G.J. Tatlock

Platinum and the oxidation behavior of a nickel based superalloy

The oxidation behavior in air of a platinum containing nickel based superalloy (RJM2012) is compared with that of a similar alloy (IN792+Hf) without platinum. The distinct improvement in the oxidation resistance of the platinum containing alloy at high temperatures (1100°C) is explained in terms of the oxide morphologies and oxidation kinetics. In particular, it appears that platinum has a small but significant effect on the diffusion of other species in the alloy, and a model is presented to account for the complex sequence of oxide development, which culminates in the formation of a protective alumina scale.

Phase transformations in yttrium –aluminium oxides in friction stir welded and recrystallised PM2000 alloys

Abstract PM2000 is an Fe –Cr – Al oxide dispersion strengthened (ODS) alloy containing 0.5wt% of fine, uniformly dispersed, yttrium oxide particles in a ferritic matrix. The alloys are attractive candidates for high temperature applications since nano-dispersoids improve the creep resistance of the alloys at high temperatures. Friction stir welding (FSW) has been used successfully for the joining of PM2000 sheet without oxide particle agglomeration and significant change in the microstructure. However, it has been reported that the initial Y2O3 particles may sometimes oxidise the aluminium from the surrounding matrix to form mixed Y–Al oxides. Hence, in this study, we have been using extraction replication plus high-spatial resolution scanning transmission electron microscopy (STEM) to investigate phase transformations and oxidation of Y–Al during FSW processing and recrystallisation treatments (1380°C, 1 hour in laboratory air). High-resolution SuperSTEM images indicate that the Y2O3 can transform to Y3Al5O12 garnet (YAG) and YAlO3 perovskite (YAP) particles even in the consolidated PM2000. These dispersoids appear to be stable during the FSW process, but most of the Y2O3 or YAG particles transform into YAP particles after the high temperature recrystallisation treatment at 1380°C. In some cases partially transformed particles were observed and these may enable the details of the oxidation/transformation mechanisms to be elucidated.

The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000

Abstract Preliminary studies have been undertaken on cyclic and isothermal oxidation at 1,300°C of thin (125 μm) samples of commercial ODS alloy PM2000 for up to 350h in two different oxidising environments; dry and moist air. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) have been used to study the influence of such environments on alumina scale formation and growth. Initial mass gain observations showed that the alumina scale, which formed on the samples oxidised in air+2.5vol% H2O grew faster in the early stages of oxidation than in the case of dry air. However the SEM analysis revealed that the scale morphologies in both dry air and air+2.5vol% H2O were similar. In both cases the scales consisted of equiaxed grains at the scale–gas interface with Ti-rich particles in the outermost part of the scale. The major factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in moist air than in dry air

The effect of aluminium depletion on the oxidation behaviour of FeCrAl foils

Abstract In thin FeCrAl foils, the formation of a chromia layer within or underneath the alumina layer has been observed after consumption of the aluminium from the alloy. For Aluchrom I SE, the growth law of the alumina-forming step has been evaluated and an activation energy for oxygen diffusion of 383 ± 36 kJ mol−1 has been determined. For the growth mechanism of the chromia layer, three models are introduced and discussed. In agreement with a model proposed by H. E. Evans, the measurements of the aluminium content resulted in complete aluminium consumption before the beginning of chromia formation. Because of strong deformation of the thin samples during oxidation, a model is proposed to calculate the alloy thickness based on the amount of aluminium consumption. From a comparison of these calculated values with the measured thicknesses, the elongation of the sample due to creep processes could be determined.

The effect of heating rate variations on secondary phases in YBa2Cu3O7−δ

Abstract During melt texturing, secondary phases such as 2-1-1 particles and Ba-Cu-rich platelets are generated. We describe a new method based on rapid heating for controlling the final 2-1-1 particle size and the nature of the Ba-Cu-rich platelets. Using this new technique, 1-2-3 with a very finely dispersed 2-1-1 phase has been produced. Optical and electron microscopy have been used to characterise the final microstructure.

The Application of a Wedge-Shaped Sample Technique for the Study of Breakaway Oxidation in Fe–20Cr–5Al Base Alloys

The evolution of the breakdown of protective alumina scales on Fe–20Cr–5Alalloys has been studied by the use of taper-sectioned samples. The oxidationin air of two model alloys is compared with that of two commercial alloys,Kanthal APM and MA 956 at 1350°C. It is shown that the length of thebreakaway-oxidation region along the taper may be used to rank theperformance of the alloys and that the wedge geometry allows a detailedstudy to be made of the chemical processes involved in the degradation ofthe oxide scale just prior to breakaway oxidation.

The Effects of Zr Doping on the Optical, Electrical and Microstructural Properties of Thin ZnO Films Deposited by Atomic Layer Deposition

Transparent conducting oxides (TCOs), with high optical transparency (≥85%) and low electrical resistivity (10−4 Ω·cm) are used in a wide variety of commercial devices. There is growing interest in replacing conventional TCOs such as indium tin oxide with lower cost, earth abundant materials. In the current study, we dope Zr into thin ZnO films grown by atomic layer deposition (ALD) to target properties of an efficient TCO. The effects of doping (0–10 at.% Zr) were investigated for ~100 nm thick films and the effect of thickness on the properties was investigated for 50–250 nm thick films. The addition of Zr4+ ions acting as electron donors showed reduced resistivity (1.44 × 10−3 Ω·cm), increased carrier density (3.81 × 1020 cm−3), and increased optical gap (3.5 eV) with 4.8 at.% doping. The increase of film thickness to 250 nm reduced the electron carrier/photon scattering leading to a further reduction of resistivity to 7.5 × 10−4 Ω·cm and an average optical transparency in the visible/near infrared (IR) range up to 91%. The improved n-type properties of ZnO: Zr films are promising for TCO applications after reaching the targets for high carrier density (>1020 cm−3), low resistivity in the order of 10−4 Ω·cm and high optical transparency (≥85%).

Effect of annealing temperatures on the secondary re-crystallization of extruded PM2000 steel bar

The ferritic oxide dispersion‐strengthened alloy PM2000 is an ideal candidate for high‐temperature applications as it contains uniform nano‐oxide dispersoids, which act as pinning points to obstruct dislocation and grain boundary motion and therefore impart excellent creep resistance. The development of the microstructure during re‐crystallization of oxide dispersion‐strengthened alloys has been discussed by a number of authors, but the precise mechanism of secondary re‐crystallization still remains uncertain. Hence, this work is aimed at investigating the re‐crystallization behaviour of extruded PM2000 bar for different annealing temperatures, using electron backscatter diffraction, in particular, to determine grain orientations, grain boundary misorientation angles, etc. The results show that the as‐extruded bar microstructure comprises both low‐angle grain boundaries pinned by oxide particles and high‐angle boundaries that will have inherent boundary mobility to allow boundary migration. In addition, dynamical re‐crystallization was found in the outer region of the non–heat‐treated PM2000 bar, which suggested that deformation heterogeneities can be introduced during thermo‐mechanical processing that enhance the nucleation of re‐crystallization. Subsequent heat treatments promote and stimulate secondary re‐crystallization, giving rise to large grains with few sub‐grain boundaries.

A comparison of breakaway oxidation in wedge-shaped and parallel sided coupons of FeCrAl alloys

Abstract At elevated temperatures, FeCrAl based alloys go into non-protective breakaway oxidation when the aluminium content drops below a critical level and α-alumina ceases to form. For 2-mm thick coupons, even at 1,300°C, this may take several thousand hours of isothermal oxidation and it can be quite difficult to obtain samples in the critical period immediately before breakaway commences. An alternative approach has been developed, which involves the oxidation of wedge-shaped samples with a 7° taper. The surface to volume ratio varies along the wedge, hence the aluminium reservoir is depleted first at the thin end of the sample, where breakaway commences, and then moves progressively up to the wedge. There is always a region on each sample which can be studied next to the breakaway area. Although the test can be used to rank alloys by the ease with which breakaway progresses, there is clearly a difference between a parallel sided coupon just prior to breakaway and a wedge where one adjacent region has already gone into breakaway. Electron microscopy and analysis have been used to explore these differences and preliminary results are reported.

Stacking fault structures in melt-processed YBa2Cu3O7-δ superconductors

An electron microscopy study of melt-textured (1-2-3) superconductors has allowed us to characterize several distinct types of stacking faults which form as a direct consequence of the processing method. Extrinsic faulted 1/6 -type dislocation loops have a tendency to form in regions where the local stoichiometry of the system has been upset by the formation of 2-1-1 particles. These faults correspond to the insertion of extra CuO planes into the 1-2-3 crystal structure parallel to the basal plane