D. Dye

Effect of low transformation temperature weld filler metal on welding residual stress

Abstract The effect of weld filler metal austenite to acicular ferrite transformation temperature on the residual stresses that arise during the gas metal arc welding of a low carbon steel has been examined using a finite element model. It was found that the stress levels in the weld can be tailored by the appropriate selection of the filler metal and compressive, near zero or tensile residual stresses produced. Reasonable agreement was obtained between the model and the stresses measured using neutron diffraction both in welds using conventional and low transformation temperature filler metal.

Production of NiTi via the FFC Cambridge Process

The FFC Cambridge process is a direct electrodeoxidation process used to reduce metal oxides to their constituent metals in a molten CaCl2 salt bath. NiTiO3 was used as a precursor (the first stable oxide to form upon blending and sintering NiO and TiO2 powders) and was successfully reduced using the FFC Cambridge process at 1173 K and a constant cell voltage of -3.1 V to produce a NiTi alloy. This work builds on the literature work [Chinese Science Bulletin, 51, 2535 (2006)] through: (i) a predominance diagram calculated to show the regions of phase stability throughout the usable potential window of the CaCl2 salt; (ii) the investigation of a wide range of reduction times for a fixed cell voltage, elucidating several additional stable phases, to yield a complete and detailed reduction pathway. The reduction pathway for NiTiO3 was identified through the analysis of a series of partial reductions, with fully reduced NiTi formed after a period of 24 h. The first stage of the reaction involved the rapid formation of Ni and CaTiO3. The reduction then proceeded via the formation of the intermediate compounds Ni3Ti and Ni2Ti4O. All the NiTiO3 and Ni were consumed after a period of 6 h, while the intermediate compounds remained until the reaction was near completion. The experimental results related well to the thermodynamic predictions of the predominance diagram. A small variation in stoichiometry of the produced NiTi observed from the edge to the core of the samples was attributed to redeposition of Ti on the sample surface from the salt and a slightly Ti-rich NiTiO3 precursor material. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2987739] All rights reserved.

Characterization of the FFC Cambridge Process for NiTi Production Using In Situ X-Ray Synchrotron Diffraction

To date, the characterization of the reduction pathway for the Fray Farthing Chen (FFC) Cambridge process has been achieved through ex situ studies, leading to some ambiguities. This study employs a synchrotron X-ray diffraction technique to monitor in situ the FFC reduction of NiTiO3 to NiTi, yielding an unmatched level of detail on the electrochemical and chemical reactions involved. The reduction pathway consists of rapid initial reduction of NiTiO3 to form CaTiO3 and Ni, then the transformation of Ni to Ni3Ti, and finally the consumption of CaTiO3 and Ni3Ti to produce NiTi. The phases observed agree with thermodynamic predictions [J. Electrochem. Soc., 155, E171 (2008)] and allow the mapping of the reduction pathway on an electrochemical predominance diagram. Ni3Ti is a short-lived transient phase in the reduction pathway. Ni3Ti was found in significant quantities in ex situ studies [J. Electrochem. Soc., 155, E171 (2008); Chin. Sci. Bull., 51, 2535 (2006)]. The authors propose that Ni3Ti forms during furnace cooling to ambient temperature if extracted before a complete reduction. Neither Ni2Ti4O nor CaO was observed. The progression of the reduction front is clearly in line with existing observations and models [Metall. Mater. Trans., B, Process Metall. Mater. Proc. Sci., 35, 223 (2004); J. Phys. Chem. B, 109, 14043 (2005)].

Shape memory alloys: Towards practical actuators

Shape memory alloys have been developed that are free of functional fatigue, a key step in obtaining versatile actuators.

Insights into microstructural interfaces in aerospace alloys characterised by atom probe tomography

Atom probe tomography (APT) is becoming increasingly applied to understand the relationship between the structure and composition of new alloys at the micro- and nanoscale and their physical properties. Here, we use APT datasets from two modern aerospace alloys to highlight the detailed information available from APT analysis, along with potential pitfalls that can affect data interpretation. The interface between two phases in a Ti–6Al–4V alloy is used to illustrate the importance of parameter choice when using proximity histograms or concentration profiles to characterise interfacial chemistry. The higher number density of precipitates and large number of constituent elements in a maraging steel (F1E) present additional challenges such as peak overlaps that vary across the dataset, along with inhomogeneous interface chemistries.

Isothermal omega formation and evolution in the Beta-Ti alloy Ti-5Al-5Mo-5V-3Cr

Abstract The isothermal phase of Ti-5Al-5Mo-5V-3Cr wt.% is formed within a heat treatment at and identified by atom probe tomography as Ti-rich/solute lean precipitates. The composition and size remain essentially constant during ageing, although the volume fraction increases to 9.5% after ageing for 8 h. This is consistent with an ongoing transformation process of athermal to isothermal . The / interface becomes enriched with oxygen. This may be of significance as oxygen strongly stabilizes the phase, and the / interface has previously been suggested as the nucleation site for subsequent formation.

The Effects of Plate Dimensions on Residual Stresses in Welded Thin Steel Plates

Residual stress distributions have been measured on thin (4 mm thick) plates made from a ferritic steel designated grade DH-36 using the neutron diffraction technique. The welded specimens include two large (1 m × 1 m) butt welded plates, onto one fillet welded stiffeners were also added, and a smaller (0.5 m × 0.5 m) fillet welded stiffened plates. The large butt and fillet welded stiffened plate has also been cut to smaller dimensions and the strain relaxation due to cutting quantified by strain gauges. The residual stress distributions have been re-measured after specimen cutting and the relaxation quantities compared to the strain gauge values. The influence of specimen size is also examined by comparing measurements at the base of the fillet welds in the large and smaller plates. The results are interpreted to identify plate size effects on the residual stress levels of the welds.Copyright

Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale

Abstract Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.

Hydrogen in Ti and Zr alloys: industrial perspective, failure modes and mechanistic understanding

Titanium is widely used in demanding applications, such as in aerospace. Its strength-to-weight ratio and corrosion resistance make it well suited to highly stressed rotating components. Zirconium has a no less critical application where its low neutron capture cross section and good corrosion resistance in hot water and steam make it well suited to reactor core use, including fuel cladding and structures. The similar metallurgical behaviour of these alloy systems makes it alluring to compare and contrast their behaviour. This is rarely undertaken, mostly because the industrial and academic communities studying these alloys have little overlap. The similarities with respect to hydrogen are remarkable, albeit potentially unsurprising, and so this paper aims to provide an overview of the role hydrogen has to play through the material life cycle. This includes the relationship between alloy design and manufacturing process windows, the role of hydrogen in degradation and failure mechanisms and some of the underpinning metallurgy. The potential role of some advanced experimental and modelling techniques will also be explored to give a tentative view of potential for advances in this field in the next decade or so. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

Using transmission Kikuchi diffraction to characterise α variants in an α+β titanium alloy

Two phase titanium alloys are important for high‐performance engineering components, such as aeroengine discs. The microstructures of these alloys are tailored during thermomechanical processing to precisely control phase fractions, morphology and crystallographic orientations. In bimodal two phase (α + β) Ti‐6Al‐2Sn‐4Zr‐2Mo (Ti‐6242) alloys there are often three microstructural lengthscales to consider: large (∼10 μm) equiaxed primary α; >200 nm thick plate α with a basketweave morphology; and very fine scaled (<50 nm plate thickness) secondary α that grows between the larger α plates surrounded by retained β. In this work, we utilise high spatial resolution transmission Kikuchi diffraction (TKD, also known as transmission‐based electron backscatter diffraction, t‐EBSD) and scanning electron microscopy (SEM)‐based forward scattering electron imaging to resolve the structures and orientations of basketweave and secondary α in Ti‐6242. We analyse the α variants formed within one prior β grain, and test whether existing theories of habit planes of the phase transformation are upheld. Our analysis is important in understanding both the thermomechanical processing strategy of new bimodal two‐phase titanium alloys, as well as the ultimate performance of these alloys in complex loading regimes such as dwell fatigue. Our paper champions the significant increase in spatial resolution afforded using transmission techniques, combined with the ease of SEM‐based analysis using conventional electron backscatter diffraction (EBSD) systems and forescatter detector (FSD) imaging, to study the nanostructure of real‐world engineering alloys.