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Local stresses and microstructures

 

During phase transformations in the solid state, stresses are generated at the phase scale due to transformation strain. In many cases, the material tends to minimize the stresses by adapting the morphology of the precipitates and their spatial arrangement. This research theme led to different studies between 1990 and 2000. Novel studies were led on this theme, using on the one hand XRD in situ, which allowed the evolution of average stresses in the phases to be characterized in certain cases, and using on the other hand FE micro-mechanical calculations or FFT methods and approaches of field phase coupled to mechanics.

In the case of nickel-based super-alloys (in collaboration with the LEM3), we studied the relaxation coherence stresses of the gamma-gamma’ microstructure between a 3D volume and a thin layer element. The results obtained by calculations using finite elements on model and real microstructures (TEM) showed that free edges caused strong gradients within the matrix corridors in contrast to the case of a 3D structure. Furthermore, these studies allowed the interpretation of measures of distortion gradients determined by TEM (CBED) (G. Brunetti thesis, LEM3).

In the case of MMC, micro-mechanical calculations (2D strain on a general plane) were carried out to analyze the evolution of the constraint fields in a composite during cooling ( stresses generated by differences in the thermal dilation coefficient and by distortions of phase transformation). These analyses were confronted with measurements at room temperature, along with evolutions of average parameters in the phases obtained by XRD in situ. The characterization of local fields associated with temperature evolution and phase transformations and their micromechanical modelling will be developed further.

<font size="1"><i>Figure 1: experimental device for rapid transformations </font></i>
<font size="1"><i>Figure 2: successive diffractograms showing the formation of metastable phases during heating (5°C/s)</font></i>
<font size="1"><i>Figure 3: characterization of rapid transformation kinetics of martensitic or bainitic phases </font></i>
<font size="1"><i>Figure 4: evolution of austenite cell parameters and of the martensitic phase fraction during cooling. The discrepancy compared to linear behaviour is characteristic of the development of stresses in the parental phase.</font></i>

Study of phase transformations by synchrotron diffraction.

 

Since 2004, the "Microstructures and stresses" GRoup has used high energy X-ray diffraction to study the evolutions of the microstructure that are generated during solid-solid phase transformations. High energy XRD has allowed a bulk analysis, with a low exposure time (0.2 s) and has led to accurate characterizations of the nature of phases, their fraction and average cell parameters.

The simultaneous analysis of these data provided fundamental knowledge for the characterization of transformation mechanisms and stresses generated during transformations. Different experimental devices were used for the study of transformations during cooling or in isothermal conditions and these allowed heating speeds of 0.2°C/s to 10°C/s. Experiments were carried out on ID15B and ID11 ESRF beamlines, using high energy radiation with a rapid acquisition of diffraction data from bulk samples (Fig. 1).

The continued 2D Debye-Scherrer acquisition led to a succession of diffraction data that could be quantified (Fig. 2).

The methodology for the study of transformations in steels was developed and improved: ferrous transformation into austenite upon heating, precipitation or dissolving of carbides and nitrites, martensitic and bainite transformations (fig. 3), along with precipitation in aluminium and titanium alloys (dissolution and transformation upon cooling or ageing), transformations in metallic matrix composites. Depending on the transformation conditions, we could highlight the diffusible nature of the transformation (bainite, titanium alloys) or the constraints generated during martensitic transformation upon cooling (Fig. 4) or the relaxation of stresses during martensitic ageing.

<font size="1"><i>Evolution of strain and the phase fractions during cooling at 0.5°C/s of a TiC particle re-enforced metallic matrix composite. The data are from high energy in situ diffraction experiments </font></i>
<font size="1"><i>Devices for in situ thermal treatment for high energy beamlines </font></i>