E. Mohandas

Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9 wt-%Cr low carbon ferritic steels

Abstract The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-%Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr–1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1–100 K min−1. In addition to the accurate determination of Ac1, Ac3, M23C6, MX carbide dissolution and δ-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823–873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr–1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4–5 K min−1 for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature Ms, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.

Microstructural, nanomechanical and antibacterial properties of magnetron sputtered nanocomposite thin films of CrN/Cu

Abstract Synthesis of nanocomposite thin films of CrN/Cu on (100) Si and titanium modified stainless steel (D-9 alloy) substrates by pulsed magnetron sputtering from a composite target of Cr–Cu using sputtering gas mixture of argon and nitrogen is investigated. X-ray diffraction analysis of the films deposited at 773 K and 10 sccm of nitrogen flowrate indicated that the films are nanocrystalline and biphasic (fcc CrN and fcc Cu). The films showed a peak hardness of ∼15 GPa and a Young’s modulus of ∼200 GPa for those with a copper content of 15·1 at-%, and these values were found to decrease significantly with a Cu content of ⩾18·4 at-%. The evaluation of the antibacterial activity as a function of copper content in the range of 15·1–38·5 at-% indicated excellent antibacterial properties for CrN/Cu films with the copper content of 18·4 at-%. The phase stability of CrN of the nanocomposite is also discussed in relation to pure CrN thin films.

Structure imaging and vanadium substitution in cubic TiCr2 Laves phase

Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr2 Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces.

Electron Microscopy Studies on Oxide Dispersion Strengthened Steels

The 9Cr ODS steel, a candidate material for fast fission and fusion reactor applications, derives its superior irradiation performance due to the dispersoids in ferrite matrix. Electron microscopy studies on mechanically alloyed Fe- Y2O3-Ti model alloys and 9Cr yttria/Ti dispersion strengthened (ODS) ferritic steels are discussed in this paper. The size distribution of dispersoid in the consolidated model alloy and the ODS steel were found to be peaking at ~ 15 nm and 5 nm, respectively. The porosity in the ODS steel was greatly reduced in comparison to the model alloy owing to a superior milling process. The dispersoids were identified as Y-Ti-O complexes. An orientation relationship between the yttria dispersoid and the ferrite grains, in which they are embedded, was observed in the model alloy. Microtexture analysis on sections of consolidated ODS alloy rods showed a [110] fiber texture typical of rods of bcc metals. The ODS steel tube however contained randomly oriented ferrite grains.

Characterization and Performance of Magnetron-Sputtered Zirconium Coatings Deposited on 9Cr-1Mo Steel

Zirconium coatings of different thicknesses have been deposited at 773xa0K on 9Cr-1Mo steel substrate using pulsed DC magnetron sputtering. These coatings were heat treated in vacuum at two different temperatures (1173 and 1273xa0K) for one hour. X-ray diffraction (XRD) analysis of Zr-coated samples revealed the formation of α-phase (HCP structure) of Zr. XRD analysis of heat-treated samples show the presence of Zr3Fe and Zr2Fe intermetallics. The lattice parameter of these coatings was calculated, and it matches with the bulk values when the thickness reached 2µm. In order to understand this, crystallite size and strain values of these coatings were calculated from XRD plots employing Williamson-Hall method. In order to assess the performance of the coatings, systematic corrosion tests were carried out. The corrosion current density calculated from the polarization behavior showed that the corrosion current density of the uncoated 9Cr-1Mo steel was higher than the coated sample before and after the heat treatment. Studies using electrochemical impedance spectroscopy confirmed that the coated steel has higher impedance than the uncoated steel. The corrosion resistance of 9Cr1Mo steel had improved after Zr coating. However, the corrosion resistance of the coating after heat treatment decreased when compared to the as-deposited coating. The microstructure and composition of the surface oxide film influence the corrosion resistance of the Zr-coated 9Cr1Mo steel.

Plasma plume behavior of laser ablated cerium oxide: Effect of oxygen partial pressure

This paper describes the spatial and temporal investigation of laser ablated plasma plume of cerium oxide target using Langmuir probe. Cerium oxide target was ablated using a KrF (λ ~ 248xa0nm) gas laser. Experimental studies confirmed that oxygen partial pressure of 2xa0×xa010 −2 mbar is sufficient enough to get good quality films of cerium oxide. At this pressure, plume was diagnosed for their spatial and temporal behavior. Spatial distribution was investigated at a distance of 15xa0mm, 30xa0mm, and up to a maximum distance of 45xa0mm from the target, whereas temporal behavior has been recorded in the range of 0 to 50xa0µS with an interval of 0.5xa0µS. The average electron densities are found to be maximum at 30xa0mm from the target position and the plasma current of the laser ablated ceria is found to be maximum at 22xa0µS.

Structural and optical properties of electron beam evaporated yttria stabilized zirconia thin films

Yttria stabilized zirconia (10 mole % Y2O3) thin films were deposited on quartz substrates using electron beam physical vapor deposition at the substrate temperatures in the range 300 – 973u2005K. XRD analysis showed cubic crystalline phase of YSZ films with preferred orientation along (111). The surface roughness was found to increase with the increase of deposition temperatures. The optical band gap of ∼5.7 eV was calculated from transmittance curves. The variation in the optical properties is correlated with the changes in the microstructural features of the films prepared as a function of substrate temperature.

Microstructural, nanomechanical and tribological properties of ZrAlN thin films prepared by pulsed DC magnetron sputtering

An investigation on the structural, nano-mechanical and tribological properties of thin films of ZrAlN deposited on (100) Si and D-9 alloy substrates prepared by pulsed DC magnetron is reported. Zr1-xAlxNy films have been studied with the Al concentration range of 0<;x<;0.36 Al. It was observed that values of hardness and Youngs modulus of the alloyed ZrAlN films were in the range 9-18 GPa and 235-365 GPa, respectively. Tribological studies of ZrAlN thin film showed that the coefficient of friction of alloyed films of ZrAlN was lower for the steel ball (100Cr6 steel) than that for the Al2O3 ball up to 36 at.% of Al.

Characterisation of ZrAl and ZrAlN Thin Films Prepared by Pulsed DC Magnetron Sputtering

Thin films of ZrAl and ZrAlN were deposited on Si(100) substrates in the temperature range 300–773xa0K and with varying nitrogen flow rate from 0 to 10xa0sccm by co-sputtering of Zr (99.9xa0% purity) and Al (99.9xa0% purity) targets using a single pulsed power supply. At substrate temperature of 773xa0K, ZrAl thin films crystallized in hcp structure. The nitrogen flow rate was found to significantly influence the phase formation in ZrAlN films with the formation of fcc structure with a nitrogen flow rates in the range 1–2xa0sccm. The electrical resistivity of ZrAl films decreased from 62 to 24xa0μΩxa0cm with increase of substrate temperature, while it increased with nitrogen flow rate as a consequence of change in crystal structure and decrease in the crystallite size. Nanohardness measurements indicated a maximum hardness and elastic modulus of 20 and 369xa0GPa, respectively for ZrAlN films deposited at 773xa0K and 1xa0sccm of nitrogen flow rate.

X‐Ray Diffraction Analysis of Defects in Cold Worked Type 316 Stainless Steel

In the present study, the dislocation densities of the cold worked type 316 stainless steel were calculated on the basis of the crystallite size and strain present in the samples using Williamson–Smallman approach. The samples annealed at 1323 K, 30 minutes and then furnace cooled were cold rolled to various percentages in the range 5–40percent;. Dislocation density has been found to increase with increasing percentage of cold work. The dislocation density values were correlated with hardness values of the cold worked samples.