Microstructure of high-tensile strength brasses containing silicon and manganese
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I.
INTRODUCTION
H I G H - t e n s i l e strength brasses containing silicon and manganese have a high wear resistance and good bearing properties, tl,2] The two most commonly used alloys are C67300 and C67400 (U.S.A. Specification); the range of the composition of C67400 is similar to that of British Standard 2872/4 CZ135. The microstructures of these alloys, which do not appear to have been studied in any great detail, are reported to consist of fcc a phase, ordered bcc (B2) /3' phase, and a manganese silicide, t3] The manganese silicide is believed to have a high hardness and to give the alloys its good wear resistance. The object of the present work was to study the microstructures of alloys C67300 and C67400 in the extruded, cast, and heat-treated conditions and to investigate the distributions, compositions, and crystal structures of the phases present. The development of microstructure on continuous cooling from temperatures approaching the solidus was also examined. II.
EXPERIMENTAL
Three alloys were supplied in the extruded condition. In each case, the extrusion temperature was 750 ~ The compositions of these alloys, as determined by spectroscopic analysis, are listed in Table I. Alloy SIB 1 contained higher Mn and Si concentrations than alloy SIB 2. The compositions of both alloys were in the specified range of the composition of C67400 and BS 2872/4 CZ135. The main difference in the compositions of alloys SIB 3 and SIB 1 and 2 was the aluminum content.
Alloys SIB 1 and 2 contained about 1.5 wt pct A1, while alloy SIB 3 was essentially aluminum-flee (Table I). The latter alloy had a composition in accordance with specification C67300. For optical microscopic examination, the specimens were metallographically polished following standard procedures and etched in a solution of 1 g FeE13 and 20 ml HC1 in 100 ml water. In order to improve the contrast in the optical microscope, the technique of differential interference contrast (DIC) microscopy was used. Thin foil specimens for transmission electron microscopy were prepared by electropolishing with 33 pct Nital at - 4 0 ~ using 10 to 12 V. Some thin foil specimens were cleaned at room temperature for ten minutes in an Ion Tech Super Microlap apparatus using 4 kV argon ions to remove any surface film formed during electropolishing. The thin foil specimens were examined with either a PHILIPS EM301 or EM400T* transmission *PHIL1PS Instruments.
EM301
and EM400T
electron microscope. Microanalyses of individual phases in thin foil specimens were carried out using the ratio technique and experimental kxs i values, as discussed elsewhere. [4'51 Absorption corrections were made assuming a foil thickness of 80 to 120 nm. The crystal structure of the manganese silicide was determined by both electron diffraction from thin foil specimens and X-ray diffraction from the powder residue extracted by dissolving the matrix electrolytically in 10 pct Nital.
III. Y.S. SUN, formerly with the Manchester Materials Science Centre, is Lecturer, Department of Materials Science
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