Annotation of the results obtained in 2023
The approaches, methods, techniques, specialized testing devices, experimental configurations, and software developed within the framework of the project "Multiscale Experimental Analysis of Stresses in Lightweight Aviation Materials and Alloys" have been successfully applied to nickel heat-resistant alloys KhN75MBTYu, KhN58MBYu (VZhn159) used in aeroengine manufacturing, and polymer matrix composite materials based on epoxy resins reinforced with carbon fibers employed in the production of critical components of the airframe, such as wings and tail surfaces. In the chamber of Tescan Solaris S9000 FIB-SEM, necessary sample preparation techniques and ion etching methodologies were developed and applied. Using ion etching of circular trenches in these materials, residual stresses (RS) were determined at the scale of 1 to 30 micrometers, comparable to the average grain size (20 to 30 micrometers in nickel alloys), carbon fiber diameter (7 to 8 micrometers in composite materials), and the thickness of the epoxy resin layer between individual fibers (3 to 5 micrometers) and between layers with different orientations of reinforcing fibers. In the normalized nickel alloy KhN75MBTYu, RS were determined in various local configurations of the grain structure (in individual large grains and groups of grains), including during operando stretching of samples with a U-shaped notches in a micromechanical testing rig placed inside the FIB-SEM chamber. In the undeformed alloy, although no pronounced dependence of RS on the local grain configurations could be deduced, the magnitude of RS itself varied significantly (-150 to +150 MPa). In the vicinity of holes obtained by mechanical drilling, the RS variation increased to -400 to +450 MPa. In the nickel alloy KhN58MBYu (VZhn159) obtained by selective laser melting, RS were determined in various local configurations of the grain structure in the cross-section of a single track line and in the adjacent zone of intensive thermal exposure. It was found that RS generally varied within -200 to +200 MPa with a tendency to decrease in absolute value away from the single track. However, they significant negative values (up to -500 MPa) were found closer to the center of the single track in local configurations corresponding to relatively small elongated grains. During operando stretching of samples with a U-shaped notch subjected to micromechanical testing inside the FIB-SEM chamber, effective resulting stresses (the sum of RS at various scales) were determined before, during, and after plastic deformation. It was discovered that effective resulting stresses on the surface of the samples were close in value to the nominal applied external stress. Probably as a world’s first, determination of RS was carried out at micron resolution inside polymer matrix composite material reinforced with carbon fibres. The exceptionally important practical result were obtained for individual carbon fibers (in cross-section and along the fiber) and in the epoxy resin matrix between individual fibers and between separate layers reinforced with carbon fibers with different orientations. It was established that along the carbon fiber, mainly compressive RS were present (up to -4000 MPa), exceptionally high absolute values, significantly exceeding the strength of the composite material as a whole. It was found that in the epoxy resin, mainly tensile RS were present (+70 to +170 MPa). A hypothesis was put forward that the presence of such stresses contributes (by adding to tensile external stresses) to the destruction of the epoxy matrix in the space between fibers and the formation of delaminations between layers reinforced with carbon fibers with different orientations. Using a specially designed testing device placed within the FIB-SEM chamber, the process of destruction of a layered polymer matrix composite material based on epoxy resins reinforced with carbon fibers was visualized during operando micromechanical testing. Local strain fields were mapped by correlating digital images, and direct observation revealed that mechanical failure develops through a series of mechanisms: a) elongation and breaking of individual carbon fibers; b) delamination within the body of the epoxy matrix within one layer of the composite (mainly in the space between individual carbon fibers); c) interlayer cracking of the matrix (with different orientations of fibers relative to the stretching axis). Simultaneously, residual stresses (RS) in such composite materials near the indentation mark from a steel ball were assessed at the scale of 1 to 5 mm using speckle interferometric deformation determination after mechanical drilling. It was demonstrated that tensile RS do not exceed +142 MPa near the indentation mark and attenuate to values ranging from +30 to +47 MPa at distances corresponding to 3 to 5 diameters of the indentation. Devices developed for operando micromechanical testing in the SEM chamber are versatile and have been used for operando testing (tensile and bending) in chambers of devices for X-ray structural analysis and tomography with fine focus X-ray sources, allowing the acquisition of digital images for diffraction, includingg wide-angle (WAXS) and small-angle (SAXS) scattering, as well as high-resolution tomographic and radiographic imaging (voxel size of 10 to 50 micrometers). For samples of polymer matrix composite material based on epoxy resins reinforced with carbon fibers, sets of high-resolution digital images were obtained to map deformations at various load levels. Algorithms and software tools were developed in the Python environment for synchronizing ion etching processes and obtaining (capturing) a stream of high-resolution digital images with optimal contrast, suitable for determining deformations by correlation of digital images during RS relaxation through ion etching. Additionally, they were used for automatic analysis of a set of digital images and calculation of RS at the location of the circular incision. The main results of the second and third years of the project were presented at the international conference "The 13th International Conference on Key Engineering Materials" on March 24-26, 2023, in Istanbul, Turkey, through oral presentations by Ph.D. candidate E.S. Statnik and Professor A.M. Korsunsky. They were also published as scientific articles in the refereed journal (Q2 according to https://www.scimagojr.com/) Materials Today: Proceedings, Elsevier. The titles of the articles are: 1. Eleonsky S.I, Kazantsev D.A., Pisarev V.S., Statnik E.S. "Influence of plate thickness on the results of residual stresses determination by through hole drilling in orthotropic composites of different fiber orientation," Materials Today: Proceedings (2023), Elsevier; and 2. Korsunsky A.M., Sadykova I.A., Statnik E.S., Salimon A.I. "Operando mechanical microscopy of materials," Materials Today: Proceedings (2023), Elsevier. In addition, at the time of writing this report, an article by Eleonsky S.I., Statnik E.S., Pisarev V.S., Salimon A.I., Korsunsky A.M. "Residual stress determination by blind hole drilling and local displacement mapping in aluminum alloy aerospace components" is under review for publication in the refereed journal (Q1 according to https://www.scimagojr.com/) Mechanics of Materials, Elsevier.

 

Publications

---

Annotation of the results obtained in 2021
A complex of traditional and new developed techniques for multiscale experimental analysis (on a scale from 0.1 μm to 10 mm) of residual stresses (RS) was systematically applied on samples of the VT6 (Russian designation for Ti-6Al-4V) titanium alloy widely used in aviation with various microstructures. Various types of microstructure were formed in VT6 as a result of various thermomechanical treatments: 1) coarse-grained Widmanstätten structure of the hcp phase with an average grain size of more than 70 μm after hot rolling and quenching; 2) fine-grained structure of collective recrystallization (average grain size about 10 microns) of hcp and bcc phases, 3) bimodal grain structure with subsystems of grains with sizes less than 2 microns and 10 ... 20 microns of primary recrystallization of the hcp phase. RS were purposefully created in prismatic specimens with such microstructures due to: a) indentation of a steel ball Ø16, b) three-point plastic bending, and c) shot peening (multiple impacts) of the surface with balls Ø1.5 made of bearing steel, respectively. Samples VT6 130x30x6 with a Widmanstätten structure with a dimple of Ø7 from a pressed-in ball Ø16 were purposefully created for the assessment of RS by methods of different size scales and spatial resolution: by the method of mechanical drilling of holes with speckle-interferometric registration of unloading deformations (scale 1 ... 10 mm), by the contour method after ED cutting (scale 250 ... 500 μm) with contact profilometry, X-ray diffraction (sin2φ) (scale 1 ... 5 mm in the plane of the sample and up to 20 μm in depth) and analysis of digital images in the process of ion and plasma-ion etching of annular notches using dual beam scanning microscopes (scale 1 ... 100 µm in the plane of the sample and 0.1 ... 0.5 µm in depth). On the edge of the indentation and on the edge of the bulge on the reverse side of the sample, RS close to zero are observed. At a distance of 2 ... 4 mm from the edge of the imprint and the edge of the bulge, the maximum values of RS are observed with an absolute value of up to 900 MPa (stretching or compressing on different sides) in the radial and tangential directions. As the distance from the edge of the indentation and the bulge increase to 20 mm, the absolute R values decrease to 200 MPa. On this characteristic type of samples, the idea of a multiscale experimental stress analysis with a spatial resolution of 1 μm to 10 mm was fully implemented, and the RS estimates obtained by different methods and on different scales agree with each other with respect to the sign, the nature of the spatial distribution, and the absolute value, taking into account the measurement error of each method. This fundamental scientific result is of a general meaning, since it was also obtained for VT6 samples with a surface hardened by ultrasonic shot peening, where RS was also determined by the above methods with a dimensional scale from 1 μm to 10 mm. By the method of ion etching of annular notches, RS was determined near the holes obtained by mechanical or ED drilling. It has been experimentally proven that in the latter case their level corresponds to the average for the annealed sample (-200 ... + 200 MPa), while mechanical drilling forms increased RS (-500 ... + 500 MPa) in the vicinity of hole edge. A prismatic specimen-beam of VT6 25x10x5 alloy after three-point plastic bending was used to develop new and improve existing methods for assessing RS on a scale of 1 ... 100 µm using (plasma-)ion etching of annular notches. Perhaps for the first time in the world, RS was measured on one sample in comparison using ion etching with gallium ions (scale 10 ... 15 μm) and plasma-ion etching with xenon ions (scale 35 ... 50 μm), and it was shown that the results of both methods were in good agreement. The standard algorithm for RS determination by the digital image correlation method after its modification to eliminate macro drift and correction of overspray effects turned out to be applicable for plasma-ion etching with xenon ions (after the selection of etching modes), which opens up the possibility of a wider use of this technique for accelerated determination of OH at inaccessible to others. dimensional scale methods. In addition, a modified algorithm for determining RS was developed, tested and recognized as correct, based on mapping deformations during relaxation of RS in the peripheral zone of the core-ring notch. The stresses are calculated from the values of the relaxation strains, taking into account the average elastic modulus and are compared with the calculated values from the solution of the Kirsch problem for RS, determined by the standard algorithm. The resulting residual is used to calculate the correction factor to the values obtained by the standard algorithm. Samples VT6 with OH formed by plastic bending and shot peening were subjected to nanoindentation with an indentation depth of up to 5 μm. A correlation was established between the values of hardness and RS. RS affects the amount of deformation work required to achieve a given indentation depth in such a way that the presence of compressing RS requires additional deformation work, which leads to an increase in the measured value of hardness. This model of the established correlation was validated by means of computer modeling and can be used in future with certain precautions as an additional, simple and affordable means of assessing RS with a spatial resolution of 50 ... 200 µm, which is a significant scientific result of the project. Using a Tescan Solaris S9000 high-resolution dual beam scanning electron microscope, III type RS (on a scale of one grain) were evaluated and mapped in purposefully selected large grains of the Widmanstätten structure of the VT6 hcp phase with crystallographic orientations (0001), (01-10) and (-12- ten). On a scale of 10 ... 12 microns in grains with an orientation (0001) RS does not exceed -120 MPa, (01-10) - up to -220 MPa, (-12-10) - up to -350 MPa, which is consistent with the hypothesis of predominant accumulation unrelaxed intrinsic deformations (eigenstrains) in prismatic planes. It was also found that when the scale changes from 10 ... 12 microns to 2 microns (compressive and tensile RS from -800 to + 500 MPa), the RS variations significantly increase - from 85 to 300%. Using the Alemnis device, equipped with a Berkovich indenter, in the Tescan Solaris S9000 chamber, targeted nanoindentation of the (0001), (01-10) and (-12-10) grains was carried out. The hardness values for these grains were 7.12 ± 0.05, 5.23 ± 0.57, and 4.55 ± 0.87 GPa, while the statistical analysis of the results of random nanoindentation (180 indentations) reveals a lognormal distribution of hardness values with a median of 4.885 ± 0.45 GPa. Mechanical tests of miniature VT6 samples were carried out in a Tescan Vega 3 LMU scanning electron microscope chamber after the integration of the Deben MT 1kN universal testing device into it. Simultaneously with stretching in the operando mode, large sets of high-resolution images were obtained for analysis by the digital image correlation method, which made it possible to visualize microdeformation processes, map deformation fields, and reveal the localization of deformation preceding fracture in a narrow band. The stress-strain curves obtained in SEM were used to determine the elastic modulus, yield strength and tensile strength, and elongation till rupture and were compared with the results of macro-mechanical tests in tension, compression, and bending, which were carried out for VT6 specimens using universal testing machines. Methodological aspects of mechanical tests in an SEM chamber and the effect of RS on the mechanical characteristics of light aviation materials measured in this way are discussed in the published article https://doi.org/10.3390/met11081175 (Q1 at www.scimagojr.com), dedicated to the aluminum alloy used in Yarilo satellite design.

 

Publications

---

Annotation of the results obtained in 2022
A multiscale analysis of light aircraft materials (VT6 titanium alloy, D16 aluminum alloy and MA2 magnesium alloy) was aimed at systematic comparison of data from new and developed experimental methods that estimate residual stresses (RS) and mechanical properties of these materials on a scale from 1 to 30 µm, s the results of applying traditional methods on a scale of 0.5…10 mm. Methods that provide understanding of the features of RS distribution and the development of microdeformation processes on a scale of 10–30 µm are being developed by conducting unique experiments on mechanical tests in the chamber of a two-beam scanning microscope. After the development of methods for ion etching of microcolumns in the VT6 alloy with a diameter of 2 μm and a height of 4 μm in individual large grains (70 μm) and ensembles of small grains in the SEM chamber, more than 80 microcolumn compression curves (up to 40% of the column height) were plotted with a blunt conical diamond indenter devices of the Alemnis device integrated with the Tescan Solaris S9000 dual-beam electron microscope. At the same time, thanks to the developed software for synchronizing and automating the acquisition of high-resolution digital images in a stream, the formation of single and multiple shear bands was visualized and their activation stresses, as well as additional competitive shear bands, were determined. A significant dependence of the yield strength and the maximum compressive stress achieved on crystallographic was shown. When compared with the macromechanical properties of VT6, the average values of micromechanical properties over a variety of crystallographic orientations show similar values, however, there is a strong variation relative to the average depending on the crystallographic orientation - the yield strength, depending on the crystallographic orientation of the grain, can differ up to 5 times (from 500 to 2500 MPa) , and also differ for the hexagonal or cubic phase. After the development and manufacture of special miniature samples, grippers and connecting elements in the chamber of a two-beam scanning microscope Tescan Solaris S9000, unique experiments were carried out to determine, using the method of ion etching with gallium ions, ring-shaped cuts of the effective resulting stresses directly in the process of micromechanical testing (operando) using a Deben device MT 1kN of miniature samples of titanium alloy VT6, aluminum alloy D16 and magnesium alloy MA2. Miniature samples, incl. with concentrators in the form of holes, U and V-shaped notches, were subjected to tension until failure, and the effective resulting stresses (the result of the superposition of residual stresses of various size scales and the applied external stress) were determined at 4 levels of the applied external stress as an average of 3 ring-shaped notches. It is found that the effective resultant stresses change in a complex way as the external applied stress increases, but in general, they noticeably decrease (and even the appearance of local compressive effective resultant stresses at a nominal tensile external applied stress) by the time of the onset of intense plastic flow, especially near the fracture zone. . In this case, the effective resulting stresses change little in miniature specimens with round holes as the external applied voltage increases. To simulate the practical spatial distribution of favorable compressive residual stresses (RS) around the holes, a special loading device was developed and manufactured for coaxial two-sided indentation of balls into the plates, which made it possible to purposefully form light aviation materials in the plates (VT6 titanium alloy, D16 aluminum alloy and magnesium alloy MA2) compressive residual stresses in the area around the impressions from the pressed balls. The RS formed by coaxial bilateral indentation of balls were studied by mechanical drilling (and subsequent speckle interferometry of relaxation deformation), electric spark cutting (contour method), X-ray method sin2ψ and ion etching of annular notches; multiscale (10 μm ... 10 mm) was carried out. For X-ray studies, a sharp-focus source was used at the XEUSS 3.0 facility at JINR, Dubna, which ensures the size of the illumination spot on the sample of 150 x 150 μm x μm, which significantly increased the spatial resolution. It has been experimentally shown that the formation of noticeable compressive RS near the indentations from the balls on all materials takes place on a scale of 100 μm ... 10 mm, while on a scale of 10 ... 30 μm RS showing generally the same spatial distribution may occur tensile inheriting perhaps the history metal plastic deformation. Methods for multi-scale experimental analysis of residual stresses developed and applied in the works of the first and second years of the project are implemented, and the results are published in the article “Comparative Multi-Modal, Multi-Scale Residual Stress Evaluation in SLM 3D-Printed Al-Si-Mg Alloy (RS -300) Parts". Metals MDPI, Volume 11 Issue 12 Article number 2064, 1-19. Eugene S. Statnik, Fatih Uzun, Svetlana A. Lipovskikh, Yuliya V. Kan, Sviatoslav I. Eleonsky, Vladimir S. Pisarev, Pavel A. Somov, Alexey I. Salimon, Yuliya V. Malakhova, Aleksandr G. Seferyan, Dmitry K Ryabov, Alexander M. Korsunsky. doi: 10.3390/met11122064 (Q1 at www.scimagojr.com), dedicated to the aluminum alloy used in the construction of the Yarilo satellite. Similarly, the article "FIB-DIC Residual Stress Evaluation in Shot Peened VT-6 Alloy Validated by X-ray Diffraction and Laser Speckle Interferometry". Nanomaterials MDPI, 2022, Volume 12, Article number 1235, pages 1-13. Pavel A. Somov, Eugene S. Statnik, Yuliya Kan, Vladimir S. Pisarev, Svyatoslav I. Eleonsky, Dmitry Yu. Ozherelkov, Alexey I. Salimon. doi: 10.3390/nano12071235 (Q1 at www.scimagojr.com) presents in full the data (obtained by various methods on conjugate dimensional scales) on the magnitude and nature of the distribution of residual stresses in a widely used aviation light alloy. Article "Micro-scale residual stress and deformation analysis in bimetal bronze-stainless steel samples produced by laser powder bed fusion technology". Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing (2022). Volume 858, page 144110-144127. Statnik E.S., Somov P.A., Zherebtsov D.D., Saprykin D.L., Saprykin L.G., Chernovolov V.V., Polozov N.A., Salimon A.I. doi: 10.1016/j.msea.2022.144110 (Q1 at www.scimagojr.com) contains the most significant results of the study and for the first time provides experimental data on the magnitude of residual stresses in a critical product obtained by 3D printing by selective laser alloying near the interface of immiscible metals.

 

Publications

---