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Ramavath P.,International Advanced Research Center for Powder Metallurgy And New Materials | Biswas P.,International Advanced Research Center for Powder Metallurgy And New Materials | Kumar R.S.,International Advanced Research Center for Powder Metallurgy And New Materials | Mahendar V.,International Advanced Research Center for Powder Metallurgy And New Materials | And 4 more authors.
Ceramics International

Zinc sulphide (ZnS) ceramics synthesized by chemical vapour deposition (CVD) were subjected to post CVD thermal treatments at 850 and 1050 °C in inert atmosphere under pressureless conditions. The samples were found to undergo cubic (sphalerite) to hexagonal (wurtzite) crystallographic transformation at around 1020 °C as confirmed by dilatometric and X-ray diffraction studies. The paper reports the effect of transformation in terms of structure - both crystallography and microstructure. Further, the effects of this transformation on the optical and mechanical properties are also analyzed. The increase in grain size was found to be beneficial for the IR transmission of sphalerite (cubic) phase while, the presence of wurtzite (hexagonal) was found to reduce the transmission significantly. A detailed evaluation of the nature and characteristics of the fracture revealed that the ZnS ceramics failed predominantly by low energy, quasi - cleavage fracture. It was also confirmed that the mechanical properties of this material vary with the extent (area fraction) of quasi - cleavage facets. © 2011 Elsevier Ltd and Techna Group S.r.l. Source

Sundararajan G.,International Advanced Research Center for Powder Metallurgy And New Materials | Biswas P.,International Advanced Research Center for Powder Metallurgy And New Materials | Eswara Prasad N.,Regional Center for Military Airworthiness Materials
Experimental Mechanics

Technological advancements in ceramic powder synthesis, shaping and sintering have made it possible to tailor the microstructural, mechanical and optical property relationships in the case of advanced transparent ceramic materials. Transparent polycrystalline alumina (TPCA) is the hardest known transparent ceramic and one of the emerging candidate materials for transparent armour applications. The prerequisites for obtaining transparency with the high hardness, is to achieve the sintered average grain sizes <1 μm in combination with density close to the theoretical value. This paper outlines the processing of TPCA by an environmentally benign methyl cellulose based thermal gel casting (MCTG) process, which is employed for the first time in shaping of the TPCA. The green specimens shaped through this technique were pressureless sintered (PLS) to >96 % density at an optimum temperature of 1350 °C. The post sintering by Hot Isostatic Pressing (HIP) at an optimum temperature of 1350 °C and a pressure of 195 MPa resulted in >99. 5 % of the theoretical density and a grain size of 0. 7 μm. For the sake of comparison, conventional polycrystalline alumina samples (non-transparent) were also processed by sintering at 1550 °C under PLS condition with nearly the same densities (designated as PCA). The TPCA thus developed exhibit a combination of high hardness of 21 GPa, flexural strength of 550 MPa and excellent fracture resistance properties as compared to conventional PCA samples. © 2012 Society for Experimental Mechanics. Source

Majila A.N.,Gas Turbine Research Establishment | Fernando D.C.,Gas Turbine Research Establishment | Babu S.N.N.,Gas Turbine Research Establishment | Patnaik B.V.A.,Gas Turbine Research Establishment | Prasad N.E.,Regional Center for Military Airworthiness Materials
Defence Science Journal

In the present technical paper an iso-symmetrical forging in titanium alloy, i.e., Ti-6Al-4V is chosen for cut-up evaluation and study of mechanical properties and their correlation with microstructural characteristics. Tensile test specimens were extracted from rim, web and bore regions of the forgings aligned in radial and tangential directions. Test specimens varying from various locations were extracted to conduct the tests (ASTM E8) at various temperatures from room temperature to 300 °C. Statistical analyses of the tested data were carried out to quantify the variation in tensile properties along rim, web, and bore regions at room temperature. Effects of radial and tangential alignments of specimens at room temperature was also studied. Among the different test specimens, the specimen that exhibited mechanical properties close to average values were further subjected to microstructural and fractographic investigations using optical and scanning electron microscopes. These studies revealed that there is a marginal inhomogenity in the microstructure of the forgings and this variation controls the mechanical properties and fracture characteristics of the material. Microstructure marginally varies from rim to bore region. Similarly, along the thickness of the forging, there is a small variation in the microstructure. The aforementioned correlations have established the fact that the microstructure variations from different locations and among different specimen orientations have resulted in mild variation in the tensile properties. © 2015, DESIDOC. Source

Ramavath P.,International Advanced Research Center for Powder Metallurgy And New Materials | Mahender V.,International Advanced Research Center for Powder Metallurgy And New Materials | Hareesh U.S.,International Advanced Research Center for Powder Metallurgy And New Materials | Johnson R.,International Advanced Research Center for Powder Metallurgy And New Materials | And 2 more authors.
Materials Science and Engineering A

Fracture behaviour of zinc sulphide ceramics prepared by chemical vapour deposition (CVD) followed by hot isostatic pressing (CVD+HIP) was investigated in terms of flexural strength (σ f), plane-strain fracture toughness (K Ic), even conditional fracture toughness (K IQ), R-curve behaviour (variation of total fracture energy release rate, J c with crack extension, δ/δ c) and fracture mode. The corresponding Knoop Hardness number (KHN) and its correlations to flexural strength (σ f) are also evaluated and reported. The present study showed that the zinc sulphide (ZnS) ceramics processed by CVD exhibited higher fracture resistance compared to ZnS processed by CVD+HIP condition. This observation is principally attributed to higher grain size associated with post-CVD HIPing process. In both conditions, the ZnS materials exhibited conditional fracture toughness (K IQ) that decreased moderately with increased crack length due to the change in fracture mode form grossly tensile to predominant shear. A constantly rising R-curve behaviour was indicated in both the materials with significant increase in total fracture energy release rate (J c with the normalised displacement (δ/δ c), a parameter representing crack extension. © 2011 Elsevier B.V. Source

Shekar K.C.,Vignan Institute of Technology and Science | Priya M.S.,Mahatma Gandhi Institute | Subramanian P.K.,Mahatma Gandhi Institute | Kumar A.,Advanced Systems Laboratory | And 2 more authors.
Bulletin of Materials Science

Advanced materials such as continuous fibre-reinforced polymer matrix composites offer significant enhancements in variety of properties, as compared to their bulk, monolithic counterparts. These properties include primarily the tensile stress, flexural stress and fracture parameters. However, till date, there are hardly any scientific studies reported on carbon fibre (Cf) and carbon nanotube (CNT) reinforced hybrid epoxy matrix composites (unidirectional). The present work is an attempt to bring out the flexural strength properties along with a detailed investigation in the synthesis of reinforced hybrid composite. In this present study, the importance of alignment of fibre is comprehensively evaluated and reported. The results obtained are discussed in terms of material characteristics, microstructure and mode of failure under flexural (3-point bend) loading. The study reveals the material exhibiting exceptionally high strength values and declaring itself as a material with high strength to weight ratio when compared to other competing polymer matrix composites (PMCs); as a novel structural material for aeronautical and aerospace applications. © Indian Academy of Sciences. Source

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