Incoherent Precipitate Structure

Fig. 2.3. Coherent and Incoherent Precipitates3

Incoherent Precipitate Structure

Fig. 2.3. Coherent and Incoherent Precipitates3

mechanical history of the product. The progression of precipitation hardening in the aluminum-copper system is:

Super Saturated Solid Solution ^ Clustering ^ GP Zones ^ 6'' ^ 6' ^ 6

The GP zones will normally develop on aging at room temperature. During heating, the GP zones develop an intermediate precipitate (6''), which has a tetragonal structure that forms as plates that maintains coherency with the matrix and further increases the strain in the matrix, providing peak strength levels. On still further heating, 6'' is replaced by a second intermediate precipitate 6' which is not coherent with the matrix and the strength starts to decrease, and the alloy is now termed overaged. However, in the highest strength condition, both 6 and 6 are generally present. Both the precipitate particles themselves and the strains they produce in the lattice structure inhibit dislocation motion, and thus both contribute to strengthening. Further heating of the alloy causes 6' to transform to the equilibrium precipitate 6, which is stoichiometric CuAl2. The other heat treatable aluminum alloys behave similarly but the precipitates are, of course, different. The progression of the aging process is shown schematically in Fig. 2.4. Both the underaged and overaged conditions have lower strengths and hardness levels than the peak aged condition.

The wrought heat treatable 2XXX alloys generally contain magnesium in addition to copper as an alloying element; the significance being that these alloys can be aged at either room temperature or at elevated temperature. Other significant alloying additions include titanium to refine the grain structure during ingot casting and transition element additions (manganese, chromium, and/or zirconium) that form dispersoid particles (Al20Cu2Mn3, Al18Mg3Cr2, and Al3Zr) which help control the wrought grain structure. Iron and silicon are considered impurities and are held to an absolute minimum, because they form intermetallic

Fig. 2.4. Typical Aging Curve for Aluminum Alloys

Fig. 2.4. Typical Aging Curve for Aluminum Alloys compounds (Al7Cu2Fe and Mg2Si) that are detrimental to both fatigue and fracture toughness.

The wrought heat treatable 7XXX alloys are even more responsive to precipitation hardening than the 2XXX alloys and can obtain higher strength levels. These alloys are based on the Al-Zn-Mg(-Cu) system. The 7XXX alloys can be naturally aged but are not because they are not stable if aged at room temperature, i.e. their strength will gradually increase with increasing time and can continue to do so for years. Therefore, all 7XXX alloys are artificially aged to produce a stable alloy.

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