Materials Science and Technology

Sources of anisotropy in metals are discusssed as well as ways of interpreting and modelling this behaviour. A number of case studies are presented relating to mechanical and magnetic properties.

In this investigation, the effective mechanical properties of fullerene nanocomposites considering interface effects were characterised. Load transfer in nanocomposite materials is achieved through the fullerene/matrix interface. Thus, to determine nanocomposite mechanical properties, the interface behaviour must be determined. A single fullerene and the surrounding polymer matrix are modelled. Two cases of perfect bonding and an elastic interface are considered. Two models are suggested for elastic interface. The first elastic interface model consists of a thin layer of an elastic material surrounding the fullerene. In the second elastic interface model, a series of spring elements are used as the fullerene/matrix interface. The results of numerical models indicate the importance of adequate interface bonding for a more effective strengthening of polymer matrix by fullerene. Also, Young's modulus prediction for fullerene in epoxy matrix is compared to experimental data investigated by Rafiee et al. (2011), and good agreement is observed.

The effect of carbon on the microstructure and mechanical properties of lightweight steel based on Fe–7 wt-Al produced by air induction melting with flux cover is investigated. The ingots were hot worked to plates and were characterised. Steel containing 0.02 wt-C exhibited a single phase microstructure Fe–Al(α), whereas steel containing 0.5 and 1.0 wt- carbon exhibited a two-phase microstructure containing significant amounts of Fe₃AlC_0.5 precipitates in Fe–Al(α) matrix. Microhardness of the matrix decreases with increasing carbon content due to depletion of aluminium from the matrix to form Fe₃AlC_0.5 carbides. The bulk hardness, room temperature strength increases and tensile elongation decreases with increasing carbon content. However, at 873 K the improvement in strength as well as creep properties with increasing carbon content is marginal.

Surface textured Si doped TiCN coatings were synthesised on Ti–6Al–4V alloy by laser cladding technique. Phase constituent examination by X-ray diffraction revealed the formation of similar phases of TiC_0.2N_0.8 and Ti₅Si₄ within all coated samples. Laser coated samples presented much higher surface free energy compared to Ti–6Al–4V control due to the textured structure, which in turn demonstrated a better wettability and improved biomineralisation. Variation of silica content presented no significant influence on surface free energy, indicating that the silica content can be varied in a large range. The mineralised samples obtained after immersion in simulated body fluid were characterised to understand the mechanism and kinetics of Ca–P precipitation. The results confirmed that the precipitation kinetics of Ca–P was influenced by the substitution of silica.

Eglin steel is a new ultrahigh strength steel that has been developed at Eglin Air Force Base in the early 2000s. This steel could be subjected to a variety of processing steps during fabrication, each with its own thermal history. This article presents a continuous cooling transformation diagram developed for Eglin steel to be used as a guideline during processing. Dilatometry techniques performed on a Gleeble thermomechanical simulator were combined with microhardness results and microstructural characterisation to develop the diagram. The results show that four distinct microstructures form within Eglin steel depending on the cooling rate. At cooling rates above ∼1°C s⁻¹, a predominately martensitic microstructure is formed with hardness of ∼520 HV. Intermediate cooling rates of 1 to 0.2°C s⁻¹ produce a mixed martensitic/bainitic microstructure with a hardness that ranges from 520 to 420 HV. Slower cooling rates of 0.1 to 0.03°C s⁻¹ lead to the formation of a bainitic microstructure with a hardness of ∼420 HV. The slowest cooling rate of 0.01°C s⁻¹ formed a bainitic microstructure with an apparent carbide constituent at the prior austenite grain boundaries.

Novel friction stir welding (FSW) technique, characterised by big concave upper and small convex lower shoulders, for aluminium hollow extrusion was studied. Assisted with the lower shoulder, root flaws due to the lack of tool penetration have been eliminated. The tensile strength increased with increasing welding speed. As the welding speed increases from 50 to 200 mm min⁻¹, the width of the welding nugget zone (WNZ) decreases, and the ductile fractured location occurred at WNZ instead of heat affected zone (HAZ) adjacent to thermomechanically affected zone (TMAZ). The interface between the TMAZ and HAZ exhibited the lowest microhardness. The results indicated that the novel FSW method has the potential to join tubular structures and hollow profiles widely used in transportation industries.

Microstructure evolution for an Al–Zn–Mg–Cu alloy has been investigated under different non-linear cooling processes from the solution temperature. Combined with in situ electrical resistivity measurements, transmission electron microscopy, hardness tests and Newtonian cooling equations, a full continuous cooling precipitation diagram for the experimental alloy has been developed. The critical cooling rate is between 23 and 46°C s^–1 at the transformation temperature and the average cooling rate, , is between 20 and 35°C s^–1. The phase transformation during the non-linear cooling process can be concluded as: S phase precipitated in the high temperature transformation stage, η phase heterogeneously precipitated on Al₃Zr dispersoids and grain boundaries during the medium temperature transformation stage and homogeneously nucleated η′/η phase precipitated at the low temperature transformation stage.

New magnesium nanocomposites reinforced with copper–graphene nanoplatelet hybrid particles have been prepared through the semipowder metallurgy method. Compared with the monolithic Mg, the Mg–1Cu–xGNPs nanocomposites exhibited higher tensile and compressive strength. In tension, nanocomposites revealed substantial enhancement in elastic modulus, 0.2 yield strength, ultimate tensile strength and failure strain (up to +89, +117, +58 and +96 respectively) compared to monolithic Mg. In compression, the nanocomposites showed the greatest improvement in 0.2 yield strength, and the ultimate compressive strength and failure strain () (up to +34, +59 and +61 respectively), whilst the compressive elastic modulus first increases and then decreases with an increase in the graphene nanoplatelets (GNPs) contents. The enhanced strength of the composites is likely to result from strengthening mechanisms invoked by the addition of Cu–GNPs hybrids.

In this work, the 18Cr–2Mo ferritic stainless steel was treated with and without warm rolling at 573 K with 66.5 reduction after conventional hot rolling process. It was shown that ductile-to-brittle transition behaviour could be closely related to deformation twinning and parameters affecting critical temperature for twinning would also inevitably affect ductile-to-brittle transition temperature. This correlation between ductile-to-brittle transition behaviour and twinning was in good agreement with the Cottrell–Petch model. A lowered transition temperature and an improved toughness after introducing warm rolling process and corresponding annealing process could be mainly explained in terms of refining the recrystallised grains and reducing the volume fraction of grains having orientations favourable for twinning, which decrease the critical temperature at which twins form.

An in situ magnetic device was employed to quantify the deformation induced martensite in a Fe–19Cr–3Mn–4Ni–0.15C–0.15N (wt-) steel during tensile testing in the temperature range of −40 to 22°C. The new device consists of an electromagnetic field which serves to magnetise the martensite phase as it forms during tensile loading and a second coil to detect the effective electrical potential difference induced by the magnetisation of tensile specimens. To implement the in situ measurement system, a correlation was necessary between the induced electrical potential difference and the deformation induced martensite fractions during uniaxial static tensile tests. The correlation procedure was found to require only the quantification of deformation induced martensite content in a tensile specimen strained until fracture using an ex situ magnetic saturation unit.

‘Second Generation’ ductile iron with a silicon content of up to 4.3 wt- exhibits a fully ferritic matrix, which is solution strengthened by silicon. Outstanding advantages of these ductile iron grades result in their strongly increasing demand. However, due to a presumed formation of a silicon long range order, the maximum strength is limited to 600 MPa at 4.3 wt- silicon. At higher silicon content, the mechanical properties dramatically decrease. In order to increase the maximum achievable strength, the potential of additional solution strengthening elements is subject of present research. Initially, the effects of cobalt and nickel on matrix, graphite shape and nodule count are investigated. Cobalt and nickel are identified as promising candidates for further solid solution hardening.

Small punch tests (SPTs) have been carried out at room temperature to correlate the microstructural variation of Ti–6Al–4V alloy with that of SPT parameters. Microstructural variation in terms of different volume fractions of primary alpha phase of Ti–6Al–4V alloy has been introduced as a result of solution annealing at different temperatures followed by thermal aging. Small punch test parameters, i.e. total area under the load vs displacement curve, area under the zone of elastic bending, plastic bending and plastic instability have been found to increase from the content of 10 primary alpha phase to 20 primary alpha phase and then these are decreasing from the content of 20 primary alpha phase to 30 primary alpha phase.

Microstructure observation and texture analysis of two AA3xxx hotbands made by twin belt casting route and a proprietary pellet casting and compacting process were carried out respectively with the aid of scanning electron microscopy and orientation distribution function from X-ray diffraction. The textures and microstructures of the two alloys made by the two processes were systematically studied and discussed, which showed significant differences through hotband thickness in individual alloys. The results as obtained in the present investigation may provide practically useful reference for the microstructures and textures control and optimisation in production of hotband of aluminium alloys and final downstream rolling products.

Titanium dioxide nanostructures have been synthesised by annealing the Pd loaded titanium foil at 680°C under the atmosphere of Ar or Ar–H₂ gas mixture. The microstructure and production of titanium dioxide are found to depend critically on the concentration of Pd catalyst and the Ar–H₂ atmosphere. The catalyst of Pd and the introduction of 5H₂ gas are the key factor for the formation of the long (about 100 μm in the average) and smooth titanium dioxide nanowires at low temperature. Scanning electron microscope and X-ray diffraction show that they are all rutile phase. The further result shows that the titanium dioxide nanowires possess good crystallinity and high surface photovoltage response.

Recently, valuable combinations of mechanical properties with strength of 1.9 GPa accompanied by very decent ductility of 19 and toughness of 31 J, have been achieved in a set of nanostructured bainitic steels. However, it is necessary to elucidate the significance of various microstructural features responsible of that extraordinary mechanical response in more detail. Thus, using two steels, with different Mn, Ni and V contents, and changing the nanostructured bainite isothermal transformation temperatures (200–300°C), has led to a plethora of subtle and essential microstructural variations, necessary to explain how the mechanical response of nanostructured bainite is attained.