Cellular decomposition in a Cu-25Ni-15Co side-band alloy
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INTRODUCTION
II.
THE cellular
transformation conventionally has been defined as the transformation in which cells with a lamellar mixture of the a phase and the precipitate phase of equilibrium composition nucleate at grain boundaries and grow concomitantly into the matrix, l However, Miki and Amano recently reported a different type of cellular transformation in Cu-Ni rich Cu-Ni-A1 side-band alloys. 2'3 In these alloys, neither the a nor the precipitate phase in the cells had the equilibrium compositions during the early stage of cell growth. Their composition changed gradually to the equilibrium values with the progress of aging. Moreover, the microstructure in cells was not pearlitic but consisted of a very fine set of fibers. The fibers formed an aligned periodic structure, but distinct X-ray side bands were not detected from these cells. In order to ascertain if a periodic structure exists in the cells of these alloys and to elucidate the structure of these cells, the present work was performed o n C u - N i - C o 4-9 and Cu-Ni-Fe '~ alloys, which are known to be typical sideband alloys. We will show the similar results on Cu-Ni-Fe alloy in a later paper. Several papers 7'8'9 have been published on the agehardening or intragranular transformation in Cu-Ni-Co alloys, but a detailed study by TEM and X-ray on the cellular transformation in these alloys has not been reported. Moreover, how the intragranular transformation in these alloys relates to the spinodal microstructure has not been clarified yet, though the X-ray side bands have been observed in the aged specimens. Therefore, this point was also investigated in the present work. Table I.
Nominal Cu-25Ni- 15Co
Cu bal.
The chemical compositions of the alloy used in this work are presented in Table I. The section of Cu-Ni-Co phase diagram at 25 mass pct Ni is shown in Figure l . 4 The
1873
1673 1&73
Ot
~ A
~1273 1073
~873 I-- 673 I !
I I I
473 273 o
l I
10
I
I
i
I
I
20
30
40
50
60
I
I L i
70 75
Copper Content (moss*l,) Fig. 1 --Vertical section of the Cu-Ni-Co phase diagram for a constant Ni content of 25 mass pct (Ref. 4). The solution treatment and aging temperatures of the alloy used in this work are denoted by the dots A and B, respectively.
Chemical Compositions of Alloy (Mass Pct)
Ni 24.41
MASAMICHI MIKI, Lecturer, Department of Materials Science, Himeji Institute of Technology, Himeji, Japan, is now Visiting Researcher, Department of Metallurgical Engineering and Materials Science, CarnegieMellon University, Pittsburgh, PA 15213. DAVID E. LAUGHLIN is Professor, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted December 28, 1984. METALLURGICALTRANSACTIONS A
EXPERIMENTAL PROCEDURES
Co 15.72
Fe 0.016
A1, Ti, Si, Pb trace
composition of the alloy used in this work and its solution treatment and aging temperatures are denoted in this figure. The aging temperature is about 150 K below the estimated mean field spinodal. The alloy was prepar
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