C. R. Das

Surface optical modes in GaN nanowires

We investigate the optical phonons in crystalline GaN nanowires using Raman spectroscopy. Reduced phonon lifetime in the nanostructures is attributed to the increased anisotropy in lattice vibrations. Apart from the group theoretically allowed optical phonons, new phonon modes around 652 cm−1 and 691 cm−1 have been observed. In view of its good agreement with values in GaN, the observed phonon mode is assigned as surface optical (SO) phonon. This could be attributed to the surface modulation along the GaN nanowire diameter and it is quantitatively evaluated with observed surface morphology and the calculated dispersion relation corresponding to SO phonon modes. The modulation in the surface morphology, observed in the present study, is typical of the vapour-liquid-solid growth process. The instability in the surface phonon potential activates the SO phonon modes, which is well explained in this present study.

Direct observation of amophization in load rate dependent nanoindentation studies of crystalline Si

Indentation at very low load rate showed region of constant volume with releasing load in crystalline (c-)Si, indicating a direct observation of liquidlike amorphous phase which is incompressible under pressure. Signature of amorphization is also confirmed from load dependent indentation study where increased amount of amorphized phase is made responsible for the increasing elastic recovery of the sample with increasing load. Ex situ Raman study confirmed the presence of amorphous phase at the center of indentation. The molecular dynamic simulation has been employed to demonstrate that the effect of indentation velocities has a direct influence on c-Si during nanoindentation processes.

Weldability of 17-4PH stainless steel in overaged heat treated condition

Abstract Studies on the weldability of 17-4PH stainless steel, in the 621°C overaged condition, showed that Creq/Nieq ratio higher than 1·5 resulted in primary ferritic mode of solidification in the weld metal. Post-weld aging treatment at 482°C enhanced the strength of the weld joint with corresponding reduction in impact toughness of the weld metal while post-weld aging at 621°C caused marginal reduction in strength of the weld joint with significant increase in impact toughness of the weld metal.

Characterisation of nickel based hardfacing deposits on austenitic stainless steel

Abstract Nickel based hardfacing alloys Colmonoy-5 and Colmonoy-6 deposited on type 316L austenitic stainless steel substrate using the gas tungsten arc welding process, were characterised for their microstructure and hardness variations across the deposit/substrate interface, and for identification of the principal precipitates present in these hardface deposits. Hardness measurements and electron probe microanalysis (EPMA) across the interface revealed the presence of a dilution zone 2·5 mm wide in the Colmonoy-6 deposit and 1·5 mm wide in the Colmonoy-5 deposit. EPMA and X-ray diffraction studies of the precipitates showed that he blocky type precipitates were chromium borides and the needle-like precipitates were chromium carbides. Also, stress relief heat treatment at 1123 K for 4 h was found to have no significant effect on the hardness of these hard face deposits.

Recrystallization of epitaxial GaN under indentation

We report recrystallization of epitaxial (epi-) GaN(0001) film under indentation. Hardness value is measured as ∼10GPa using a Berkovich indenter. “Pop-in” burst in the loading line indicates nucleation of dislocations setting in plastic motion of lattice atoms under stress field for the recrystallization process. Micro-Raman studies are used to identify the recrystallization process. Raman area mapping indicates the crystallized region. Phonon mode corresponding to E2(high) ∼570cm−1 in the as-grown epi-GaN is redshifted to stress free value ∼567cm−1 in the indented region. Evolution of A1(TO) and E1(TO) phonon modes are also reported to signify the recrystallization process.

Fatigue failure of a fillet welded nozzle joint

Abstract A fillet welded joint in a nozzle of a storage tank, made of AISI 304L stainless steel (SS), had failed very close to the weld fusion line during transportation. Visual examination of the weld showed deposition of excessive weld metal in the joint. Scanning electron microscope observation of the fracture surface revealed beach and ratchet marks on the fracture surfaces. The observation of in-situ metallography of the heat-affected zone (HAZ) anticipated sensitisation of the HAZ. Bending of the nozzle to 15° anticipated the final failure, which occurred due to overloading during transportation.

Assessment of Sensitization in AISI 304 Stainless Steel by Nonlinear Ultrasonic Method

A nonlinear ultrasonic technique has been developed to evaluate sensitization in Type 304 stainless steel. In order to achieve different degree of sensitization (DOS), specimens have been subjected to heat treatment at 675 °C at varying soaking time (0.5, 1.0, 2.0, 3.0 and 4.0 h). Heat treated specimens were subjected to intergranular corrosion tests as per ASTM standards A262 and G108. Sensitization in longer soaked material has been confirmed through ditch microstructures, cracks on the bend tested specimens and higher degree of sensitization. Nonlinear ultrasonic studies showed variation in the nonlinearity parameter with soaking time which also confirms sensitization. A good correlation was observed between the degree of sensitization measured by the electrochemical potentiokinetic reactivation test and the ultrasonic nonlinearity parameter. This study clearly demonstrated that nonlinear ultrasonic technique can be used as a potential technique for non-destructive characterization of sensitization in austenitic stainless steel.

Effect of minor change in composition on toughness of weldmetal for repair of turbine blades made of martensitic stainless steel

Abstract Repair welding procedure for cracked turbine blades, made of 13Cr–2˙6Ni–1˙1Mo martensitic stainless steel, has been developed using gas tungsten arc welding process and a twin wire filler metal. The twin wire consists of a 1˙5 mm diameter ER 16-8-2 and a 2˙0 mm diameter ER 410 filler wires tack welded along the length of the two filler wires. A two stage post-weld heat treatment at 675°C for 2 h and 615°C for 4 h, such that the first heat treatments is above the Ac 1 temperatures of the weld metal and the second is just below its Ac 1 temperature; has been found to be suitable for obtaining good mechanical properties for the weldment. The weldment has a good combination of transverse weldment strength and weldmetal toughness, with its room temperature yield strength and Charpy V notch impact toughness being similar to that of the turbine blade material.

Joining of galvannealed steel and aluminium alloy using controlled short circuiting gas metal arc welding process

Abstract Controlling the Fe–Al intermetallic layer thickness along the joint interface has remained a critical challenge in gas metal arc welding of galvanised steel and aluminium alloys. An attempt is presented here to join galvannealed steel and aluminium alloy sheets using a novel gas metal arc welding process that allows controlled short circuiting to reduce the rate of heat input significantly. The real time current and voltage transients during the process are monitored to estimate the rate of heat input and its influence on the formation of intermetallic phases and layer thickness. The results show that the intermetallic layer thickness can be controlled and good joint strengths can be achieved when the rate of heat input remains lesser than 130 J mm− 1 for the typical lap joint configuration considered here.

Tribo-induced phase transformation and associated evolution of friction of NiCrB alloy

In the present study, the tribological behavior of NiCrB coating at various loading conditions was investigated. At lower load, evolution of amorphous α-Fe2O3, Fe3O4, and Cr2O3 phases were observed in wear scars, whereas at higher loading conditions the growth of these phases was prominent. Among the different phases formed, the crystalline Cr2O3 phase dominated at higher loading conditions. Transformation of these phases occurred with a change in loading conditions, which is attributed to the growth and oxidation of oxide scales. The morphology of wear scars was found to change with a change in loading conditions. These changes significantly damaged the oxide scale by cracking and fretting at higher loading conditions. Evolution of amorphous and crystalline Cr2O3 gives rise to a coefficient of friction value comparable to that of the lubricant phase of α-Fe2O3.