Browsing by Author "Yaqoob, Khurram"
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- Ceramic-reinforced HEA matrix composites exhibiting an excellent combination of mechanical propertiesPublication . Mehmood, M. Adil; Shehzad, Khurram; Mujahid, M.; Yaqub, Talha Bin; Godfrey, Andy; Fernandes, Filipe; Muhammad, F. Z.; Yaqoob, KhurramCoCrFeNi is a well-studied face centered cubic (fcc) high entropy alloy (HEA) that exhibits excellent ductility but only limited strength. The present study focusses on improving the strength-ductility balance of this HEA by addition of varying amounts of SiC using an arc melting route. Chromium present in the base HEA is found to result in decomposition of SiC during melting. Consequently, interaction of free carbon with chromium results in the in-situ formation of chromium carbide, while free silicon remains in solution in the base HEA and/or interacts with the constituent elements of the base HEA to form silicides. The changes in microstructural phases with increasing amount of SiC are found to follow the sequence: fcc → fcc + eutectic → fcc + chromium carbide platelets → fcc + chromium carbide platelets + silicides → fcc + chromium carbide platelets + silicides + graphite globules/flakes. In comparison to both conventional and high entropy alloys, the resulting composites were found to exhibit a very wide range of mechanical properties (yield strength from 277 MPa with more than 60% elongation to 2522 MPa with 6% elongation). Some of the developed high entropy composites showed an outstanding combination of mechanical properties (yield strength 1200 MPa with 37% elongation) and occupied previously unattainable regions in a yield strength versus elongation map. In addition to their significant elongation, the hardness and yield strength of the HEA composites are found to lie in the same range as those of bulk metallic glasses. It is therefore believed that development of high entropy composites can help in obtaining outstanding combinations of mechanical properties for advanced structural applications.
- Design and development of NbTiVZr porous high entropy alloys for energy applicationsPublication . Siddique, Ayesha; Abid, Talha; Akram, M. Aftab; Yaqub, Talha Bin; Karim, M. Ramzan Abdul; Fernandes, Filipe; Khan, Rizwan; Yaqoob, KhurramPorous NbTiVZr refractory high entropy alloys have been developed by a novel method that is based on the addition of yttrium to the base high entropy alloy and its subsequent removal by electrochemical dealloying process. Microstructure and crystal structure characterization of as-cast alloys confirmed the segregation of yttrium at interdendritic regions. Successful removal of yttrium from interdendritic region has been confirmed by microstructure and crystal structure characterization of dealloyed samples. NbTiVZr foams with varying levels of porosity were successfully developed and characterized. Electrochemical performance of the developed foams was investigated by cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS). High areal capacitance of 82.66 mF cm 2 at scan rate of 2 mV s 1 was exhibited by one of the developed electrodes. The electrode displayed capacitance retention of 104 % after 5000 cycles at current density of 1 mA cm 2. The excellent electrochemical performance demonstrated by the NbTiVZr refractory high entropy foams highlights their potential as suitable candidates for supercapacitor electrode.
- Design and development of porous CoCrFeNiMn high entropy alloy Cantor alloy with outstanding electrochemical propertiesPublication . Abid, Talha; Akram, M. Aftab; Karim, M. Ramzan Abdul; Fernandes, Filipe; Fernandes, Filipe; Zafar, Muhammad Farooq; Yaqoob, KhurramCoCrFeNiMn high entropy alloys are among the most well-studied high entropy alloys that exhibit reasonable strength and outstanding ductility. In the present study, porous CoCrFeNiMn foams have been developed by the addition of copper in the base high entropy alloy by arc melting followed by its removal through an electrochemical dealloying process. Microstructure characterization of the as-cast samples confirmed limited solubility of copper in the matrix while the majority of the copper was found to segregate to interdendritic areas. Removal of copper from the interdendritic areas was successfully carried out by an electrochemical dealloying process which resulted in the development of foams with interconnected porosity. CoCrFeNiMn foams with different levels of porosities were successfully developed by varying the amount of added copper in the base HEA and its removal by a dealloying process. The electrochemical performance of the developed foams was assessed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). One of the developed foams was found to exhibit an areal capacitance of 1.56 F cm 2 at 2 mA cm 2 which is more than 2x times higher than the value reported for recently developed porous AlCoCrFeNi high entropy foam. Developed foam, besides showing excellent values of areal capacitance, demonstrated capacitance retention of 114.6% after 5000 cycles at 8 mA cm 2. The excellent electrochemical performance of the developed high entropy foams exhibits their potential to be used as electrode materials for supercapacitor applications and was attributed to the insertion of interconnected porosity in the base HEA.
- Development of eutectic high entropy alloy by addition of W to CoCrFeNi HEAPublication . Abdullah, M.; Mukarram, Muhammad; Yaqub, Talha Bin; Fernandes, Filipe; Yaqoob, KhurramHigh entropy alloys have shown a remarkable combination of physical and mechanical properties. The introduction of eutectic microstructure, consisting of a tough fcc phase, and a hard-intermetallic phase, can help in obtaining even better synergy of strength and ductility. The presence of multiple principal alloying elements in HEAs and absence of corresponding multicomponent phase diagrams makes designing of eutectic high entropy alloys a tedious task. In the present study, systematic investigation of CoCrFeNi-Wx system has been carried out for the development of eutectic microstructure. Experimental results validated the presence of eutectic reaction in the calculated phase diagram. CoCrFeNi-Wx HEAs remained single fcc phase alloys at smaller amount of W (x = 0.1) but changed to hypoeutectic (x = 0.25, 0.5, 0.75) and hypereutectic alloys (x = 1.0) with increase in the amount of tungsten. It has been shown that calculated pseudo binary phase diagrams can provide a very good starting point for the development of eutectic HEAs. Mechanical characterization of the developed HEAs revealed that development of eutectic mixture of a soft (fcc) and a hard phase (intermetallic/bcc) can help in obtaining outstanding combination of mechanical properties.
- Effect of Annealing Heat Treatment on the Composition, Morphology, Structure and Mechanical Properties of the W-S-N CoatingsPublication . Yaqub, Talha Bin; Al-Rjoub, Abbas; Khalid, Hafiza Ayesha; Yaqoob, Khurram; Fernandes, Filipe; Cavaleiro, AlbanoAlloyed-transition metal dichalcogenide (TMD) coatings have been under investigation as multi-environment lubricants for the past few decades. These coatings display very low coefficient of friction properties at elevated temperatures. Studies on the annealing of these low-friction coatings are missing in the literature. For the first time, in this study, the annealing of the W-S-N dry lubricant coatings was carried out to study its effects on the composition, morphology, crystal structure and hardness of the coatings. The W-S-N coatings were deposited by direct current (DC) reactive magnetron sputtering. The analysis was carried out for as-deposited, 200 °C and 400 °C annealed coatings. The as-deposited coatings have N content in the range of 0–25.5 at. %. The coatings are compact and the densification increased with the increase in N-alloying. All the coatings are crystalline except the highest N-alloyed coating which is X-ray amorphous. A maximum hardness of 8.0 GPa was measured for the coating alloyed with 23 at. % N. Annealing did not affect the composition and morphology of the coatings, while some variations were observed in their crystal structure and hardness. The maximum hardness increased from 8 GPa to 9.2 GPa after 400 °C annealing of the 23 at. % N-alloyed coating.