloy. The cryorolled Al alloy after 90% thickness reduction exhibits ultrafine grain structure as observed from its TEM micrographs. It is observed that the strength and hardness of the cryorolled materials (CR) at different percentage of thickness reductions are higher as compared to the room temperature rolled (RTR) materials at the same strain due to suppression of dynamic recovery and accumulation of higher dislocations density in the CR materials.

Structural,Wettability and Optical Investigation of Titanium Oxynitride Coatings:Effect of Various Sputtering Parameters

The objective of the present work is to investigate the effect of various sputtering parameters such as nitrogen flow rate,deposition time and sputtering pressure on structural,wettability and optical properties of titanium oxynitride films deposited on glass substrate by reactive magnetron sputtering.The X-ray diffraction graphs of titanium oxynitride films show evolution of various textures of TiO_xN_y and TiN phases with increasing nitrogen flow rate and deposition time,but an increase in sputtering pressure from 4.0 to 8.0 Pa results in decline of various textures observed for TiO_xN_y and TiN phases.The stress and strain calculated by sin~2Ψ method are compressive,which decrease with increasing nitrogen flow rate from 55 to 100 sccm(standard cubic centimeter per minute) and increase with increasing deposition time from 80 to 140 min due to atomic penning effect and increasing thickness of the deposited films.The titanium oxynitride films have contact angle values above 90 deg.,indicating that films are hydrophobic.The maximum contact angle of 109.1 deg.is observed at deposition time of 140 min.This water repellent property can add value to potential protective,wear and corrosion resistant application of titanium oxynitride films.The band gap decreases from 1.98 to 1.83 eV as nitrogen flow rate is increased from 55 to 100 sccm;it decreases from 1.93 to 1.79 eV as deposition time is increased from 80 to 140 min as more nitrogen incorporation results in higher negative potential of valence band N2p orbital.But it increases from 2.26 to 2.34 eV for titanium oxynitride films as sputtering pressure increases from 4.0 to 8.0 Pa.

Effect of sputtering pressure and temperature on DC magnetron sputtered CrN films

Abstract Chromium nitride thin films were deposited on Si(100) substrate by using DC magnetron sputtering and the influence of process parameters such as substrate temperature, pressure and power on their microstructural characteristics were investigated in the present work. The CrN films were characterised with X-ray diffraction and it was observed that the films exhibit (111) preferred orientation but it transforms in to (200) orientation with increasing working pressure. The preferred orientations of CrN thin films are strongly influenced by sputtering conditions, thickness and the induced microstrain in the thin films. Field emission scanning electron microscopy and atomic force microscopy were used to characterise the morphology and surface topography of the CrN thin films respectively. The as deposited CrN films exhibited columnar morphology and its surface roughness values were influenced by the working pressure and temperature. Similarly, the microstrain in the films exhibited a strong dependence on deposition conditions.

Effect of annealing and aging treatment on mechanical properties of ultrafine grained Al 6061 alloy

Abstract A comparative study of aging and a combined treatment of short annealing and aging on mechanical properties and microstructure of cryorolled (CR) Al 6061 alloy is investigated in the present work by using tensile tests, hardness tests, electron backscattered diffraction and transmission electron microscope. The pre-CR solid solution treatment combined with post-CR short annealing (200°C, 5 min) followed by aging treatment (100°C, 57 h) of the Al 6061 alloy showed an improved ductility and well defined ultrafine grain structure as compared to the samples subjected to pre-CR solid solution treatment followed by post-CR aging treatment (100°C, 60 h).

Microstructures and impact toughness behavior of Al 5083 alloy processed by cryorolling and afterwards annealing

The influence of rolling at liquid nitrogen temperature and annealing on the microstructure and mechanical properties of Al 5083 alloy was studied in this paper. Cryorolled samples of Al 5083 show significant improvements in strength and hardness. The ultimate tensile strength increases up to 340 MPa and 390 MPa for the 30% and 50% cryorolled samples, respectively. The cryorolled samples, with 30% and 50% reduction, were subjected to Charpy impact testing at various temperatures from −190°C to 100°C. It is observed that increasing the percentage of reduction of samples during cryorolling has significant effect on decreasing impact toughness at all temperatures by increasing yield strength and decreasing ductility. Annealing of samples after cryorolling shows remarkable increment in impact toughness through recovery and recrystallization. The average grain size of the 50% cryorolled sample (14 μm) after annealing at 350°C for 1 h is found to be finer than that of the 30% cryorolled sample (25 μm). The scanning electron microscopy (SEM) analysis of fractured surfaces shows a large-size dimpled morphology, resembling the ductile fracture mechanism in the starting material and fibrous structure with very fine dimples in cryorolled samples corresponding to the brittle fracture mechanism.

Evaluation of hot corrosion behaviour of HVOF sprayed Ni–5Al and NiCrAl coatings in coal fired boiler environment

Abstract Hot corrosion was recognised as a serious problem in connection with the degradation of fireside boiler tubes in coal fired steam generating plants. It is one of the most deleterious forms of surface degradation which can lead to the loss of mechanical strength and catastrophic failure of structural and engineering components. In the present investigation, an attempt has been made to evaluate the hot corrosion behaviour of high velocity oxyfuel (HVOF) sprayed Ni–5Al and NiCrAl coatings on iron based superalloy in an actual coal fired boiler environment. The coated specimens were hung in the low temperature superheater zone (∼700°C) of a coal fired boiler. The studies were carried out for 10 cycles each cycle consisting of exposing the coated specimens for 100 h followed by 1 h cooling to ambient conditions. NiCrAl coated specimen provided better resistance to hot corrosion than Ni–5Al coated specimen in the given environment.

Hot corrosion behaviour of D-gun sprayed NiCoCrAlYTa coated superalloys at 900°C in molten salt environment

Abstract Detonation-gun sprayed NiCoCrAlYTa coatings deposited on superalloys have been investigated for their hot corrosion behaviour at 900°C in molten salt environment under cyclic conditions. The weight change measurements made on the bare and coated superalloys were used to determine the kinetics of hot corrosion. X-ray diffraction, X-ray mapping and field emission scanning electron microscope/EDS were used to characterise the corroded products of the coated and bare superalloys and substantiate the corrosion mechanisms. The formation of scales such as NiCr2O4, NiAl2O4 and Al2O3 on the coated superalloys provided a higher hot corrosion resistance as observed in the present work.

Effect of Cryorolling Strain on Precipitation Kinetics of Al 7075 Alloy

The effect of rolling strain on precipitation kinetics of Al 7075 alloy processed at liquid nitrogen temperature has been investigated in the present work. The Al 7075 alloy plates were solutionized and cryorolled with thickness reduction of 35% and 90%. The microstructural characterizations of the bulk and cryorolled Al alloy samples were carried out by electron backscatter diffraction analysis (EBSD) and transmission electron microscopy (TEM), respectively. The cryorolled Al alloys upon 90% thickness reduction exhibit ultrafine grained microstructure. The DSC results of cryorolled Al 7075 alloys obtained at different heating rates are used to calculate activation energies for the evolution of precipitates. The influence of different reduction rates on activation energy of precipitate formation in the cryorolled Al 7075 alloys was analyzed. The present study has shown that an ultrafine-grained Al 7075 alloy exhibits a higher driving force for the precipitation formation when compared to that of its bulk Al alloys.

Effect of annealing on microstructure and mechanical properties of Al 6061 alloy processed by cryorolling

Abstract An ultrafine grained (UFG) structure developed in precipitation hardenable Al alloys through cryorolling by suppression of dynamic recovery followed by low temperature aging has received great research interest because of its high strength and very good ductility. In the present work, Al 6061 alloy was solution treated and deformed by cryorolling up to an effective true strain of 2·6 and then subjected to annealing at the temperature range from 150 to 350°C to study the effect of annealing on the microstructure and mechanical properties. The evolution of microstructure and precipitates was investigated by employing X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Vickers hardness and tensile testings were performed at room temperature to evaluate the effect of annealing on the mechanical properties. It was observed that the strength and ductility increased upon annealing at 150°C, and further annealing at high temperatures (200–350°C) results in reduction in hardness and strength but increase in ductility. A significant improvement in strength observed at low temperature annealing (150°C) is due to the precipitation of metastable phase β″. It overcompensates the reduction in hardness that occurred due to the softening caused by the recovery effect. It was found that the cryorolled Al 6061 alloy with UFG structure is thermally stable up to temperatures 250°C with slight grain coarsening. At this temperature, the TEM studies revealed that second phase Mg2Si particles are effectively pinning the grain boundaries due to the Zener drag effect. Owing to the presence of heterogeneities in the material, a duplex structure was observed upon annealing at temperatures 150 and 200°C. Abnormal grain growth was observed after annealing at high temperatures (300°C).

Tribological behaviour of nanostructured Al2O3 coatings

Abstract Nanostructured alumina coatings have been deposited on SS 304 substrate using atmospheric plasma spray process. In the present work, nanostructured feed powder is obtained by manual granulation of nano-Al2O3 powder particles followed by exhaustive sieving, which is helpful in their proper consolidation. The tribological performance of the nano-alumina coatings, such as dry sliding, slurry erosion and cavitation erosion, has been investigated and compared with conventional coatings. The superior wear and erosion resistance of nanostructured coatings as compared to conventional coatings are due to its higher hardness and effective hindrance to crack propagation. Wear mechanism was explained based on their microstructure and worn surface morphologies. The microhardness and porosity of the two coatings were also experimentally investigated.