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A critical problem encountered in the use of highest strength aluminum alloys (Al-Zn-Mg type) rests with an increased susceptibility to stress corrosion cracking as maximum strength levels are reached by conventional aging. Owing to the insidious nature of stress-corrosion failure, a compromise is made in the strength level at which the alloy is used, resulting in severe weight penalties, particularly in airframe applications.
A critical problem encountered in the use of highest strength aluminum alloys (Al-Zn-Mg type) rests with an increased susceptibility to stress corrosion cracking as maximum strength levels are reached by conventional aging. Owing to the insidious nature of stress-corrosion failure, a compromise is made in the strength level at which the alloy is used, resulting in severe weight penalties, particularly in airframe applications.
In the past, variation of alloy-chemistry, solidification parameters, processing techniques and heat treatment have resulted in varying degrees of success toward product development in terms of strength and structural reliability. Of these, thermomechanical processing appears to be a most promising to the solution of this problem.
This article will only review the literature of thermo mechanical processing (TMP) of Al-Zn-Mg type alloy with particular emphasis on mechanical properties such as strength, ductility, fatigue and fracture toughness A systematic approach to a better understanding of the various stages of processing can be obtained through a correlation of microstructure and properties.
It is well known that there are strong interactions between the solute atoms and defects in aluminum alloys that result in structural instabilities variation solute profi1es and changes in solute diffusion rates. Investigations into repeated yielding in 7075 and other Al alloys by D’Antonio and others have shown that mobile dislocations are pinned by enhanced solute diffusion during tensile straining giving rise to the Portevin-LeChatelier (P-L) effect. This phenomenon appears in aluminum alloys over a range of temperatures above and below room temperature.
The amount of strain required for enhanced diffusion is a function of the temperature at which the straining is effected, and the strain-rate. Around room temperature, the diffusion coefficient of solute atoms in undeformed alloys is too low to result in their migration to dislocations.
If the diffusion of the solute atoms in a substitutional alloy is control1ed by a vacancy mechanism, then the diffusion coefficient, D, can be written as
D ≈ a2υZ Cv exp (-Em/kT)
where a is the interatomic distance, u is the average vibrational frequency of the atom, Z is the coordination number, Cv is the vacancy concentration, Em is the effective vacancy migration energy and k and T are the Boltzman constant and absolute temperature respectively.
To account for the enhanced diffusion, it has been proposed, that the diffusion coefficient is increased by the creation of non-equilibrium number of vacancies, Cv, due to plastic deformation, Cv can be related to the plastic strain Eo by the semiempirical relation
Cv = KEom
where K and m are material constants.
Plastic deformation not only increases the mobility of solute atoms in aluminum alloys but can cause clustering at dislocations. These clusters can as nucleation sites for subsequent strengthening precipitates. This phenomenon can be caused to occur at temperatures substantially below conventional aging temperatures and at which vacancy migration rates are low. This allows for wide variations in thermomechanical treatments and subsequent precipitate distribution and morphology.
Previous attempts to strengthen Al-Mg-Zn alloys thermomechanically have involved plastic deformation of solution-treated alloys prior to, during and subsequent to the aging process below, at and above room temperature.
McEvily have shown that 50% deformation at room temperature prior to aging produced slightly increased strength and greatly retarded crack growth rate during stress-corrosion. Morris found that large prestrains at room temperature produced instabilities that enhanced strengthening on subsequent aging. Post-aging deformation, i.e., cold working after stable precipitates have formed, also leads to strengthening.
DiRusso and others have found enhancement in both strength and ductility by deforming at an intermediate stage prior to attaining maximum strength in the aging process. Sommer tried to plastically deform in the peak hardened condition (T6) at an elevated temperature (above G.P. zone solvus temperature) and then overage to bring the strength level to T6 condition. The application of this TMP resulted in a material with the strength and fracture toughness of conventional T6 temper, possessing stress-corrosion cracking resistance equivalent to the overaged T73 condition.
The work on the effect of high temperature deformation in partially aged and in peak hardened condition toward property enhancement is quite extensive. However, none of these investigations have assessed in detail the effect of room temperature deformation on microstructure and properties in the underaged condition. These have led to a thermomechanical treatment yielding a product with superior physical and mechanical properties.
Osterman reported an increase of 25% in the 107 cycles-to-fai1ure stress level in 7075 Al. The above increase, over that of the conventionally aged alloy (T65l condition) was attained by thermomechanical treatment, where by the alloy is deformed at room temperature in a partially aged condition, and then post-deformation aged.
A great deal of controversy presently exists concerning the role of particular microstructural features on the stress-corrosion susceptibility of high strength alloy of 7075 type. Studies on stress-corrosion attack in Al-Zn-Mg type alloys have focused on three principal microstructural features:
a) matrix precipitate structure,
b) grain-boundary precipitate structure and
c) precipitate-free-zone (PFZ) which forms adjacent to the grain boundaries.
Work of Kent supports the Unwin and Nicholson contention that grain-boundary precipitate structure is of overriding importance to stress-corrosion attack. Other investigators contend that the nature of matrix precipitate is of importance to the deformation mode and thereby susceptibility. The planar slip mode is associated with susceptibility whereas materials deforming with a dislocation tangle structure is non-susceptible.
The importance of precipitate-free-zone has also been a point of controversy. The results of early work indicated that PFZ enhanced stress-corrosion susceptibility. More recent studies show that wide zones are preferred since they are weaker and undergo preferential deformation. Others contend that PFZ is not significant to stress-corrosion attack. All of the above three important features can be effected through thermomechanical treatments and hence have been used to improve stress-corrosion resistance.
Thermomechanicat treatment involved 10% deformation of partially aged (3 days at room temperature and 5 hours at 120°C after solution treatment and water quench) samples and an aging treatment for 10 hours at 120°C. Samples which had been thermomechanically processed using the above sequence contained no precipitate-free-zones, precipitates of a size comparable to those in overaged condition and a stable and uniform dislocation substructure. Fracture toughness of the TMT material appears to be better than T6 but not as good as T73 condition.