PSI - Issue 40

M.V. Nadezhkin et al. / Procedia Structural Integrity 40 (2022) 321–324 M.V. Nadezhkin at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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evaluating the characteristics of metals is ultrasonic monitoring (Ding X. et al. (2014); and Kobayashi M. (2010)). It enables, specifically, to track the material state under plastic deformation and to study the influence of structural changes on acoustic parameters long before the emergence of macro-cracks. Based on the interaction between the elastic wave and the metal structure, it is possible to identify the main aspects affecting the acoustic characteristics of the latter. In particular, these include any strain-induced changes in the micro-uniformity of the material, which can be associated with dislocation and vacancy densities, accumulation of micropores and microcracks, contact between strengthening particles and metal matrix, effective elastic moduli in local zones due to microstresses and so on. All these factors impact the scattering and absorption energies of the ultrasonic waves, as well as their propagation speed, which is one of the parameters widely used in the material state diagnosis (Torello D. et al. (2015); Marcantonio V. et al. (2019)). In general, the ultrasonic wave velocity during loading exhibits a non-monotonic behavior (Murav'ev V.V. et al., 1996). The relationship between the ultrasound velocity and the mechanical properties of metals is shown in Ref. (Barannikova S.A. et al., 2016). The dependences of the ultrasound speed on the composition, structure and state of metals and alloys were described in detail in work (Murav'ev V.V. et al., 1996). However, there is still a lack of knowledge about the correlations between the characteristics of ultrasound, in particular, its propagation speed in metals, and the physical and mechanical properties of the material in a wide range of low temperatures (Kobayashi M. (2010); Ding X. et al. (2014); Lunev A.G. et al. (2018)). Therefore, the current study is aimed at establishing the dependences of the ultrasound propagation velocity V and the attenuation coefficient α on the total strain to failure and the tensile strength for the Fe – 18 wt % Cr – 10 wt % Ni alloy in a wide temperature range. 2. Materials and methods Samples were polycrystalline Fe-18 % Cr -10 % Ni alloys with work piece dimensions of 40x5x2 mm and grain size of ~12.5 μm. The uniaxial tensile testing of the specimens was performed on an Instron -1185 universal testing machine at a strain rate of 3.3·10 – 4 s -1 . To induce the direct γ→α’ martensitic transformation in the material, the tests were implemented in a temperature range of 180 K ≤ T ≤ 318 K (Talonen J. et al., 2005). For this, the purge rate of nitrogen vapor in the working chamber was properly adjusted by a heating element mounted inside the Dewar vessel. In turn, the temperature was controlled by a chromel-alumel thermocouple whose junction was in contact with the sample. The velocity of Rayleigh waves in the alloys was measured using a dual-element sensor consisting of CTS-19 piezoelectric radiating and receiving converters with a resonant frequency of 5 MHz in a housing (Lunev A.G. et al., 2018). In particular, periodic excitation of the radiating piezoelectric converter induced the formation of a surface acoustic wave (Rayleigh wave) in the sample, allowing one to record the acoustic parameters of the medium. The wave propagating from the emitting to the receiving element was converted into an electrical signal that was afterwards received at the input of the Rigol DS2072A digital oscilloscope. The high sampling rate of the oscilloscope made it possible to measure the propagation time of the acoustic signal with an accuracy of 10 4 -10 5 . The velocity of the acoustic wave propagating throughout the surface of the working part of the sample was found as the ratio of the distance between the emitting and receiving converters to the time of signal propagation in the sample. According to the experimental evaluation of the error caused by the instability of the acoustic contact of the sensor with the sample, the margin of error in determining the speed was not higher than ±3 m/s. The data were afterwards processed using conventional statistical methods. 3. Results The stress-strain curves of Fe-Ni-Cr alloys cover the ranges of elastic and plastic deformations and fracture. The indicator tension curves initially recorded in the σ (stress)- ε (strain) coordinates were first transformed into the true stress ( s )-true strain ( e ) dependences (Pelleg J., 2013) and then into the destruction diagrams s ( e 1/2 ) (Fig. 1a, curves 1 – 8). The inflection points could be clearly observed between the yield strength and the ultimate strength regions, which corresponded to the transition from the elasto-plastic to the plastic-damage stage due to the appearance and accumulation of microcracks in the material. As seen from the inset to Fig. 1a, the inflection point (referred to as D (destruction) point) in the destruction diagram is attributed to the deformation and strength characteristics (stress S D