P. Mukherjee

Scaling behavior of the Portevin-Le Chatelier effect in an Al-2.5%Mg alloy

The scaling behavior of the Portevin-Le Chatelier (PLC) effect was studied by deforming an Al-2.5%Mg alloy for a wide range of strain rates. To reveal the exact scaling nature, the time series data of true stress versus time, obtained during deformation, were analyzed by two complementary methods: the finite variance scaling method and the diffusion entropy analysis. From these analyses we could establish that, in the entire span of strain rates, the PLC effect showed the Levy-walk property.

Characterisation of microstructural parameters in oxygen-irradiated Zr–1.0%Nb–1.0%Sn–0.1%Fe

Abstract We have characterised the microstructural parameters like domain size, microstrain within the domain, dislocation density and stacking fault probabilities in Zr–1.0%Nb–1.0%Sn–0.1%Fe alloy, irradiated with 116 MeV O 5+ ion at different doses, by X-ray diffraction line profile analysis using modified Rietveld technique. The analysis revealed that there is a significant decrease in domain size but increase in microstrain and dislocation density with dose of irradiation. These microstructural parameters have been compared with our earlier observations on the same alloy irradiated at different doses with 15 MeV proton. We find significant changes of these parameters in oxygen-irradiated samples as compared to the proton-irradiated samples. The variations of microstructures in these samples have been explained in the light of the mechanism of defect evolution for light and heavy ion irradiation.

The study of microstructural defects and mechanical properties in proton-irradiated Zr–1.0%Nb–1.0%Sn–0.1%Fe

Abstract We had earlier analysed point defects in irradiated Zr–1.0%Nb–1.0% Sn–0.1%Fe samples by positron annihilation lifetime spectroscopy (PALS). The studies revealed the generation of irradiation induced di- and tri-vacancy clusters. In the present work, we have carried out X-ray diffraction line profile analysis (XRDLPA) and studies of mechanical properties like ultimate tensile strength (UTS), yield strength (YS) and percentage elongation (%E) on the same irradiated samples at different doses. XRDLPA reveals that order of dislocation density remains almost unchanged up to the experimental limit of irradiation dose. Attempts have also been made to correlate the changes in mechanical properties with irradiation-induced defects. YS of the alloy increases with irradiation due to locking, or pinning of dislocations with vacancy clusters. However, there is an anomaly in YS at a dose of 5×10 20 protons/m 2 which has been explained in the light of interaction of vacancies with solute atoms. Percentage elongation on the other hand, shows a monotonic fall with increasing dose.

Recurrence analysis of the Portevin–Le Chatelier effect

Abstract Tensile tests were carried out by deforming polycrystalline samples of Al-2.5%Mg alloy at room temperature in a wide range of strain rates where the Portevin–Le Chatelier (PLC) effect was observed. The experimental stress-time series data have been analyzed using the recurrence analysis technique based on the Recurrence Plot (RP) and the Recurrence Quantification Analysis (RQA) to study the change in the dynamical behavior of the PLC effect with the imposed strain rate. Our study revealed that the RQA is able to detect the unique crossover phenomenon in the PLC dynamics.

Proton irradiation effects in Zr-1.0 Nb-1.0 Sn-0.1 Fe probed by positron annihilation

Samples of a new zirconium-based alloy (zirlo) were proton-irradiated and the structural defects were characterised by positron annihilation. The lifetimes of positrons trapped in the defects generated at different doses of irradiation appeared to suggest significant interaction of the defects with the substitutional solute atoms. The isochronal recovery of the sample irradiated at the highest dose showed three distinct stages. The initial annealing of free vacancy clusters was followed by a solute atom-vacancy dissociation stage above 673 K. The vacancies thus released annealed out at 973 K.

Lattice Resistance to Dislocation Motion at the Nanoscale

In this Letter, we propose a model that demonstrates the effect of a free surface on the lattice resistance experienced by a moving dislocation in nanodimensional systems. This effect manifests in an enhanced velocity of dislocation due to the proximity of the dislocation line to the surface. To verify this finding, molecular dynamics simulations for an edge dislocation in bcc molybdenum are performed, and the results are found to be in agreement with the numerical implementations of this model. The reduction in this effect at higher stresses and temperatures, as revealed by the simulations, confirms the role of lattice resistance behind the observed change in the dislocation velocity.

The study of defects, transport properties and dissipative flux motion in proton irradiated textured polycrystalline Bi2Sr2CaCu2O8+δ and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ superconductors

Abstract Textured polycrystalline Bi2Sr2CaCu2O8+δ (Bi-2212) and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ (Bi-2223) samples have been irradiated with 15 MeV protons at different fluences (1×1015 protons/cm2 to 1×1016 protons/cm2). Positron annihilation spectroscopy studies show that divacancies and monovacancies are the predominant trapping sites of positrons in unirradiated Bi-2212 and Bi-2223, respectively. In Bi-2212, divacancies agglomerate to trivacancies at moderate doses, while at higher doses, vacancies reduce to monovacancy and defect concentration increases. There is no appreciable change in size of vacancies in Bi-2223 with dose. Resistivity under magnetic field (up to 7 T) oriented in the direction of irradiation, has been studied to analyze the dissipative flux motion based on thermally activated flux creep model. The sharp fall in activation energy U(0,H) in Bi-2223 at high dose has been observed compared to that of Bi-2212.

A STUDY OF SUPERCONDUCTING (Y1-XCAX)BA2CU3OY

Abstract We have investigated the effects of hole doping in the YBaCuO system by substituting calcium(II) in the Y(III)-site. We have analysed the resistivity, paraconductivity, interlayer coupling strength ( J ) as a function of dopant concentration. T c and J are found to decrease with increasing calcium content. We have also studied surface binding energies of various components in a Ca doped sample through secondary ion mass spectrometry (SIMS). The binding energy of CuO in a CuO 2 sheet does not increase as compared to that in undoped YBCO, indicating that the extra hole doped in the Y site does not bring positive charge to the conducting CuO 2 layer. Doping of calcium perhaps destroys the ordering of oxygen in the CuOCu chain, which affects superconductivity.

An X-ray diffraction study of defect parameters in a Ti-base alloy

Detailed studies based on the well established method of Fourier line shape analysis have been made on the X-ray diffraction profile of hexagonal titanium alloy of nominal composition Ti-6.58% Al-3.16% Mo-1.81% Zr-008% Fe-0.012% N-0.0078% C. While deformation fault probability, α, has been found to be quite high compared to that of pure titanium, the deformation growth fault parameter, β, shows a negative value ruling out the presence of growth fault in this alloy in the deformed state.

Acoustic emission studies on welded and thermally treated AISI 304 stainless steel during tensile deformation

The present investigations are planned to study the influence of prior martensites formed due to cold treatment as 77K in AISI 304 SS welded specimens, on strain-induced martensites occurred during tensile deformation using AE technique. AE parameters like count rate and root mean square (r.m.s.) voltage have been used to characterize AE activities generated during tensile deformation process in as-welded and welded-treated samples. Frequency spectrum analysis of AE signals captured from the samples has been done to understand the dynamic behavior of the martensite phase formation. Tensile properties of these samples have also been reported. Volume fraction of the magnetic phase (martensite and delta ferrite) formed in these samples are measured before and after straining. X-ray diffraction (XRD) technique has been used to support the presence of delta ferrite (formed during welding) and martensite in the weld region.