Hardening and softening of aluminum alloys under load at 135?150�C

Abstract

1. Semifinished products of wrought heat resistant aluminum alloys D20-1, D16, and VAD23 undergo hardening and softening at 150°, 135°, and lower temperatures under the influence of tensile stresses; hardening is due to additional precipitation of hardening phase and evidently to phase strain hardening; softening is due to coalescence of the precipitated phase. 2. Less aged semifinished products tend to harden more under high stress equal to the long-term strength for 100 h; intensive hardening of semifinished products of alloys D20-1 and D16 occurs at 135°; hardening in semifinished products of alloys AK4-1 and VAD23 also beings at 135 and 150°. 3. With increasing temperature and decreasing stress the coalescence of particles predominates, the acceleration of which in semifinished products of alloys D20-1 and D16 leads to precipitation of the less heat resistant CuAl2 phase and a heterogeneous partially recrystallized structure in extruded semifinished products of alloy D16. 4. Extruded semifinished products with a large-grained recrystallized structure tend to harden more. The long-term strength of these semifinished products may be more consistent than that of products with a fine-grained equiaxed structure. In the direction along the height these products are hardened less and have a lower long-term strength than those with a fine-grained equiaxed structure. 5. The tendency of the alloy to harden and soften during long-term strength tests can be judged from the change in the long-term ductility — the larger the slope of the curve, the greater the hardening. An increase of the reduction in section during long-term strength tests characterizes intensive softening of the alloy during necking.