Pradyumna Kumar Parida

Residual stress relaxation mechanism at low homologous temperature in nanocrystalline iron thin film deposited on Si (1?0?0) substrate

The effect of thermal stress and the role of defects in residual stress relaxation have been investigated in a nanocrystalline iron thin film coated on Si (1 0 0) substrate using glancing incidence x-ray diffraction (GIXRD) and depth resolved positron beam studies. The film has been annealed isothermally at a low homologous temperature of 400 K (~0.2T m) for different time intervals. The change in residual stress, micro-strain and grain size as a function of annealing time has been deduced using GIXRD. The change in vacancy defects as a function of the annealing time has been investigated using defect-sensitive line shape S-parameter from depth-resolved positron beam studies. It is observed that the residual stress relaxes by the creation of defects at the film surface. A physical model has been proposed based on the atomic diffusion through the grain boundaries, which explains the observed results well. The proposed model confirms the residual stress relaxation by the transport of atoms and corresponding point defects between the free surface of the film and the grain boundaries indicating the stress relaxation is mediated by atomic diffusion.

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.

Structural characterisation of Y2Ti2O7 dispersoids in ODS alloys

Oxide dispersion strengthened (ODS) steels are being developed as promising core component structural materials for future generation fast breeder and fusion reactors because of their better high temperature thermal stability and neutron irradiation void swelling resistance as compared to currently used austenitic steels. The type of oxide dispersoids, their size distribution in metallic matrix and stability at adverse service conditions (such as high temperature along with fast neutron irradiation) governs the physical and mechanical properties of the steel. Yttria (Y2O3), is the most preferred oxide dispersoid being used in the ODS steels, because of its superior thermal and neutron irradiation stability. However there are reports that show that these oxides either dissolve or dissociate or even become amorphous during mechanical milling and reprecipitate as coarse particles during high temperature consolidation process, in absence of Ti in a model Fe-15Y2O3 system [1]. It is believed that Ti can inhibit the growth of nano-dispersoid during annealing by formation of Y-Ti-O complex oxides such as Y2Ti2O7 or Y2TiO5 or YTiO3 [2]. The Y2O3 to Ti weight ratio in the alloy is critical in determining the chemical composition of the dispersoid and the Y2Ti2O7 oxides are finer and most stable oxide among all combinations of Y-Ti-O complex oxides whose size which varies in the range of 2-15 nm in ODS steel. Synchrotron XRD is used to characterize the dispersoids in the ODS steel, with 0.35 wt% of yttria and 0.2 wt% of Ti, due to the low volume fraction. TEM has been found more suitable for complete characterization of the nano-sized (~2-5 nm) dispersoids w.r.t size, distribution, morphology, chemical composition and crystal structure. However, characterisation of samples prepared by conventional methods for TEM studies continues to be difficult owing to magnetic nature of ferritic steel. Hence FIB was employed to extract electron transparent samples which are of micrometer dimensions. In order to understand the structural evolution of the Y2Ti2O7 oxide in ODS steel, a concentrated alloys of Fe-15wt%Y2O3-Xwt%Ti (X=0, 5, 10, 15) were synthesized by mechanical milling and subsequently annealed. Figure 1(a) and (b) represents the typical bright field (BF) TEM micrographs of Fe-15wt%Y2O3-15wt%Ti model ODS after 60h of milling and subsequent annealing at 1273K respectively, the corresponding SAD patterns are shown as inset. The analysis of SADP reveals amorphisation of yttria upon milling and recrystallisation of Y2Ti2O7, in annealed alloy powder. Interestingly, it was observed when the Y2O3 to Ti weight ratio is 1:1, the oxide phase formed upon annealing is only Y2Ti2O7 and are very finer in size (varies in the range of 2-30 nm). Details of these studies will be presented in the paper. 1. Pradyumna Kumar Parida, Arup Dasgupta, K. Jayasankar, M. Kamruddin, S. Saroja, J. Nucl. Mater. 441 (2013) 331–336. 2. P.K. Parida, A. Dasgupta, K.G. Raghavendra, K. Jayasankar, S. Saroja, Trans IIM (2016) 1-7.

Structural and nano-mechanical characterization of TiN / Ti 1−x Al x N multilayered thin films

Nanostructured multilayers have high interface area densities along with other factors such as periodicity, single layer thickness, sharpness, in general lead to improved mechanical properties. In this study, we have selected a combination of two metallic hard materials, such as cubic TiN and metastable cubic TiAlN sub-layers to synthesize a periodic multilayer thin films. Advantage of TiAlN as a sub-layer is the stability against oxidation due to the formation of dense Al2O3, which prevents further oxidation. These periodic TiN/TiAlN multilayers were synthesized by reactive magnetron co-sputtering technique on SS 304 LN substrates with TiN as a starting sub-layer and TiAlN as next with bi-layer thicknesses 7 nm to 60 nm. GIXRD results confirm the formation of metastable TiAlN with NaCl structure which is similar to TiN with a shift in the peak position. The individual layers were resolved by X-TEM and Secondary Ion Mass Spectroscopy to understand the architecture. Depth sensing nanoindentation was used to study the surface hardness which clearly explains the reverse Hall-Petch relationship (change in the periodicity). The maximum surface hardness of 34 GPa was obtained for a bi-layer thickness of 30 nm.

Texture, Grain Boundaries, Defects and Location of Substitutional Atoms in Cryo-Mechanically Processed Ti-5Ta-1.8Nb Alloy

An α-β alloy (β~8% in the stress relieved condition) of Ti-5Ta-1.8Nb has been subjected to severe plastic deformation (SPD) by cryo-rolling. The grain size of α-Ti could be reduced significantly from ~ 8µm to 100 nm and less by cryo-rolling. Extensive plastic deformation leads to grain fragmentation through the formation of defect clusters. The fragmented grains exhibit deformation texture. High resolution transmission electron microscopy (HRTEM) confirmed the presence of low and high angle grain boundaries. The role of substitutional atoms (Ta, Nb) in producing lattice strains and altering the projected potential from the atomic columns has been discussed. Although, the minor phase, β (bcc-Ti) is evident in the starting alloy, it was not observed after SPD, possibly due to extended solid solution formation (Gibbs–Thomson effect) in the fine grains or due to the stress induced transformation of the α-Ti phase.