In situ micro tensile testing of He+ ion irradiated single crystal nickel film
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Date
2015-11-01
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Microscopy and Microanalysis Society
Abstract
Introduction :
Radiation damage can cause increase in strength and decrease
in ductility, thus reducing the service life of structural parts
in reactors. Ion beam irradiation has been a method of choice
to simulate the effects of neutron irradiation in a reactor
for some time now [1], since it enables the attainment of
reasonable doses within hours, instead of years inside a
reactor. A major problem in this method is that the damaged
region is very shallow, and mechanical testing of such thin
layers is extremely difficult. In this study, we have used in situ
micro-tensile testing in the scanning electron microscope
(SEM) to understand the effects of high energy ion beam
irradiation on the mechanical properties of a single crystal
nickel thin film.
Experiments
Single crystal Nickel foils, ~12.8-13.1 μm thick, were
irradiated with 6 MeV He+ ions in the Tandetron “STAR”
accelerator at ANSTO. The samples were irradiated to two
different fluences – (i) 2 x 1017 ions/cm2 (peak damage of
~ 10 displacements per atom or dpa), and (ii) 3.8 x 10 17
ions/cm2 (peak damage of ~19 dpa). Damage profiles
calculated using the SRIM software [2], showed that there
is a long, low tail of the profile beginning at the entry face
and extending to approximately 9-10 μm depth, after which
the damage rises sharply (Fig. 1(a)). Micro-tensile samples
of approximate dimensions 25-30 μm (l) x 10 μm (w) x
12-13 μm (h) were fabricated using a Zeiss® Auriga 60™
Cross-Beam™ instrument. The free end of the sample was
milled to obtain a rectangular hole which was used as a grip.
The end of the tensile device, shaped as an L shaped hook,
was inserted into the aforementioned rectangular hole. The
sample was then subjected to tension by applying a voltage
to a piezo-electric device attached to the tensile head, causing it to move at a rate of ~20 nm/sec. SEM images
were taken at regular intervals, and the strain measured
using two fiducial markers, one on each side of the gauge
length.
Results :
An image of a typical tensile sample used in these tests is
shown in Figure 1(b), before the start of the test. The SEM
image in Figure 1(c) shows the unirradiated sample after
a tensile strain of e ~ 56%. The sample had a Y.S. of ~70-
100 MPa, and an U.T.S. of ~240 MPa (see Fig. 2). There
was significant strain hardening up to the U.T.S., and
subsequently it underwent plastic strain with large slip
bands passing on two major sets of planes in an alternate
manner. The formation of these slip bands was accompanied
by small drops in the stress and increases in strain. A post-
test SEM image of a sample irradiated with 6 MeV He+
ions to a fluence of ~2e17 ions/ cm2 and a peak damage of ~
10 dpa is presented in Fig. 1(d), showing slip bands passing
through the whole thickness of the sample and fracture at
the lower surface, which in this case is the “exit surface” of
the ions. This sample had a Y.S. of ~ 195-230 MPa, and
a peak strength of ~358 MPa before first rupture at the
surface near peak damage, at a strain of about 1.9% (Fig.
2). A post-test SEM image of the sample fabricated from
the foil irradiated with He+ ions to a total fluence of 3.8e17
ions/ cm2 and a peak damage of ~ 19 dpa is shown in Figure
1(e). This sample showed a Y.S. of ~ 400 MPa and a peak
strength of ~ 500 MPa before first rupture at the exit surface
of the ions, which is the top surface in this case.
Conclusions:
The effect of He+ ion irradiation on the tensile strength of
Ni single crystals was measured successfully by in situ micro-
tensile testing of FIB-fabricated samples which included
the damaged layers. The results showed increase in average
strength of up to ~118 MPa for a total fluence of 2e17 ions/
cm2 and ~260 MPa for a peak damage of ~3.8e17 ions/cm2. Brittle fracture was observed in the irradiated samples at the
surface nearer to the peak damage layer.
Description
Keywords
In-situ processing, Tensile properties, Helium, Irradiation, Films, Testing, Reactors, Damage, ANSTO
Citation
Bhattacharyya, D., Reichardt, A., Ionescu, M., Davis, J., Hosemann, P., Edwards, L., & Harrison, R. P. (2015). Paper presented at AMAS XIII : the 13th Biennial Australian Microbeam Analysis Symposium, University of Tasmania, Hobart, 9-13 February, 2015. In situ micro tensile testing of He+ ion irradiated single crystal nickel film. In Goemann, K., Danyushevsky, L. & Thompson, J. (Eds), AMAS XIII : the 13th Biennial Australian Microbeam Analysis Symposium : program and abstracts, (pp.40-41).