Atom probe characterisation of high temperature materials

Abstract

An atom probe is capable of quantitatively analysing materials at the atomic level. Modern atom probes are derived from the field ion microscope, and are coupled with time-of-flight mass spectrometers, permitting identification of individual atoms. The introduction of position-sensitive detectors enables the reconstruction of a small volume of the sample owing to simultaneous determination of the x, y, and zcoordinates and the mass to charge ratios of individual atoms. This paper focuses on the application of atom probe techniques to the microstructural analysis of high temperature materials. Illustrations include carbide precipitation in creep resistant power plant steels and analyses of model and commercial multicomponent nickel based superalloys. It is demonstrated that atom probe field ion microscopy and atom probe tomography are valuable techniques in the development and understanding of technologically important alloys for high temperature service.     

Dynamic materials modelling concepts applied to analysis of hot rolling of commercial aluminium alloys

Abstract

A typical industrial hot rolling operation applied to a commercial Al–1%Mg alloy has been analysed in terms of some concepts from dynamic materials modelling (DMM), particularly the so called dissipator co-content J and the efficiency of power dissipation through microstructural changes η. The calculation of the parameter η for every deformation condition of the hot rolling schedule has been conducted assuming that this variable depends not only on the mean deformation temperature and strain rate but also on the strain applied to the material. All the analysis has been conducted on the basis of a constitutive equation previously determined for this material on a rational basis, which is capable of describing the strain, strain rate, and temperature dependence of the flow stress. It has been determined that, for this material, η can be significantly dependent on strain under certain deformation conditions, particularly low deformation temperatures and relatively high strain rates. It has also been shown that for the materials analysed, η is much more dependent on deformation temperature than on strain rate. A comparative analysis carried out with aluminium of various purities indicates that η is much lower for the alloy when it is deformed under similar conditions. In this sense, it has been suggested that such results could be interpreted in terms of the impurity content of the material and the interaction of the alloying atoms with the dislocation structure developed during high temperature deformation. However, this is only possible if the hypothesis advanced by Prasad and co-workers, that the power dissipation efficiency is associated with the dynamic microstructural processes that occur during deformation, is considered valid. The present results indicate that, contrary to previous findings, in terms of DMM concepts there are no specific conditions of temperature and strain rate for the optimum processing of the materials investigated.     

Influence of oxygen in plasma nitriding

Abstract

The development of low temperature nitriding treatments to enhance wear resistance over the past 40 years is reviewed. The progress achieved is illustrated with evidence from extensive laboratory investigations and industrial trials on textile machine components and other parts. It is demonstrated that small additions of an oxidising component to gaseous nitrocarburising or plasma nitriding/nitrocarburising atmospheres have a beneficial effect on layer formation and on the wear properties of the treated components. The presence of oxygen increases layer formation rates and stabilises the ?-compound layer. The role of oxygen on surface activation is important for chromium containing steels but also on plain carbon or alloyed steels and to neutralise the passivating effect of finishing procedures. Applying a post-oxidation step after nitrocarburising can significantly increase the corrosion resistance of the surface but close control of gas composition and treatment parameters is necessary.     

37—THE WRINKLING AND BENDING OF FUSIBLE INTERLININGS

A series of apparel fabrics were bonded both to a cotton base cloth and to a worsted base cloth. The fabrics were then wrinkled by the AKU tester under wet and standard atmospheric conditions, and their wrinkle severity was determined by means of the Sivim Wrinklemeter. Buckling and cantilever tests were also made, and the wrinkling behaviour and bending performance were compared.     

Wear mechanism in contact tube

Abstract

Stable sliding contact is the most important factor in the stable arc discharge of solid wire. If the contact tube wears seriously, arc discharge will become unstable and the wire position will be shifted. Ordinary non-Cu-coated solid wire causes much wear of the contact tube, about 10 times that caused by Cu-coated solid wire. The mechanism of contact tube wear is not only friction at high temperature, but also a molten bridge formed between the contact tube and wire surface. A complex of superfine particles decreases the amount of contact tube wear. For example, the mixture of graphite and magnetite reduces the amount of contact tube wear by 80%. These particles form an interface layer on the inner surface of the contact tube, and the layer prevents the transfer of molten Cu to the wire surface. The interface layer mainly consists of FeO and complex oxides.     

Causes for the Improved Water Resistance in Pine Wood Linear Welded Joints

Linear vibration welding of good quality pine (Pinus sylvestris) wood from Sweden containing a small proportion of a native mixture of terpenoic acids, known under the collective name of rosin, has been shown to yield joints of much upgraded water resistance. This has been shown to be due to the protecting influence the molten rosin from the wood itself has on the welded interphase, because of the water repellency of rosin. Joints of unusually high percentage wood failure but modest strength were obtained, rosin apparently reinforcing the welded interphase to yield weldline strengths always much higher than the strength of the surrounding wood.     

Ultrasensitive technique for detection of hydrogen emanating from steel and other solid surfaces

Abstract

Measurement of diffusible hydrogen in steel by destructive testing is, in many cases, impractical. A new non-invasive surface measurement technique is presented, whereby hydrogen emanating from a well defined steel surface is captured in a stream of air and analysed in situ. The apparatus embodying the technique is evaluated with respect to critical measurement parameters, indicating, for a high sensitivity version, flux resolvable to <2 pL cm^-2 s^-1 hydrogen at 298 K and 1013 mbar, and linear measurement of flux to 1000 pL cm^-2 s^-1. Flux measurements were obtained under ambient conditions by magnetically attaching the apparatus to one face of coupons expected to contain a low and uniform concentration of hydrogen, and 2 mm steel plate cathodically charged at the opposing face. The results suggested that hydrogen emanated into air at a rate of 50–30 000 pL cm^-2 s^-1, and was controlled by hydrogen diffusion through the steel bulk. The reproducibility and responsivity of the technique is also illustrated.     

Ni3Al based composite interlayers for wide gap transient liquid phase bonding of NiAl–Hf single crystals to a nickel base superalloy

Abstract

The formation of undesirable second phases, for example σ and/or carbides, represents a major problem during transient liquid phase (TLP) bonding of NiAl alloys to nickel base superalloys. Unlike many other TLP bonding systems, the formation of these second phases does not appear to be associated directly with the presence of the interlayer. Instead, the formation of second phases in the bonds results from a combination of extensive interdiffusion between the two substrates and the low solubility, in NiAl, of common alloying additions to nickel base alloys (such as Cr, Mo, and W). The use of a composite interlayer consisting of NiAl (as a non-melting constituent) and copper (as a liquid former) allows very short bonding times and this has been found previously to suppress the formation of σ and carbides, during bonding. In contrast, the use of NiAl–Cu composite interlayers had little effect on the formation of second phases during post-bond heat treatment and this represents a considerable problem. Hence, the present paper investigates the use of an alternate, Ni₃Al–Cu, interlayer, which is able to prevent the entry of Cr, Mo, and W into the NiAl alloy and hence suppress the formation of undesirable second phases (such as σ) during post-bond thermal exposure.     

Modelling of the GMAW system in free flight and short circuiting transfer

Abstract

An efficient model of gas metal arc welding (GMAW) systems was developed. The model accounts for the short circuiting transfer with free flight transfer, with finite droplet height included. Furthermore, we have analytically solved the energy equation of the wire, including the contact resistance between contact tip and wire as a boundary condition of the energy equation. The model predicts the time variable properties of GMAW systems from short circuiting to free flight transfer mode. Model prediction results have been compared with experimental results, and have shown good agreement.