APPLICATION OF TEM TO STUDY THE CHANGES IN SUB-SURFACE DEFECTS IN TUNGSTEN SAMPLES AS A FUNCTION OF ANNEALING TEMPERATURE

摘要: Transmission Electron Microscope (TEM) is a powerful instrument that can be used for identifying and studying sub-surface defects in materials.‘STEM (scanning transmission electron microscope) bright field’mode of TEM, utilizing the diffraction contrast, was used for identifying the dislocations in Tungsten (W) samples. Annealing of cold rolled W foil samples was carried out at 773 K, 1173 K, 1373 K, 1838 K, and 2038 K in a vacuum furnace, for two hours each, under reducing atmosphere. The effect of annealing temperature on dislocation density and micro-structural changes in cold rolled W foils have been investigated well below and above the recrystallization threshold temperature (~ 1600 K) of bulk tungsten. Electron transparent specimens required for TEM study have been carefully prepared. Dislocations have been identified and the changes in nature of these dislocations with annealing temperature were studied using a FEI make 300 kV TEM (Model: Tecnai G2 F30). TEM images and their analysis has shown that the average dislocation density has reduced and the length of individual dislocations increased, with increased annealing temperature. Calculations indicated that the average dislocation density was reduced by two orders of magnitude in the samples annealed at 2038 K, when compared with that of cold rolled samples. Further, in the samples annealed at temperatures above recrystallization threshold temperature, TEM images have clearly shown the disappearance of elongated grains in place of which stress free new grains were formed.