Microstructural changes during superplastic deformation of an Al-Li-Cu-Mg-Zr alloy

Abstract

Fine grain size is one of the essential conditions for polycrystalline materials to have superplasticity. Several methods are available for grain refinement. Recrystallization has been used extensively for the grain refinement of superplastic quasi-single-phase aluminium base alloys. One of the characteristics of superplastic deformation is that the fine grain sizes do not change substantially even after several hundreds of per cent strain [1, 2]. The grain size stabilization provided by the precipitate pinning has been widely used as an explanation of the above characteristic. An early suggestion has also been made that recrystallization plays an important role in grain size stabilization during the deformation of superplastic alloys [3, 4]. Deform-induced recrystallization during the initial stage of superplastic deformation of a warm-rolled AI-Li base alloy has been well known [5, 6]. Chokshi et al. [7] have found that the coarse surface grains formed by annealing before the tensile testing of an AI-Li base alloy recrystallized dynamically to produce a fine-grained microstructure on the surface of the specimen. This letter deals with the microstructural changes of a warm-rolled AI-Li-Cu-Mg-Zr alloy during superplastic deformation by means of transmission electron microscopy (TEM). It is demonstrated that the fine recrystallized grains formed by deformationinduced recrystallization during the initial stage of superplastic deformation recrystallize dynamically and repeatedly during the following stable superplastic deformation. The AI-Li base alloy and the deformation conditions used in this study were the same as those described in [8]. Microstructural studies were conducted on the specimens which were unloaded from superplastic tensile tests at a constant strain rate of 3 x 10-4s -1 by interrupting the test. For TEM observation the specimens were prepared by the conventional double-jetting method. A Philips CM12/STEM electron microscope was used to study the internal structure of the specimen. The misorientations of the grain boundaries of different specimens were measured by a simple and convenient method innovated by Liu [9], and more than 100 boundaries were measured for one specimen. Histograms were plotted to show the changes of misorientations occurring during superplastic deformation. The microstructure of the warm-rolled alloy is shown in Fig. la. On heating to the superplastic test temperature (515 °C) for about 5 rain, a subgrain structure was developed which was approximately 2/xm in average diameter (Fig. lb) and the misorientations between the subgrains was within a few degrees (Fig. lc). Fig. 2 shows the changes of the misorientations of grain boundaries with the strain during superplastic deformation at a strain rate of 3 x 10 .4 s -1. These histograms of misorientations indicate a progressive shift to higher misorientations with strain, with an