• N Eswara Prasad

      Articles written in Sadhana

    • Mechanical behaviour of aluminium-lithium alloys

      N Eswara Prasad A A Gokhale P Rama Rao

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      Aluminium-lithium alloys hold promise of providing a breakthrough response to the crying need for lightweight alloys for use as structurals in aerospace applications. Considerable worldwide research has gone into developing a range of these alloys over the last three decades. As a result, substantial understanding has been developed of the microstructure-based micromechanisms of strengthening, of fatigue and fracture as well as of anisotropy in mechanical properties. However, these alloys have not yet greatly displaced the conventionally used denser Al alloys on account of their poorer ductility, fracture toughness and low cycle fatigue resistance. This review aims to summarise the work pertaining to study of structure and mechanical properties with a view to indicate the directions that have been and can be pursued to overcome property limitations.

    • In-plane anisotropy in tensile deformation and its influence on the drawability of Nimonic C–263 alloy sheets

      K Ankamma D V V Satyanarayana G Chandramohan Reddy M Komaraiah N Eswara Prasad

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      An effort has been made to comprehensively evaluate and rationalize the in-plane anisotropy in tensile properties and the effect of aging on the nature of deformation (strain hardening behaviour) and formability characteristics, especially the limit drawing ratio and forming limit diagram. Despite weak crystallographic texture and excellent ductility and high work hardening exponents, the alloy sheets of C-263 exhibit significant extent of in-plane anisotropy in its tensile properties and yield loci. The absolute magnitudes of yield stress and the exact nature of anisotropy that can be predicted from the tensile part of the yield locus need to be employed with caution. This is because when the magnitudes of the yield stresses, obtained from yield locus are more than 5 times higher as compared to the yield and ultimate tensile strength values and the nature and degree of in-plane anisotropy under tensile loading matches with that of only the compressive yield stresses of yield locus. The alloy sheet, due to weak crystallographic texture and relatively high strengths, is found to be far more suitable for structural applications, rather than for deep drawing applications, which was reflected in low limit drawing ratio values (1.34 for CR+ST and 1.23 for peak aged conditions). Further, the study conducted reveals that the safer forming limits in strain space is higher for CR+ST condition; while, the safer forming limits in stress space are higher for peak aged (CR+ST+1073 K/8 h) condition. Finally, detailed studies are outlined to arrive at suitable microstructural and textural characteristics that provide significantly enhanced drawability in the Nimonic C-263 alloy sheets.

    • Synthesis of carbon fibre-reinforced, silicon carbide composites by soft-solution approach

      N Padmavathi P Ghosal N Eswara Prasad J Subramanyam K K Ray

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      The aim of the present work centers on synthesizing and characterizing carbon fibre (Cf) reinforced, silicon carbide matrix composites which are considered to have potential applications in aerospace and automobile industry. A series of composites, namely the Cf-SiC, Cf-(SiC+ZrC), Cf-(SiC+ZrB2), and Cf-(SiC+ZrO2), have been prepared by a proposed soft-solution approach. This approach involves the use of water-soluble precursors of colloidal silica, sucrose, zirconium oxychloride, and boric acid as sources of silica, carbon, zirconia, and boron oxide, respectively to achieve the desired matrices through drying, carbonization and carbothermal reduction. The prepared powders and the composites were characterized by thermal analysis, X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses to assess the phase formation and microstructure of the materials, apart from assessment of their tensile properties. The study shows that the soft-solution process yields matrices with finer crystallite sizes, having homogeneous distribution of the constituent phases of either the powders or of the composite matrices. The role of the additional phases on the tensile properties of the composites has been discussed using consideration of thermal stresses at fibre-matrix interface; whereas the role of the carbothermal reduction temperature in determining these properties has been explained using the interfacial characteristics of the fibre-matrix. Addition of ZrO2 in the matrix of SiC has shown to improve the properties of Cf-SiC composites considerably. The results of this investigation unambiguously demonstrate that aqueous solution-based processing can be used for fabrication of these composites in relatively shorter time in an environmental friendly manner without using any expensive equipment. The approach is capable of yielding composites with different phases in the matrix by simple variation of precursor materials and solutions. The small crystallite sizes, fine particle distribution and low carbothermal reduction temperatures are some of the specific merits of the proposed method.

    • Optimization of burnishing parameters and determination of select surface characteristics in engineering materials

      P Ravindra Babu K Ankamma T Siva Prasad A V S Raju N Eswara Prasad

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      The present study is aimed at filling the gaps in scientific understanding of the burnishing process, and also to aid and arrive at technological solutions for the surface modifications based on burnishing of some of the commonly employed engineering materials. The effects of various burnishing parameters on the surface characteristics, surface microstructure, micro hardness are evaluated, reported and discussed in the case of EN Series steels (EN 8, EN 24 and EN 31), Aluminum alloy (AA6061) and Alpha-beta brass. The burnishing parameters considered for studies principally are burnishing speed, burnishing force, burnishing feed and number of passes. Taguchi technique is employed in the present investigation to identify the most influencing parameters on surface roughness. Effort is also made to identify the optimal burnishing parameters and the factors for scientific basis of such optimization. Finally, a brief attempt is made to construct the Burnishing maps with respect to strength level (in this case, average micro hardness of unburnished material).

    • Indigenous development and airworthiness certification of 15–5 PH precipitation hardenable stainless steel for aircraft applications

      Ashok Kumar Y Balaji N Eswara Prasad G Gouda K Tamilmani

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      In this paper, we discuss the optimization of chemical composition, processing (forging and rolling) and heat treatment parameters to obtain the best combination of mechanical properties in case of a Fe–15Cr–5Ni–4Cu precipitation hardenable stainless steel. The 𝜀-copper precipitates that form during aging are spherical in shape and coherent with the matrix and principally provide strengthening in this alloy. The orientation relationship is found to be Kurdjumov–Sachs (K–S), which is common in fcc–bcc systems. Results obtained from metallurgical evaluation (mechanical property and metallography) on 15–5 PH alloy during type certification on 3 different melts were used for the optimization, attempted in this study. The mechanical properties following strain deformation has been carried out using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). In the aged conditions, the 15–5 PH alloy exhibited brittle failure with extensive cleavage and/or quasicleavage fracture. This paper reports all results and also factually shows that indigenously developed and produced 15–5 PH stainless steel matches in its properties with the equivalent aeronautical grade precipitation hardening stainless steels globally produced by internationally renowned manufactures.

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