The application of high strength aluminum alloys, particularly the Al-Zn-Mg-Cu-based 7000-series alloys, could be extended if it were not for their relatively low fracture toughness, and considerable research has been conducted on the factors affecting toughness of these alloys.
It is well established that one major variable is strength, since as strength is increased there is an accompanying decrease in toughness. There is also evidence that fracture is sensitive to heat treatment, and particularly that the toughness associated with an underaged structure is superior to that of an overaged structure having the same yield stress.
The application of high strength aluminum alloys, particularly the Al-Zn-Mg-Cu-based
7000-series alloys, could be extended if it were not for their relatively low fracture
toughness, and considerable research has been conducted on the factors affecting toughness
of these alloys.
It is well established that one major variable is strength, since as strength is
increased there is an accompanying decrease in toughness. There is also evidence that
fracture is sensitive to heat treatment, and particularly that the toughness associated
with an underaged structure is superior to that of an overaged structure having the
same yield stress.
However, considerable use is being made commercially of over-aged tempers, particularly
two-step overaged tempers such as T73; although these were developed primarily to improve
stress-corrosion resistance, alloys thus aged have significantly greater toughness than
those aged to maximum strength. The published information is hence somewhat confusing
and little attempt has been made to relate these effects to microstructural differences.
Where microstructure and fracture modes have been studied, it has been shown that fracture
in these alloys may be complex. Fracture by transgranular dimple rupture, by brittle or
ductile intergranular failure, and by mixtures of these modes has been observed. Several
authors have suggested that transgranular fracture is facilitated by inclusion particles.
Precipitation reactions in copper-free Al-Zn-Mg alloys have been extensively studied and
several attempts have been made to relate ductility or toughness to the precipitate-free
zones that are commonly observed adjacent to grain boundaries. They showed that toughness
was strongly dependent on fracture mode; the toughness associated with intergranular
fracture was considerably lower than that observed for transgranular fracture.
This paper reports the variation of both toughness, as measured by tear resistance,
and fracture path with aging treatment for a commercial 7075 aluminum alloy, with the
aim of clarifying and explaining the dependence of toughness on microstructure.
The inverse relation between propagation energy and yield stress is to be expected;
as the yield stress is increased, the extent to which the stress concentration ahead
of a crack or notch may be relaxed by plastic deformation is decreased. It is, therefore,
one of these processes which is affecting the observed toughness.
It has been postulated that the mechanical properties of Al-Zn-Mg based alloys should
be sensitive to matrix microstructures. G.P. zones, being coherent, are sheared by
dislocations and this process favors the formation of coplanar dislocation arrays
and pile-ups at boundaries. Although η’ plates are thought to be partially coherent
and hence should permit considerable shearing, the deformation of η’ dispersions
has been shown to occur in bands rather than by coplanar flow. Stress concentrations
at boundaries should hence be greater for G.P. zone dispersions than for η’
The close similarity between the propagation energies for 120° and 150°C underaged
structures indicates that, in the present case, matrix microstructure does not have
a major effect on toughness. This is supported by the two-stage η’ structure (4 hr
at 120° + 1 hr at 177°C) having similar propagation energy to the G.P. zone
structure (24 hr at 120°C). The apparent insensitivity to microstructural variations
is most probably due to the dispersoid particles. These, acting as barriers to dislocation
motion, break up the coplanar flow and hence greatly decrease the local stress
concentrations in the structure. In 7075, deformation, even in the structures aged at
120°C, occurs in bands rather than along single planes.
Similarly there is no first order effect of precipitate-free zone width. The zone at
120°C is much smaller than that at 150°, but no significant effect on toughness
is observed. Also the precipitate-free zone width is constant with aging time and cannot
be responsible for the underaged/overaged differences.
Fractography indicates that the decrease in toughness at a given strength level due to
overaging is associated with a transition from transgranular to intergranular fracture.
The transgranular dimples have the same characteristics in all conditions. Their size
and distribution fluctuate considerably across the fracture surface as does the spacing
of the dispersoid particles in the infrastructure. This together with the shape
similarity between the dispersoid particles and the particles observed in the dimples
supports the suggestions that transgranular fracture is nucleated by the dispersoid
particles, most probably by decohesion of the particle/matrix interface. As further
support, it has been shown that removing the chromium from 7075 raises the transgranular
propagation energy considerably.
The intergranular fracture surfaces vary considerably with aging treatments. Their
dimple size increases with increasing aging time and temperature and the dimple depth
appears to increase with increasing temperature. It is suggested that fracture proceeds
by the nucleation of voids at the grain boundary MgZn2-particle/matrix
interface and the subsequent coalescence of these voids. Hence the dimple size
increases as the MgZn2 particles coarsen; the dimple depth may be related
to the precipitate-free zone width, a wide zone permitting more ductile flow than a
narrow zone as suggested by Ryum. The effect on toughness is, however, minor.
The observed variation of propagation energy with heat treatment may be explained if
the coarsening of grain boundary precipitation results in a steady decrease in the
intergranular fracture stress relative to the transgranular fracture stress. If this
relative decrease occurs at a constant yield stress then the fracture stress and
fracture mode will vary.
The transgranular stress should be independent of aging time but may decrease slightly
if the precipitation of MgZn2 on the dispersoid interface facilitates
fracture. The intergranular fracture stress is presented as a band on graphic, because
the fracture stress for a particular boundary will depend on its orientation relative to
the applied stress-the band represents the difference between the most favorable
orientation for fracture (normal to applied stress) and the least (parallel to applied
stress). Varying the yield stress will vary toughness, but, at a given strength,
this effect will be the same for underaged and overaged structures.
The only grain boundary microstructural variable which may be correlated with a steady
decrease in intergranular fracture stress is the grain boundary precipitate size.
Similarly, that the directly quenched specimens have coarser grain boundary precipitates
than identically aged specimens which have been quenched to 0°C; the directly quenched
specimens have significantly lower toughnesses.
A rationalization of the effect of grain boundary precipitate size may be developed
if the particles are considered as cracks. The observation of particle-nucleated dimples
is generally held to be due to void initiation ahead of the main crack; if the
MgZn2 particle interfaces fracture ahead of the crack, secondary cracks are
produced with the same length as the particles. Crack propagation then proceeds by the
ductile fracture of the grain boundary regions between the main crack and the secondary
cracks. These regions will be subjected to a stress concentration due to the secondary
cracks, in addition to the concentration caused by the main crack.
An array of brittle particles along a grain boundary may be considered as an array
of colinear cracks. Paris and Sih have used linear elastic analysis to describe the
stress concentration effects of regularly spaced, identical, colinear cracks; they
showed that the stress concentration factor of such an array was approximately
proportional to the square root of the length of the individual cracks.
While there is no available analysis which describes the effects of a similar array
within the plastic zone of a larger crack, it is probable that any additional stress
concentration effect due to the array will show a similar dependence upon crack length.
From the close analogy between brittle particles and cracks, it is suggested that the
coarsening of the grain boundary particles leads to an increase in the stress
concentration due to these particles. Hence intergranular void coalescence would
occur under an increasing stress concentration as aging proceeds.
Similar considerations do not apply for the transgranular fracture mode as there is
no significant coarsening of the dispersoid particles during aging. It is to be expected,
therefore, that the effect of increasing the aging time would be to lower the
intergranular fracture stress relative to that of the transgranular mode, consistent
with the observed transition of fracture mode.
It can be concluded that:
- The toughness of aged 7075, as measured by crack propagation energy, decreases
as yield stress increases.
- At the same yield stress, the underaged structure has greater toughness than
the overaged structure.
- There is a transition from predominantly transgranular fracture to predominantly
intergranular fracture with increasing aging time.
- Both transgranular and intergranular fracture proceed by dimple rupture processes.
The transgranular dimples are nucleated at the interfaces of chromium-rich dispersed
particles. Intergranular dimples are nucleated at the interfaces of grain boundaries
- The fracture mode transition from underaged to overaged structures may be correlated
to a decrease in the intergranular fracture stress due to coarsening of the grain
- The fracture of 7075 is not primarily controlled by either precipitate-free
zone width or the nature of the matrix precipitate.