Volume Change in Ternary Zn-Al-Cu Alloy
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VOLUME CHANGE
IN TERNARY Zn-Al-Cu ALLOY
N. Mykura, S. Murphy, and Y.H. Zhu University of Aston, Gosta Green,
Birmingham,
England,
B4 7ET
INTRODUCTION The alloys of the ternary system Zn-Al-Cu are characterised by dimensional changes that occur on ageing in conjunction with certain phase transformations and changes in solid solubility [1]. The phase transformation that is suggested to he responsible is the four phase reaction a + 6
T'I +
-
that occurs below 273°C [2]. The average density of the products of this reaction was hypothesised to be less than that of the reactants resulting in an increase in volume. Previous work [3] had indicated that the lattice parameters of the zinc structure cell varied considerably on ageing and this change was attributed to the change in solubility of substitutionally dissolved copper. On ageing it is hypothesised that copper tends to come out of solution in zinc, contributing to the total increase in volume. This copper may come out as c-phase which may react to form T'-phase with a resulting change in volume. Experimental A 70% :n, 15% Al, 15% Cu (weight per cent) alloy was produced. Specimens of the alloy were soluti 8 n treated at 3500C for 6 days and then equilibrated at 280 C for 6 days. After quenching to room temperature the specimens were aged
1.0
C .2 "• 0"5 0-5 -J
0.1
10 Ageing tine
100
1000
(hours)
FIG. 1. Linear exBansion of 70% Zn, 15% Al, 15% Cu alloy homogenised at 280 C and aged at 175°C. Mat. Res.
Soc. Symp.
Proc. Vol. 21 (1984) QElsevier Science Publishing Co.,
Inc.
842
During ageing the change in linear dimensions at 175 0 C. of the specimens were measured (± 0.005 mm) (Fig 1). Using an x-ray diffractometer the change in phases during ageing was observed from the variation of the integrated area of characteristic x-ray peaks (Fig 2).
a (111) " (002)
o o
C (100) . T'Oi o) x 0
? E
E
0
S
T-
C0
I 1( ×
i -
4---
10
0-1
Ageing time
FIG. 2. 70% Zn,
• ,•-° -
(hours)
Effect of ageing on the phases present in 15% Al, 15% Cu alloy.
aged
From the x-ray diffractometer results the changes and 'a' parameters of the zinc (q) phase were the 'c' calculated (Fig 3).
in
From these results the volume of the zinc structure was calculated (Fig 4). The volume fraction of phases present at 2800C was determined by optical microscopy and the composition of these phases measured using quantitative energy dispersive x-ray From the volume fraction of phases, the mass microanalysis. phases a, c and T' was calculated. fraction of the initial This and x-ray diffraction data enabled the volume fraction and mass fraction of phases present at the end of ageing to This led to a determination of the mass be calculated. fractions involved in the reaction a + E
-.0
TI
+
which were compared with the expected results phase diagram using the lever rule.
obtained from the
843
4.96
C
4.95 4-94
0
a
4-93
2-67 0
4-92
QD
2-66
4-91
2-65 0o 2"64
4-90
10 Ageing
FIG. zinc
3. (n)
1000
100 time
(hour.)
Effect of ageing at 175 C on the 'c' and 'a'
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