Faculty Publications
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Item Effect of heat treatment on microstructure, corrosion, and shape memory characteristics of laser deposited NiTi alloy(Elsevier Ltd, 2018) Marattukalam, J.J.; Balla, V.K.; Das, M.; Bontha, S.; Kalpathy, S.K.The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm2, and lenticular or plate-like martensite in the samples processed at 80 J/mm2. The XRD results revealed relatively high concentration of martensite (B19?) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing. © 2018 Elsevier B.V.Item Effect of zinc and rare-earth element addition on mechanical, corrosion, and biological properties of magnesium(Cambridge University Press, 2018) Kottuparambil, R.R.; Bontha, S.; Ramesh, M.R.; Arya, S.; Jana, A.; Das, M.; Balla, V.K.; Amrithalingam, S.; Prabhu, T.R.The present work aims to understand the effect of zinc and rare-earth element addition (i.e., 2 wt% Gd, 2 wt% Dy, and 2 wt% of Gd and Nd individually) on the microstructure evolution, mechanical properties, in vitro corrosion behavior, and cytotoxicity of Mg for biomedical application. The microstructure results indicate that the Mg-Zn-Gd alloy consists of the lamellar long period stacking ordered phase. The electrochemical and immersion corrosion behavior were studied in Hanks balanced salt solution. Enhanced corrosion resistance with reduced hydrogen evolution volume and magnesium (Mg2+) ion release were estimated for the Mg-Zn-Gd alloy as compared to the other two alloy systems. At the early stage of corrosion, formation of the oxide film inhibited the corrosion propagation. However, at the later stages, the breaking of the oxide film leads to shallow pitting mode of corrosion. The ultimate tensile strength of Mg-Zn-Gd-Nd is better than the other two alloys due to the uniform distribution of the Mg12Nd precipitate phase. The moderate strength in the Mg-Zn-Gd alloy is due to the low volume fraction of the secondary phase. The MTT (methylthiazoldiphenyl-tetrazolium bromide) assay study was carried out to understand the cell cytotoxicity on the alloy surfaces. Studies revealed that all three alloys had significant cellular adherence and no adverse effect on cells. © 2018 Materials Research Society.Item Comparative investigation of coating and friction stir processing on Mg-Zn-Dy alloy for improving antibacterial, bioactive and corrosion behaviour(Elsevier B.V., 2021) Rokkala, U.; Jana, A.; Bontha, S.; Ramesh, M.R.; Balla, V.K.Magnesium based alloys are well-known materials for temporary implant applications. However, failures due to early degradation and bacterial infection are limiting their applications. To overcome these problems, in the present work a Mg-Zn-Dy alloy based composite surface was prepared using coating and friction stir processing (FSP) techniques. Herein, hydroxyapatite (HA) and silver (Ag) particles were deposited on Mg-Zn-Dy alloy to obtain HA and Ag coated surface (C-HAg). Later, FSP was carried out on the C-HAg surface to develop a Mg-Zn-Dy alloy based composite surface (F-HAg). Field emission scanning electron microscope (FESEM) and energy dispersive X-ray analysis (EDS) confirm the mixing of HA and Ag particles with the Mg-Zn-Dy substrate. Antibacterial studies reveal that both C-HAg and F-HAg samples inhibit Escherichia coli and Staphylococcus aureus bacteria. In vitro cytotoxicity study indicates that the both samples are non-toxic in nature. Results of in vitro corrosion study reveal a significant reduction (72%) in corrosion rate of F-HAg sample when compared to C-HAg sample. The F-HAg samples showed simultaneous improvement in corrosion resistance and antibacterial properties with good biocompatibility. The results of this study indicate that the developed composite surface is a promising material for antibacterial and biodegradable implant applications. © 2021 Elsevier B.V.Item Influence of friction stir processing on microstructure, mechanical properties and corrosion behaviour of Mg-Zn-Dy alloy(Springer, 2023) Rokkala, U.; Bontha, S.; Ramesh, M.R.; Balla, V.K.In the present study, friction stir processing (FSP) was carried out on as-cast Mg-Zn-Dy alloy to tailor grain size and texture which alter the mechanical properties and corrosion behaviour. The grain size of the as-cast alloy was reduced from 60 ± 2 µm to 3 ± 0.1 µm after FSP due to dynamic recrystallization. The effect of grain size, crystallographic orientation and fine precipitates on mechanical properties were investigated using field emission scanning electron microscope (FESEM) and electron back scattered diffraction (EBSD). The ultimate tensile strength, yield strength, % elongation and hardness of FSPed alloy improved by 55%, 60%, 53% and 46% when compared to as-cast alloy. The FSPed Mg-Zn-Dy alloy exhibited a 79% decrease in corrosion rate when compared to as-cast alloy which can be attributed to grain refinement, uniform distribution of secondary precipitates and strong basal texture. The surface of FSPed sample after immersion corrosion exhibited calcium phosphate rich minerals which help in apatite formation on the sample surface. Cytotoxicity studies using MTT assay revealed more than 80% cell viability for both as-cast and FSPed alloy illustrating non-toxic nature of both the samples. The results of this study indicate that FSPed Mg-Zn-Dy alloy is a potential material for biodegradable implants due to its high strength, corrosion resistance and biocompatibility. Graphical Abstract: [Figure not available: see fulltext.]. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item Multi-step fabrication of bioactive Mg–Zn–Dy–AlO3/HA composites: exploring the synergistic effects of plasma spray and friction stir processing(Springer, 2024) Rokkala, U.; Bontha, S.; Ramesh, M.R.; Balla, V.K.Magnesium (Mg) alloys are gaining more attention in recent times as biodegradable materials. However, two major problems with Mg alloy implants are bacterial infections and poor corrosion resistance. In this context, a composite surface (Mg–Zn–Dy–Al2O3/HA) is developed using surface modification techniques. First, Al2O3 + HA composite powder is coated on Mg–Zn–Dy alloy to attain coated surface (C-AHa). Next, the C-AHa surface is subjected to friction stir processing to develop composite surface (F-AHa). Microstructural characterization reveals that, the Al2O3 + HA particles were distributed evenly into the Mg–Zn–Dy substrate. Antimicrobial activities against Escherichia coli and Staphylococcus aureus reveal low adhesion of bacteria on the F-AHa sample surface due to low surface energy (37.83 ± 0.22 mN/m) and low surface roughness (0.36 ± 0.1 µm). Further, the cytotoxicity tests confirm that the F-AHa sample shows significant improvement in cell viability (98%) after 7 days and non-toxic against the mouse osteoblast cells. In Vitro corrosion study observations demonstrate that the corrosion rate for the F-AHa sample is decreased by 72% compared to the C-AHa sample. Thus, the results of this study for the fabricated composites are promising for antimicrobial, biocompatible and bioabsorbable temporary implants. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024.Item Microstructural Evolution of Mg-Zn-Gd Alloy Using Equal Channel Angular Pressing to Enhance Mechanical and Corrosion Properties(Springer, 2025) Rokkala, U.; Patil, A.; Bontha, S.; Ramesh, M.R.; Balla, V.K.; Srinivasan, A.Equal channel angular pressing (ECAP) was used on the Mg-Zn-Gd alloy in this study to improve its corrosion and mechanical properties. Microstructural and phase analysis reveal that, after ECAP, a substantial grain refinement occurred, and secondary phases were observed. The grain size of the as-cast (AC) sample is reduced from 20 ± 1 to 0.88 ± 0.6 µm, attributed to dynamic recrystallization. The mechanical properties of the ECAP sample were significantly improved when compared to the AC sample. An improvement in the microhardness (43%), ultimate tensile strength (73%), yield strength (76%), and ductility (50%) were observed for the ECAP sample. A decrease in the corrosion rate was observed for ECAP sample (9 ± 1 mm/year) compared to the AC (16 ± 2 mm/year) sample. The grain refinement and crystallographic orientation of the ECAP samples contributed to the enhancement of corrosion resistance. © ASM International 2025.