X-ray diffraction for diagnosis at the nanoscale

Abstract The creep phenomenon stems from a continuous increase of atomic thermal vibration degenerating to plastic deformation triggering a number of correlated effects: -variations of lattice macrostrains, -microstructures assets and related grain boundary variation, -diffusion of atoms and vacancies and related variation of atomic flows -dynamic recombination of atomic aggregations thus resulting in nucleation and growth of new chemical compounds (secondary phases). The problem rise when the diagnosis for the early stage recognition of creep is carried on in laboratory with using the traditional diffractometers; unavoidably specimen manipulations and laboratory ambient conditions have an influence on the results of the analyses. The alternative is the use of XRD directly on the site where the components are in service. The capability of instruments, to detect contemporaneously the creep governing physical mechanisms and related chemical secondary effects, brings to consider the “on site” inspection via NDT-XRD at the highest level of the inspection effectiveness, in principle though. A preliminary prototypal technology (DifRob®) has been implemented to demonstrate such a capability. When used to identify the effects on the diffraction signal from alloys and other materials, such a technology has already demonstrated very promising performances, it is still running at the stage of concept proofs though. The paper outlines the basic principles of the x-ray diffraction method; the justifications to state that XRD measures can be among the NDT methods with the highest effectiveness for early stage diagnoses at the nano scale; the creep nucleation and propagation is considered with special attention; the compliance between technology requirements and value proposition of XRD is also considered when observing creep phenomenon, i.e. the material induced modification when working under cyclic thermal loads; the significance of measure and related results along with the warning and limitations when such measures and results shall be compared with coherent reference materials and benchmark values. Moreover the paper gives overview of: - the evolution from the traditional Bragg Brentano diffractometers configuration - the state of the art of the new technology for “on site XRD”, its hardware functionalities, the software platforms on which the programming codes have been based to implement: i) the instrument set up and measures processes, ii) the interpretation of results. - examples are presented of the observations from surfaces at different roughness, microstructures and cold working effects. Said proofs of concept have been obtained from using DifRob® by collecting data from materials on which lattice deformations were induced by: o macrostrains or residual stress from thermal alterations of welding; o variation of microstructure and chemical composition of alloys o mechanical or chemical treatment for roughness and rectification processes of surfaces. - future developments of the technology to improving the robotic attitude for near remote sensing usability of the technology.