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Papers by Oluwatoyin E Jegede

Research paper thumbnail of Reduced gravity processing of Cu-Co metastable monotectic alloy via drop-tube processing

The metastable monotectic alloy Co-Cu has been subject to rapid solidification under reduced grav... more The metastable monotectic alloy Co-Cu has been subject to rapid solidification under reduced gravity conditions via drop-tube processing, with the resulting particles being classified into 9 size fractions between 850 m and 38 m, giving cooling rates between 850 K s-1 and 85,000 K s-1. Two alloy compositions were studied, Co-50 at.% Cu and Co-31.5 at.% Cu, with the undercooling required for binodal decomposition estimated as 41 K and 143 K respectively. Liquid phase separation was observed in both alloys, as were fully spherical core-shell morphologies. A statistical analysis of > 3000 particles across both compositions and all size fractions was undertaken. It was found that for both alloys the incidence of liquid phase separation increased with increasing cooling rate (decreasing particle size). For the Co-31.5 at.% Cu alloy the incidence of spherical core-shell morphologies also increased with increasing cooling rate but for the Co-50 at.% Cu alloy it was found that the incidence of spherical core-shell morphologies peaked in the 106-75 m size fraction, wherein  40 % of all particles displayed this morphology. However, with further increases in cooling rate the incidence of core-shell structures decreased rapidly. This is thought to be due to the limited time available for migration of the two melts following liquid phase separation.

Research paper thumbnail of Metastable liquid phase separation in Co - Cu alloys

I would not have been able to complete this Ph.D. journey if not for their kindness and support. ... more I would not have been able to complete this Ph.D. journey if not for their kindness and support. I have learnt so much under them and will forever be grateful for this experience that I will carry with me throughout my life. Mrs Diane Cochrane and Rob Simpson will forever be in my prayers. It was difficult initially for me to use the metallographic prepping equipment but you both helped me in finding my way around it. I would also like to say a very big thank you to Rob for always helping me with my wheelchair without which all these would have been impossible. I cannot thank you enough Mrs Diane Cochrane for ever willing to help and for all your kind words of encouragement which had gotten me through tough times especially at the beginning and when I lost my father. I cannot forget Dr. Tim Bigg as well for his help producing the drop tube powders Stuart and John of LEMAS, thank you so much for your patience and constant support in using the SEM. My few friends, Tarun, Suraya, Kahier and my colleagues (Nafis, Lekan and Frank) thank you for being there and making my time in Leeds interesting. Abbey, my one and only sister, thank you for being like a mother to me and for your constant prayers and support. I am so fortunate to have you as my family. Lastly, to Jesus, who keeps me going when everything else fails.

Research paper thumbnail of Size distribution and solidification of Cu-rich dispersed particles in the core region of stable core shell microstructures of metastable alloy

Journal of Materials Science

Research paper thumbnail of Thermal transitions in metastable Cu – 68.5 at. % Co alloy

Canadian Journal of Chemistry, 2021

Arc melted and drop tube processed Cu – 68.5 at. % Co alloy has been subjected to differential th... more Arc melted and drop tube processed Cu – 68.5 at. % Co alloy has been subjected to differential thermal analysis (DTA). The liquidus temperature determined from the DTA curves in the arc melt sample (1664 K) was found to be close to phase diagram estimate of 1662 K. In contrast as a result of liquid phase separation in the drop tube samples, the values obtained in the powders were much lower mainly because the compositions of the demixed phases vary from that of the parent melt. The liquidus temperature of the 850 + μm powders was 1632 K while that of the < 38 μm sieve size powder was 1616 K. This variance is due to the asymmetric nature of the metastable phase diagram of the system.

Research paper thumbnail of The Development of Plate and Lath Morphology in Ni5Ge3

Physics of Metals and Metallography, 2021

Drop-tube processing was used to solidify rapidly a congruently melting, single phase intermetall... more Drop-tube processing was used to solidify rapidly a congruently melting, single phase intermetallic Ni5Ge3. This process results in the production of powders with diameters that are between 850-53 m. After etching that occurs at low cooling rates (850-150 m diameter particles, 700-7800 K s-1), an isolated plate and lath microstructure in what is an otherwise featureless matrix constitutes the dominant solidification morphology. By contrast, when the cooling rates are higher (150-53 m diameter particles, 7800-42000 K s-1), it is isolated hexagonal crystallites within a featureless matrix, which constitute the dominant solidification morphology. The TEM analysis of selected area diffraction shows that plate and lath microstructures are variants of -Ni5Ge3, which are partially ordered. By contrast, the isolated hexagonal crystallites are revealed to be disordered. However, the featureless matrix of both microstructures are the fully ordered variant of the same compound. The plate & lath has a very different EBSD and GOS signatures to the hexagonal crystallites structure. Histogram of the correlated grain orientation angle distribution across grain orientation in plate & lath microstructure sample from the 300-212 μm fraction showing predominance of low angle grain boundaries. However, grain orientation in isolated hexagonal crystallites from 150-106 μm revealing the distribution typical of random grain orientation.

Research paper thumbnail of Presence of έ and ε crystal structures in rapidly solidified intermetallic compound Ni5Ge3

Journal of Alloys and Compounds

Research paper thumbnail of Relationship between cooling rate and SDAS in liquid phase separated metastable Cu–Co alloys

Journal of Alloys and Compounds

Research paper thumbnail of Solidification transformations in liquid phase separated metastable monotectic Cu-50 at. % Co alloy

Canadian Journal of Chemistry

Metastable monotectic Cu-50 at. % Co alloy produced by arc melting has been processed under micro... more Metastable monotectic Cu-50 at. % Co alloy produced by arc melting has been processed under micro gravity condition using a drop tube and subjected to differential thermal analysis (DTA). Microstructural evidence from the as solidified sample revealed that rapid cooling of the arc melt process was enough to incite liquid phase separation in the alloy. In the drop tube samples, the melting temperature of the β-phase (Cu-rich) was determined to be 1294.8 K, whereas that of the α-phase (Co-rich) was found to vary with cobalt content.

Research paper thumbnail of Dynamics of core–shell particle formation in drop-tube processed metastable monotectic alloys

Research paper thumbnail of Mechanical Properties of Rapidly Solidified Ni3Ge and Ni5Ge3 Intermetallic Compounds

Journal of Nanoscience and Nanotechnology

The congruently melting, single phase, intermetallic compounds β-Ni3Ge and ε-Ni5Ge3 were produced... more The congruently melting, single phase, intermetallic compounds β-Ni3Ge and ε-Ni5Ge3 were produced by arc melt. Each was subject to rapid solidification via drop-tube processing. Each compound remained fully single phase (either β-Ni3Ge or ε-Ni5Ge3) irrespective of the imposed cooling rate. In the investigation of β-Ni3Ge compound, droplets spanning the size range ≥850 to ≤38 μm diameter particles, with corresponding cooling rates of <700 to >54500 K s−1, were subject to microstructural investigation using SEM. Six dominant solidification morphologies were identified with increasing cooling rate, namely; (i) spherulites, (ii) mixed spherulites and dendrites, (iii) dendrites-orthogonal, (iv) dendrites-non-orthogonal, (v) recrystallized, and (vi) dendritic seaweed, are observed imbedded within a featureless matrix. For the ε-Ni5Ge3 compound, four dominant solidification morphologies were observed, namely; (i) partial plate and lath, (ii) plate and lath microstructure (iii) isolat...

Research paper thumbnail of Metastable monotectic phase separation in Co–Cu alloys

Journal of Materials Science

The liquid phase separation behaviour of metastable monotectic Co-Cu alloys was investigated as a... more The liquid phase separation behaviour of metastable monotectic Co-Cu alloys was investigated as a function of cooling rate using a 6.5 m drop tube facility. A range of liquid phase separated morphologies were observed including stable two-layer core-shell, evolving core-shell and dendritic structures. It was found that in the core-shell structures the core was always the higher melting point (Co-rich) phase, irrespective of the core and shell volume fraction. In Cu-50 at. % Co alloy, high cooling rates were observed to yield two episodes of liquid phase separation, corresponding to binodal, followed by spinodal decomposition. The resulting structure comprised a core-shell structure in which the Co-rich core contained a very fine dispersion of Cu-rich particles with a Cu-rich shell which may, or may not, contain a similar dispersion of Co-rich particles.

Research paper thumbnail of Reduced gravity processing of Cu-Co metastable monotectic alloy via drop-tube processing

The metastable monotectic alloy Co-Cu has been subject to rapid solidification under reduced grav... more The metastable monotectic alloy Co-Cu has been subject to rapid solidification under reduced gravity conditions via drop-tube processing, with the resulting particles being classified into 9 size fractions between 850 m and 38 m, giving cooling rates between 850 K s-1 and 85,000 K s-1. Two alloy compositions were studied, Co-50 at.% Cu and Co-31.5 at.% Cu, with the undercooling required for binodal decomposition estimated as 41 K and 143 K respectively. Liquid phase separation was observed in both alloys, as were fully spherical core-shell morphologies. A statistical analysis of > 3000 particles across both compositions and all size fractions was undertaken. It was found that for both alloys the incidence of liquid phase separation increased with increasing cooling rate (decreasing particle size). For the Co-31.5 at.% Cu alloy the incidence of spherical core-shell morphologies also increased with increasing cooling rate but for the Co-50 at.% Cu alloy it was found that the incidence of spherical core-shell morphologies peaked in the 106-75 m size fraction, wherein  40 % of all particles displayed this morphology. However, with further increases in cooling rate the incidence of core-shell structures decreased rapidly. This is thought to be due to the limited time available for migration of the two melts following liquid phase separation.

Research paper thumbnail of Metastable liquid phase separation in Co - Cu alloys

I would not have been able to complete this Ph.D. journey if not for their kindness and support. ... more I would not have been able to complete this Ph.D. journey if not for their kindness and support. I have learnt so much under them and will forever be grateful for this experience that I will carry with me throughout my life. Mrs Diane Cochrane and Rob Simpson will forever be in my prayers. It was difficult initially for me to use the metallographic prepping equipment but you both helped me in finding my way around it. I would also like to say a very big thank you to Rob for always helping me with my wheelchair without which all these would have been impossible. I cannot thank you enough Mrs Diane Cochrane for ever willing to help and for all your kind words of encouragement which had gotten me through tough times especially at the beginning and when I lost my father. I cannot forget Dr. Tim Bigg as well for his help producing the drop tube powders Stuart and John of LEMAS, thank you so much for your patience and constant support in using the SEM. My few friends, Tarun, Suraya, Kahier and my colleagues (Nafis, Lekan and Frank) thank you for being there and making my time in Leeds interesting. Abbey, my one and only sister, thank you for being like a mother to me and for your constant prayers and support. I am so fortunate to have you as my family. Lastly, to Jesus, who keeps me going when everything else fails.

Research paper thumbnail of Size distribution and solidification of Cu-rich dispersed particles in the core region of stable core shell microstructures of metastable alloy

Journal of Materials Science

Research paper thumbnail of Thermal transitions in metastable Cu – 68.5 at. % Co alloy

Canadian Journal of Chemistry, 2021

Arc melted and drop tube processed Cu – 68.5 at. % Co alloy has been subjected to differential th... more Arc melted and drop tube processed Cu – 68.5 at. % Co alloy has been subjected to differential thermal analysis (DTA). The liquidus temperature determined from the DTA curves in the arc melt sample (1664 K) was found to be close to phase diagram estimate of 1662 K. In contrast as a result of liquid phase separation in the drop tube samples, the values obtained in the powders were much lower mainly because the compositions of the demixed phases vary from that of the parent melt. The liquidus temperature of the 850 + μm powders was 1632 K while that of the < 38 μm sieve size powder was 1616 K. This variance is due to the asymmetric nature of the metastable phase diagram of the system.

Research paper thumbnail of The Development of Plate and Lath Morphology in Ni5Ge3

Physics of Metals and Metallography, 2021

Drop-tube processing was used to solidify rapidly a congruently melting, single phase intermetall... more Drop-tube processing was used to solidify rapidly a congruently melting, single phase intermetallic Ni5Ge3. This process results in the production of powders with diameters that are between 850-53 m. After etching that occurs at low cooling rates (850-150 m diameter particles, 700-7800 K s-1), an isolated plate and lath microstructure in what is an otherwise featureless matrix constitutes the dominant solidification morphology. By contrast, when the cooling rates are higher (150-53 m diameter particles, 7800-42000 K s-1), it is isolated hexagonal crystallites within a featureless matrix, which constitute the dominant solidification morphology. The TEM analysis of selected area diffraction shows that plate and lath microstructures are variants of -Ni5Ge3, which are partially ordered. By contrast, the isolated hexagonal crystallites are revealed to be disordered. However, the featureless matrix of both microstructures are the fully ordered variant of the same compound. The plate & lath has a very different EBSD and GOS signatures to the hexagonal crystallites structure. Histogram of the correlated grain orientation angle distribution across grain orientation in plate & lath microstructure sample from the 300-212 μm fraction showing predominance of low angle grain boundaries. However, grain orientation in isolated hexagonal crystallites from 150-106 μm revealing the distribution typical of random grain orientation.

Research paper thumbnail of Presence of έ and ε crystal structures in rapidly solidified intermetallic compound Ni5Ge3

Journal of Alloys and Compounds

Research paper thumbnail of Relationship between cooling rate and SDAS in liquid phase separated metastable Cu–Co alloys

Journal of Alloys and Compounds

Research paper thumbnail of Solidification transformations in liquid phase separated metastable monotectic Cu-50 at. % Co alloy

Canadian Journal of Chemistry

Metastable monotectic Cu-50 at. % Co alloy produced by arc melting has been processed under micro... more Metastable monotectic Cu-50 at. % Co alloy produced by arc melting has been processed under micro gravity condition using a drop tube and subjected to differential thermal analysis (DTA). Microstructural evidence from the as solidified sample revealed that rapid cooling of the arc melt process was enough to incite liquid phase separation in the alloy. In the drop tube samples, the melting temperature of the β-phase (Cu-rich) was determined to be 1294.8 K, whereas that of the α-phase (Co-rich) was found to vary with cobalt content.

Research paper thumbnail of Dynamics of core–shell particle formation in drop-tube processed metastable monotectic alloys

Research paper thumbnail of Mechanical Properties of Rapidly Solidified Ni3Ge and Ni5Ge3 Intermetallic Compounds

Journal of Nanoscience and Nanotechnology

The congruently melting, single phase, intermetallic compounds β-Ni3Ge and ε-Ni5Ge3 were produced... more The congruently melting, single phase, intermetallic compounds β-Ni3Ge and ε-Ni5Ge3 were produced by arc melt. Each was subject to rapid solidification via drop-tube processing. Each compound remained fully single phase (either β-Ni3Ge or ε-Ni5Ge3) irrespective of the imposed cooling rate. In the investigation of β-Ni3Ge compound, droplets spanning the size range ≥850 to ≤38 μm diameter particles, with corresponding cooling rates of <700 to >54500 K s−1, were subject to microstructural investigation using SEM. Six dominant solidification morphologies were identified with increasing cooling rate, namely; (i) spherulites, (ii) mixed spherulites and dendrites, (iii) dendrites-orthogonal, (iv) dendrites-non-orthogonal, (v) recrystallized, and (vi) dendritic seaweed, are observed imbedded within a featureless matrix. For the ε-Ni5Ge3 compound, four dominant solidification morphologies were observed, namely; (i) partial plate and lath, (ii) plate and lath microstructure (iii) isolat...

Research paper thumbnail of Metastable monotectic phase separation in Co–Cu alloys

Journal of Materials Science

The liquid phase separation behaviour of metastable monotectic Co-Cu alloys was investigated as a... more The liquid phase separation behaviour of metastable monotectic Co-Cu alloys was investigated as a function of cooling rate using a 6.5 m drop tube facility. A range of liquid phase separated morphologies were observed including stable two-layer core-shell, evolving core-shell and dendritic structures. It was found that in the core-shell structures the core was always the higher melting point (Co-rich) phase, irrespective of the core and shell volume fraction. In Cu-50 at. % Co alloy, high cooling rates were observed to yield two episodes of liquid phase separation, corresponding to binodal, followed by spinodal decomposition. The resulting structure comprised a core-shell structure in which the Co-rich core contained a very fine dispersion of Cu-rich particles with a Cu-rich shell which may, or may not, contain a similar dispersion of Co-rich particles.