An Al-Cu-Li-Ag-Mg alloy, Weldalite (trademark) 049, was recently introduced as an ultra-high strength alloy (7000 MPa yield strength in artificially aged tempers) with good weldability. In addition, the alloy exhibits an extraordinary natural aging response (440 MPa yield strength (YS) in the unstretch condition) and a high ductility reversion condition which may be useful as a cold-forming temper. In contrast to other Al-Li alloys, these properties can essentially be obtained with or without a stretch or other coldworking operation prior to aging. Preliminary studies have revealed that the T4 temper (no stretch, natural age) is strengthened by a combination of GP zones and delta prime (Al3Li). The T6 temper (no stretch, aged at 180 C to peak strength) was reported to be strengthened primarily by T(sub 1) phase (Al2CuLi) with a minor presence of a theta prime like (Al2Cu) phase. On the other hand, a similar but lower solute containing alloy was reported to contain omega, (stoichiometry unknown), theta prime, and S prime in the peak strength condition. The purpose of this study is to further elucidate the strengthening phases in Weldalite (trademark) 049 in the unstretched tempers, and to follow the development of the microstructure from the T4 temper through reversion (180 C for 5 to 45 minutes) to the T6 temper

Gayle, Frank W.

Heubaum, Frank H.

Pickens, Joseph R.

English

NASA Technical Reports Server

N91-24406
II. STRUCTURE AND PROPERTIES DURING AGING
OF AN AI-Cu-Li-Ag-Mg ALLOY, WELDALITEm049
Frank W. Gayle, Frank H. Heubaum, and Joseph R. Pickens
27
https://ntrs.nasa.gov/search.jsp?R=19910015093 2020-03-19T18:25:19+00:00Z
1. INTRODUCTION
An AI-Cu-Li-Ag-Mg alloy Weldalite TM 049 was recently introduced
as an ultra-high strength alloy (700 MPa yield strength in artificially
aged tempers) with good weldability (1). In addition, the alloy exhibits
an extraordinary natural aging response (440 MPa yield strength (YS) in
the unstretched condition) and a high ductility reversion condition
which may be useful as a cold-forming temper (1,2). In contrast to
other AI-Li alloys, these properties can essentially be obtained with or
without a stretch or other coldworking operation prior to aging.
Preliminary studies have revealed that the T4 temper (no stretch,
natural age) is strengthened by a combination of GP zones and 5' (AI3Li)
(2). The T6 temper (no stretch, aged at 180°C to peak strength) was
reported to be strengthened primarily by the T1 phase (AI2CuLi) with a
minor presence of a e'-Iike (AI2Cu) phase (3). On the other hand, a
similar but lower solute-containing alloy was reported to contain _,
(stoichiometry unknown), e', and S' in the peak strength condition (4).
The purpose of the present study is to further elucidate the
strengthening phases in Weldalite TM 049 in the unstretched tempers,
and to follow the development of the microstructure from the T4
temper through reversion (180°C for 5-45 minutes) to the T6 temper.
2. EXPERIMENTAL PROCEDURE
The alloy was examined in two product forms, sheet and extruded
bar, with the compositions given in Table 1. The alloys were heat
treated as follows: solutionized at 503°C for 1 hour, quenched in cold
water, and allowed to naturally age at room temperature to the T4
condition. (The T4 temper was defined as >1000 hours at ambient
temperature and was used as the starting condition for all artificially
aged tempers.) Artificial aging was carried out by heating specimens
in a circulating air furnace at 180°C. The sheet specimens were used
for the T4 and reversion studie.s, whereas extrusions were examined in
other aged conditions.
Specimens for transmission electron microscopy (TEM) analysis
were twin-jet electropolished at -20°C and 12 volts in a solution of
30% HNO3 in methanol. After polishing, the specimens were dipped in a
solution of 50% HNO3 in H20 to remove superficial Ag which had re-
deposited from the electrolyte during polishing.
28
TABLE 1
Alloy Compositions (wt.%)
Form (;;U Li Mg Ag Zr $i Fe
Sheet 6.0 1.3 0.4 0.4 0.18 0.04 0.05
Extruded bar 4.50 1.16 0.36 0.37 0.13 0.01 0.03
3. RESULTS
The aging response was monitored using Rockwell B hardness
measurements. Natural aging resulted in a very high T4 hardness,
RB80, corresponding to a tensile strength of about 585 MPa. A short
artificial aging period, e.g., 5-45 minutes at 180°C, then led to a
reversion with associated loss of hardness (see Figure 1). Further
aging at 180°C for 16-24 hours resulted in peak hardness. Also shown
in Figure 1 (dashed curve) is the room temperature aging response after
a reversion treatment (45 minutes at 180°C. It is clear that after
reversion the alloy re-ages at room temperature back to the T4
hardness level, albeit only after an extended incubation period. Typical
tensile properties for various conditions are extremely high, as shown
in Table 2.
TABLE 2
Longitudinal Tensile Properties
(25 mm gauge; _ = 3x10 -4 s-1)
Aging YS UTS El.
Temper Practice MPa MPa
T4 RT/1000 h 438 585 15.5
Reversion 180°C/15 min 280 480 25.5
Revert/re-age 180°C + RT/104h 440 610 11.0
T6 180°C/24 h 680 720 4.0
RT - room temperature
29
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The T4 structure exhibited a remarkably well-developed GP zone
structure (Fig.2a) compared with other aluminum alloys aged at room
temperature. Continuity of <100> streaks in the diffraction patterns
indicates that these are classical GPI zones consisting of monolayers
of Cu lying in disks on {100} matrix planes (5). 8' was also present,
both as 1-2 nm spheres in a very fine uniform distribution, and as
characteristic envelopes surrounding the Zr-rich oC similar to that seen
in Zr-bearing higher-lithium alloys (6,7).
The reversion treatment was found to dissolve 8' and coarsen the
GP zone structure. Re-aging at room temperature reprecipitated (5' and
additional very fine GP zones (Fig. 3), resulting in a complex
microstructure. The GP zones, all lying on {100} planes, exist in three
distributions with two major populations:
1) homogeneously nucleated zones, 12-15 nm / remaining after
2) zones nucleated on e_', 12-15 nm t reversion
3) homogeneously nucleated zones, 2.5-5 nm, precipitated during
re-aging.
The 8' phase, although present as a small volume fraction, also assumed
at least three distributions:
1) uniformly distributed, 1-2 nm spheres
2) heterogeneously nucleated envelopes on o_'
3) coating the faces of larger GPI zones.
Artificial aging beyond 45 minutes brought the alloy out of the
reversion well. After aging at 180°C for two hours (Figure 4), the
structure contained a wealth of precipitate phases: GPI zones (minor
volume fraction), e' (AI2Cu plates lying on {100}), T1 (AI2CuLi plates
lying on {111}), and S' (AI2CuMg laths on {210} extended in <001>
directions). Continued aging to peak strength and beyond (16-48 hours)
dissolved the GP zones leaving e', T1, and S' in substantial numbers,
with T1 appearing as the largest volume fraction. In the T6 condition,
the precipitates remain quite thin and in a good dispersion, with T1
typically one unit cell in thickness, and e' and S' less than 2-3 nm in
thickness.
An additional unidentified precipitate was observed (Fig. 5) for
aging times from 2 to 48 hours. This precipitate lies on {110}matrix
31
ORIGINALPAGE
BLACKAND WHITE PHOTOGRAPH
Figure 2 T4 structure, showing advanced GP zone structure (TEM BF).
Continuous streaks in the SADP indicate the zones are GPI
zones.
a
Figure 3
_ ;=_ !i!_¸¸¸
Microstructure after reversion (180°C/45 min) and re-
aging at room temperature for 6400 hours, a) Bright field
image showing very fine, duplex GP zone structure, b)
Superlattice dark field image showing various
distributions of 6', including the precipitation of 5' on the
faces of larger GP zones (arrows). Large L12-ordered o_'
dispersoid is also imaged brightly.
32
(") i-'_ I I_$,,! F" ,6, L PAC_ _
B;,-_C_, AI_D V_I'Hi]I: PHOTOGRAPH
Figure 4 Aged 180°C/2 hours, showing T1 lying on {111}, e' lying on
{100}, and laths extending in the [100] direction (Beam
direction B=[011]).
33
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
( oo)
e' (420) ,_ '1 (110) B=[O01]
Figure 5 Aged 180°C/2 hours, showing S', lying on {210} planes, and
another lath-like phase (arrows) lying on {110} planes (e'
plates on [100] planes are also visible. Wide, dark bands are
T1 plates viewed at an oblique angle.) (B=[001]).
34
planes, appearing as laths extending in <001> matrix directions. The
precipitate remained very small for all aging times from 2 to 48 hours,
with cross sections of 1.5 x 4.5 nm when observed end on.
A summary of the precipitate distribution as a function of aging
condition is given in Figure 6. For heuristic purposes, the proposed new
phase described above is designated by the Greek letter v.
4. DISCUSSION
Weldalite TM 049 shows extraordinary T4 hardness and strength,
comparable to many aerospace alloys artificially aged to peak strength.
The strength is derived from a very well developed GP zone structure
and the precipitation of 5'. Accelerated zone formation has been
reported in AI-6Cu-0.3Mn alloys with 0.02-0.12 Mg (8) and 0.18 Mg (9)
additions, and was attributed to enhanced nucleation at Mg-vacancy
complexes, as previously proposed by Kelly and Nicholson (5). It is
likely that Mg produces a similar effect in Weldalite TM 049. That is,
the Mg atoms enhance the equilibrium vacancy concentration during
solution heat treatment and help retain the vacancies through the
quenching operation. These vacancies may provide a dual beneficial
effect on T4 properties -- enhanced nucleation and also increased
diffusivity. Whether this natural aging process is enhanced by the
presence of Ag is being addressed in another study.
Room temperature precipitation of GP zones and 5' after the
reversion treatment is in part made possible by the reduction in
solubility of Cu and Li upon cooling from 180°C to room temperature.
The long delay before a significant increase in strength upon
subsequent natural aging is unexpected at first glance, considering the
rapid aging response following solution heat treatment. However, the
annealing out of quenched-in excess vacancies during the original aging
and reversion treatments, and solute depletion associated with the
coarser GP zones present after reversion likely contribute to the long
incubation period.
It should be noted that all GP zones observed in this alloy are
believed to be GPI zones based on the continuous nature of <100>
streaks in the diffraction patterns. In the classical view (5), GPI zones
are monolayers of nearly pure Cu lying in disks on {100} matrix planes.
To our knowledge, this alloy system is the first one in which GPI zone
nucleation on the Zr-rich o_' dispersoid is reported. The unusual post-
35
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reversion precipitation of 5' on the faces of the larger GP zones during
room temperature aging is not unique; in addition to the Weldalite TM
alloy system (2), De Jesus and Ardell (11) have recently seen similar
behavior in the higher Li-containing alloy 2090 (AI-2.7Cu-2.3Li-0.12Zr)
after natural aging. These observations are but two new additions to
the many examples (2) of heterogeneous nucleation of coherent phases
in AI-Li-(Cu,Mg,Zr) systems.
The 2 hour aging treatment yields a transition state in which GP
zones are still present along with the various strengthening phases
that are present in the T6 temper. Longer artificial aging times
produce the T6 structure, which is strengthened by a combination of
several phases, including e', S', and T1. The T6 condition may indeed
represent a metastable six-phase equilibrium between the aluminum
alloy matrix, several strengthening precipitates (e', T1, S' and v), and
the dispersoid o_'. (Six phases in stable or metastable equilibrium over
a range of temperature is the maximum allowed by Gibbs' phase rule in
a six component system.) T1 is likely the dominant strengthening
phase, based on apparent number density, morphology and distribution.
In AI-Cu-Mg alloys S' is reported to assume two different habits.
In the first, the precipitates occur as laths lying on {210} planes,
extending in <001> directions (11, 12). Corrugated precipitate sheets
approximating a {110} plane may form from two or more variants lying
on {210} planes with a common <100> direction. The other habit is
rodlike, extending in {100} directions, with some tendency to cluster on
{110} planes (13).
The proposed v phase has characteristics similar to S'. Both
phases are lath-like with the longest dimension extending in the
<100> matrix direction. However, a clear distinction between the two
phases exists, as shown in Figure 5. The laths visible in this B=[100]
micrograph are all viewed end-on, with the length projected into the
plane of the photomicrograph. The S' is visible both as individual laths
lying on {210} planes, and as the occasional corrugated sheet which is
composed of two {210} variants. This corrugated sheet approximates a
{110} habit plane. Also seen are laths which lie on {110} planes. These
laths are 1.5 nm in thickness by 4.5 nm in width, and appear as
individual laths with a {110} habit rather than corrugated composites
of S'. Therefore, these individual laths with a {110} habit are either a
new phase, or else S' with a new habit and/or a new orientation
relationship with the matrix. For very small precipitates, e.g., 1-5 unit
37
cells in the smallest dimension, interfacial energy is a predominant
factor in determining the morphology. Generally, one habit will be at
least slightly energetically more favorable than other possible habits
and will predominate to the exclusion of the others. For similar
thermodynamic reasons, it is unlikely that there is more than one
orientation relationship with the matrix. Thus the observation of two
habit planes ({210} and {110}) in a single microstructure wherein the
precipitates are on the order of 2-3 unit cells in thickness suggests
the presence of two distinct lath-like phases• Consideration has also
been given to the possibility that the (110) precipitate is due to
impurity effects, for example that v is high enough that it is unlikely
that trace impurities are responsible. Clearly, further work must be
undertaken to determine whether a new phase is indeed present.
5. CONCLUSIONS
The AI-Cu-Li-Ag-Mg alloy Weldalite TM 049 exhibits a series of
unusual and technologically useful combinations of mechanical
properties in different aging conditions:
• Natural aging without prior cold work to high strength (438
MPa YS, 585 MPA UTS),
. A reversion temper of lower yield strength and unusually
high ductility (25% elongation),
. Room temperature re-aging of the reversion temper
eventually leading to original T4 hardness, and
o Ultra-high strength T6 properties (680 MPa YS, 720 MPA
UTS).
Corresponding microstructures found to be responsible for these
properties are:
, Unusually well developed GP zone structure coupled with 5'
in T4 temper,
. Dissolution of 8' and coarsening (and some dissolution) of
GP zones in reversion temper,
38
. Reprecipitation of fine GP zones and 5' upon reversion plus
natural aging,
. A combination of several strengthening precipitates,
including S', e', and T1 in the T6 temper.
In addition there is evidence of a fourth, previously unidentified
precipitate, present as very thin laths on {110} planes, in the T6-type
conditions. The persistence of these phases (plus the Zr-rich a') in the
slightly overaged condition suggests the T6 condition may represent a
metastable equilibrium between six phases.
6. ACKNOWLEDGEMENTS
Some of the data included in this section were obtained under
sponsorship of the Martin Marietta Emerging Technology Fund and The
National Institutes for Science and Technology (NIST).
39
7. REFERENCES
[1] J.R. Pickens, F.H. Heubaum, T.J. Langan, and L.S. Kramer, p.1397,
Aluminum-Lithium Alloys (Proceedings of the Fifth International
Aluminum-Lithium Conference, T.H. Sanders and E.A.Starke, eds.,
MCE Publications, Ltd, Birmingham, U.K., 1989).
[2] F.W. Gayle, J.R. Pickens, and F.H. Heubaum, ibid., p. 701.
[3] T.J. Langan and J.R. Pickens, ibid., p.691.
[4] I.J. Polmear and R.J. Chester, Scr. Metall. 23, 1213 (1989).
[5] A. Kelly and R. Nicholson, Progress in Materials Science, 10, 149
(1963).
[6] F.W. Gayle and J.B. VanderSande, Scr.Metall. 18, 473 (1984).
[7] P.J. Gregson and H.M. Flower, J.Mat.Sci.Letters 3, 829 (1984).
[8] A.M.Drits, N.A. Vorob'yev, A.M. Zinden, O.1. Voroshilova, Russ.
Metall. 4, 142 (1981).
[9] R.K. Wyss and R.E. Sanders, Met.Trans.A 19A, 1988,pp.2523-2530.
[10] R. DeJesus and A.J. Ardell, Aluminum-Lithium Alloys, op.cit.,
p.661.
[11] A. Kelly and R. Nicholson, "Precipitation Hardening," Progress in
Materials Science 10,pp. 149-392.
[12] R.N. Wilson and P.G. Partridge, Acta Metall. 13, 1321 (1965).
[13] V. Radmilovic, G. Thomas, G.J. Shiflet, and E.A. Starke, Scr. Metall.
23, 1141 (1989).
[14] A.K. Gupta, P. Gaunt, and M.C. Chaturvedi, Phil.Mag.A 55, 375
(1987).
4O


Structure and properties during aging of an Al-Cu-Li-Ag-Mg alloy, Weldalite (tm) 049

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