Microstructure and its development in Cu-Al-Ni alloys
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INTRODUCTION
R E C E N T investigations of cast commercial nickelaluminum bronze (NAB Cu-10A1-5Ni-5Fe) have revealed several intermetallic phases/precipitates and a complicated sequence of microstructural development. t~'21In an attempt to understand the effects of combined additions of Ni and Fe to Cu-10A1 alloys, microstructural studies have been made on Cu-10A1-3Fe t31 and Cu-10A1-Ni ternary alloys. Some of the studies of the Cu-10A1-Ni alloys are described in the present paper. The original investigations of the Cu-AI-Ni system were carried out by Alexander t41 and Bradley and Goldschmit. tSj Alexander t41examined Cu-Ni alloys containing up to 35 wt pct A1; Figure 1 is the vertical section of the Cu-A1-Ni system taken from his work. At a composition of about 10 wt pct A1, there are two eutectoid reactions:/3 to a + fl~ (NiAI) and /3 to t~ + "/2 (Cu9A14). In 1948, Krster e t al. t6] carded out a more extensive investigation covering the entire ternary system; part of the 500 ~ isothermal section based on their work is shown in .Figure 2. In the copper-rich corner, there are four phases: a, /3~, 3'2, and/x; the/x phase only occurs in alloys outside the composition range of interest in the present work. More information on the copper-rich corner of the ternary system was obtained by Thomson and Edwards. [71Although they observed some differences in the positions of the phase boundaries, there was general agreement with the previous work. The characteristics of the equilibrium phases in Cu-9 to 10 wt pct A1-0 to 5 wt pct Ni alloys are listed in Table I. The microstructures of both eutectoidal products, t~ + '/2 and a + fl~, tend to be very fine and metallograph-
Y.S. SUN, formerly Research Associate with the Manchester Materials Science Centre, is Lecturer with the Department of Materials Science, Nanjing Institute of Technology, Nanjing, People's Republic of China. G.W. LORIMER and N. RIDLEY, Readers, are with the Manchester Materials Science Centre, University of Manchester/ UMIST, Grosvenor Street, Manchester M1 7HS, United Kingdom. Manuscript submitted July 26, 1988. METALLURGICAL TRANSACTIONS A
ically similar. According to Brezina, I81 it is not possible to distinguish between the two microstructures by optical microscopy. The lattice parameter of '/2 is 3 times that of/3~ so that many peaks of the X-ray diffraction patterns of "/2 and fl~ are superimposed. Nevertheless, it is important to identify the two phases because "/2 has been reported to have a deleterious effect on mechanical properties and corrosion resistance, tsl In the present work, the distribution, morphologies, compositions, and crystal structures of the phases present in copper-base alloys containing 9 to 10 wt pct AI and up to 5 wt pct Ni, which are close to the a + /3~ and a + "/2 + fl~ phase boundary in the ternary system, have been investigated. The development of microstructure on continuous cooling has also been examined. Various metallographic techniques have been used, and these include optical microscopy, scanning electron micr
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