High Resolution Electron Microscopy Observations of Structural Changes in Iron Nitride Films Annealed in Vacuum

Iron-nitride films were prepared by reactive sputtering, and the effect of annealing treatment on the structures was investigated by means of in-situ electron microscopy and high resolution electron microscopy (HREM). As-deposited films were observed to be a mixed structure of a few ultrafine epsilon-Fe2-3N particles existing in the amorphous matrix. it was found that the structure-relaxation in the amorphous occurred at 473 K, and the ultrafine grains began to grow at the higher annealing temperatures. The transition of the amorphous to epsilon-Fe2-3N was almost completed at 673 K. It is considered that the formation of the ideal epsilon-Fe3N is originated from the ordering of the nitrogen atoms during the annealing in vacuum. On the other hand, gamma'-phase (Fe4N) was seen to precipitation of epsilon-phase at 723 K. Two possible modes are proposed in the precipitation of gamma'-phase, depending on the heating rate and crystallographic orientation relationships, i.e. [121](epsilon)//[001](gamma), (2(1) over bar0$)(epsilon)//(110)(gamma) and [100](epsilon)//[110](gamma), (001)(epsilon)//(111)(gamma). In addition, alpha-Fe particles were observed to form from the gamma'-phase at high temperatures. We assumed that these structural changes are due to the diffusion of nitrogen and iron atoms during the annealing, except for the case of the precipitation of the gamma'-phase as depicted above. The results obtained in this work are in a good agreement with the assumption.

In-situ transmission electron microscopy observation of the evolution of dislocation loops and gas bubbles in tungsten during H2+and He+dual-beam irradiation

Dislocation loop and gas bubble evolution in tungsten were in-situ investigated under 30 keV H_(2)^(+)and He^(+)dual-beam irra-diation at 973 K and 1173 K.The average size and number density of dislocation loops and gas bubbles were obtained as a function of irradiation dose.The quantitative calculation and analysis of the migration distance of 1/2<111>loops at low irradiation dose indicated that the main mechanism of the formation of<100>loops should be attributed to the high-density helium cluster inducement mechanism,instead of the 1/2<111>loop reaction mechanism.H2+and He+dual-beam irradiation induced the formation of<100>loops and 1/2<111>loops,while increasing the irradiation temperature would increase<100>loop percentage.The percentage of<100>loops was approximately 18.6%at 973 K and increased to 22.9%at 1173 K.The loop reaction between two 1/2<111>loops to form a large-sized 1/2<111>loop was in-situ observed,which induced not only the decrease of the number of 1/2<111>loops but also the significant increase of their sizes.The<100>loops impeded the movement of dislocation line and tended to escape from it instead of being absorbed.With the increase of irradiation dose,the yield strength increment(Δ_(σloop))caused by the change of loop size and density increased first and then decreased slightly,while the yield strength increment(Δ_(σbubble))caused by the change of bubble size and density always increased.Meanwhile,within the current irradiation dose range,Δ_(σloop)was much larger thanΔ_(σbubble).

Deformation Field around the Stress Induced Crack Area in Sandstone by the Digital Speckle Correlation Method

The deformation field around sub-cracks was calculated using the digital speckle correlation method. First, the uni-axial compression tests on sandstone samples containing a pre- fabricated fracture were made. Photomicrographs showing the characteristics of the sub-crack development were taken using a scanning electron microscope （SEM）. From these photomicrographs, the real-time images showing the initiation, growth and coalescence of sub-cracks and micro-cracks in the sandstone specimens were obtained and the effects of loading level as well as grain boundaries on the development of cracks were analyzed. Second, the intensity images of the sandstone specimen surface were captured from the observations of the SEM corresponding to different loading levels. Then correlation computation was carried out for the sequential pairs of intensity images to evaluate the displacement components, as well as the strain field. The results show that the deformation varies in different areas separated by sub-cracks during rock damage processes.