Faculty Publications
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Item Solid state amorphization of Mg-Zn-Ca system via mechanical alloying and characterization(Elsevier B.V., 2017) Manne, B.; Bontha, S.; Ramesh, M.R.; Krishna, M.; Balla, V.K.Magnesium based bulk metallic glasses have attracted significant attention of researchers due to better mechanical and corrosion properties when compared to their crystalline counterparts especially for biomedical applications. Scaling up the part size and production volumes of such materials through liquid metallurgy route is challenging. In this work amorphous Ca5Mg60+xZn35?x (X = 0, 3 and 7) alloys have been successfully synthesized through solid state amorphization using a high energy planetary ball mill. X-ray diffraction was used to identify the crystalline phases of the powder during reaction. Evolution of amorphous phase was analysed using a parameter involving the ratio of integral area of peaks to the integral area of background (IPB) obtained from XRD patterns. Results showed reaction time increases with decreasing Zn content in Ca5Mg60+xZn35?x (X = 0, 3 and 7) alloy to obtain maximum amorphous structure with a small amount of residual crystalline phase. Prolonged milling of these powders, to eliminate residual crystalline phases, resulted in the nucleation of Mg102.08Zn39.6 phase. The composition dependent characteristic temperatures and thermal stabilities were studied using differential scanning calorimetry. © 2016 The Society of Powder Technology JapanItem Microstructure and Adhesion Strength of Ni3Ti Coating Prepared by Mechanical Alloying and HVOF(Pleiades Publishing compmg@maik.ru, 2018) Reddy, N.C.; Ajay Kumar, B.S.; Ramesh, M.R.; Koppad, P.G.In the present work we report the development of Ni3Ti intermetallic compound by high energy ball milling of Ni and Ti powders. The ball milled powders were taken at various intervals (4, 6, 8, 10, and 11 h) to analyze the formation of NixTix intermetallic compounds. The ball milled powders were analyzed using scanning electron microscopy and X-ray diffraction. The layered shaped powder particles of Ni3Ti phase were formed after 11 h of ball milling, which was confirmed by X-ray peaks. Further High-Velocity Oxy-Fuel (HVOF) process was used to coat Ni3Ti and Ni3Ti + (Cr3C2 + 20NiCr) on MDN 420 steel. Both the coated materials displayed excellent cohesion with minimal porosity less than 2%. The tensile adhesion strength test was carried out on these coatings to check the bond strength. Out of the two the Ni3Ti coating showed excellent bond strength of 41.04 MPa compared to that of Ni3Ti + (Cr3C2 + 20NiCr) coating. © 2018, Pleiades Publishing, Ltd.Item Phase evolution and high-temperature wear behavior of non-equiatomic metastable CoCrNiTiMox HEA coatings fabricated by high-velocity oxy-fuel technique(Elsevier Ltd, 2023) Addepalli, S.N.; Joladarashi, S.; Ramesh, M.R.The current research aims to enhance the tribological performance of maraging steels at high temperatures by surface modification techniques. CoCrNiTiMox (x; molar fraction, x = 0.5, 1.5) high-entropy alloy (HEA) coatings with dense lamellar microstructures were deposited onto maraging steels using high-velocity oxy-fuel spray (HVOF). In order to achieve a uniform distribution of constituent elements for thermal spray deposition, mechanical alloying was employed to synthesize the HEA feedstock. The phases and microstructure of the synthesized HEA powder, as-sprayed coatings, and worn surfaces were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as-sprayed HEA coatings exhibited metastability, with a BCC phase solid solution, NiTiO3 spinel, and an intermetallic MoNi phase for CoCrNiTiMo0.5 and Co2Mo3 phase for CoCrNiTiMo1.5. The average microhardness of CoCrNiTiMo0.5 and CoCrNiTiMo1.5 HEA coatings were 841 ± 62 HV0.3 and 952 ± 23 HV0.3, respectively. The specific wear rate and friction coefficients of CoCrNiTiMox HEA coatings exhibited a decreasing trend with an increase in temperature, owing to the formation of tribofilms on the worn surface. X-ray diffraction studies revealed the formation of NiMoO4 spinel for CoCrNiTiMo0.5 and MoO2, Co3O4 phases for CoCrNiTiMo1.5 HEA at a wear temperature of 600 °C. The investigation of worn surfaces showed a transformation in wear mechanisms from abrasive wear at room temperature to oxidative wear with mild fatigue at elevated temperatures. © 2023 Elsevier LtdItem Elevated temperature tribological performance of non-equiatomic CoCrNiTiWx high entropy alloy coatings developed by mechanical alloying and high-velocity oxy-fuel spray(Elsevier B.V., 2024) Addepalli, S.N.; Joladarashi, S.; Ramesh, M.R.High entropy alloys (HEA) have applications in multiple fields owing to their exceptional mechanical and physical properties. In the current study, mechanical alloyed CoCrNiTiWx (x; a molar fraction, x = 0.5 and 1.5) HEA feedstock powders were deposited on maraging steel substrate using high-velocity oxy-fuel spray (HVOF). The phase evolution and the microstructure of the milled powders and as-sprayed coatings were analysed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The tribological behaviour of CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings retained the BCC solid solution phases formed during the milling stage. However, additional oxide and intermetallic phases were formed owing to the in-flight oxidation and high temperatures experienced during the HVOF deposition. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings were found to be 1.69 ± 0.32 % and 1.51 ± 0.37 % respectively.Consequently, the CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings displayed average microhardness values of 863 ± 52 HV0.3 and 1025 ± 39 HV0.3, respectively. Further, the wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, NiCr2O4 oxide tribofilms for CoCrNiTiW0.5, and CoCr2O4, NiWO4, WO3 oxides for CoCrNiTiW1.5. The specific wear rate of CoCrNiTiW0.5 HEA coating dropped by 73.6 % from 22.7 ± 2.6 × 10−6 mm3/N-m to 5.99 ± 1.9 × 10−6 mm3/N-m, while CoCrNiTiW1.5 dropped by 78.8 % from 11.86 ± 3.5 × 10−6 mm3/N-m to 2.51 ± 1.5 × 10−6 mm3/N-m, with a rise in the temperature from RT to 600 °C. Likewise, The frictional coefficients of CoCrNiTiW0.5 HEA dropped from 0.504 ± 0.015 to 0.397 ± 0.005, while CoCrNiTiW1.5 HEA dropped from 0.578 ± 0.025 to 0.471 ± 0.004, with a rise in temperature from RT to 600 °C. At room temperature, the wear mechanisms of the as-sprayed CoCrNiTiWx coatings were dominated by adhesive wear. However, at elevated temperatures, a shift towards oxidative wear was observed. © 2023 Elsevier B.V.