The major application of high strength aluminum alloys is in airframe
construction. Over the last 50 years, the degree of control of their proportion
has been increasing as now insights and test techniques have become available.
Today, the final properties of particular interest are strength, toughness,
fatigue crack growth rate and exfoliation and stress corrosion resistance.
Since strength is usually a primary consideration, it will not be treated as a
variable. In discussing the effect of microstructure on other properties, it
may be presumed that adequate strength was achieved as a necessary
Only aluminum alloys from two systems, both precipitations hardenable, are serious
contenders for aircraft structures: the Al-Zn-Mg-Cu (7000 series) and the Al-Cu-Mg
(2000 series) systems. While there is some interest in the Al-Mg-Li system, no real
benefits have been demonstrated and the potential casting problems with such a
volatile alloying element as lithium are not encouraging.
Of the two major systems, 7000 series alloys are usually chosen for their high
strength, while 2000 series alloys are generally selected where fatigue is a
problem and for applications where higher service temperatures may be encountered.
The alloys 7075 and 2024 are, by far, the most often used, and offer very useful
properties. Improved versions of these alloys, such as 7475 and X2048, have been
recently introduced for improved toughness. For specific applications other alloys
have been developed, such as 2618 for elevated service temperatures or 2219 for
The effects of processing on stress corrosion or exfoliation performance are
well known, although considerable doubt still exists as to the mechanisms involved.
The most important microstructural features are grain shape and orientation with
respect to the applied stress, degree of recrystallization and electrochemical
differences between grain boundary precipitates and the matrix. It is worth
mentioning at this point that the corrosion resistance of Al-Zn-Mg alloys containing
approximately 1.5% or more Cu can usually be assured when aging is carried on to a
sufficiently overaged (T73) condition. Electrical conductivity has been found to
be a very good measure of degree of overaging, and is incorporated in many corrosion
resistant temper specifications.
Effect of Microstructural Features on Mechanical Properties
We will first consider which microstructural features influence the most important
properties, particularly toughness.
The important microstructural features, as far as toughness is concerned, are
second phase particles and grain structure. The second phase particles of concern
are of 3 types:
Other microstructural features, such as precipitate free zone, may be important,
but often they are difficult to dissociate from other effects.
- Coarse insoluble particles formed during casting or coarse particles of
normally soluble phases formed during casting or subsequent processing,
- Smaller intermediate particles formed during homogenization, and
- Aging precipitates.
Grain orientation also plays a large part in determining toughness of alloys
containing coarse particles. In a rolled product, the insoluble phases are broken
up and strung out in the rolling plane. Thus, when fracture toughness is measured
in the short transverse direction or, to a lesser extent, in the long transverse
direction, toughness is greatly reduced due to the presence of the aligned weak
The Effect of Intermediate Particles and Grain Structure. The
principal role of the intermediate size Cr-, Zr- or Mn-bearing particles is to
retard recrystallization and grain growth. In fact, little is known about the
intermediate particles acting on their own. The most effective elements are
Cr or Zr in Al-Zn-Mg alloys and Mn
in Al-Cu-Mg alloys. The Cr, Zr or Mn
additions remain in solution during casting and are precipitated during homogenization
or other high temperature treatment. Their size and spacing, which are largely
controlled by temperature and time, have important effects.
The Effect of Hardening Precipitates. In commonly used tempers,
the greatest size difference in hardening precipitates occurs in 2000 series
alloys between the T3 and T8 tempers. In the former temper, aging is carried
out at room temperature and results in a fine distribution of GP zones. The T8
temper is achieved by aging T3 temper material at elevated temperatures,
and contains coarse, partially coherent laths of S’ (metastable Al2(CuMg)).
Smaller effects of the hardening precipitates have been noted for 7000 series.
Toughness decreased as the peak of the aging curve was approached, as might be
expected on the basis of increasing yield strength.
The effect of age hardening precipitates on stress corrosion behavior of Al-Zn-Mg
alloys is complex; but, in general, the presence of the metastable M’ hardening
precipitates produced by overaging is preferable to the GP zones present in an
underaged condition. The presence of the M’ particles appears to mainly
improve the deformation mode and electrochemical homogeneity.
The deformation mode in the underaged condition is one of narrow slip bands and
coplanar arrays of dislocations due to shearing GP zones. These result in dislocation
pile-ups at grain boundaries and, consequently, high local stresses which may assist
preferential fracture at the boundaries. In the overaged condition, such differences
are small, and a more uniform pitting attack, rather than stress corrosion cracking,
Copper content and quench rate after solution heat treatment play important roles
in determining stress corrosion resistance. Only above about 1.0% Cu
is good resistance to stress corrosion achieved in all grain orientations in an
overaged condition. The effect of quench rate is markedly different for high- and
low-Cu Al-Zn-Mg alloys. The stress corrosion performance of low-Cu alloys (below
about 1.0 pct) benefits from as slow a quench rate as is consistent with strength
requirements. However, these alloys are only resistant to stress corrosion when
their grain structure is not equiaxed and when the applied stress direction is
not normal to a continuous high-angle grain boundary path.
In summary, it appears that coarser particles of all types and recrystallized
coarse grain structures both lead to lower toughness. The actual effects of the
intermediate particles is uncertain since it appears that they influence toughness
more by their effects on grain size than due to their own size per se. The effect
of microstructure on fatigue is less well understood, though in the case of 2000
series alloys, the presence of coarser age hardening precipitates is undesirable.
Stress corrosion and exfoliation resistance are fairly well understood. They depend
primarily on the state of aging though quench rate and grain structure can also
play an important role.
Control of Microstructure
Composition. Obvious gains in toughness can be obtained by
reducing Fe, Si and other trace elements. Limits
to such reductions are set by cost and availability of high purity materials.
Alternatively, minor benefits can be derived from achieving a more uniform distribution
of whatever second phase particles are present.
In a unilaxially-worked wrought product, the second phase particles break up
during working and appear in the final product as rows of particles or
"stringers". Such stringers tend to have their worst effect on deformation
and fracture when stresses are applied normal to them, i.e. particularly in the short
More homogeneous properties can be obtained by not allowing these stringers to form.
The reduction of Fe and Si should be treated with
some caution since it may also affect the precipitation process in some not very
well understood manner. It has been observed that Fe and
Si can influence the nucleation of new phases or change
More readily attainable reductions in the amount of second phase particles can be
achieved by control of the major alloying elements to avoid exceeding the solubility
limit. The composition limits of 2024, for example, have been fixed for years, yet
approximately half its composition range lies above the solubility limit.
As detailed above, composition is important if stress corrosion resistant tempers
are to be produced in 7000 series alloys. An alloy with a Cu level
below about 1% will not fully respond to overaging for SCC resistance. For such an
alloy, it is desirable to reduce the quench rate and to seek an unrecrystallized
grain structure for improved longitudinal (L) and long transverse (LT) stress
corrosion resistance. Short transverse tensile stresses in exposed surfaces must
Quench sensitivity of 7000 series alloys can be influenced by composition.
Specifically, the minor addition elements (Cr, Zr,
Mn), which are added to control recrystallization, cause a loss
in strength on reducing quench rate. Cr results in the greatest
In heavy plate applications, quench rate is naturally slow and it is often desirable
to quench at an even slower rate to minimize residual stresses. To do this and still
achieved adequate strength, Zr can be substituted for Cr
since it allows the attainment of strength at much lower quench rates and yet is a very
effective recrystallization retardant.
Casting. Faster freezing rates would be desirable to reduce
the dendrite arm spacing and, hence, facilitate the solutionizing of soluble
second phases during homogenization. Also, by faster freezing, larger amounts
of the low solubility phases can be kept in solution. These will subsequently be
precipitated, but as relatively fine particles. Usually, however, there is little
that can be done to increase freezing rates since the ingot must be of sufficient
thickness for the economical production of a wrought material and the avoidance of
ingot cracking is often achieved by reduced cooling. For certain products, such
as those produced by powder metallurgy or from small ingots, it may be possible
to significantly alter the distribution of insoluble particles.
Homogenization. It is desirable to achieve adequate homogenization,
though this is not always necessary since, to eliminate soluble second phase
particles, it is the ability to return the soluble elements to solution during
solution heat treatment that is important. However, good solutionizing is more
readily achieved if the soluble elements have been in solution at some point
during processing. For thinner products, this may not be necessary since coarse
particles may be broken up during fabrication, and so can be more readily returned
to solution during an intermediate anneal or during solution heat treatment. A
homogenization or intermediate anneal is also necessary to precipitate
Cr, Zr or Mn from solid solution.
This is an important processing step since the size and distribution of these
particles determines final degree of recrystallization.
Fabrication Practices. In general, while wishing to avoid hot
shortness, it is desirable to keep a high working temperature. This helps to minimize
recrystallization after solution heat treatment, which benefits strength and stress
A combination of quench rate and the presence of subgrains or hot working structure
can influence strength. They showed that the outer layers of a press quenched extrusion,
although not recrystallized, had lower properties than the core. This they attributed
to the nucleation of MgZn2 on high-angle boundaries in the outer regions
during quenching. That is, the outer layers were more quench sensitive than the core.
Solution Heat Treatment and Aging. For optimum toughness, solution
heat treatment should be carried out at as high a temperature as possible, though
undue melting should be avoided. Either increased time or temperature can be used to
increase the solutionizing of the soluble elements. Generally, after a few hours,
further improvements are not accomplished. More solutionizing can generally be
gained from using higher temperatures rather than longer times.
Small, coherent precipitates are the most desirable, particularly for Al-Cu-Mg
alloys. If, however, over-aged tempers are necessary for good stress corrosion
and exfoliation resistance, there is little choice of aging practice. Some benefits
may be expected from using as low an aging temperature as possible, consistent
with a reasonable aging time. For Al-Zn-Mg-Cu alloys, a two-step aging practice