• N K MUKHOPADHYAY

      Articles written in Bulletin of Materials Science

    • Ordering transformation in icosahedral quasicrystals and related crystalline phases

      N K Mukhopadhyay K Chattopadhyay S Ranganathan

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      Arcs of diffuse intensity appear in various shapes and positions in the diffraction patterns from the icosahedral phase, violating the parity rule for simple icosahedral (SI) symmetry. In the process of annealing treatment, the diffuse spots also evolve in the centre of the arcs and become sharp. These extra diffuse spots change the symmetry of the quasilattice fromP-type toF-type. The ordered and disordered structures in quasicrystal have been linked to the ordered and disordered structures present in the crystalline α (Al-Mn-Si) and α (Al-Fe-Si) alloys.

    • Studies on indentation fracture toughness on ceramic and ceramic composite using acoustic emission technique

      A K Ray G Das N K Mukhopadhyay D K Bhattacharya E S Dwarakadasa N Parida

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      This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SiCw) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.

    • Electron microscopy study of striation contrast in Al-Cu-Co-Si decagonal quasicrystals

      N K Mukhopadhyay G V S Sastry G C Weatherly

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      Detailed transmission electron microscopy study was carried out in single crystals of a decagonal phase in the Al-Cu-Co-Si quaternary system. X-ray diffraction and convergent beam electron diffraction patterns of the powder samples confirmed the structures to be decagonal quasicrystals. No microcrystalline nor crystalline phases could be identified. Thin slices normal to the 10-fold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots were observed in the diffraction patterns. Bright field images and dark field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. The striation contrast appears to be originating from anti-phase boundary (APB) in the decagonal superstructures. The diffuse streaks seem to be a characteristic feature of a partially ordered decagonal superlattice structure. The atomic rearrangement or phasonic movement in certain symmetric directions along the pentagrids or Ammann lines in the structure has obviously caused the type of contrast observed in the images. The evolution of rhombic domains consisting of APBs in localized regions can be understood as one of the signature of an intermediate structural state formed prior to a superstructure formation.

    • Leaching of rapidly quenched Al$_{65}$Cu${20}$Fe$_{15}$ quasicrystalline ribbons

      S S MISHRA T P YADAV N K MUKHOPADHYAY O N SRIVASTAVA

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      In the present work, Al$_{65}$Cu$_{20}$Fe$_{15}$ alloy has been synthesized by melting of pure elements (e.g., Al (99.96%),Cu (99.99%) and Fe (99.98%)), using a radiofrequency induction melting furnace. The as-prepared alloy was subjected torapid solidification by melt spinning technique at $\sim$3500 rpm speed on a copper disk of diameter 14 cm. As a result of themelt spinning, nearly 2 mm wide, 30−40 $\mu$m thick and 4–5 cm long ribbons were formed. The structural and microstructuralcharacterizations were carried out by X-ray diffraction and transmission electron microscopy techniques.We have performedleaching operation using 10 mol NaOH aqueous solution on the surface using a pipette. Leaching was performed for variousdurations ranging from 30 min to 8 h. After leaching, the reflectivity reduces and the surface looks reddish brown. Themicrostructure of the 8 h leached sample shows a breakdown of the quasicrystalline phase but with the evolution of othermetallic phases. Copper (Cu) particles are found to be present on the surface of quasicrystal after 4 h of leaching and relativelymore iron (Fe) evolves during further leaching of 8 h. This low-cost method to prepare a distribution of nano-Cu/Fe metalparticles encourages their uses in catalytic reactions, indicating the possibility of use of quasicrystals as the industrial catalysts.

    • Fe/Fe$_3$O$_4$ nanocomposite powders with giant high magnetization values by high energy ball milling

      V RAMYA A GANGWAR S K SHAW N K MUKHOPADHYAY N K PRASAD

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      The present work reports on the relatively higher saturation magnetization values of Fe/Fe$_3$O$_4$ nanocomposites. For example, the nanocomposites of Fe obtained after milling for 10 h with 5, 10 and 15 wt% of Fe$_3$O$_4$ had displayed saturation magnetization values of 210, 238 and 216 Am$^2$ kg$^{−1}$, respectively, in contrast to 218 Am$^2$ kg$^{−1}$ of bulk Fe. Similarly, themaximum magnetization values for the nanocomposites after 20 and 30 h of milling were 215 and 190 Am$^2$ kg$^{−1}$ for the sample containing 5 and 15 wt% of Fe$_3$O$_4$, respectively. The values of $H_{\rm C}$ and $Mr$ suggest that nanocomposites exhibit soft ferromagnetic behaviour. The ball milling also reduced the crystallite and particle size of Fe from microndimension tonanometres. This was confirmed from X-ray diffraction, transmission electron microscopy and scanning electron microscopy analyses. The crystallite size of pure Fe decreased to 35, 20 and 19 nm, respectively, for the samples having 5, 10 and 15 wt% of Fe$_3$O$_4$ after 10 h of milling. The crystallite size decreased further with increased milling time.

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