Improved fatigue resistance of Al-Zn-Mg-Cu (7075) alloys through thermomechanical processing
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T o d e c r e a s e the accumulation of d a m a g e during long-life low-stress cyclic loading, m i c r o structures must accommodate inelastic deformation by homogeneous or " d i s p e r s e d " slip r a t h e r than by localized slip concentrations. In age-hardening aluminum alloys this r e q u i r e ment can be met by introducing a dense and uniform dislocation forest through suitable thermo-mechanical treatments. Such a treatment was developed for A1-Zn-Mg-Cu (7075) alloys, involving a process cycle of solution annealing, partial aging, mechanical working and final aging. The fatigue properties (S-N curves) of c o m m e r c i a l and high-purity 7075TMT a r e compared with conventional 7075-T651 properties; w i t h zero mean s t r e s s the a l t e r nating s t r e s s t o c a u s e f a i l u r e in 107 cycles is more than 25 pct h i g h e r for c o m m e r c i a l p u r i t y 7075TMT and almost 50 pct h i g h e r for high-purity 7075TMT. The r e s u l t s emphasize the importance of microstructural control when high fatigue resistance is required.
A qualitative, yet detailed, picture has e m e r g e d from the efforts of many investigators ~-16 describing v a r i o u s microstructural changes associated with the a c c u m u lation of fatigue d a m a g e in age-hardened aluminum alloys. While one might d e s i r e a more quantitative correlation between microstructural properties and the fatigue processes, it is worthwhile t o attempt t o synthesize microstructures which should be more resistant t o fatigue d a m a g e u s i n g the presently a v a i l able information. In a r e c e n t review of thermomechanical processing and fatigue of age-hardened aluminum alloys Ostermann and Reimann~7 proposed r e q u i r e m e n t s for microstructures of age-hardened aluminum alloys resistant t o the accumulation of fatigue d a m a g e in the high-cycle fatigue r e g i m e . The principal requirement for high-cycle fatigue resistance is the ability of the alloy t o deform by homogeneous or dispersed slip r a t h e r than by slip concentrating in narrow, isolated bands. Homogeneous deformation of age-hardened alloys r e q u i r e s a h o m o geneous microstructure, i . e . absence of strain-concentrating nonmetallic and intermetallic inclusions, and precipitale-free zones. The precipitate-strengthened m a t r i x must also be stable during cyclic loading and r e s i s t reversion and overaging of the second-phase particles due t o dislocation motion and other slip induced lattice defects. Deformation of such an i d e a l , homogeneous alloy structure, however, will not be necessarily homogeneous, s i n c e the initiation of slip in a highly precipitation-strengthened m a t r i x is quite difficult and slip bands, once they are formed, remain narrow~s-2° and thereby localize further deformation during cyclic loading. Thermomechanical treatment (TMT) of age-hardened alloys~7 offers the possibility of dispersing the slip and broadening slip bands by introducing abundant slip initation sites through mechanical working. For
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