The morphology, crystallography, and chemistry of phases in as-cast nickel-aluminum bronze
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I.
INTRODUCTION
1000
N I C K E L - a l u m i n u m bronze (NAB) with a nominal composition Cu-10 wt pct AI-5 wt pct Ni-5 wt pct Fe is widely used to produce cast components for marine applications. However, the alloy shows a variable resistance to corrosion in sea water, and this has been attributed to the complex microstructure which is affected by the thermal history of the casting and by its chemical composition. Under normal casting conditions, the alloy has a microstructure consisting of an fcc Cu-rich solid solution or a-phase, several intermetallic phases collectively referred to as K-phase, and some "retained/3-phase". Figure 1 shows the phase-relationships in the Cu-A1-Ni-Fe quaternary system at 5 wt pct Ni and 5 wt pct Fe, as determined by Cook e t a l . l T h e work of Cook e t a l L suggested that the K-phase was an Ni-Fe-AI complex with an ordered bcc (B2) structure. Weill-Couly and Arnaud 2 classified the K-phases into four types on the basis of the various morphologies observed in the optical microscope. Recently, microstructural studies have been made using electron beam instruments, and some information about the chemistry of the K-phases has been obtained by the microanalysis of both bulk and thin specimens. 3'4 The object of the present work was to investigate the crystal structures of the various K-phases (precipitates), to determine their orientation relationships with the a-matrix, and to examine their chemical compositions. The microanalysis and diffraction data have been combined to produce a classification of the K-phases based on their chemistry and crystallography, rather than on morphology alone.
II.
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w
-
C.§ 13+'Y'-'6
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1
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I
._ I
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B
1
I
10
6
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11
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At , w t - ~ Fig. 1 - - Vertica/section of the Cu-A1-5Ni-5Fe equilibrium diagram, after Cook et al.l
Table I.
Elements Alloy-I Alloy- II
C h e m i c a l C o m p o s i t i o n of Alloys, W t Pct
Cu 80.0 80.1
A1 9.4 9.0
Ni 4.9 4.4
Fe 4.4 5.1
Mn 1.2 1.4
Si 0.07 0.07
EXPERIMENTAL
The compositions of the alloys examined, as determined by wet chemical analysis, are given in Table I. Most of the investigation was concentrated on Alloy I and the results are E HASAN and A. JAHANAFROOZ, Graduate Students, and G. W. LORIMER and N. RIDLEY, Senior Lecturers, are all with Joint University of Manchester/UMIST, Department of Metallurgy, Grosvenor Street, Manchester MI 7HS, England. Manuscript submitted September 23, 1981. METALLURGICAL TRANSACTIONS A
from this alloy unless stated otherwise. For optical microscopy the specimens were polished using conventional procedures and etched in a solution of 10 pet ferric nitrate in water. Carbon extraction replicas of polished and etched specimens were prepared using standard techniques. Thin foil specimens were prepared either by electropolishing in 7 pct perchloric acid in acetic acid at room temperature using a potential of 30 volts, or by ion-beam-thinning with
ISSN 0360-2133/82/0811-1337500.75/0 9 1982 AMERICAN SOCIETY FOR M
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