Seminar by Kristián Máthis ” Synergy of in-situ methods in the investigation of key deformation processes in magnesium alloys and composites “


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Date | Time
12/12/2018 | 14 h 00 min

Location
LEM3

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Seminar by Kristián Máthis

Synergy of in-situ methods in the investigation of key deformation processes in magnesium alloys and composites

Charles University, Faculty of Mathematics and Physics, Department of Physics of Materials, Ke Karlovu 5, 121 16 Prague, Czech Republic

Owing to their hexagonal close packed (HCP) structure and c/a ratio close to ideal value the deformation behavior of magnesium alloys differ from the other, e.g. fcc of bcc metals. The basal slip has the lowest value of critical resolved shear stress (CRSS), followed by prismatic slip and first-order pyramidal slip systems. All these slips provides deformation in 〈a〉 (i.e. 〈1120〉) direction and their combination provides only 4 independent slip systems. Therefore, the von Mises criterion requiring five independent slip systems for homogenous deformation is not fulfilled and activation of second-order pyramidal system or mechanical twinning is necessary.

The experimental study of the deformation mechanisms includes both ex-situ (e.g. optical light, scanning (SEM) or transmission electron microscopy (TEM)) and in-situ techniques (e.g. diffraction methods, acoustic emission etc.). The main drawback of microscopy methods in studying twinning and dislocations is the relatively small observed volume in the specimen. On contrary, the irradiated volume in the diffraction experiments provides statistically representative data. The X-ray line profile analysis, pioneered by Ungár et al. [1], was successfully used for ex-situ analysis of the temperature dependence of the dislocation structure evolution during uniaxial tensile test of magnesium alloys [2; 3]. The neutron diffraction (ND) technique was used first by Gharghouri et al. for study of twinning in Mg-Al alloy [4]. In this type of experiment, the overall twinned volume can be determined from the intensity variations of particular peaks, caused by the crystal lattice reorientation during twinning [4-6]. Furthermore, the activity of a particular slip system manifests as a deviation of the lattice strains from the ideally elastic response [7]. Muránsky [8] introduced acoustic emission (AE) as a useful complementary experimental technique to ND during in-situ testing of the wrought Mg alloy, ZM20. The main advantages of the method are the high time resolution and sensitivity to twin nucleation and collective dislocation motion [9]. A recent statistical method worked out by Pomponi & Vinogradov [10] was successfully applied for the determination of the dominant deformation mechanisms from the AE signal at the various stages of deformation.

In the talk the influence of various microstructural (composition, orientation of the reinforcement) and experimental (loading direction, temperature) parameters on the deformation mechanisms are elucidated using combined usage of AE, ND and SEM. Owing to their hexagonal close packed (HCP) structure and c/a ratio close to ideal value the deformation behavior of magnesium alloys differ from the other, e.g. fcc of bcc metals. The basal slip has the lowest value of critical resolved shear stress (CRSS), followed by prismatic slip and first-order pyramidal slip systems. All these slips provides deformation in 〈a〉 (i.e. 〈1120〉) direction and their combination provides only 4 independent slip systems. Therefore, the von Mises criterion requiring five independent slip systems for homogenous deformation is not fulfilled and activation of second-order pyramidal system or mechanical twinning is necessary.

The experimental study of the deformation mechanisms includes both ex-situ (e.g. optical light, scanning (SEM) or transmission electron microscopy (TEM)) and in-situ techniques (e.g. diffraction methods, acoustic emission etc.). The main drawback of microscopy methods in studying twinning and dislocations is the relatively small observed volume in the specimen. On contrary, the irradiated volume in the diffraction experiments provides statistically representative data. The X-ray line profile analysis, can be successfully used for ex-situ analysis of the temperature dependence of the dislocation structure evolution during uniaxial tensile test of magnesium alloys. In the neutron diffraction experiments, the overall twinned volume can be determined from the intensity variations of particular peaks, caused by the crystal lattice reorientation during twinning. The acoustic emission (AE) is a useful complementary experimental technique to ND during in-situ testing of the magnesium alloys. The main advantages of the method are the high time resolution and sensitivity to twin nucleation and collective dislocation motion.

In the talk the influence of various microstructural (composition, orientation of the reinforcement) and experimental (loading direction, temperature) parameters on the deformation mechanisms are elucidated using combined usage of AE, ND and SEM.

References

  1. Ungár, T., Micro structural parameters from X-ray diffraction peak broadening. Scripta Materialia. 51, 777-781 (2004).
  2. Máthis, K., Chmelík, F., et al., Investigating deformation processes in AM60 magnesium alloy using the acoustic emission technique. Acta Materialia. 54, 5361-5366 (2006).
  3. Gubicza, J., Mathis, K., et al., Inhomogeneous evolution of microstructure in AZ91 Mg-alloy during high temperature equal-channel angular pressing. Journal of Alloys and Compounds. 492, 166-172 (2010).
  4. Gharghouri, M.A., Weatherly, G.C., et al., Study of the mechanical properties of Mg-7.7at.% Al by in-situ neutron diffraction. Philosophical Magazine A. 79, 1671-1695 (1999).
  5. Muransky, O., Barnett, M.R., et al., On the correlation between deformation twinning and Luders-like deformation in an extruded Mg alloy: In situ neutron diffraction and EPSC.4 modelling. Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing. 527, 1383-1394 (2010).
  6. Agnew, S.R., Mulay, R.P., et al., In situ neutron diffraction and polycrystal plasticity modeling of a Mg-Y-Nd-Zr alloy: Effects of precipitation on individual deformation mechanisms. Acta Materialia. 61, 3769-3780 (2013).
  7. Agnew, S.R., Brown, D.W., et al., Validating a polycrystal model for the elastoplastic response of magnesium alloy AZ31 using in situ neutron diffraction. Acta Materialia. 54, 4841-4852 (2006).
  8. Muránsky, O., Barnett, M.R., et al., Investigation of deformation twinning in a fine-grained and coarse-grained ZM20 Mg alloy: Combined in situ neutron diffraction and acoustic emission. Acta Materialia. 58, 1503-1517 (2010).
  9. Lou, X.Y., Li, M., et al., Hardening evolution of AZ31B Mg sheet. International Journal of Plasticity. 23, 44-86 (2007).
  10. Pomponi, E., Vinogradov, A., A real-time approach to acoustic emission clustering. Mech Syst Signal Pr. 40, 791-804 (2013).

Email: mathis@met.mff.cuni.cz