Understanding the apparent fracture toughness of microcantilevers / Hydrogen-dislocation interaction in tungsten due to deuterium plasma exposure: Insights from nanoindentation and micropillar compression tests

Wall Forum

  • Datum: 06.03.2019
  • Uhrzeit: 15:30 - 16:30
  • Vortragende(r): Dr. Steffen Brinckmann and Dr. Xufei Fang
  • MPI für Eisenforschung, Düsseldorf
  • Ort: Garching
  • Raum: Seminarraum D3
This issue of the Wall Forum consists of two shorter talks.

Understanding the apparent fracture toughness of microcantilevers

Steffen Brinckmann, Kurt Matoy, Christoph Kirchlechner, Gehrhard Dehm

1Max-Planck-Institut für Eisenforschung, Germany
2Infineon Technologies Austria AG, Austria

In the past decade, micrometer cantilevers were used frequently to quantify the fracture toughness of single phases and of particular grain and phase boundaries. The fracture toughness quantification relies on the cantilever dimensions and the critical force. The geometry and force enter analytical models, which are based on simulations that use isotropic elastic material models, a perfect beam geometry and in many cases a two-dimensional configuration. However, the vast majority of materials exhibit plasticity and are anisotropic. Moreover, the intentionally prepared pre-crack is seldom straight due to the focused ion beam (FIB) production method. This study uses thousands of 3D finite element method (FEM) simulations to investigate the influence of anisotropy, imperfect pre-crack shape and plasticity on the apparent fracture toughness of the material.

Initially, we will discuss the influence of anisotropy on the apparent fracture toughness. To that end, the ratio of anisotropy between the cantilever axis and both transversal axes is varied to establish the anisotropy effect on the apparent fracture toughness. Moreover, we investigate the influence of Poisson's ratio and beam geometry because these values interact with the anisotropy. We find that typical metals - with an anisotropy ratio less than 3 and typical cantilever geometries with an l/h ratio of more than 4 - are only slightly affected by the anisotropy. We present view-graphs that allow the user to determine the anisotropy influence for other cantilevers and highly anisotropic materials.

Secondly, we investigate the influence of material bridges that are frequently used to ensure straight pre-cracks at the start of crack propagation. Additionally, we investigate the influence of pre-crack front rounding, i.e. the material bridges are omitted. We varied the geometry of the material bridge and crack front rounding to establish the influence of the geometry on the apparent fracture toughness. We discuss the difference between load controlled experiments and displacement controlled experiments. We argue about the geometric requirements to ensure stable crack growth and the optimal experiments.

We close with a discussion on the influence of plasticity on the fracture toughness and the applicability of the plastic zone size for microcantilever experiments.

References:
[1] K. Matoy, H. Schönherr, T. Detzel, T. Schöberl, R. Pippan, C. Motz, G. Dehm, “A comparative micro-cantilever study of the mechanical behavior of silicon based passivation films”, Thin Solid Films 518(1) (2009) 247-256.
[2] S. Brinckmann, C. Kirchlechner, G. Dehm, Stress intensity factor dependence on anisotropy and geometry during micro-fracture experiments, Scripta Materialia 127, pp. 76-78, 2017.
[3] S. Brinckmann, K. Matoy, C. Kirchlechner, G. Dehm, On the influence of microcantilever pre-crack geometries on the apparent fracture toughness of brittle materials. Acta Materialia 136, S. 281 - 287 (2017)



Hydrogen-dislocation interaction in tungsten due to deuterium plasma exposure: Insights from nanoindentation and micropillar compression tests

Xufei Fang1, Marcin Rasinski2, Arkadi Kreter2, Christoph Kirchlechner1, Christian Linsmeier2, Gerhard Dehm1, Steffen Brinckmann1

1Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany
2
Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung – Plasmaphysik, 52425 Jülich, Germany

The understanding of hydrogen embrittlement (HE) in metals is important for the failure prediction in components. Some recent studies in HE have been focusing on the dislocation-hydrogen interaction. In this talk, we present some recent results on the tungsten-deuterium interaction based on micromechanical testing methods, including nanoindentation and micropillar compression. The nanoindentation tests show that exposure to deuterium (D) plasma causes a decrease in pop-in load and an increase in hardness of tungsten (W) in comparison to the unexposed reference sample. Micropillar compression tests demonstrate an increased apparent strain hardening rate as well as an increased multitude of slip traces after D exposure when compared to the reference pillars. These outcomes are attributed to the presence of D that facilitates the dislocation nucleation while may at the same time suppress the dislocation mobility. Varying the loading rates in micropillar compression provides further proof for the competing mechanisms of dislocation mobility and D diffusion velocity. These findings help to shed light on the hydrogen embrittlement mechanisms of W during plasma exposure.

References:
[1] X. Fang, et al., Journal of Materials Research 33(20) (2018) 3530-3536.
[2] X. Fang, et al., Scripta Materialia 162 (2019) 132-135.


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