X-ray-diffraction approach for studies on fatigue and creep

It is well known that X-ray diffraction is one of the most powerful means for investigating the microscopic structure of crystalline materials. X-ray diffraction is advantageous when it is applied to metallic materials; it responds very sensitively to changes in the metal's crystalline structure. Another characteristic advantage of the X-ray-diffraction approach is its nondestructive nature in the measurement of crystalline-material parameters, enabling us to observe the process of mechanical phenomena of metals, such as fatigue and creep. The X-ray-diffraction patterns obtained on a deformed material include a great deal of information covering the microscopic and macroscopic characters consistent with the nature of the existing material. Residual stress measured by means of X ray is called the macroscopic-material parameter. It is evaluated by measuring the shift of the peak of a diffraction profile. The diffusiveness of the profile corresponds to the irregularity in microscopic structure of deformed crystalline material and it is noted as the submacroscopic material parameter. The X-ray-microbeam diffraction technique supplies information on the change in microscopic structure such as subgrain size, misorientation and microlattice strain. Profile analysis is another way to evaluate the microscopic-material parameters: particle size and microscopic strain. By appropriately combining these techniques in the study of mechanical behavior of materials, the parameters that control the phenomena may be extracted to facilitate discussion of their mechanism. In this lecture, X-ray-diffraction techniques to evaluate the macroscopic, submacroscopic and microscopic-material parameters are presented and the approach is demonstrated by exhibiting a case of studies on fatigue and creep of carbon steels at room and elevated temperature, where phenomena are discussed in terms of the change in the material parameters. Initiation and propagation of fatigue crack in steel at room temperature, the change in microstructure during isothermal and thermal fatigue, and also that in creep at elevated temperature under variational load are presented.