Yao Mei

On deformation-induced continuous recrystallization in a superplastic AlLiCuMgZr alloy

Abstract The microstructural changes of a warm rolled AlLi alloy occurring during static annealing and superplastic deformation at 515°C were studied by means of transmission electron microscopy. Deformation induces a continuous recrystallization with a rapid subgrain growth and a rapid increase in boundary misorientations. The higher strain rate results in a faster subgrain growth and a finer recrystallized grain size. The increasing rate of boundary misorientations and the strain at which the average misorientation reaches about 20° increase with increasing strain rate. The increase in boundary misorientations is proportional to the subgrain growth during the whole static annealing process. Deformation results in a more rapid increase in boundary misorientation with subgrain size than static annealing. Dislocation gliding plays an important role before the formation of high angle grain boundaries during superplastic deformation. The absorption of dislocations into subgrain boundaries results in a more rapid increase in boundary misorientation during deformation. Thus, the mechanism of the deformation-induced continuous recrystallization is suggested to be the generation of dislocations in grains and the absorption of gliding dislocations into subgrain boundaries.

New method for determining grain boundary parameters

Abstract With the aid of a TEM double-tilt holder, a new method for determining grain boundary parameters, the misorientation axis, misorientation angle and the boundary normal is developed. Using the new method, three zone-axis directions from each of both grains are aligned to the beam direction and the corresponding tilt angles are obtained from the meter reading of the holder. After inputting these data, the misorientation axis and angle can be calculated precisely and rapidly with the aid of computer software. For determining the boundary normal, using the new method two vectors lying in the plane of boundary are determined by measuring the change of the projections of two vectors from two micrographs of the boundary taken before and after an adjustment of the double-tilt holder. The boundary normal is the cross product of these two vectors. Thus, the new method is considered to be simpler and more convenient when compared to other methods since no Kikuchi pattern needs to be taken.

Anomalous strain rate dependence of the flow stress in TiAl intermetallic compounds

Plastic deformation of TiAl and TiAl-V intermetallic compounds has been stu- died by compression experiment at various temperatures and strain rates. Results show that the plastic deformation in distinct temperature range is principally controlled by the mechanisms of Perierls-Nabarro, cross slip and creep of dislocation. For TiAl-V alloy deformed at a range of 600--700 K, the negative strain rate dependence of flow stress was observed, i. e., the more the plastic strain is, the more the negative dependence will be. A possible mechanism of the anomaly could be interpreted by thermal activation of dislocation cross slipping. The effects of tempera- ture and strain rate on work-hardening exponent were also studied and discussed.

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

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

TEM method for rapid and precise evaluation of continuous recrystallization

Abstract With the aid of a double-tilt holder in a transmission electron microscope, a new method for a rapid and precise determination of the misorientations of a large number of subgrain or grain boundaries is given in this article. By use of the method, the continuous recrystallization can be rapidly and precisely evaluated when compared to the other conventional methods.