ROOT-MEAN-SQUARE DEVIATIONS OF ATOMIC POSITIONS - NANOSCALE INDICATOR OF SURFACE CONDITION
The study relevance is associated with the need of increasing the reliability of welded joints in steam power plants, hence with a rational use of product materials due to improvement and development of methods for studying the physical characteristics.Purpose of the study is to introduce the new nanoscale structural criteria of the third kind to assess the surface condition.Object of the study is specimens of heat-resistant steel tubular heating surfaces of steam boilers.Methods include physical modeling of operating conditions by thermal cycling in the MIMP-10UE electric furnace, cyclic cold deformation by means of a hydraulic press, roentgenometry of specimens with evaluation of the root-mean-square deviations and internal structural stresses using the DRON X-ray diffractometers, analysis of surface morphology with the portable electron microscope PENSCKOPE with20x magnification.Results. Experimental studies, related to quantitative evaluation of the amplitude of atomic vibration and its evolution during cyclic thermaland mechanical loads, resulted in introduction of a new diagnostic parameter - the amplitude of atomic vibration �̅2, reflecting the funda-mental nature of interatomic bonds. Correlation between the root-mean-square deviations of atoms (the microscale level of properties),which are the microstructural characteristics of the third kind, and the macromechanical characteristics 𝜎0.2 and (the macroscale levelof properties) was demonstrated. As the offset yield point and ultimate strength point are related to strength and breaking, then the root-mean-square deviations of atoms can be used as an indicator for damage and an indicator of limit states. The parameter �̅2 can also be acharacteristic of the current physical state of metal. The new method for assessment of current and limit states was suggested. It wasshown that structure restoration is monitored at the atomic microscale level, using the vibration amplitude and the stresses of the first kind.