Electrical double-layer formed at the electrode/electrolyte interface in combination with electron-transfer reaction can lead to many important applications of electrochemistry, including energy storage devices, namely, batteries, fuel cells and electrochemical supercapacitors. Electrochemical supercapacitors are characterized by their higher power density as compared to batteries and higher energy density than the conventional electrostatic and electrolytic capacitors. Thus, supercapacitors are useful as auxiliary energy storage devices along with primary sources such as batteries or fuel cells for the purpose of power enhancement in short pulse applications. These are expected to be useful in hybrid devices together with batteries or fuel cells, in electric vehicle propulsion systems. Among the various materials studied for electrochemical supercapacitors, carbonaceous materials, transition metal oxides and conducting polymers are important. Carbon in various forms is used as a double-layer capacitor material, which stores charge by electrostatic charge separation at the electrode/electrolyte interface. The specific capacitance (SC) of high surface area activated carbon is about 100 F g-1 in aqueous electrolytes. Transition metal oxides have attracted considerable attention as electrode materials for supercapacitors because of the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and convenience in handling. Hydrated RuO2 prepared by sol-gel process exhibited a SC as high as 720 F g-1. However, high cost, low porosity and toxic nature of RuO2 limit its commercialization in supercapacitors. On the otherhand, MnO2 is an attractive electrode material as it is electrochemically active, cheap, environmentally benign, and its resources are abundant in nature. In an early report on the capacitance properties of MnO2 by Lee and Goodenough [J. Solid State Chem. 144 (1999) 220], amorphous hydrous MnO2 synthesized by co-precipitation method exhibited a SC of 203 F g-1 in 2 M KCl electrolyte. According to the charge-storage mechanism of MnO2 involving MnO2 + M+ + e- ↔ (MnOO)-M+ (where M+ = Li+, Na+, K+ etc.), a SC of 1110 F g-1 is expected over a potential window of 1.0 V. However, SC values in the range of 100-200 F g-1 are reported in the literature. The low values of SC are because of the charge-storage is confined to surface region of MnO2 particles or films. It is desirable to enhance the SC of MnO2 to a value close to the theoretical value. In view of this, attempts are made to enhance the SC of MnO2 by adopting different synthetic procedures such as electrochemical method for depositing MnO2 and also nanostructured mesoporous MnO2 by polyol route, hydrothermal route and sonochemical method in the present studies. As the charge-storage mechanism of MnO2 involves the surface insertion/deinsertion of cations from the electrolyte during discharge/charge processes, respectively, the capacitance properties of MnO2 are studied in various aqueous electrolytes containing monovalent (Na+), bivalent (Mg2+, Ca2+, Sr2+ and Ba2+) and trivalent (La3+) cations. The mass variation occurring at the electrode during the charge/discharge of MnO2 is examined by electrochemical quartz crystal microbalance (EQCM) study. In addition to this, the kinetics of electrodeposition and capacitance properties of Mn(OH)2 are studied by employing EQCM. Also, properties of asymmetric capacitors assembled with Mn(OH)2 as the positive electrode and carbon as the negative electrode are studied and compared with symmetric Mn(OH)2 capacitors. Furthermore, attempts are made to increase the potential window of Co(OH)2 in alkaline and neutral electrolytes. The contents of the thesis by Chapter-wise are given below. Chapter 1 introduces the importance of electrochemistry in energy storage and conversion, basics of electrochemical power sources, importance of some electroactive materials in electrochemical energy storage, different synthetic procedures for MnO2 and its application in electrochemical supercapacitors. Transition metal oxides are widely studied because of their variable oxidation states, high electrochemical activity, abundance in nature and environmental compatibility. Various reports appeared in the form of open publications on supercapacitor studies of transition metal oxides such as RuO2, MnO2, Fe3O4, Co(OH)2, Ni(OH)2, NiO, etc., are briefly reviewed. The chapter ends with statements on objectives of the studies carried out and reported in the thesis. Chapter 2 provides experimental procedures and methodologies used for the studies reported in the thesis. Different experimental routes adopted for synthesis of MnO2, Mn(OH)2 and Co(OH)2 used for the studies are described. Also included are brief descriptions of various physicochemical and electrochemical techniques employed for the investigations. In Chapter 3, MnO2 samples synthesized by various routes such as electrochemical method, polyol route, hydrothermal route and sonochemical method are studied. MnO2 and Mn(OH)2 are simultaneously electrodeposited on the anode and the cathode, respectively, in a galvanostatic electrolysis cell consisting of aqueous Mn(NO3)2 electrolyte. MnO2/SS and Mn(OH)2/SS electrodes are used as the negative and the positive electrodes, respectively, in an asymmetric Mn(OH)2//MnO2 supercapacitor. MnO2 samples are prepared at room temperature and in hydrothermal method at a temperature of 140 ◦C by reduction of KMnO4 with poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) or P123 as a reductant. Also, MnO2 is prepared from KMnO4 by hydrothermal method without using any reducing agent. This procedure requires a temperature of 180 ◦C and 24 h duration. MnO2 is also synthesized with an ultrasonic aided procedure. The electrochemical capacitance properties of MnO2 samples synthesized by various routes are investigated. A maximum SC of 264 F g-1 is obtained at a current density of 0.5 mA cm-2 (1.0 A g-1) for MnO2 prepared by sonochemical method. The capacitance properties of MnO2 are generally studied in neutral aqueous Na2SO4 electrolytes. In Chapter 4, electrolytes of NaNO3, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2 and also La(NO3)3 are studied and the results are compared with Na2SO4 electrolyte. Among the alkaline earth salt solutions, higher SC values are obtained in Mg(NO3)2 and Ca(NO3)2 electrolytes than in the rest of the electrolytes. Furthermore, MnO2 exhibits capacitance behaviour in La(NO3)3 solution with enhanced SC in comparison with NaNO3 and Mg(NO3)2 solutions. The SC increases with an increase in charge on the cation (Na+, Mg2+ and La3+). The values of SC measured in Na+, Mg2+ and La3+ electrolytes are 190, 220 and 257 F g-1, respectively at a c.d. of 0.5 mA cm-2 (1.0 A g-1). Rate capabilities are also found to be different in different electrolytes. Specific energy and specific power are calculated and presented as Ragone plots. The presence of divalent and trivalent cations inserted onto MnO2 is identified by X-ray photoelectron spectroscopy. EQCM is employed to monitor the increased mass variations that accompany reversible adsorption/desorption of Na+, Mg2+ and La3+ ions onto MnO2. In Chapter 5, EQCM has been used to study the kinetics of electrochemical precipitation of Mn(OH)2 on Au-crystal and its capacitance properties. From the EQCM data, it is inferred that NO3- ions get adsorbed on Au-crystal, and then undergo reduction resulting an increase in pH near the electrode surface. Precipitation of Mn2+ occurs as Mn(OH)2, resulting an increase in mass of the Au-crystal. On charging, Mn(OH)2 undergoes oxidation to MnO2, which exhibits electrochemical supercapacitor behaviour on subjecting to cycling in aqueous Na2SO4 electrolyte. EQCM data indicates the mass variations corresponding to surface insertion/extraction of Na+ ions during discharge/charge cycling of Mn(OH)2 in aqueous Na2SO4 electrolyte. In Chapter 6, Mn(OH)2 synthesized by precipitation of MnSO4 with NH4OH solution is studied for capacitance properties. A SC of 141 F g-1 is obtained for the Mn(OH)2 at a c.d. of 0.66 A g-1 in 1.0 M Na2SO4 electrolyte in the potential range of 0-1.0 V vs. standard calomel electrode (SCE). Also, carbon electrode made from high surface area carbon exhibits a SC of 158 F g-1 at a c.d. of 0.81 A g-1 in the potential range of 0 to -1.0 V vs. SCE. Asymmetric capacitors are assembled by combining Mn(OH)2 as the positive and carbon as the negative electrodes. The asymmetric capacitor has a SC of 39 F g-1 at a c.d. of 0.42 A g-1 in the operating voltage of 1.8 V. However, a symmetric capacitor consisting of two Mn(OH)2 electrodes provides a SC of 11 F g-1 only at a c.d. of 0.24 A g-1 in an operating voltage of 1.2 V. In Chapter 7, MnO2 synthesized by reduction of KMnO4 using ethylene glycol is used for fabrication of large area electrodes. Stainless steel (SS) mesh of 3 cm x 3 cm with geometrical area of 18 cm2 is used as current collector. Three symmetrical electrochemical supercapacitors (capacitance of about 100 F per each at a current of 0.2 A) are assembled, each with 11 electrodes positioned in parallel. Six alternate electrodes are stacked as the negative terminal and the other five as the positive terminal. The electrochemical properties of MnO2 supercapacitors are studied by galvanostatic charge-discharge cycling and ac impedance in 1.0 M Na2SO4 electrolyte. Also, the capacitors are combined in parallel as well as in series and the capacitance is evaluated. The practical application of the electrochemical supercapacitors is shown by demonstrating the running of a toy fan connected to the charged capacitor as well as the glowing of LED cell connected to charged supercapacitors connected in series. A parallel combination of batteries and capacitors is also demonstrated. Capacitor studies of Co(OH)2 over a limited potential window in alkaline electrolytes are reported in the literature. A high potential window of a capacitor material is desirable for using in a device. In Chapter 8, experiments are conducted to understand the reason for a low potential window for Co(OH)2 as a capacitor material and also to increase its potential window. Experiments are conducted in aqueous NaOH and Na2SO4 electrolytes of various concentrations using electrochemically precipitated Co(OH)2 on stainless steel current collectors in an aqueous Co(NO3)2 electrolyte. Based on the potential window, specific capacitance and specific energy, it is found that 0.05 M NaOH electrolyte is more appropriate for capacitor properties of Co(OH)2 than the rest of the electrolytes studied. Using a Co(OH)2 electrode with a specific mass of 1.0 mg cm-2 in 0.05 M NaOH, a SC of about 380 F g-1 is obtained with a potential window of 0.85 V at a charge-discharge c.d. of 10 A g-1 (10 mA cm-2). The work presented in this thesis is carried out by the candidate as a part of Ph. D. training program and most of the results have been published in the literature. A list of publications of the candidate is enclosed below. It is hoped that the studies reported here will constitute a worthwhile contribution.

Les couches minces épitaxiales de Ni-Mn-Ga ont attiré une attention considérable, car ils sont des candidats prometteurs pour les capteurs et actionneurs magnétiques dans des microsystèmes électromécaniques. Des informations complètes sur les caractéristiques de la microstructure et de la cristallographie des films NiMnGa et leur relation avec les contraintes du substrat sont essentielles à l'optimisation des propriétés. Dans le présent travail, les couches minces épitaxiale de Ni-Mn-Ga ont été produites par pulvérisation cathodique magnétron à courant continu et ensuite caractérisées par la technique de diffraction des rayons X (XRD) et la diffraction d'électrons rétrodiffusés dans un microscope électronique à balayage équipé d’analyse EBSD (MEB-EBSD). Des couches minces épitaxiales avec NiMnGa de composition nominale Ni50Mn30Ga20 et d'épaisseur 1,5 µm ont été fabriquées avec succès sur le substrat monocristallin de MgO par pulvérisation cathodique magnétron DC, après l'optimisation des paramètres tels que la puissance de pulvérisation cathodique, la température du substrat et de la couche d'ensemencement dans le cadre du présent travail. Les mesures de diffraction DRX montrent que les couches minces épitaxiales NiMnGa sont composées de trois phases: austénite, martensite NM et martensite modulée 7M. Avec les géométries de mesure optimisées, le nombre maximum possible de pics de diffraction des phases relatives, en particulier compte tenu de la basse symétrie de la martensite 7M, sont acquis et analysés. Les constantes de réseau de l'ensemble des trois phases dans le cadre des contraintes du substrat dans les films sont entièrement déterminées. L’analyse SEM-EBSD en profondeur du film a permis en outre de vérifier la situation de coexistence de trois phases constitutives: austénite, 7M martensite et martensite NM. La martensite NM se trouve près de la surface libre du film, l'austénite au-dessus de la surface du substrat, et la martensite 7M dans les couches intermédiaires entre l'austénite et la martensite NM. La caractérisation de microstructure montre que la martensite 7M et la martensite NM ont une morphologie de plaque et sont organisées en deux zones caractéristiques décrites avec des bas et haut contraste en images d’électrons secondaires. Des plaques de martensite locales similaire en orientation morphologique sont organisées en groupes de plaques ou colonies ou variantes de colonies. Une caractérisation plus poussée en EBSD indique qu'il existe quatre plaques de martensite distinctes dans chaque colonie de variante à la fois pour la martensite NM et 7M. Chaque plaque de martensite NM est composée de variantes lamellaires majeures et mineures en termes d’épaisseurs appariées et ayant une interface interlamellaire cohérente, alors que chaque plaque de martensite 7M contient une variante d'orientation. Ainsi, il existe quatre variantes d'orientation de martensite 7M et huit variantes d’orientation de martensite NM dans une colonie de variantes. Selon l'orientation cristallographique des martensites et des calculs cristallographiques, pour la martensite NM, les interfaces inter-plaques sont constituées de macles de type composées dans des plaques adjacentes de martensite NM. La distribution symétrique des macles composées résulte dans des interfaces de plaques longues et droites dans la zone de contraste relatif faible. La répartition asymétrique conduit à la modification de l’orientation d'interface entre les plaques de la zone de contraste relativement élevé. Pour la martensite 7M, à la fois les interfaces de type I et de type II sont à peu près perpendiculaires à la surface du substrat dans les zones à faible contraste relatif. Les paires de macles de type-I apparaissent avec une fréquence beaucoup plus élevée, par comparaison avec celle des macles de type II. Cependant, il y a deux traces d’interface de macles de type II et une trace d’interface de macles de type I dans les zones de contraste relatifs élevés. [...] / Epitaxial Ni-Mn-Ga thin films have attracted considerable attention, since they are promising candidates for magnetic sensors and actuators in micro-electro-mechanical systems. Comprehensive information on the microstructural and crystallographic features of the NiMnGa films and their relationship with the constraints of the substrate is essential for further property optimization. In the present work, epitaxial Ni-Mn-Ga thin films were produced by DC magnetron sputtering and then characterized by x-ray diffraction technique (XRD) and backscatter electron diffraction equipped in scanning electron microscope (SEM-EBSD). Epitaxial NiMnGa thin films with nominal composition of Ni50Mn30Ga20 and thickness of 1.5 µm were successfully fabricated on MgO monocrystalline substrate by DC magnetron sputtering, after the optimization of sputtering parameters such as sputtering power, substrate temperature and seed layer by the present work. XRD diffraction measurements demonstrate that the epitaxial NiMnGa thin films are composed of three phases: austenite, NM martensite and 7M martensite. With the optimized measurement geometries, maximum number of diffraction peaks of the concerning phases, especially of the low symmetrical 7M martensite, are acquired and analyzed. The lattice constants of all the three phases under the constraints of the substrate in the films are fully determined. These serve as prerequisites for the subsequent EBSD crystallographic orientation characterizations. SEM-EBSD in film depth analyses further verified the co-existence situation of the three constituent phases: austenite, 7M martensite and NM martensite. NM martensite is located near the free surface of the film, austenite above the substrate surface, and 7M martensite in the intermediate layers between austenite and NM martensite. Microstructure characterization shows that both the 7M martensite and NM martensite are of plate morphology and organized into two characteristic zones featured with low and high relative second electron image contrast. Local martensite plates with similar plate morphology orientation are organized into plate groups or groups or variant colonies. Further EBSD characterization indicates that there are four distinct martensite plates in each variant groups for both NM and 7M martensite. Each NM martensite plate is composed of paired major and minor lamellar variants in terms of their thicknesses having a coherent interlamellar interface, whereas, each 7M martensite plate contains one orientation variant. Thus, there are four orientation 7M martensite variants and eight orientation NM martensite variants in one variant group. According to the crystallographic orientation of martensites and the crystallographic calculation, for NM martensite, the inter-plate interfaces are composed of compound twins in adjacent NM plates. The symmetrically distribution of compound twins results in the long and straight plate interfaces in the low relative contrast zone. The asymmetrically distribution leads to the change of inter-plate interface orientation in the high relative contrast zone. For 7M martensite, both Type-I and Type-II twin interfaces are nearly perpendicular to the substrate surface in the low relative contrast zones. The Type-I twin pairs appear with much higher frequency, as compared with that of the Type-II twin pairs. However, there are two Type-II twin interface trace orientations and one Type-I twin interface trace orientation in the high relative contrast zones. The Type-II twin pairs are more frequent than the Type-I twin pairs. The inconsistent occurrences of the different types of twins in different zones are originated from the substrate constrain. The crystallographic calculation also indicates that the martensitic transformation sequence is from Austenite to 7M martensite and then transform into NM martensite (A→7M→NM). [...]

A new materials system of a single layered Co(W) barrier/liner coupled with a Cu(Mn) alloy seed was investigated. Atom probe tomography visualized the sub-nanoscale structure of Cu(Mn)/Co(W) system, and thereby revealed Cu diffusion behavior of Co(W). Grain boundaries of Co were found to be the diffusion path, and successfully stuffed by W. Mn in Cu(Mn) also segregated to stuff the grain boundaries of Co. Combination of these two additives enabled high barrier property against Cu diffusion of Cu(Mn)/Co(W). Foreseeing tiny and high-aspect-ratio Cu interconnect features, Cu(Mn)/Co(W) was fabricated by ALD/CVD processes. To maximize the performance, minor impurities of the film incorporated from the ligand of the precursors were controlled by precursor selection. Thin, conformal, and smooth films were finally demonstrated onto a trench substrate.

Im Legierungssytem Ni-Mn-Ga tritt bei Zusammensetzungen nahe der stöchiometrischen Zusammensetzung Ni2MnGa der magnetische Formgedächtniseffekt auf. Darunter versteht man die Dehnung durch Bewegung von Zwillingsgrenzen im Magnetfeld. Einkristalle aus Ni-Mn-Ga mit einer tetragonalen 5M-Martensitstruktur zeigen magnetisch erzeugbare Dehnungen von bis zu 6 %. Diese großen Dehnungen verbunden mit der schnellen Schaltfrequenz von Magnetfeldern machen den Effekt interessant für technische Anwendungen z. B. als Aktoren. Derartige Einkristalle sind schwierig und teuer herzustellen, weshalb für technische Anwendung Polykristalle von Interesse sind. Diese lassen sich im Allgemeinen leichter und preiswerter herstellen. Um den magnetischen Formgedächtniseffekt in Polykristalle einzustellen, werden grobkörnige, texturierte Proben mittels des Verfahrens der gerichteten Erstarrung hergestellt. Die Gefügeuntersuchungen erfolgen mit metallographischen Schliffen und die Kornorientierungen werden mit der EBSD-Technik bestimmt. Um das Gefüge zu vergröbern, werden Glühungen nach einer aufgebrachten Warmverformung untersucht. Zur Verringerung der für die Bewegung der Zwillingsgrenzen notwendigen Spannung (Zwillingsspannung) werden die Proben im Druckversuch mechanisch trainiert. Dabei kann die Zwillingsspannung teilweise unter die magnetisch erzeugbare Spannung auf die Zwillingsgrenzen (Magnetospannung) abgesenkt werden. Eine weitere Absenkung der Zwillingsspannung wird durch eine plattenförmige Probengeometrie mit Dicken im Bereich der Korndurchmesser erreicht. An derartigen Proben wird magnetisch rückstellbare freie Dehnung durch Zwillingsgrenzenbewegung erzielt.

Manganese (Mn) doped III-V dilute ferromagnetic semiconductors (DFSs) are a candidate materials for semiconductor spintronics due to their intrinsic ferromagnetism mediated by holes. In this thesis, Mn doped III-V dilute ferromagnetic semiconductors (DFSs), including (Ga,Mn)As, (In,Mn)As, (Ga,Mn)P, and (In,Ga,Mn)As have been successfully prepared by ion implantation and pulsed laser melting. All (In,Ga,Mn)As films are confirmed to be well recrystallized and ferromagnetic while their Curie temperatures depend on the Ga concentration. (Ga,Mn)As and (Ga,Mn)P have an inplane easy axis, while an out-of-plane easy axis for (In,Mn)As is observed. However, all of them do not present strong in-plane uniaxial anisotropy between [110] and [110] directions, which always occurs in low temperature molecular beam epitaxy (LT-MBE) grown (Ga,Mn)As samples. The reason is ascribed to the fact that the ultra-fastrecrystallization induced by pulsed laser melting weakens the formation of Mn-Mn dimers along the [100] direction which occurs in LT-MBE grown (Ga,Mn)As. Then selected samples were co-doped with Zn or irradiated with He ions. The Zn co-doping leads to the increase of conductivity of (Ga,Mn)P, however both the Curie temperature and magnetization decrease, which is probably due to the suppression of active Mn substitution by Zn co-doping. By using Rutherford Backscattering Spectroscopy and Particle-Induced X-ray Emission, the substitutional Mn atoms in (Ga,Mn)As are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites. This is consistent with first-principles calculations, showing that the formation energy of substitutional Zn and interstitial Mn is 0.7 eV lower than that of interstitial Zn and substitutional Mn. For ion irradiated (Ga,Mn)As, (In,Mn)As and (Ga,Mn)P, both Curie temperature and magnetization decrease due to the hole compensation. However, the compensation effect is the strongest in (In,Mn)As and the least in (Ga,Mn)P. This is due to the different energy level of the produced defect relative to the band edges in different semiconductors. The results in the thesis point to an important issue: the difference in the band alignment and the hole binding energy of Mn dopants in different III-Mn-V dilute ferromagnetic semiconductors have strong influence on their magnetic properties and should be taken into account in the material design.

Aluminium is usually added to the continuous hot-dip galvanizing bath to improve coating ductility and adhesion through the rapid formation of a thin Fe-Al intermetallic layer at the substrate-liquid interface, thereby inhibiting the formation of brittle Fe-Zn intermetallic compounds. On the other hand, Mn is essential for obtaining the desired microstructure and mechanical properties in advanced high strength steels, but is selectively oxidized in conventional continuous galvanizing line annealing atmospheres. This can deteriorate reactive wetting by the liquid Zn(Al,Fe) alloy during galvanizing and prevent the formation of a well developed Fe-Al interfacial layer at the coating/substrate interface, resulting in poor zinc coating adherence and formability. However, despite Mn selective oxidation and the presence of surface MnO, complete reactive wetting and a well developed Fe-Al interfacial layer have been observed for Mn-containing steels. These observations have been attributed to the aluminothermic reduction of surface MnO in the galvanizing bath. According to this reaction, MnO is reduced by the bath dissolved Al, so the bath can have contact with the substrate and form the desired interfacial layer. Heat treatments compatible with continuous hot-dip galvanizing were performed on four different Mn-containing steels whose compositions contained 0.2-3.0 wt% Mn. It was determined that substrate Mn selectively oxidized to MnO for all alloys and process atmospheres. Little Mn surface segregation was observed for the 0.2Mn steel, as would be expected because of its relatively low Mn content, whereas the 1.4Mn through 3.0Mn steels showed considerable Mn-oxide surface enrichment. In addition, the proportion of the substrate surface covered with MnO and its thickness increased with increasing steel Mn content.A galvanizing simulator equipped with a He jet spot cooler was used to arrest the reaction between the substrate and liquid zinc coating to obtain well-characterized reaction times characteristic of the timescales encountered while the strip is resident in the industrial continuous galvanizing bath and short times after in which the Zn-alloy layer continues to be liquid (i.e. before coating solidification). Two different bath dissolved Al contents (0.20 and 0.30 wt%) were chosen for this study. The 0.20 wt% Al bath was chosen as it is widely used in industrial continuous galvanizing lines. The 0.30 wt% Al bath was chosen to (partially) compensate for any dissolved Al consumption arising from MnO reduction in the galvanizing bath.The Al uptake increased with increasing reaction time following non-parabolic growth kinetics for all experimental steels and dissolved Al baths. For the 0.20 wt% dissolved Al bath, the interfacial layer on the 1.4Mn steel showed the highest Al uptake, with the 0.2Mn, 2.5Mn and 3.0Mn substrates showing significantly lower Al uptake. However, increasing the dissolved bath Al to 0.30 wt% Al resulted in a significantly increased Al uptake being observed for the 2.5Mn and 3.0Mn steels for all reaction times. These observations were explained by the combined effects of the open microstructures associated with the multi-phase nature of an oxide-containing interfacial layer and additional Al consumption through MnO reduction. For instance, in the case of the 1.4Mn steel, the more open interfacial layer structure accelerated Fe diffusion through the interfacial layer and increased Al uptake versus the 0.2Mn substrate for the same bath Al. However, in the case of the 2.5Mn and 3.0Mn substrates and 0.20 wt% Al bath, additional Al consumption through MnO reduction caused the interfacial layer growth to become Al limited, whereas the very open structure dominated growth in the case of the 0.30 wt% Al bath and resulted in the changing the growth kinetics from mixed diffusion-controlled to a more interface controlled growth mode. A kinetic model based on oxide film growth (Smeltzer et al. 1961, Perrow et al. 1968) was developed to describe the Fe-Al interfacial layer growth kinetics within the context of the microstructural evolution of the Fe-Al interfacial layer for Mn-containing steels reacted in 0.20 wt% and 0.30 wt% dissolved Al baths. It indicated that the interfacial layer microstructure development and the presence of MnO at the interfacial layer had significant influence on the effective diffusion coefficient and interfacial layer growth rate. However, in the cases of the 2.5Mn and 3.0Mn steels in 0.20 wt% Al bath, the kinetic model could not predict the interfacial layer Al uptake, since the Fe-Al growth was Al limited. In fact, in these cases, additional Al was consumed for reducing their thicker surface MnO layer, resulted in limiting the dissolved Al available for Fe-Al growth. / Dissertation / Doctor of Science (PhD)

Medium Mn (med-Mn) third generation advanced high strength steels (3G AHSSs) are promising candidates for meeting automotive weight reduction requirements without compromising passenger safety. However, the thermal processing of these steels should be compatible with continuous galvanizing line (CGL) processing capabilities as it provides cost-effective, robust corrosion protection for autobody parts. Hence, the main objective of this Ph.D. research is to develop a CGL-compatible thermal processing route for a prototype 0.2C-6Mn-1.5Si-0.5Al-0.5Cr-xSn (wt%) (x = 0 and 0.05 wt%) med-Mn steel that will result in the 3G AHSS target mechanical properties (24,000 MPa%  UTS × TE  40,000 MPa%) and high-quality galvanized coatings via enhanced reactive wetting. It was found that the starting microstructure, intercritical annealing (IA) time/temperature, and Sn micro-alloying had a significant effect on the retained austenite volume fraction and stability and, thereby, the mechanical properties of the prototype med-Mn steel. For the as-received cold-rolled (CR) starting microstructure, the intercritical austenite nucleated and grew on dissolving carbide particles and resulted in blocky retained austenite. However, Sn micro-alloying significantly effected the intercritical austenite chemical stability by segregating to the carbide/matrix interface and retarding C partitioning to the intercritical austenite. This resulted in lower volume fractions of low stability retained austenite which transformed to martensite (via the TRIP effect) at low strains, thereby quickly exhausting the TRIP effect and resulting in a failure to sustain high work hardening rates and delay the onset of necking. Consequently, the Sn micro-alloyed CR starting microstructure was unsuccessful in achieving 3G AHSS target mechanical properties regardless of the IA parameters employed. Contrastingly, the CR starting microstructure without Sn micro-alloying was able to meet target 3G mechanical properties via intercritical annealing at 675 °C × 60 s and 120 s, and at 690 °C × 60 s owing to sufficiently rapid carbide dissolution and C/Mn partitioning into the intercritical austenite such that it had sufficient mechanical and chemical stability to sustain a gradual deformation-induced transformation to martensite and maintain high work hardening rates. On the other hand, the martensitic (M) starting microstructure produced higher volume fractions of chemically and mechanically stable lamellar retained austenite regardless of Sn micro-alloying. Intercritical annealing at 650 °C × 60 s and 675 °C × 60 s and 120 s produced 3G AHSS target mechanical properties. It was shown that the stable lamellar retained austenite transformed gradually during deformation. Furthermore, deformation-induced nano-twin formation in the retained austenite was observed, suggesting the TWIP effect being operational alongside the TRIP effect. As a result, a continuous supply of obstacles to dislocation motion was maintained during deformation, which aided in sustaining a high work hardening rate and resulted in a high strength/ductility balance, meeting 3G AHSS target properties. Based on these results, the martensitic starting microstructure without Sn micro-alloying and the M-675 °C × 120 s IA condition were chosen for the selective oxidation and reactive wetting studies. The selective oxidation study determined the effect of a N2-5H2-xH2O (vol%) process atmosphere pO2 (–30, –10, and +5 °C dew point (Tdp)) on the composition, morphology, and spatial distribution of the external and internal oxides formed during the austenitizing and subsequent intercritical annealing cycles. The objective of this study was to identify the process atmosphere for the promising M-675 °C × 120 s heat treatment that would result in a pre-immersion surface that could be successfully galvanized in a conventional galvanizing (GI) bath. The austenitizing heat treatment (775 °C × 600 s) used to produce the martensitic starting microstructure resulted in thick (~ 200 nm) external oxides comprising MnO, MnAl2O4, MnSiO3/Mn2SiO4, and MnCr2O4, regardless of the process atmosphere pO2. However, intermediate flash pickling was successful in dissolving the external oxides to a thickness of approximately 30 nm along with exposing metallic Fe in areas which contained relatively thin external oxides. Furthermore, extruded Fe nodules that were trapped under the external oxides were revealed during the flash pickling process. Overall, flash pickling resulted in a surface consisting of dispersed external oxide particles with exposed metallic substrate and extruded Fe nodules. This external surface remained unchanged during IA owing to the multi-micron (~ 2–8 µm) solute-depleted layer that formed during the austenitizing heat treatment. Subsequent galvanizing in a 0.2 wt% (dissolved) Al GI bath with an immersion time of 4 s at 460 °C was successful in achieving high-quality, adherent galvanized coatings through multiple reactive wetting mechanisms. The dispersed nodule-type external oxides along with exposed substrate and extruded Fe nodules on the pre-immersion surface facilitated direct wetting of the steel substrate and promoted the formation of a robust and continuous Fe2Al5Znx interfacial layer at the steel/coating interface. Additionally, oxide lift-off, oxide wetting, bath metal ingress, and aluminothermic reduction were operational during galvanizing. The galvanized med-Mn steels met 3G AHSS target mechanical properties. Overall, this Ph.D. research showed that it is possible to employ a CGL-compatible thermal processing route for med-Mn steels to successfully produce 3G AHSS target mechanical properties as well as robust galvanized coatings. / Thesis / Doctor of Philosophy (PhD) / One of the largest challenges associated with incorporating the next generation of advanced high strength steels into the automotive industry lies in processing these steels in existing industrial production lines. In that regard, a two-stage heat treatment with an intermediate flash pickling stage and process atmosphere compatible with existing industrial continuous galvanizing line technology was developed for a prototype medium-Mn steel. The heat-treated prototype steel met the target mechanical properties outlined for the next generation of advanced high strength steels. Furthermore, the heat treatment and process atmosphere utilised in this research produced a surface that facilitated the successful galvanizing of the prototype medium-Mn steel. This adherent and high-quality galvanized coating will provide robust corrosion protection if the candidate medium-Mn steel is used in future automotive structural applications.

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