Philipp Frankel

Comparison of residual stresses in Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo linear friction welds

Abstract In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α–β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d 0 variation across the weld has been evaluated using the biaxial sin2Ψ technique with laboratory X-rays. It was found that failure to account for the d 0 variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (∼750 MPa) than for Ti-64 (∼650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+β titanium alloy.

Studies regarding corrosion mechanisms in zirconium alloys

Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415°C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360°C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre- and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360°C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.

Identifying suboxide grains at the metal-oxide interface of a corroded Zr-1.0%Nb alloy using (S)TEM, transmission-EBSD and EELS.

Here we report a methodology combining TEM, STEM, Transmission-EBSD and EELS to analyse the structural and chemical properties of the metal-oxide interface of corroded Zr alloys in unprecedented detail. TEM, STEM and diffraction results revealed the complexity of the distribution of suboxide grains at the metal-oxide interface. EELS provided accurate quantitative analysis of the oxygen concentration across the interface, identifying the existence of local regions of stoichiometric ZrO and Zr3O2 with varying thickness. Transmission-EBSD confirmed that the suboxide grains can be indexed with the hexagonal ZrO structure predicted with ab initio by Nicholls et al. (2014). The t-EBSD analysis has also allowed for the mapping of a relatively large region of the metal-oxide interface, revealing the location and size distribution of the suboxide grains.

Understanding Crack Formation at the Metal/Oxide Interface During Corrosion of Zircaloy-4 Using a Simple Mechanical Model

It has been established in previous works that corrosion kinetics in primary water of various zirconium alloys are periodic. Each period is associated with a layer of cracks parallel to the metal-oxide interface. These observations have been made either in autoclave or in pile. This indicates that corrosion processes in autoclave and under irradiation are of similar nature though their absolute kinetics might be different. Taking advantage of this correlation between cracks and corrosion kinetics, the present work aims at identifying the main microstructural parameters controlling cracks appearance in the oxide layer under well-controlled conditions. In order to achieve this, Zircaloy-4 was heat-treated to obtain various metallurgical states (stress-relieved versus recrystallised with different grain sizes) followed by corrosion tests in primary water. The key metallurgical parameters for the various conditions have been analysed (texture, precipitate sizes and grain sizes and distributions) using electron microscopy and synchrotron X-ray diffraction techniques. Corrosion kinetics of the various Zircaloy-4 microstructures are distinct as expected from the literature. Crack morphology in the oxide layer has been analysed and quantified using a dual beam scanning electron microscope/focused ion beam. Crack layers are evident even at small scale of observation. Three dimensional (3D) images of the oxide structure are presented. Cracks observed in this way are typically penny-shaped with a radius of about 100 nm. Near the metal-oxide interface, they are mainly found at the top of metal protrusions in the oxide. The roughness of the metal-oxide interface was measured. It does not exhibit any periodicity. The residual stresses in the oxide layers were measured by high energy (44 keV) synchrotron X-ray diffraction in transmission mode. Large compressive stresses (∼−1 GPa), changing with the metallurgical state and through the oxide scale thickness, were measured. The residual stresses in the oxide layers were measured by high energy (44 keV) synchrotron X-ray diffraction in transmission mode. Large compressive stresses (∼−1 GPa), changing with the metallurgical state and through the oxide scale thickness, were measured. A model of the oxide breaking at the point of transition has been developed. It is based on mechanical considerations and the existence of compressive stress in the oxide layer.

Autoclave study of zirconium alloys with and without hydride rim

Abstract Autoclave corrosion experiments were conducted on a number of zirconium alloys in different heat treatment conditions. The alloys tested in the present work were Zircaloy-4, ZIRLO® (ZIRLO is a registered trademark of Westinghouse Electric Company LLC in the USA and may be registered in other countries throughout the world. All rights reserved. Unauthorised use is strictly prohibited.) and two variants of ZIRLO with significantly lower Sn levels, referred to here as A-0·6Sn and A-0·0Sn. Typical corrosion kinetics with a change from pre- to post-initial transition was observed with ZIRLO and Zircaloy-4 displaying the shortest time to the initial transition after 120–140 days of autoclave exposure, followed by A-0·6Sn materials after 140–260 days. A-0·0Sn materials showed no sign of transition even after 360 days although one sample tested to 540 days had gone through transition. Material in the stress relieved condition generally experienced initial transition earlier than the same alloy in the recrystallised condition. Pretransition samples had a universally black oxide layer, which eventually developed grey patches when transition occurred. Practically, all non-hydrogen charged alloys showed a strong trend towards cubic oxide growth rates. Cathodic hydrogen charging was conducted to simulate end of life condition of cladding tubes, forming a hydride rich rim region at the outer surface of the cladding tubes. Hydrogen charged materials generally experienced accelerated corrosion of different degrees with the exception of recrystallised A-0·0Sn and partially recrystallised A-0·6Sn showing no sign of acceleration. It therefore seems that increasing tin levels has a negative impact on autoclave corrosion behaviour for materials with and without a hydride rich rim. In developing advanced alloys for use in cladding, this effect has been balanced against the benefits that Sn is known to provide in-reactor, including robustness in corrosion behaviour and reduced irradiation growth. It was noted that most materials with a hydride rich rim exhibit parabolic corrosion kinetics with decreased initial weight gain but increased overall weight gain.

A method for accurate texture determination of thin oxide films by glancing-angle laboratory X-ray diffraction

The present article describes a modification to the standard method of glancing- angle X-ray diffraction for accurate measurement of the texture of thin oxide films. The technique resolves the problems caused by overlapping diffraction peaks originating from multiphase materials with asymmetric unit cells and the peak broadening associated with sample tilt during glancing-angle texture measurement. The entire 2? range of interest is recorded as a function of sample orientation, and the integrated intensities from different crystallographic planes are extracted from fitted diffraction profiles. The technique allows for pole figures to be plotted from diffraction peaks that could otherwise not be resolved and separates contributions from neighbouring peaks, leading to a more accurate representation of the existing oxide texture. The proposed method has been used for determining texture in a 3 mm layer of monoclinic/tetragonal zirconium oxide grown during aqueous corrosion testing and has been verified by additional synchrotron X-ray diffraction measurements.

Effect of Sn on Corrosion Mechanisms in Advanced Zr-Cladding for Pressurised Water Reactors

The desire to improve the corrosion resistance of Zr cladding material to allow high burnup has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. While the detrimental effect of Sn on high temperature aqueous corrosion performance is widely accepted, the reason for it remains unclear. High-Energy synchrotron X-ray diffraction was used to characterise the oxides formed by autoclave exposure on Zr-Sn-Nb alloys with tin concentrations ranging from 0.01 to 0.92 wt.%. The alloys studied included the commercial alloy ZIRLO® and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy during autoclave testing at 360°C was investigated by cross-sectional Scanning and Transmission Electron Microscopy (SEM & TEM). Non-destructive synchrotron X-ray diffraction analysis on the oxides revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on both phase and alloy. Additional in-situ Synchrotron X-ray diffraction experiments during oxidation at 550°C provided further confirmation of the trends seen for autoclave tested samples and demonstrated the presence of elevated levels of tetragonal phase in the initial stages of oxidation. In-situ and ex-situ measurements demonstrate unambiguously that the amount of tetragonal phase present and, more importantly, the degree of transformation from tetragonal to monoclinic oxide both decrease with decreasing tin levels, suggesting that tin stabilises the tetragonal phase. It is proposed that in Zr-Nb-Sn alloys with low Sn, the tetragonal phase is mainly stabilised by very small grain size and therefore remains stable throughout the corrosion process. By contrast, in alloys with higher tin levels larger, stress stabilised, tetragonal grains can form initially, but then become unstable as the corrosion front progresses inwards and stresses in the existing oxide relax.

Residual Stress Analysis Around Foreign Object Damage Using Synchrotron Diffraction

The current study compares the residual strain around foreign object damage (FOD), measured using synchrotron diffraction, to the strain predicted by a plastic model with power-law dependence. It is shown that the measured strains are significantly lower than those predicted by the model. This may be explained in part, by the inability of the model to account for damage mechanisms such as micro-cracking and shear band formation.

Investigating the effect of zirconium oxide microstructure on corrosion performance: a comparison between neutron, proton and non-irradiated oxides

18th International Symposium on Zirconium in the Nuclear Industry 1Materials Performance Centre, School of Materials, The University of Manchester, Manchester M13 9PL, UK 2Studsvik Nuclear AB, 611 82 Nykoping, Sweden 3Westinghouse Electric Company, Hopkins, SC 29061, USA 4Rolls Royce, Derby DE21 7XX, UK 5Amec Foster Wheeler, Walton House, Faraday Street, Birchwood Park, Risley, Warrington WA3 6GA, UK

Developments in Large Volume 3D Analysis via P-FIB: EBSD & EDS

The integration of electron backscattered diffraction (EBSD) and energy dispersive spectroscopy (EDS) on scanning electron microscopes (SEM) is an increasingly common method of characterising materials. EDS offers chemical quantification and spatial distribution of the elements whilst EBSD enables microstructural characterisation. The integration of these two techniques with simultaneous acquisition, as in the AZtec platform, enables full material characterisation and data correlation within a single user interface.