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To improve the corrosion resistance and impart the bioactivity and antibacterial functionalities to titanium alloys for biomedical applications, α-(Nb1-xAgx)5Si3 compounds, with three different Ag contents, were deposited on Ti–6Al–4V by double cathode glow discharge method. The new α-(Nb1-xAgx)5Si3 coatings, each about 16 μm in thickness, exhibited a compact uniform structure, composed of equiaxed nanocrystalline grains. Nanoindentation measurements revealed that Ag alloying reduced the hardness (H) and elastic modulus (E) values of the α-(Nb1-xAgx)5Si3 coatings. Compared to Ti–6Al–4V, the α-(Nb1-xAgx)5Si3 coatings exhibited improved corrosion resistance in Ringer's physiological solution, as well as newly added in-vitro bioactivity and antibacterial ability against both E. coli and S. aureus. A MTT assay experiment shows that the two α-(Nb1-xAgx)5Si3(x = 0 and 0.05) coatings were as non-cytotoxic as bare Ti-6Al-4V. However, the MC3T3-E1 osteoblast-like cell viability for the α-(Nb0.9Ag0.1)5Si3 coating was slightly reduced, suggesting that the proliferation of MC3T3-E1 osteoblast-like cell was affected to some degree by excess Ag added to α-Nb5Si3. Overall, the α-(Nb0.95Ag0.05)5Si3 coating provides an attractive combination of good bioactivity, high corrosion resistance and new antibacterial function, and thus demonstrates great potential for applications in orthopedic devices.

Structural thin film coatings have enabled technological advances in a range of industrial applications by providing protection against wear and corrosion for tools used in machining and forming. In this thesis, the microstructure, mechanical properties and tribological behaviours of a range of nitride and high entropy alloy coatings, deposited by magnetron sputtering, were investigated. First, the influence of Ni content, controlled by the NiCr alloy target current (INiCr), on the structure and properties of CrAlNiN coatings was studied. These coatings exhibited a columnar structure at lower INiCr currents (< 3 A), but this changed to a more equiaxed structure at higher currents (3-5 A). In addition, increasing Ni content in the coatings led to a decrease both in hardness (H) and elastic modulus (Er). Effects associated with high residual compressive stresses, solid solution hardening and grain refinement were suggested to account for the high hardness (25-28 GPa) values observed in the low-Ni-content (< 7 at. %) coatings. Furthermore, the coatings deposited at lower INiCr currents exhibited higher H/Er and H3/Er2 ratios and exhibited improved scratch toughness, adhesion strength and superior wear resistance. In contrast, coatings deposited at higher INiCr currents were more susceptible to grain pull-out and less wear-resistant. Finally, a series of high entropy alloy FeMnNiCoCr coatings were deposited with either varying substrate bias voltage (-20, -60, -120 V) or N2 flow rate (4, 8, 15, 25 sccm), where the latter coatings formed nitride-based structures. It was demonstrated that an increasing substrate bias voltage caused a microstructural evolution from a fine elongated structure to coarse V-shaped columnar grains. Increasing bias voltage also promoted a fcc-to-hcp phase transformation. Coatings deposited at -120 V exhibited a high hardness of ~9.1 GPa and excellent damage-tolerance properties, attributed to the formation of nanotwins within a dual-phase struct

Thermal spray describes a broad family of coating techniques, which is used for surfacing and resurfacing of engineering components without compromising the substrate’s material properties. In thermal spray, the coating particles impact the substrate surface either in the molten or semi-molten state, depending on the spray process used. The bonding and adherence of the spray particles (called splats) to the substrate surface significantly influence the overall quality of the finished product. This thesis investigates the deposition behaviour of various feedstock particles, including Ni, Ni-20Cr, Ni-5Al and a Fe-based metallic glass sprayed on to different substrates, including mild steel, stainless steel and chromium subjected to different surface pre-treatments (polishing, oxidation and thermal). The splat-substrate bonding mechanisms were investigated across different spray methods such as plasma spray, high-velocity oxy-fuel (HVOF) and cold spray (CS) through a range of microscopy techniques. Substrate preheating, prior to deposition, lowered the moisture contents on the substrate surface and reduced splashing and fragmentation and promoted the formation of disk-shaped splats by improving substrate wettability. Substrate surface chemistry and thermal properties (in particular thermal conductivity) influenced the splat solidification process and hence, the splat morphology. In the plasma spray process, the feedstock particles impacted the substrate in a fully molten state, so splat-substrate interfacial bonding was metallurgical and localised substrate melting, along with interdiffusion, was observed between the splat and the substrate. The addition of Cr to Ni promoted better adherence of the splat to the substrate and modified splat formation by reducing splat fragmentation. Substrate melting resulted in ‘jets’ of substrate material entrapped within the splat. Local oxidation of the splats occurred in flight resulted in oxide layers around splats. Whereas, in th

© 2020, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature. The mechanical size effect of nanostructured, dual-phase CrCoNi medium-entropy alloy (MEA) was investigated by combining in-situ micro-compression testing with post-mortem electron microscopy analysis. The alloy possesses a superior yield strength up to ∼4 GPa, primarily due to its hierarchical microstructure including column nanograins, preferred orientation, a high density of planar defects and the presence of the hexagonal close packed (HCP) phase. While the yield strength of the alloy has shown size-independency, the deformation behaviour was strongly dependent on the sample size. Specifically, with decreasing the pillar diameters, the dominant deformation mode changed from highly localized and catastrophic shear banding to apparently homogeneous deformation with appreciable plasticity. This transition is believed to be governed by the size-dependent critical stress required for a shear band traversing the pillar and mediated by the competition between shear-induced softening and subsequent hardening mechanisms. In addition, an unexpected phase transformation from HCP to face-centered cubic (FCC) was observed in the highly localized deformation zones, leading to strain softening that contributed to accommodating plasticity. These findings provide insights into the criticality of sample dimensions in influencing mechanical behaviors of nanostructured metallic materials used for nanoelectromechanical systems.

More than 37,000 new aircraft are required by 2037 as air traffic has been predicted to double every two decades. Improved fuel efficiency is crucial for next generation aircraft engines to extend aircraft range and speed. Aircraft engine turbine discs are commonly made of Alloy 718, a polycrystalline, precipitation-strengthened nickel-based superalloy. High-temperature strength, creep resistance and corrosion resistance allow operational temperatures of 650 °C for service cycles of several 100,000 h under high mechanical workloads.Conventional processing of Alloy 718 turbine discs involves multi-step forging, solution annealing, and dual ageing. Direct ageing is an alternative process to achieve yield strength increments of > 10 % by omitting solution annealing. However, the detailed processing-microstructure-property relationships during direct ageing of Alloy 718 are not fully understood.The microstructure of Alloy 718 contains a γ- (FCC) matrix with a grain size of ASTM 10 for grain boundary and solid solution strengthening. Micronscale δ- (Ni3(Nb), D0a) phase platelets decorate grain- and twin-boundaries for optimal creep strength. High dislocation densities in the grain interior provide work hardening. Nanoscale γ'- (Ni3(Al,Ti), L12) and γ''- (Ni3(Nb),D022) precipitates in duplet and triplet morphology provide the high-temperature strength of Alloy 718.This study provides a correlative high-resolution microscopy approach to reveal the microstructural evolution in Alloy 718. Comparing conventional versus direct ageing demonstrates that high retained dislocation densities and Nb concentrations in solid solution yield inverted precipitate morphologies. Atom probe microscopy techniques such as the enhanced iso-surface method and the interface method are developed to quantify γ'- and γ''-precipitate volume fractions, particle radii, number densities, and co-precipitate morphologies. Slices of forged turbine disc materials are characterized to clarify the occurrence

In this study, a β-Ta2O5 coating was deposited onto a Ti–6Al–4V substrate using a double-cathode glow discharge plasma technique. The hardness and elastic modulus values of the β-Ta2O5 coating were almost 4 times and 1.8 times greater than that of the Ti–6Al–4V substrate, respectively. Potentiodynamic polarisation curves indicated that the current densities of the β-Ta2O5 coated sample are least one order of magnitude less than that observed for the uncoated substrate under a solid/liquid slurry flow. Electrochemical impedance spectroscopy results showed that the resistance values for the β-Ta2O5 coating are of the order of 106 Ω cm2, and at the same erosion time, the resistance value for the β-Ta2O5 coating is one order of magnitude greater than that of the Ti–6Al–4V.

Engineering microstructures in high strength low alloy (HSLA) steels via advanced thermo-mechanical processing (aTMP) is a promising approach to overcome challenges around low work hardenability and toughness in ultrafine-grained mild steels. Thus, the applicability of advanced thermo-mechanical processing for achieving multi-scale hierarchical microstructures in a HSLA steel was studied. The microstructural evolution during warm deformation of a martensitic/bainitic starting microstructure using a Gleeble 3500 thermo-mechanical simulator at 600°C followed by a direct aging step was investigated. A strain rate of 10 s-1 during a single pass plane strain compression led to strain localisation and, therefore, the formation of a macroscopic shear band. Hence, an optimised advanced multi-hit thermo-mechanical process for achieving homogenous hierarchical microstructures without strain localisation was developed. Ultrafine crystallites confined by a mixture of high angle gain and subgrain boundaries are formed, decorated by two types of precipitates. Large FeMnC-rich cementite particles are found on grain boundaries and smaller TiNbC-rich precipitates on dislocations and subgrain boundaries. The further aim of this study was the investigation of the mechanical properties by upscaling the process developed in the Gleeble using a Hille 100 rolling mill. It was found that rolling to reduction of thickness of 55% at a temperature around 650°C can lead to an ultimate tensile strength (UTS) of 650 MPa, a yield ratio of 0.95 and a total elongation of 14% in the as-rolled condition. Delaminations did occur in the lower temperature region of the Charpy impact testing in longitudinal and transversal directions. Finally, a similar processing was implemented to invoke grain refinement coupled with strengthening arising from microalloying. It was found that Mo leads to an increase in hardness of ~20% compared to the base alloy, whereas Cr provides only a minor hardening increment. It

Hard thin film coatings play an important role in many industrial applications, such as cutting tools and biomedical devices. Magnetron sputtering, as one important deposition method, has been widely applied to a wide range of coating types. In this thesis, the microstructure and mechanical properties of a number of different families of coatings, deposited by magnetron sputtering, were investigated in detail. First, the influence of reactive gas ratio on the microstructure and mechanical properties of Ni-Ti-O-N nanocomposite coatings was studied. It was shown that an increase in reactive gas flow ratio resulted in a transformation from a dense structure with columnar grains into a porous structure with equiaxed grains, accompanied by a degradation in mechanical properties. It was shown that intergranular porosity had a detrimental effect on both hardness and elastic modulus. Further, the effect of substrate bias on the TiAlSiN coatings deposited at ~ 500°C was investigated. Variations in substrate bias had little impact on the chemical concentration of the coatings, but significantly influenced the microstructure, phase composition and mechanical properties. An evolution of microstructure from coarse to fine columnar grains with increasing bias voltage was identified, together with enhanced hardness and elastic modulus. Further, a transformation from a mixed fcc TiN + fcc AlN phases into a single fcc TiAlN phase was observed with increasing substrate bias. Finally, a series of equiatomic medium entropy alloy CoCrNi coatings, including both monolayer coatings and a tri-layered CoCrNi/Ti coating, was deposited onto M2 substrates. It was found that the monolayer CoCrNi coatings were composed of mostly a fcc CoCrNi phase, together with a small amount of a hcp Co-based phase. Interestingly, for the tri-layered coating, the outmost layer exhibited a fcc structure, while a bcc structure was observed in both the bottom and middle layers. The transition from fcc to bcc phas