Abstract
We discuss and demonstrate the application of recently developed spherical nanoindentation stress-strain protocols in characterizing the mechanical behavior of tungsten polycrystalline samples with ion-irradiated surfaces. It is demonstrated that a simple variation of the indenter size (radius) can provide valuable insights into heterogeneous characteristics of the radiation-induced-damage zone. We have also studied the effect of irradiation for the different grain orientations in the same sample.
Highlights
Materials with modified surfaces – either as a consequence of a graded microstructure or due to an intentional alteration of the surface such that its physical, chemical or biological characteristics are different from the bulk of the material – are of increasing interest for a variety of applications such as enhanced wear and corrosion resistance, superior thermal and biomedical properties, and higher fracture toughness[1,2]
Reactor conditions can be mimicked using ion beams where large amounts of radiation damage (several displacements per atom) are imparted in relatively short time spans of hours or days that would require months or years to achieve in reactor conditions[8,9,10]
A key challenge becomes: “How can we study the mechanical response of materials with varying degrees of damage over scales of only a few hundreds of nanometers in such a way that the data can be related to bulk values?” The very small thickness of irradiated material, high level of damage heterogeneity, sensitivity to sample preparation techniques, and the time and effort needed for sample preparation and testing, often preclude the application of many of the commonly used nano-mechanical test techniques; these include the use of focused ion beams (FIB) to fabricate micro-pillars or other small scale test geometries[5,6,7,11,12,13,14,15,16]
Summary
The first step in the analysis process is an accurate estimation of the point of effective initial contact in the given data set, i.e., a clear identification of a zero-point that makes the measurements in the initial elastic loading segment consistent with the predictions of Hertz’s theory[59,84,85]. The concept of an effective point of initial contact allows one to de-emphasize any artifacts created at the actual initial contact due to the unavoidable surface conditions (e.g., surface roughness, presence of an oxide layer) and imperfections in the indenter shape It has to be interpreted as the point that brings the initial elastic loading segment to as close an agreement as possible with Hertz theory. The rigorous derivation of Eq 2 directly from Hertz theory makes the estimates of contact radius from the measured CSM signals highly trustworthy
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