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The impact velocity of particles during the cold spray process is crucial to the optimisation of coating quality and spraying costs. In the present investigation, both underexpanded and overexpanded impinging jets are employed to accelerate Aluminium particles towards a substrate. The impact velocity and angle statistics are generated by injecting polydisperse particles into the jet and the particle dynamics are characterised using the velocity and trajectories of the particles. The optimum particle size corresponding to the maximum impact speed is recast in terms of the Stokes number and shown to have a value of approximately one. Finally, a normal shock model is proposed which may be employed to estimate the particle impact speed using the nozzle exit conditions. It is shown that owing to artificial viscosity associated with the total variation diminishing scheme, this model tends to underestimate the speed.

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements.

This article discusses the competing mechanisms of martensite formation vs eutectoid decomposition via pearlitic or bainitic mechanisms during continuous cooling of a Ti-5 wt pct Cu hypoeutectoid alloy, which falls under the category of active eutectoid systems. Faster cooling rates result in a mixed microstructure of nanoscale bainite consisting of a far-from-equilibrium Ti 2 Cu phase and martensitic alpha plates, as determined from three-dimensional atom probe (3DAP) coupled with energy-filtered transmission electron microscopy (EFTEM). Slower cooling resulted in near-equilibrium eutectoid-based microstructures.

An austenitic Ni-30 wt.% Fe alloy, with a stacking-fault energy and deformation characteristics similar to those of austenitic low-carbon steel at elevated temperatures, has been used to examine the defect substructure within austenite deformed by single-pass strip rolling and to identify those features most likely to provide sites for intragranular nucleation of ultrafine ferrite in steels. Samples of this alloy and a 0.095 wt.% C-1.58Mn-0.22Si-0.27Mo steel have been hot rolled and cooled under similar conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction, and x-ray diffraction. Following a single rolling pass of approx40% reduction of a 2 mm strip at 800 deg C, three microstructural zones were identified throughout its thickness. The surface zone (of 0.1-0.4 mm in depth) within the steel comprised a uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation cells, with an average diameter of 0.5-1.0 mu m. The scale and distribution and, thus, nucleation density of the ferrite grains formed in the steel were consistent with the formation of individual ferrite nuclei on cell boundaries within the austenite. In the transition zone, 0.3-0.5 mm below the surface of the steel strip, discrete polygonal ferrite grains were observed to form in parallel, and closely spaced \"rafts\" tranversing individual grains of austenite. Based on observations of the equivalent zone of the rolled Ni-30Fe alloy, the ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within the deformed austenite during rolling. Within the central zone of the steep strip, a bainitic microstructure, typical of that observed after conventional hot rolling of this steel, was observed following air cooling. In this region of the rolled Ni-30Fe alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions.

In some alloy systems, e.g., Cu-Co, prior formation of clusters does not affect the nucleation of precipitates, whereas in others, e.g., Al-Cu-X, where X can be Mg, Ag, Cd, In, Sn, Si, or Ge (singly or in combination), clusters can not only accelerate the nucleation kinetics of the first phase to form but can also change the identity of this phase.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Ultrafine ferrite grain sizes were produced in a 0.11C-1.6Mn-0.2Si steel by torsion testing isothermally at 675 °C after air cooling from 1250 °C. The ferrite was observed to form intragranularly beyond a von Mises equivalent tensile strain of approximately 0.7 to 0.8 and the number fraction of intragranular ferrite grains continued to increase as the strain level increased. It is proposed that high-energy jogged regions surrounding intersecting microbands provide potential sites for ferrite nucleation at lower strains, while at higher strains, the walls of the microbands may also act as nucleation sites.

A summary is presented of recent attempts to model the effects of precipitate shape, orientation, and distribution on yield strength and age-hardening response, using appropriate versions of the Orowan equation and models of precipitation strengthening developed for Al alloys containing a single uniform distribution of rationally oriented plate- or rod-shaped precipitates, which are either shearable or shear resistant. It is demonstrated that these models of particle strengthening are capable of predictions that are in execellent quantitative agreement with experimental observations that high tensile yield strength is associated with microstructures containing a high density of intrinsically strong, plate-shaped precipitates with {111} sub alpha or {100))0 sub alpha habit planes and large aspect ratio. The authors predict that further improvement in the strength of existing Al alloys might be achieved by increasing the number density and/or aspect ratio of rationally oriented precipitate plates.

The orientation and structure of planar facets on the gamma-m massive phase formed in a Ti-46.5 at. pct Al alloy are characterized using a combination of TEM and electron diffraction. The planar gamma-m/alpha2 interfaces are irrational with respect to both the alpha2 matrix phase and the gamma-m phase, and there is neither evidence of a rational orientation relationship across such facets nor resolution of a linear defect structure within the interface planes in conventional electron diffraction patterns and amplitude-contrast images, respectively. However, when imaged parallel to a particular direction in the interface, these irrationally oriented interfaces are invariably parallel to the Moire plane, which is defined by the intersection between two sets of closely-packed planes in the gamma-m and alpha2 phases. The relationship is such that there is an effective continuity of these lattice planes across the interface and a 1D coherency within the planar interface. This is interpreted to imply that such interface facets adopt a configuration of reduced energy and that they are not random and fully incoherent, as often described. It is suggested that these planar facets may migrate in a glissile manner normal to the interface plane by nucleation and rapid lateral movement within the interface plane of interfacial defects that have the form of Moire ledges, which are defined by the spacing of the Moire pattern that may be formed by overlap of the relevant crystal planes across the interface. (Author)

This paper presents the results of an experimental investigation into the stress corrosion cracking (SCC) of an engineering cast iron (namely a spheroidal graphite (SG) cast iron), in a highly caustic solution (namely synthetic Bayer liquor (SBL)) at high temperature. In order to ascertain experimental conditions under which plain iron - carbon materials may fracture predominantly by SCC in a caustic environment, slow strain rate testing (SSRT) was performed on carbon steel specimens, employing various combinations of strain rates and temperatures, in SBL and an inert environment of liquid paraffin. Under the conditions identified to be most conducive for caustic SCC of mild steel, specimens of the SG cast iron were subjected to SSRT in SBL and liquid paraffin, and the fracture behaviour was investigated by detailed fractography and microstructural characterisation.