Grain Growth in Tungsten Rod

Details: Category: Tungsten Information; Published on Tuesday, 23 September 2014 15:03

Samples were deformed at elevated temperatures in compression and then annealed. The microstructure was examined by optical metallography, scanning electron microscopy, and electron backscattering diffraction patterns (EBSP) in the scanning electron microscope. The results showed that when the samples were deformed at a high crosshead speed at 1200, 1400, and 1600°C, the microstructure consisted of grains that had developed a high aspect ratio in the regions of the sample that had received the most deformation. When these samples were annealed, a more equiaxed structure was produced. Samples deformed at 1800°C developed very fine recrystallized grains during the mechanical testing. These remained during the anneal. When the crosshead speed was reduced, these fine grains could be observed even after testing at 1200°C. EBSP revealed little, if any, crystallographic texture in the recrystallized grain structure. These results are compared with previous work on tungsten rod to develop a more complete description of this process.

For many years there has been a great interest in understanding grain growth in aliminum-potassium-silicon (AKS) doped tungsten because of its importance in the lighting industry. The non-sag struture of long-life lamp filament is produced by s process secondary recrystallization (abnormal grain growth) in which the final grain shape is controlled by the rows of potassium bubbles present in the material. Also, during the processing of tungsten to make wire, it is often necessory to anneal. Grain growth in these rods occurs by a process in which a few grains grow and consume all others. The mechanism could be described as secondary recrystallization, but it has been suggested that the process is better described as strain annealing, since the few grains that grow nucleate in a matrix that is polygonized and thus is similar to the lightly deformed matrix in which the large grains grow during strain annealing.

One question that has not been addressed in whether or not there is a crystallographic component to this process. That is, do the few grains that grow in the rod and consume the others have a particular orientation relative to the sample orientation or the way that it was deformed. In this paper, we investigate this process through the use of backscattered Kikuchi patterns formed in the scanning electron microscope. Our results will show that there may be a slight crystallographic preference in the grains which initially begin to grow, but that when grain growth is complete the orientation of the grains is random. We also show that strain rate slow enough, small grains can form by dynamic recrystallization during deformation. Finally, we suggest that the formation of the new grains is controlled by the number of nucleation sites in the material and in some cases the resulting grain size may be smaller than the original grain size ans in other cases larger, Thus, trying to classify the process as recrystallization, grain growth, abnormal grain growth, or strain annealing may not be important; rather having a good understanding of the factors that control grain growth is what will allow engineers to develop material with the desired properties.

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