Phase Transformations and Ageing Phenomena in Copper-Nickel-Manganese Alloys
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PHASE TRANSFORMATIONS AND AGEING PHENOMENA IN COPPER-NICKEL-MANGANESE ALLOYS
A.E. WEATHERILL AND R.A. BUCKLEY Department of Metallurgy, University of Sheffield, Sheffield,
U.K.
INTRODUCTION The pseudo-binary alloy system extending between copper and the intermetallic NiMn was first shown by Dean et al. [1,2] to yield a series of age hardenable alloys. The alloy containing 20% each of Ni and Mn has the best combination of strength and toughness and has been the subject of more recent studies [3,4]. The phase transformation associated with ageing in quenched Cu-NiMn alloys is precipitation of the ordered tetragonal (Llo) phase, (9) based on NiMn, from an fcc solid solution. Generally three modes of transformation occur depending on composition and temperature. An intragranular precipitate of fine coherent particles of 9, or a cellular or discontinuous precipitation reaction emanating from the grain boundaries, or heterogeneous precipitation of e at the grain boundaries. The mechanism of the intragranular precipitation has been variously reported as spinodal [5] or of classical nucleation and growth type (4). The present work goes some way in presenting evidence for the latter mode of transformation in Cu-NiMn alloys. Alloys covering the whole of the pseudo-binary system have been examined, although this report will deal only with alloys within the composition range where the fcc phase is retained on quenching, or on slow cooling decomposes to form 9 and the copper-rich fcc y-phase, i.e. 20% to 80% Cu. In the range 0 to 15% Cu a bcc phase is stable at high temperatures and transforms to 9 on cooling to give a variety of structures with martensitic morphologies. This will be the subject of a separate paper.
EXPERIMENTAL A master alloy of composition 50%Ni, 50%Mn was prepared by melting under argon, portions of this alloy were then used in making ternary alloys with copper. All constituent metals were of 99.9% or higher purity. Alloys 0 of 80%, 60% and 40% Cu were soaked at 900 C and then hot rolled from 25mm to lOmm diameter. Heat treatments were carried out either in argon or fused salt, care being taken to remove any surface layers subject to loss of manganese. After all heat treatments samples were quenched to iced brine. Thin foils were made by electropolishing discs in an electrolyte of 0 70% methyl alcohol and 30% nitric acid at -30 C. Foils were studied using a Philips EM 301 transmission electron microscope at 100 kV. X-ray diffraction of solid samples was accomplished using the sample spinning stage of a Philips 1349 diffractometer and standard step-scanning techniques.
RESULTS Phase Diagram Using dilatometric, differential thermal analysis and lattice parameter measurements together with metallocraphic examination the phase boundaries for the major part of the pseudo-binary system have been determined (Fig. 1). The comnound NiMn transforms on heatino through 7000C from Llo structure (9) to bcc (ý) which in turn transforms at 9000 C to fcc (y), in agreement with Coles and Hume-Rothery [6]. The solubility
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