School of Engineering Sciences and Technology

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Browsing School of Engineering Sciences and Technology by Subject "Activation volume"
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    On the Hardness and Strain Rate Sensitivity of Electrodeposited Nanocrystalline Ni–18 wt% Co Alloy Studied by Nanoindentation
    ( 2019-01-01) Patibanda, Supriya ; Varam, Sreedevi ; Gollapudi, Srikant ; Rao, K. Bhanu Sankara ; Rajulapati, Koteswararao V.
    Ni–18wt% Co foils made by electrodeposition possesses an average grain size, computed using diffraction contrast in TEM, of about 30 nm. Vickers microindentation and depth-sensing nanoindentation have been adapted to assess the deformation parameters such as hardness, strain rate sensitivity (SRS) and activation volume. These foils with single-phase fcc-structured solid solution exhibit the hardness values of 4.5 ± 0.1 GPa and 6.2 ± 0.2 GPa measured by microindentation and nanoindentation, respectively. The dependence between hardness and applied load in the present solid solution cannot be attributed to the indentation size effect, but to the changes in internal friction and/or reduction in stacking fault energy that may have resulted from the Co additions. An elastic modulus of 193 ± 3 has been realized in these foils. Performing nanoindentation at various loading rates and subsequent analysis has resulted in a SRS of 0.017 and an activation volume of 7.6 b3. These values suggest that in these nanocrystalline Ni–18Co foils made by electrodeposition, interfaces such as grain boundaries, triple junctions and quadruple junctions play a governing role in dictating the deformation kinetics. A model reported in the literature has been successfully modified to reasonably explain the dependence of SRS on grain size for various Ni-based alloys including the one reported in the present study. However, the model fails to follow the established dependence for the materials with grain size below 10 nm as the deformation mechanisms at these extremely finer length scales (below 10 nm) are expected to be totally different from considerations applicable for the present alloy with a grain size of 30 nm.
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    On the strain rate sensitive characteristics of nanocrystalline aluminum alloys
    ( 2017-01-01) Varam, Sreedevi ; Rao, K. Bhanu Sankara ; Rajulapati, Koteswararao V.
    For structural applications, ductility is essential along with high strength in nanocrystalline (nc) materials. In general, ductility is controlled by strain hardening and strain rate sensitivity. In conventional materials which are coarse grained, the deformation is mainly dislocation based and accumulation of these dislocations results in work hardening. The deformation mechanisms that are operative in nc materials are distinct and the strain hardening ability is limited in nc materials. Strain rate sensitivity (SRS) and activation volume are the two key parameters which govern the underlying deformation mechanisms in nc materials. Higher SRS value could be an indication of better ductility levels. In general, nanocrystalline single phase fcc metals showed increased SRS, where as bcc metals showed decreased SRS. The addition of second phase effects the overall SRS of the nano composite/alloy. Since producing nc materials in bulk quantities is a challenge, nanoindentation, which can be performed on smaller sized samples, is an useful technique to study SRS and activation volume. Strain rate sensitive characteristics of Al and its alloys are reviewed in this paper. Our earlier work as well as the available literature data on these alloys showed that the nature and structure of the second phase dispersions greatly influence the SRS.
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    Strengthening mechanisms in equiatomic ultrafine grained AlCoCrCuFeNi high-entropy alloy studied by micro- and nanoindentation methods
    ( 2017-02-15) Ganji, Ramya Sree ; Sai Karthik, P. ; Bhanu Sankara Rao, K. ; Rajulapati, Koteswararao V.
    A single phase fcc based nanocrystalline solid solution in equiatomic AlCoCrCuFeNi high-entropy alloy (HEA) has been synthesized using ball milling. The milled powders were of “plate-like” morphology and possessed a precise lattice parameter of 3.641 Å. Compaction of ball milled powders into bulk components using spark plasma sintering (SPS) at 1023 K led to the precipitation of ordered bcc (B2). Detailed structural and microstructural investigations on the sintered alloy indicate the presence of bimodal grain size distribution with average grain sizes of 112 ± 46 nm and 1550 ± 500 nm, solid solutions (fcc and B2 phases), dislocations and twin boundaries. A high hardness value of 6.5 ± 0.1 GPa was measured for the sample sintered at 1023 K/15 min using Vickers microindentation. Comprehensive analysis on probable strengthening mechanisms suggests that frictional stress, Taylor hardening, Hall-Petch strengthening, solid solution strengthening and twin boundary strengthening mechanisms are responsible. The Taylor hardening arising from intersection of dislocations and grain boundary (Hall-Petch) strengthening arising from grain boundary-dislocation interactions together account for 85% of the observed flow stress. The Tabor's ratio, (H/σflow) attained a value of 2.7 which is in close agreement with that for conventional polycrystalline materials. Nanoindentation at a peak force of 8000 μN yielded a high hardness value of 8.13 ± 0.15 GPa and an elastic modulus of 172 ± 10 GPa. A low strain rate sensitivity of 0.0084 and an activation volume of 13 b3 (b is 0.23 nm) were measured, suggesting that grain boundaries, twin boundaries and interphase boundaries (fcc/B2) are influential in governing the deformation kinetics.