• C Ramachandra

      Articles written in Bulletin of Materials Science

    • Role of diffusional coherency strain theory in the discontinuous precipitation in Mg-Al alloy

      K T Kashyap C Ramachandra M Sujatha B Chatterji

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      Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. It is called discontinuous precipitation because precipitation occurs on prior matrix grain boundaries followed by grain boundary movement. The DP nodule consists of alternate lamellae of the precipitate and the matrix respectively. The chemical driving force for DP is one of solute supersaturation. Although solute supersaturation is responsible for precipitation, it has to be coupled with another driving force to explain grain boundary migration. This coupling driving force has been identified to be diffusional coherency strain which has been verified to be active in diffusion induced grain boundary migration and liquid film migration.

      To test diffusional coherency strain theory for discontinuous precipitation Mg-7Al and Mg-7Al-1Pb alloys were studied. While the fraction transformed was high at 6% in Mg-7Al alloy, dit was significantly low at 2% in Mg-7Al-1Pb alloy. The velocity of DP nodules decreased by half in alloy with Pb as compared to the alloy without Pb. Theoretical calculations also predict that the misfit parameter δth decreases with the addition of Pb. These observations are an evidence to the fact that diffusional coherency strain is the most active driving force for the movement of the grain boundaries of the DP nodules during discontinuous precipitation in Mg-Al alloy.

    • Role of work hardening characteristics of matrix alloys in the strengthening of metal matrix composites

      K T Kashyap C Ramachandra C Dutta B Chatterji

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      The strengthening of particulate reinforced metal-matrix composites is associated with a high dislocation density in the matrix due to the difference in coefficient of thermal expansion between the reinforcement and the matrix. While this is valid, the role of work hardening characteristics of the matrix alloys in strengthening of these composites is addressed in the present paper. It is found that commercial purity aluminium which has the lowest work hardening rate exhibits the highest strength increment. This effect is due to increased prismatic punching of dislocations. This relationship of decreasing work hardening rate associated with increasing prismatic punching of dislocations in the order 7075, 2014, 7010, 2024, 6061 and commercial purity aluminium leading to increased strength increments is noted.

    • A test for diffusional coherency strain hypothesis in the discontinuous precipitation in Mg-Al alloy

      K T Kashyap C Ramachandra V Bhat B Chatterji

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      Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. Although solute supersaturation is the chemical driving force for DP, this has to be coupled with another driving force for grain boundary migration. This was identified to be diffusional coherency strain ahead of the moving boundary in the case of diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM). In the present work, the validity of diffusional coherency strain hypothesis is verified in Mg-Al alloy, which exhibits discontinuous precipitation. Samples were tested with an applied stress simultaneously with discontinuous precipitation and it was found that the velocity of the boundaries both parallel and transverse to the stress axis obeys the model for diffusional coherency strain. This work can be used as a conclusive evidence for diffusional coherency strain hypothesis for the occurrence of discontinuous precipitation in Mg-Al alloys.

    • A model for two-step ageing

      K T Kashyap C Ramachandra B Chatterji S Lele

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      In commercial practice, two-step ageing is commonly used in Al-Zn-Mg alloys to produce a fine dispersion of η′ precipitates to accentuate the mechanical properties and resistance to stress corrosion cracking. While this is true in Al-Zn-Mg alloys, two-step ageing leads to inferior properties in Al-Mg-Si alloys. This controversial behaviour in different alloys can be explained by Pashley’s Kinetic model. Pashley’s model addresses the stability of clusters after two-step ageing. In the development of the model, the surface energy term between cluster and matrix is taken into account while the coherency strains between the cluster and matrix are not considered. In the present work, a model is developed which takes into account the coherency strains between cluster and matrix and defines a new stability criterion, inclusive of strain energy term. Experiments were done on AA 7010 aluminium alloy by carrying out a two-step ageing treatment and the results fit the new stability criterion. Thus it is found that the new model for two-step ageing is verified in the case of Al-Zn-Mg alloy.

    • Modeling of droplet dynamic and thermal behaviour during spray deposition

      N S Mahesh Johnson Mendonca M K Muralidhara B K Muralidhara C Ramachandra

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      Mathematical modeling of supersonic gas atomization for spray forming has been investigated. Influence of the droplet dynamic and thermal behaviour on the resultant microstructure has been studied. Analytical models have been constructed taking into account the higher Reynolds number owing to supersonic gas flow. The impact velocity profiles of the droplets lend credence to the evolution of equiaxed grain morphology through dendrite fragmentation. The thermal history profile along with the fraction solid plot could yield optimized standoff distance to obtain a mushy droplet. A comparison of secondary dendrite arm spacing obtained from the mathematical model showed good agreement with experimental observations.

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