Ravi Kumar N V

Coarsening induced reverse phase transformation of hafnia in polymer-derived Si-Hf-C-N-O ceramics was studied experimentally. X-ray diffraction and high resolution transmission electron microscopy were used to obtain information regarding nanocrystalline structure. Hafnia crystallites precipitated out from the amorphous ceramic matrix on heat treatment were observed to remain as tetragonal hafnia below a critical crystallite radius of 4 nm. Prolonged heat treatment has resulted in coarsening, as explained by Lifshitz-Slyozov-Wagner model, resulting in reverse phase transformation from tetragonal to monoclinic hafnia. © 2011 The American Ceramic Society.

Hafnium alkoxide modified polysilazane was synthesized by the drop-wise addition of hafnium tetra(n-butoxide) to polysilazane. The solid state thermolysis (SST) temperature and the ceramic yield for both the polysilazane and modified polysilazane were determined by performing thermogravimetry. Fourier transform infrared spectroscopy was performed to understand the polymer to ceramic conversion as well as the bonding characteristics of the ceramics. The modified polymer after crosslinking was subjected to SST at 800 °C at a constant heating rate of 5 °C/min for a dwell time of 2 h in atmospheric ambience. From the X-ray diffractograms, the as-thermolysed ceramics were observed to remain X-ray amorphous and on heat-treatment resulted in the crystallization of tetragonal hafnia. However, on heat-treatment at 1500 °C, reverse phase transformation from tetragonal to monoclinic hafnia was observed. Raman spectroscopy and transmission electron microscopy were employed to further understand the phase evolution. The thermal stability and the influence of amorphous matrix on the coarsening of HfO 2 were also evaluated. © 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Powder particles of polymer derived SiHfCN(O) ceramics were pulsed electric current sintered at 1300 and 1500 °C to produce amorphous and partially crystalline ceramic pellets for corrosion studies in salt (NaCl or Na2SO4) and acid (HF) environments. While, sodium dramatically accelerated phase transformation and catalyzed the crystallization process, the open porosity acted as the main cause for sodium penetration in these materials. The samples, however, were completely disintegrated during fluoride acid tests. The cristobalite and HfO2 crystalline phases were severely corroded and it was found that the SiC grains were relatively stable in comparison with other phases in the system.

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.

Thermal shock resistance of precursor derived Si-Hf-C-N(O) foams at temperatures varying from 800o-1000oC subjected to multiple thermal cycles was investigated. The as-synthesized foams possessed interconnected pores with an average cell size of 1.09 mm. The X-ray diffractograms of the foams before and after thermal cycling showed that the amorphous nature of the foams was retained. FTIR spectra exhibited that there was no change in the bonding characteristics due to thermal shock. A damage parameter (DS) based on the compressive strength was used to quantify the extent of damage. Densification was expected to occur in the first thermal cycle and the strut structures did not show any sign of cracking. However, cracking of struts occurred in the third thermal cycle which caused severe damage.

In the current work, rotary friction welding of electron beam melted (EBM) γ-TiAl alloy Ti–48Al–2Cr–2Nb (at%) was investigated. Welding experiments were conducted using cylindrical bars of 12 mm diameter and 70 mm height in as-fabricated and heat treated (1275 °C/2 h/furnace cooling) conditions. No cracking problems were encountered during friction welding of as-fabricated EBM samples. However, in as-welded condition, the joints did not perform well in tensile tests and failures were observed to invariably occur at the weld interface. Friction welded joints produced in heat treated EBM samples were also found to suffer from the same problem. In both the cases, the weld region showed fine, fully lamellar γ + α2 microstructure and relatively high hardness. In order to overcome the problem, a post-weld heat treatment (PWHT) was carried out at 1275 °C/2 h/furnace cooling. After the PWHT, the weld region showed a more desirable duplex microstructure consisting of equiaxed primary γ and relatively coarser lamellar γ + α2. The PWHTed joints showed satisfactory tensile properties, and no failures were observed at the weld interface.

Localized corrosion of stainless steel beneath the barnacle-base is an unsolved issue for the marine industry. In this work, we clearly bring out for the first time the role of the barnacle cement in acting as an etchant, preferentially etching the grain boundaries, and initiating the corrosion process in stainless steel 316L. The investigations include structural characterization of the cement and corroded region, and also chemical characterization of the corrosion products generated beneath the barnacle-base. Structural characterization studies using scanning electron microscopy (SEM) reveals the morphological changes in the cement structure across the interface of the base-plate and the substrate, modification of the steel surface by the cement and the corrosion pattern beneath the barnacle-base. Fourier transform infrared spectroscopy (FTIR) of the corrosion products show that they are composed of mainly oxides of iron thereby implying that the corrosion is aerobic in nature. A model for the etching and corrosion mechanism is proposed based on our observations. © 2010 Elsevier B.V.

Hollow glass microspheres/epoxy foams of different densities were prepared by stir casting process in order to investigate their mechanical properties. The effect of hollow spheres content and wall thickness of the microspheres on the mechanical response of these foams is studied extensively through a series of quasi-static uni-axial compression tests performed at a constant strain rate of 0.001s-1. It is found that strength of these foams decreases linearly from 105MPa (for the pure resin) to 25MPa (for foam reinforced with 60vol.% hollow microspheres) with increase in hollow spheres content. However, foams prepared using hollow spheres with a higher density possess higher strength than those prepared with a lower one. The energy absorption capacity increases till a critical volume fraction (40vol.% of the hollow microspheres content) and then decreases. Failure and fracture of these materials occur through shear yielding of the matrix followed by axial splitting beyond a critical volume fraction. © 2011 Elsevier Ltd.

The crystallization kinetics of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 and {[(Fe0.5Co0.5)0.75B0.2Si0.05]0.96Nb0.04}99.5Cu0.5 bulk metallic glasses were evaluated using differential scanning calorimetry under non-isothermal condition. The fully glassy rods with diameters up to 2 mm were obtained by copper mold injection casting. Both glasses show good thermal stability, but the addition of only 0.5% Cu completely changes the crystallization behavior. The average activation energy required for crystallization decreases from 645 kJ/mol to 425 kJ/mol after Cu addition. Upon heating, the Cu-free alloy forms only the metastable Fe23B6 phase. In contrast, two well-separated exothermic events are observed for the Cu-added bulk glassy samples. First, the (Fe,Co) phase nucleates and then (Fe,Co)2B and/or (Fe,Co)3B crystallize from the remaining glassy matrix. The Cu-added alloy exhibits a lower coercivity and a higher magnetic saturation than the base alloy, both in as-cast as well as in annealed condition. Besides, the Cu-added glassy sample with 2 mm diameter exhibits a maximum compressive fracture strength of 3913 MPa together with a plastic strain of 0.6%, which is highest plastic strain ever reported for 2 mm diameter ferromagnetic bulk metallic glass sample. Although Cu addition improves the magnetic and mechanical properties of the glass, it affects the glass-forming ability of the base alloy.

Thermodynamic modelling of Ti-Zr-N system is performed using Calphad method coupled with ab initio calculations. The energies of formation of stable and metastable end-members of sublattice formulations of solid phases in Zr-N system and enthalpy of mixing of the mixed nitride (Ti, Zr)N (δ) are calculated using ab initio method. Phonon calculations are used to compute the Gibbs energies of stoichiometric ZrN and the mixed nitride δ. With the aid of experimental thermochemical and constitutional data from the literature along with the results of ab initio calculations, thermodynamic optimization is carried out to obtain the Gibbs energy model parameters.