Materials at High Temperature Vol 16, Issue 1, 1999

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Evolution of the microstructure in a W alloy during sintering before the formation of a liquid phase

C. Chausse and F. Nardou*

LMCTS/CNRS ESA 6015 Université de Limoges, 123 avenue Albert Thomas 87060 Limoges Cedex, France

Partial densification in W-based alloy sintering occurs before liquid phase formation by solid phase diffusion. During this step W diffuses in the second phase and tungsten grains connect between them to constitute a skeleton. A dilatometric study permits to follow the shrinkage during heating. The second phase, a Ni-based alloy NiFeCo, forms with tungsten a NiFeCoW alloy. The structure is that of nickel and the lattice parameter increases with increasing W content. Sintering tests are made from 500°C to 1,400°C to know the solid phase range of W during liquid phase sintering. Limits of W% are determined until 1,400°C and are used to define the diffusion temperature range. The observation of fracture surfaces gives the evolution of the microstructure during the sintering and can be correlated to diffusion results.

Keywords: microstructure in a W alloy, sintering, liquid phase

Long-term oxidation performance of ingotproduced Fe3Al alloys

B. A. Pint, P. F.Tortorelli and I .G.Wright

Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6156, USA

The oxidation performance of ingot-produced Fe-28at.%Al-(2–5)%Cr alloys with minor oxygen-active element additions was examined in long-term exposures at 1,000–1,300°C. As expected for alumina forming alloys, additions of Zr, Y and Hf improved the oxidation behavior of these alloys. At the lower end of the temperature range, these alloys performed similarly to other iron-base alumina-formers. However, at higher temperatures, scale spallation was increased and significant deformation of the substrate was observed. This type of scale spallation was not observed on other alumina-formers. Oxidation data were used to predict times to breakaway for this temperature range.

Keywords: ODS FeCrAl, Fe3Al alloys, oxidation performance, reactive element additions, lifetime prediction

The effect of platinum additions on the oxidation of directionally-solidified Ni-Cr-Al-Y-Cr3C2 alloys at 1,100 and 1,200°C

F.H. Stott1, B. Gleeson2 and P. Castello3

1Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester M60 1QD, UK

2Formerly at School of Materials Science and Engineering, The University of New South Wales, Sydney, 2052, Australia; now at Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA

3Istituto di Chimica, Facoltà di Ingegneria, Università degli Studi di Genova, Fiera del Mare, Pad D., 16149, Genova, Italy

The effects of 2wt% platinum addition on the oxidation behaviour of directionally-solidified, yttrium containing Ni-Cr-Al-Cr3C2 alloys (Ni-12.3Cr-6.9Al-1.8C-0.2Y in wt%) at 1,100° and 1,200°C in oxygen have been investigated under isothermal and thermal cycling conditions. Owing to the presence of yttrium, aluminium oxidized externally rather than internally at both temperatures. At 1,100°C, the addition of platinum apparently favoured the formation of a continuous Al2O3 scale over primary carbides intersecting the alloy surface. The scale remained substantially adherent after isothermal oxidation, but lifted and cracked on thermal cycling, though it did not spall significantly. Overdoping with yttrium (0.7 wt%Y) had a detrimental effect on the scale adherence, probably because it altered the homogeneity of the platinum distribution by forming Pt-Y intermetallics. At 1,200°C, an external Al2O3 scale formed initially, but spalled extensively after short times under both isothermal and thermal cycling conditions. During cyclic oxidation, re-formation of an aluminium scale was possible until a critical level of aluminium depletion in the alloy was reached, with the less protective oxides forming during the subsequent oxidation cycles. The inherent lack of ductility of the Al2O3 scale over the carbide particles, coupled with the CrO3 vaporization at high temperatures, could account for the observations.

Keywords: oxidation, directionally-solidified alloys, multiphase alloys, reactive-element effect, platinum, scale adherence, scale

spallation

Crack growth under constant sustained load at elevated temperature in IN718 superalloy

C.M. Branco1, J. Baptista1 and J. Byrne2

1UME / IST, Technical University of Lisbon, 1096 Lisbon Codex, Portugal

2Department of Mechanical and Manufacturing Engineering, University of Portsmouth, Portsmouth, Hants PO1 3DJ, UK

Crack growth rate ( da/dt) in IN718 nickel base superalloy has been studied at 600 and 700°C, in both compact tension and corner crack specimens, under sustained constant load conditions, generating an intergranular fracture mode. Using the stress intensity factor, K, as the correlating parameter, slower growth rates are found for corner cracks than for the through cracks in CT specimens together with a greater degree of crack tunneling. In the CT specimens, the introduction of lateral side grooves increases the crack growth rate. Disparities found between growth rates in the CC geometry are found to correlate with the incubation time for re-initiation of crack growth, which is attributed to a crack tip blunting and creep mechanism. A limited assessment of the correlation of growth rates using the creep crack growth parameter C* is made.

Keywords: crack growth, IN718 nickel base superalloy

Managing welded high-temperature components in power plant

D. D’Angelo1, G. Fedeli1 and A. S. Pérez Ramírez2

1Enel Production Division, Engineering Department, Milan – Piacenza, Italy

2Iberdrola, Production Direction, Materials Tech. Unit, Madrid, Spain

Welds are the most common site of damage in high-temperature components. This involves large costs of control, repair and replacement. This paper discusses this issue from the viewpoint of two end users, i.e. Enel in Italy, and Iberdrola in Spain. General information on the above two organisations, and to the relevance of this subject, is given. After a brief description of the practices they apply to face relevant problems, experiences of failure analysis and testing involving welds, exposed to high temperature, are reported.

Keywords: weld, Enel, Iberdrola, high-temperature components