Materials Science and Technology

Four kinds of nanocrystalline Mo(Si_1−xAl_x)₂ coatings with differing Al contents are prepared onto a Ti–6Al–4V substrates by a double cathode glow discharge apparatus. The microstructural features of the deposited coatings were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These coatings are composed of the equiaxed C40–MoSi₂ grains with the average grain size of ∼5 nm. Nano-indentation measurements indicated that the hardness H, elastic modulus E and the H/E or H³/E² ratio of the nanocrystalline Mo(Si_1−xAl_x)₂ coatings slightly increase with the increase in Al content. The tribological behaviour of the nanocrystalline Mo(Si_1−xAl_x)₂ coating sliding against a ZrO₂ ceramic ball at room temperature has been compared using a ball-on-disc type tribometer under unlubricated conditions. Experimental results showed that the specific wear rates of the nanocrystalline Mo(Si_1−xAl_x)₂ coatings decrease with increasing Al content and are dramatically reduced by more than 1–2 orders of magnitude over the uncoated Ti–6A1–4V. The enhancement of wear resistance of the nanocrystalline Mo(Si_1−xAl_x)₂ coatings by Al additions is correlated with the increased mechanical properties and the forming oxide layer by tribochemical reactions.

An attempt to investigate solid state diffusion bonding of Ti–6Al–4V and Co–28Cr–5Mo–0·3C alloys has been made. The results showed that the joint could not be successfully bonded at 800°C, even though longer period of holding times (>1 h) was used. However, at 850°C, successful joining was achieved at holding time of 1 h. It was found that increasing temperature up to 900°C gave rise to intermetallic component formation, mainly Co₃Ti, at the joining interface, which contributed to weak bonding strength. This technique provides a reliable method of bonding of Ti–6Al–4V to Co–28Cr–5Mo–0·3C alloys, which is very helpful in the development of biomaterial industry.

Carbon nanotubes (CNTs) possess exceptional mechanical properties and are therefore suitable reinforcements for composite materials. Nanotube efficiency in reinforcing the matrix depends on the CNT alignment, volume fraction and configuration, as well as matrix properties. In this investigation, finite element method (FEM) is used to investigate the effects of nanotube waviness ratio, volume fraction, and matrix modulus on properties of CNT based polymer nanocomposites. Nanocomposite mechanical properties are evaluated using a 3D nanoscale representative volume element. Models consisting of CNTs with different waviness ratios are created to investigate the effects of nanotube configuration on nanocomposite mechanical properties. Next, the effect of nanotube volume fraction on nanocomposite moduli of elasticity is investigated. Finally, the effects of matrix modulus are investigated by analysing models consisting of matrices with different moduli. The results of this investigation are compared with those found in the literature and good agreement is observed.

In the present study, the formation process of dislocation cells in cryorolled Zr has been studied. High density dislocations were introduced into pure Zr by employing cryorolling technique. Dislocation lamellas are yielded in cryorolled Zr at a high strain rate of  = 2·24 s⁻¹, and these lamellas fragment or bend into dislocation cells during annealing processes. Instead, in the cryorolled Zr yielded at a low strain rate of  = 0·59 s⁻¹, dislocation cells form by the fragmenting or the necking of dislocation tangle regions. The formation process of dislocation cells in cryorolled Zr is dependent on dislocation configurations induced in the metal.

We have reported first principles calculations based on the density functional theory within the local density approximation to explain the structural, electronic, elastic, vibration (phonon dispersion curves and one-phonon density of states) and optical properties for NbIrSn, which is semiconductor half Heusler compound with a cubic MgAgAs type structure. The obtained results are compared with the available other theoretical data. Our band structure calculations show that NbIrSn has indirect energy band gap of ∼1 eV at Γ-X Brillouin zone. The elastic properties, namely, bulk modulus, elastic constants, shear modulus, Young's modulus, Poisson's ratio, lame constants and Kleinmann parameter, are calculated under high pressure. The pressure effects for optical properties including the real and imaginary parts of the complex dielectric constant, refractive index n(ω), extinction coefficient k(ω), energy loss function L(ω) and reflectivity R(ω) are also computed and discussed.

A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ∼250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic (γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic (γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.

The microstructural evolution of Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C (at-) alloys fabricated by centrifugal investment casting is studied. It is found that both Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C are mainly composed of γ phase and a small amount of α₂ phase, and the microstructure is mainly fully lamellar in C containing alloy but again the C free alloy is mainly nearly fully lamellar. Nb distributes homogeneously in the γ and α₂ phases but is depleted in the interdendritic regions. The addition of C reduces the lamellar spacing of Ti–45·9Al–8Nb alloy. The mean interlamellar spacings between α₂ and α₂ of Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C alloys are about 950 and 700 nm, and those between γ and α₂ lamellae are about 320 and 220 nm respectively. Oil cooling from the α phase field after 1h treatment at 1360°C leads to massive transformation to γ_m for both Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C alloys. Subsequent aging in the α+γ phase field results in α phase precipitates within the γ_m phase, which leads to convoluted microstructures. However, the C addition retards the precipitation of α₂ phase.

As rolled TC21 titanium alloy was subjected to isothermal constant strain rate tensile tests using an electronic tensile testing machine. After tensile deformation, the alloys were subjected to double annealing. Superplastic behaviour and microstructure evolution were systematically investigated. Experimental results show that as rolled TC21 alloy exhibits good superplasticity at temperatures ranging from 870 to 930°C and strain rates ranging from 3×10⁻⁴ to 3×10⁻² s⁻¹. A maximum elongation of 373·3 was obtained at 910°C and 3×10⁻⁴ s⁻¹. In addition, the alloy microstructure comprises α and β phases during plastic deformation. The primary α-grains aggregate and merge to form new crystal grains with irregular grain boundaries because of dynamic recrystallisation. Furthermore, the primary α phase content gradually decreases with increasing temperature. The resulting microstructure after deformation and double annealing is a duplex microstructure comprising a primary equiaxed α phase and a β-transformed lamellar structure. The acicular α phase transformed from the β phase is mutually interlaced as a basketweave structure after deformation at 930°C and double annealing.

The tempering resistance and stability of retained austenite in superbainitic and quenching–partitioning martensitic steels were investigated over the temperature range of 400 to 700°C. The X-ray diffraction analysis and hardness tests showed that the quenching–partitioning martensitic steel contained a considerable amount of retained austenite (26·6 vol.-) and had a relatively high hardness up to 556 HV1 after tempering at ∼600°C. In contrast, the fraction of retained austenite and hardness of superbainitic steel were considerably lower (24·5 vol.- and 385 HV1) after the same tempering cycle. The present work also showed that the quenching–partitioning steel had a higher tempering stability, probably due to the higher fraction of carbon rich retained austenite.

A stir casting process was developed to produce aluminium alloy composites containing two different sizes and volume fractions of B₄C particles up to 10 vol.-. Manufacturing of B₄C particle reinforced 2024 aluminium alloy composites was modified so as to reduce the processing temperature. In the present study, in order to improve incorporation of the B₄C particles by 2024 aluminium alloy melt, a novel pretreatment process before stir casting was attempted, and a significant improvement was gained. This finding showed that the treatment modified the surface condition of boron carbide powders via the removal of oxides. The comparison of added and incorporated particle ratios (the yield rate) indicated that the amount of incorporated particles decreased with increasing volume fraction and decreasing size of particles. Scanning electron microscopic observations of the microstructures revealed that the dispersion of the coarser sizes of particles was more uniform, while finer particles led to agglomeration of the particles and porosity. The results showed that the density of the composites decreased with increasing particle volume fraction and decreasing particle size, whereas the porosity and hardness values increased with increasing particle volume fraction and decreasing particle size.

Texture development in commercially pure titanium during equal channel angular pressing (ECAP) through route C has been studied up to four passes at room temperature. Textures were measured by X-ray diffraction, while the microstructural analyses were performed using electron backscattered diffraction as well as transmission electron microscopy. The results show that continuous dynamic recrystallisation is observed during the ECAP process and the B fibre is formed after four passes, which represents normal to shear plane.

A new integrated physically based constitutive model was developed for an age hardenable Al–Mg–Si alloy. The kinetics of precipitation during various stages of aging was modelled. The precipitate features consisted of particle radius and volume fraction obtained from the kinetics model, which was used to compute the alloy yield strength/hardness. A published multiinternal variable workhardening model was improved to take into account the effects of solute solution and precipitates on the alloy hardening capacity after performing different cycles of aging treatment. The flow curves and hardness predicted by the model were in good agreement with the experimental results. The model is able to predict the microstructure evolution such as cell/subgrain size and dislocation density in the cell interior during the plastic straining of material at different temper conditions.

To investigate the influence of temperature on nucleation and growth of Ti₃SiC₂, Ti–Si–C thin films were deposited by magnetron sputtering from elemental targets of Ti, Si and C on Si(100) and Al₂O₃ substrates at temperature <200°C. Subsequently, the as deposited films were annealed in vacuum at 800, 950, 1100 and 1200°C respectively. The as deposited films consisted of amorphous TiC, amorphous Si and free C, as determined by X-ray diffraction and X-ray photoelectron spectroscopy. Annealing in vacuum <950°C resulted in improved crystallinity of TiC and formation of SiC_x and Ti₅Si₃ phases. However, the Ti₃SiC₂ phase forms in films at 1100°C owing to the increase in Si diffusion coefficient. Moreover, the evolution of hardness and elastics modulus with annealing temperatures was determined by nanoindentation. The results showed a continuous decline of film hardness with increasing annealing temperature due to the formation of Ti₃SiC₂ and Ti₅Si₃ phases.

The production of robust joints after diffusion brazing necessitates the advanced understanding of phase transformations during the bonding process. This paper aims to investigate the solidification and the solid state precipitation during diffusion brazing of wrought IN718 nickel base superalloy using Ni–15Cr–4B (wt-) filler alloy. It was found that intermetallics containing eutectic type microconstituents were formed in the joint centreline by solidification which is controlled by segregation behaviour of B and its low solubility in Ni rich solid solution. In addition, extensive Cr–Mo–Nb rich precipitates were formed in the substrate region by solid state precipitation induced by B diffusion into base metal. The implications of the phase transformations on the joint properties are discussed.

Hysteresis, crystal structure and chemical composition of thin films deposited through reactive sputtering of titanium metal target in Ar/CH₄/N₂ gas mixture have been investigated. The transition from metallic to compound sputtering mode was clearly seen as the reactive gases (CH₄ and N₂) flowrate concentration first increased and subsequently decreased. Abrupt cathode current drop from 273 mA to reach a minimum value of 195 mA was observed upon addition of nitrogen gas from 0 to 10 flowrate concentration to the Ar/CH₄ gas mixture. This was also accompanied by an abrupt change in reactive gas partial pressure. Exploration of the deposition rate and film thickness showed that it decreased from 4·5 to 1·5 nm min⁻¹ and from 140 to 40 nm as the N₂ flowrate concentration increased from 1·5 to 7·5 at 5·5CH₄ flowrate concentration respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses of the deposited films confirmed the formation of titanium carbide and carbonitride phases as the methane and nitrogen gas concentrations in the sputtering gas were increased.

Machinability of grey cast iron parts can greatly suffer from the formation of carbide at the surface, resulting in a decrease in cutting tool life and higher production costs. Therefore, detection of the hardened layer and its hardness are the key factors in quality control and inspection processes. In the present paper, a number of metallurgical parameters (surface carbide, surface hardness and hardened depth) have been investigated using the non-destructive differential eddy current technique. The results show the high potential of the proposed method as a fast and accurate technique in inspecting and in consequent separation of undesirable parts.

Rapid water quenching experiments were first carried out according to differential scanning calorimetry results, and then the microstructure and phase composition of such water quenching Cu–8·33Ni–1·67Si (wt-) samples were investigated. Subsequently, calculation on the competition nucleation of the primary phase was also carried out. Based on the experimental and calculational results, the phase transformation behaviour of Cu–8·33Ni–1·67Si melts during the cooling process was discussed, and the results can be summarised as follows. First, the α-Cu phase formed as a primary phase due to the shorter incubation time and higher time dependent nucleation rate. Subsequently, a eutectic reaction occurred in the interdendritic residual liquid phase of the primary α-Cu grains, and its product was α-Cu+Ni₃Si. The third and fourth phase transformation could be described to the precipitation of the δ-Ni₂Si phase in the region close to the grain boundary phase and the internal of the primary α-Cu grains respectively.

Accumulative roll bonding (ARB) has been used as a severe plastic deformation process for the production of high strength materials. In the current research, multilayered copper/nickel composites were produced by the ARB process using nickel and copper strips. Tensile and magnetic behaviours of produced composites were investigated by universal tensile machine and magnetic device detector (vibrating sample magnetometers) respectively. It was observed that as passes of ARB proceeded, nickel layers were necked and fractured gradually. After five roll bonding passes, a multilayer copper/nickel composite including homogeneously distributed fragmented nickel layers in the copper matrix was achieved. Magnetic and mechanical properties of these composites were studied within different stages of the ARB process. With increasing strain during ARB passes, strength, microhardness and elongation of these composites increased. Enhancement of the strength is higher than the tensile strength of copper/copper multilayered strips produced by the ARB process. In addition, it has been found that the magnetic properties, e.g. retentivity, coercivity and magnetic saturation of the multilayer composite, were significantly influenced by the changes in microstructural and grain size.

Alloys of AA 5182 are commonly used in the automotive industry to provide weight reductions in vehicle chassis. The strength of such alloys is based on the Mg content, with Mg causing the precipitation of a β-Al₈Mg₅ phase along the length of the grain boundaries in the microstructure. These precipitates are vulnerable to intergranular corrosion (IGC) in acidic environments. For this reason, the proportions of Mg in the microstructure must be limited, with >3 wt-Mg causing intergranular attack. The alloys used in this study had an Mg content of 4·43, so were vulnerable to IGC. Standard nitric acid mass loss tests were applied to determine the level of corrosion resistance in two samples subjected to various treatments. Using ‘five parameter’ analysis, the importance of the grain boundary plane and thermomechanical history to IGC resistance was determined. ‘Special’ planes were identified in the microstructure and their impact on IGC resistance was assessed. From such analysis, it was found that a high deformation level of 50 together with a simulated batch annealing heat treatment yielded the highest proportions of ‘special’ boundaries within the microstructure. ‘Special’ boundaries in this study were identified to be low angle boundaries, <111> twist boundaries and <110> tilt boundaries, which are associated with a reduced energy compared to random high angle boundaries. As precipitation generally occurs along high energy boundaries, ‘special’ boundaries reduce precipitate formation, thus reducing IGC intensity. Future implications include the introduction of a higher proportion of ‘special’ boundaries into the microstructure, allowing for an increase in Mg content in these alloys.

Influence of microstructural evolution on the mechanical behaviour of AZ31 alloy sheet processed by flat extrusion container is analysed upon annealing treatments. The basal texture is significantly increased as a result of the formation of new grains with a largely altered c axis orientation relative to the initial basal orientation owing to static recrystallisation. The results reveal that the ductility of the alloys without annealing treatments is the highest, but the strength is the lowest, which confirms the strong anisotropy of stress–strain behaviour with respect to the initial textures. Significant differences in the stress–strain curves are observed for the as received and annealed specimens.

The microstructures and properties of fresh, artificial aged and natural aged flux cored ZnAl15 wires were investigated by scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectrometer techniques. The results of SEM showed that continuous precipitation of η_s phase occurred and the dissolved contents of both α_s phase and η_s phase reduced during the aging process of ZnAl15 wire, which led to the increase in Vickers microhardness. The results of the EDX presented that the oxygen absorption corrosion led to the embrittlement of ZnAl15 wire, which decreased the number of reverse bends. In addition, the phase transformation stress accelerated the corrosion, and finally, the natural aged ZnAl15 wire resulted in stress corrosion crack under the comprehensive effects of corrosion and phase transformation stress.