The control of grain size particles in grain boundary for superplastic aluminum alloys

Superplastic aluminum alloys have been applied to airplanes and architectures, and are now tried on automobiles. In this application, the forming speed is an important factor. Therefore, it is desired to develop aluminum alloys with high strain rate superplasticity. In I/M aluminum alloys, 7475(Al-Zn-Mg-Cu) and 8090(Al-Li) with controlled structures have been developed for superplasticity. The superplastic 7475 sheets produced by thermomechanical treatment have fine recrystallized grains of some 10 mu m in size and typically show superplasticity at a strain rate of 10(-4)-10(-3)s(-1) at 773K. On the other hand, the 8090 sheet having non-recrystallized grains is dynamically recrystallized by superplastic deformation to yield fine grains. This alloy shows superplasticity at a strain rare of 10(-3)-10(-2)s(-1) at the same temperature; this strain rate is higher than that attainable in the 7475 alloy. The author speculated that this higher strain rate is due to fine grains and clean grain boundaries with few particles and impurities due to dynamic recrystallization. This idea was applied to the superplasticity of the 7475 alloy. Two kinds of method were evaluated: one formed grain boundaries with few particles, and the other used dynamic recrystallization. The Former was carried out by adequate solution heat treatment after static recrystallization. The latter was done by pre-straining cold-rolled sheets at warm temperatures followed by superplastic deformation. As a result, the 7475 sheets produced by both methods had superplasticity at a higher strain rate of 10(-3)-10(-2)s(-1). The reason why high strain rate was achieved was because in the former, large second-phase particles that were precipitated before rolling to introduce strains around these particles, were fully solutionized, and then cavitation was inhibited during superplastic deformation. It is important to notice that higher strain rate superplasticity was achieved although the grain sizes before and after solution heat treatment were almost the same. In the latter, subgrains were formed during pre-straining at a warm temperature and fine recrystallized grains formed dynamically during superplastic deformation. In P/M aluminum alloys, many solute atoms such Zr, Or, Mn can be super-saturated in the matrix by this rapidly solidified technique. Dynamic recrystallization occurs because these elements stabilize subgrains and inhibit recrystallization at high temperatures. The 7475 PIM alloy containing 0.7%Zr shows splendid superplasticity at a high strain rate of 10(-1)s(-1). it is concluded that dynamic recrystallization is effective to improve superplasticity because it yields fine grains and produces clean grain boundaries with Few particles and impurities.