Yielding Phenomenon of an Al-3% Li Alloy

Abstract

The effect of annealing on stress-strain curves of an Al-3% Li alloy which had been overaged and then cold-rolled was studied, by comparing with the changes in lattice constant and electrical resistivity and also in the transmission electron micrographs. A sharp yield drop followed by the propagation of Lüders bands was observed when the Al-3% Li alloy was annealed in a recovery range, i.e. 200°∼380°C. The same type of yielding was also observed an Al-4.7% Cu alloy. The Al-3% Li alloy, annealed at 320°C, was deformed at various temperatures (−196°∼230°C) with various strain rates (7.4×10⁻³∼3.7×10⁻⁶sec⁻¹). At low temperatures, the specimen deformed with Lüders bands which were followed by work-hardening. In the tensile test, at room temperature with a low strain rate or at 100°C, the specimen necked and tore off in a Lüders band and showed “pseudo-brittleness”. The specimens, which showed the propagation of Lüders bands, was in a fully recovered or partially recrystallized condition. The average grain diameter was 0.9∼1.9 μ. The measurements of lattice constant and electrical resistivity revealed that partial decomposition of a dispersed second phase, i.e. AlLi, took place, when the specimen was annealed at a temperature above 220°C. This temperature was much lower than the over-aging temperature, i.e. 360°C. It was found that the decomposition of the second phase was closely related to the presence of a sharp yielding. The above results were discussed in terms of general requirements for the yielding. It was concluded that the decomposition of the precipitates, AlLi, was not always necessary for the occurrence of the sharp yielding but promoted the pinning of dislocation sources near the boundaries.