V. Pancholi

Hexagonal diamond synthesis on h-GaN strained films

Chemical vapor deposited diamond films grown on strained gallium nitride-coated quartz substrate are found to display a dominantly hexagonal diamond phase. The phase identification is done using Raman spectroscopy and orientation imaging microscopy (OIM). The presence of a 1324.4cm−1 band in the Raman spectra is attributed to a hexagonal diamond symmetry, but the unambiguous signature of the hexagonal phase is confirmed by OIM. A phase map of the sample clearly shows that 88% of the scanned sample area is hexagonal diamond.

Relating microtexture and dynamic micro hardness in an extruded AA8090 alloy and AA8090-8?vol% SiCp composite

Abstract The present study involves combined measurements of microtexture and dynamic ultra micro hardness (DUH) in hot extrudedAA8090 aluminum alloy and its composite reinforced with 8 vol% SiCp. Both the materials show strong crystallographic fiber textures—〈111〉 and 〈001〉. The dynamic micro hardness shows a clear pattern of difference between these two fiber textures, 〈111〉 oriented grains being harder and stiffer. The difference in θ/d between the fibers, where θ and d are the average cell misorientation and cell size, respectively, was marginal in the alloy and thus cannot explain the observed hardness difference. The hardness difference can be explained from the difference in Taylor factors between the respective fibers. Elastic stiffness values estimated from microtexture and DUH follow a similar trend qualitatively.

The Effect of Aging on Energy Absorption Capability of Closed Cell Aluminum Foam

The role of cell wall microstructure in influencing the mechanical behaviour of closed cell aluminium foam was investigated. The morphologies of the cell wall were characterized by using optical microscopy. An attempt has been made to describe the relationship between the microstructure of cell wall and mechanical properties primarily pertaining to the compressive response. The closed-cell foams in as-foamed and aged condition, having rather similar relative densities and cell structures but significantly different cell-wall microstructure have been examined. It was found that in as foam condition thick dendritic structure is present. On ageing at different temperatures for different duration, it was found that as ageing progresses the dendritic structure break in to finer precipitates resulting in improvement in microstructure of the cell wall. The compression tests also affirm the microstructural contribution to energy absorption capability.

Deformation Behavior of Inhomogeneous Layered Microstructure

Multipass friction stir processing (FSP) technique was used to get inhomogeneous layered microstructure in Al 5086 alloy. Different proportions of fine and coarse grain microstructures were generated using FSP. In the present work, material with inhomogeneous layered microstructure generated using FSP was subjected to deformation at temperature of 500°C and at strain rate in the range of 5×10-4 to 1×10-2 s-1. It was observed that the inhomogeneous layered microstructure with more than 50% fine grain microstructure is required to get higher elongation to failure. However material having homogeneous fine grain microstructure showed lower elongation to failure than the material having both fine and coarse grain microstructures. By varying the proportion of fine and coarse grain microstructures higher ductility was obtained at higher strain rate as compared to a material with homogeneous microstructure. It is clear that materials with inhomogeneous microstructure are not following “rule of mixture” type of behaviour.

Microstructural and Mechanical Characterization of Friction Stir Processed 5086 Aluminum Alloy

In the present investigation friction stir processing (FSP) is carried out by single and multipass FSP on a 5086 aluminum alloy to modify microstructure and mechanical properties. The processing is carried out at constant rotation speed of 1025 rpm and at a traverse speed of 30 mm/min. Inhomogeneous microstructural distribution was observed across the processed zone. EBSD analysis has been done to evaluate the microstructure. Overlapping passes is showing same grain size as in single pass FSPed material. Material processed using multi pass FSP at 30 mm/min is showing higher mechanical strength as compared to base material. The bulk material produced due to multipass seems to be good for superplastic forming applications.

Layered Microstructure Generated by Multipass Friction Stir Processing in AZ91 Alloy and Its Effect on Fatigue Characteristics

Layered microstructure with three different configurations was developed by multipass friction stir processing (FSP) on as-cast (AC) AZ91 magnesium alloy using three different tools with probe lengths 7, 5 and 4 mm. They were half thickness processed (HFSP), surface modified (SFSP) and full thickness processed (FFSP). FSP was performed at tool rotation rate of 720 rpm and transverse speed of 150 mm/min. The large β-Mg17Al12 particles with an average size of 20 μm and α-Mg matrix grains of 100 μm were reduced to approximately 1 and 2 μm, respectively, after multipass FSP. Texture of FSPed samples measured by X-ray diffraction technique had shown basal texture. Constant amplitude axial fatigue test was performed on all the microstructural configurations, with process direction parallel to loading axis. Life of the fatigue tested samples was found to increase with the increasing fraction of FSPed region in AZ91 alloy.

Role of activation energies of individual phases in two-phase range on constitutive equation of Zr–2.5Nb–0.5Cu alloy

Abstract Dominant phase during hot deformation in the two-phase region of Zr–2.5Nb–0.5Cu (ZNC) alloy was studied using activation energy calculation of individual phases. Thermo-mechanical compression tests were performed on a two-phase ZNC alloy in the temperature range of 700–925 °C and strain rate range of 10 −2 –10 s −1 . Flow stress data of the single phase were extrapolated in the two-phase range to calculate flow stress data of individual phases. Activation energies of individual phases were then calculated using calculated flow stress data in the two-phase range. Comparison of activation energies revealed that α phase is the dominant phase (deformation controlling phase) in the two-phase range. Constitutive equations were also developed on the basis of the deformation temperature range (or according to phases present) using a sine-hyperbolic type constitutive equation. The statistical analysis revealed that the constitutive equation developed for a particular phase showed good agreement with the experimental results in terms of correlation coefficient ( R ) and average absolute relative error (AARE).

Hot Deformation Behaviour and Microstructural Evaluation of Zr-1Nb Alloy

In nuclear water reactors, zirconium alloys are extensively used as fuel cladding material and in other structural applications. Uniaxial hot compression tests were performed to understand the deformation behavior of Zr-1Nb alloy. Therefore, hot compression tests were performed in the temperature range of 700-1050°C, which envelopes α-phase, (α+β) phase, and β-phase. True stress-strain curves, processing maps, microstructural observation and kinetic analysis were used to discuss the deformation behavior of Zr-1Nb alloy. Deformation at a strain rate of 10-2 s-1 reveals softening at lower temperatures and steady state behavior at higher temperatures. Processing map also reveals domain of high efficiency at 10-2 s-1 strain rate for a wide range of deformation temperatures. The flow softening and high power dissipation efficiency predicts dynamic recrystallization or dynamic recovery during the hot deformation of studied alloy.

Flow behaviour of TiHy 600 alloy under hot deformation using gleeble 3800

Abstract To understand deformation behaviour of TiHy 600 alloy at higher temperatures, hot compression tests are performed in α region (1173 K), α + β regions (1223, 1248, and 1273 K) and β region (1323 K) at strain rates (0.001, 0.01, 0.1, 1 and 10/s) for up to 50% deformation in Gleeble 3800® thermo-mechanical simulator. Flow curve plots are drawn at each strain rates and temperatures and it is observed that dominant deformation mechanism at higher temperature 1323 K (β region) and strain rates (1 and 10/s) is dynamic recovery (DRV) whereas dynamic recrystallization (DRX) is mostly observed at lower strain rates (0.001, 0.01/s) in medium temperature range of 1223 K (α region) to 1248 K (α + β region). Hyperbolic sine law equation is used to calculate the activation energy (Q) and other material sensitive parameters (A, α and n1). The activation energies for DRX in α region and DRV in β region are obtained as 384 and 251 kJ/mol. Experimental peak stress values are compared with predicted peak stress values (R2 = 96.2%) and Zener-Hollomon parameter (R2 = 94.3%). The flow stress behavior up to the peak stress is verified with Cingara equation. Finally, calculated prediction results of DRX volume fraction obtained from Avrami equation is compared with experimental observed microstructure.

Effect of Different Proportion of Coarse and Fine Grain Microstructure on Superplastic Forming Characteristics

The study was carried out to understand the effect of inhomogeneous microstructure on thickness variation in superplastically formed bulge. Friction stir processing was performed at rotational and traverse speeds of 720rpm and 155mm/min respectively on a 6mm sheet maintaining 50% overlap on the retreating side. Different probe dimensions were selected to obtain different proportions of fine grained stir zone in thickness direction. The proportions of the fine grained stir zone were 25%, 50%, 72% and, 100%. The sheets containing inhomogeneous microstructure were subjected to superplastic bulge forming under constant gas pressure up to a bulge height of 23.5mm. The sheet which was processed with 72% fine grains showed lower thickness variation from edge to apex and the bulge shape in this condition was close to the ideal spherical profile.