АКУСТОМИКРОСКОПИЧЕСКИЕ МЕТОДЫ: ИЗУЧЕНИЕ СТРУКТУРНЫХ ПАРАМЕТРОВ И ФИЗИЧЕСКИХ СВОЙСТВ МЕТАЛЛИЧЕСКИХ МАТЕРИАЛОВ

А.I. Kustov

Voronezh State Pedagogical University

I.А. Migel

Voronezh education and science center of Military Air Forces

THE   ACOUSTOMICROSCOPY   METHODS   FOR  STUDY  OF   STRUCTURAL   PARAMETRES   AND  PHYSICAL  PROPERTIES OF   METAL   MATERIALS

  Abstract     Dependences of physical-mechanical properties from structural parameters of material play very important role. The paper deals with the perspectives of the application of acoustic microscopy methods for studying the changes of  physical - mechanical properties of  materials  in a condensed  state  under external influence.  The basic principles of the methods as well as the results of the experiments of studying the structure of materials in a condensed state and its transformation upon changing the composition and types of thermomechanical treatment are given in the article. The high sensitivity to non-heterogeneity and defects upon acoustic visualization and in the regime of determining physic-mechanical properties are demonstrated. Keywords: method of visualization, acoustic microscopy, subsurface layers, nanopore density, physical-mechanical steel parameters, nondestructive evaluation.  

А.И. Kустов

Воронежский государственный педагогический университет

И.А. Mигель

Воронежский учебно-научный центр ВВА им. Н.Жуковского и Ю.Гагарина,

 

АКУСТОМИКРОСКОПИЧЕСКИЕ МЕТОДЫ: ИЗУЧЕНИЕ СТРУКТУРНЫХ  ПАРАМЕТРОВ  И ФИЗИЧЕСКИХ  СВОЙСТВ  МЕТАЛЛИЧЕСКИХ  МАТЕРИАЛОВ

  Аннотация В работе приведены результаты экспериментов по использованию методов акустомикроскопической дефектоскопии для измерения физико-механических параметров материалов в конденсированном состоянии, прежде всего сталей. Рассмотрено влияние некоторых характеристик внешних воздействий на структуру материала и изменение его свойств. Показано, что расчёт параметров материалов с использованием различных акустомикроскопических методов даёт результаты, совпадающие в пределах погрешностей. Применение методов АМД позволяет существенно расширить количество получаемой информации о материалах и их свойствах, повысить её достоверность и надёжность. Ключевые слова:  метод визуализации структуры; акустическая микроскопия; подповерхностные слои; неразрушающий контроль; предельное состояние материала; физико-механические параметры сталей.  

1. Introduction In this paper the results of experimental work of investigation of physical-mechanical properties materials with the help of scanning acoustic microscope (SAM) [1,2] are given. The essence of the subjected methods is first inlayer visualization of subsurface structures of the objects under investigation and second in the definition of velocity values of acoustic waves and calculate the elasticity constant of solid material. The nondestructive methods of investigation of structure and characteristics used have no limits by the nature of materials, including nanostructural materials. They are often different characteristics of physical – mechanical properties.

Base methods of acoustomicroscopy defectoscopy are the visualization and  V(Z)-curve methods. The image analyses assignment us possibility to calculate grain dimensions, to observe their transformation from time or with external influences. Example of the obtained by SAM image of microstructure of  10Х12Н2ВМФ steel  is represented on Fig.1. The comparative optical photo with the same magnification gives the image of a polished surface without revealing structure elements.  Structure transformation has been observed after deformational and thermal influences. In figure 2 present V(Z)-curve for 60Г steel. In according to principles as account earlier [5,6], obtained curve permit to determine the significance of velocities (u_R) of surface acoustic waves (SAW). For this calculation we use characteristic distance (DZ_N ) between maximums, situated on the right of the main one. V(Z)-method allow to determine some physical-mechanical parameters (u_R, E, G, DV/V%) [3,4]. In according to principles as account earlier [5,6], obtained curve permit to determine the significance of velocities (u_R) of surface acoustic waves (SAW). For this calculation we use characteristic distance (DZ_N ) between maximums, situated on the right of the main one. In order to calculate the value of the elastic modules in the local area of the studied material one should use the tabular values r_S of density and of  Poisson coefficient (n), or their quantity, determined by one of the certain standard methods. V(Z)-curve method allow to distinguish quantity properties of steel with different extent of  deformation  or to conduct texture characterization [7].

2. Results and discussion The main task of our investigation was elaboration and use of me-thods of structural parameter definition of metal materials, for example, grain size. Our aim was also establishment of correlation dependences with physic-mechanical  properties. The most effective methods were acoustomicroscopy defectoscopy ones. SAM of the reflective type was used to solve the task. The frequency of operation of the microscope was 400-600 MHz.

In order to calculate the value of the elastic modules in the local area of the studied material one should use the tabular values r_S of density and of  Poisson coefficient (n), or their quantity, determined by one of the certain standard methods. Then, with taking into account the definition  u_R  in  material: (2). V(Z)-curve method allow to distinguish quantity properties of steel with different extent of  deformation (Fig.3) or to conduct texture characterization [6,7].  In figure 4 present the example of exposed the texture (anisotropy of elasticity modulus in 60Г steel). One of the important parameter of solid state sample is the grain dimension (d_З). The dependences of conventional fluid point (s_0,2) from dimension  d_З  in one of low carbon steel  type are present in fig.5. The results were obtained with standard definition. As follow from that diagram, linear dependence s_0,2 from grain dimension observes in 10-150 mm interval.

 

Received dependences  s_0,2  from  d_З are in good  accordance  with  low  Hall - Peach:

                                             s_0,2  = s₀  + k  d_з^-1/2 ,                                (3)

     were k and s0 – the constant for that  material.

  However, we due to turn the attention at the fact, that the experimental curve has break by d₃ = 40  mm (with linear correlation coefficient  ~ 0,9). The nature of this break  demand further investigation.   Analogical dependence was obtained for 15Х2НМФА steel  too (fig.6). If we estimated grain dimension from acoustic image,  then we calculate critical  significance s0,2 for  this material. That significance may be to determine with method V(Z)-curve. Since change of grain dimension transforms value of SAW velocity. Dependence u_R from d_З are demonstrated in fig.7. That allow define s_0,2 and grain dimensions from u_R measuring. Obtained dependence is similar to straight line, but the approximely coefficient for logarithmic curve is some above, then it (0,97). Consequently, application of AMD methods allows to control strength properties of steel with the help of  value  dimension. The high reliability of the results is provided for using 2 parallel methods (visualization and V(Z)- curves).The first is based on d₃ dimension and using Hall-Peach law. The second is connected with using V(Z)-curves definite values u_R and solid modules (E and G) calculations. Application of these methods for steel 14 ХГС (s₀ =134 МПа,  k = 23 МПа^.мм^1/2; r = 7,83^.10³ кг/м³;  = 0,27) resulted in s_0,2 close values of 249 and 241 correspondingly.                                                                                                                                                                                d^{-1/2 .} 10 ^{3/2 .}m ^-1/2

Fig. 7.  The velocity  (u_R) dependence from grain  dimension (d_З) by 15Х2НМФА steel.

Fig. 8. The dependence of SAW velocity (u_R)  in steel (08Х18Н10Т) on the  depth  penetration.

  Besides physic-mechanical parameter material  dependences on grain dimension, the methods AMD allow to estimate the depth of penetration  to the material of diffusion substance, layer thickness with changed properties. It is also possible to apply acoustic microscopy methods based on the use of V(Z) – curves for the study of the limit state of the materials in a solid   state.  It makes it possible to calculate the values of velocities SAW and to define the quantity  of the  coefficient changes of AW fading in the material at the relative modification DV/V% of height of the main maximum of the V(Z) – curves. The control technology of  limit state with appliance of  V(Z) – curves consists of the analysis of their form transformation.  The  values of  velocities  of surface acoustic waves (SAW) and the change of the level of their fading were calculated due to the special dependencies. The limit state of material conclusion was made according to the dimension of local fluctuations of  physical – mechanical parameters. The results of the experiments illustrating the possibility to define the thickness of the modified layer caused by hydrogen diffusion in the metal materials according to the value of velocity SAW or the ratio DV/V%  were presented before. The difference between the initial and modified material according to these characteristics can reach 5 – 20%.  The results of modeling experimental for nitrogen steel are presented in fig. 8 (steel  08Х18Н10Т). Obtained experimental curve u_R are approximated by parabolic dependence with coefficient 0,986.  We can  to  ensure  definition depth of transformed layer, if shell analyses that dependence. It is possible to estimate the value and some other sample characteristics, for example the hydrogen concentration, according to the changes of the velocity SAW and the relative height DV/V% of the maximum V(Z) – curve. The depth of penetration, at which the chosen characteristics is 90% of the values inside the material, can be the criterion of the time estimation of diffusion process  ( Fig.8).The example of the process for steel 40XH is given in Figure 9. Presence the extreme on its  allows to use mathematical (computer) methods of  optimal time of influence search. The dependencies both between the hydrogen concentration and the number of the microcracks emerged, and the values of velocity SAW on depth of the hydrogen penetration into the sample, period of diffusion, etc. are received at that. The limit hydrogen concentration in this material can be evaluated concerning the value of velocity SAW from the received dependence for steel. In Fig.9 presented curve of  u_R - dependence  from  time exposure of steel (40ХН) specimens in hydrogen. The optimum  time dimensions are in interval -~55-67 h.

Fig. 9.  Dependence of SAW velocity  from time   exposure (t) of steel specimens in hydrogen.

 

3. Conclusion Thus:

- the experiment showed that SAM of the reflective type permits to visualize grains with d₃ from several micrometers  to some hundreds   micrometers; - in  accordance  d3  is calculated s0,2,  shown  the good agree with values  measured  with  V(Z)-curves method; - SAM and  the measured  characteristic uR sensetive to the degree of deformation  and  presence of  the textures; - AMD methods allows to define  the depth of penetration with changed structure and find optimal time of diffusion.

References

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