Browsing by Author "Hosemann, P"

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    In situ micro tensile testing of He+ ion irradiated single crystal nickel film
    (Australian Microscopy and Microanalysis Society, 2015-11-01) Bhattacharyya, D; Reichardt, A; Ionescu, M; Davis, J; Hosemann, P; Harrison, RP; Edwards, L
    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.
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    In situ micro tensile testing of He+2 ion irradiated and implanted single crystal nickel film
    (Elsevier B.V., 2015-11-01) Reichardt, A; Ionescu, M; Davis, J; Edwards, L; Harrison, RP; Hosemann, P; Bhattacharyya, D
    The effect of ion irradiation on the tensile properties of pure Ni single crystals was investigated using an in situ micro-mechanical testing device inside a scanning electron microscope. A 12.8 μm-thick Ni film with {0 0 1} plane normal was irradiated with 6 MeV He+2 ions to peak damage of 10 and 19 displacements per atom (dpa). Micro-tensile samples were fabricated from the specimens parallel to the plane of the film using a focused ion beam (FIB) instrument, and tested in tension along [1 0 0] direction, up to fracture. The peak strength increased from ∼230 MPa for the unirradiated material to about 370 MPa and 500 MPa for the 10 dpa and 19 dpa samples respectively, while the ductility decreased with increasing dose. The surface near the peak damage regions fractured in a brittle manner, while the regions with smaller dose underwent significant plastic deformation. Slip bands extended to the peak-damage zone in the sample with a dose of 19 dpa, but did not propagate further. Transmission electron microscopy confirmed the stopping of the slip bands at the peak-damage region, just before the high He concentration region with voids or bubbles. By removing the peak damage region and the He bubble region with FIB, it was possible to attain propagation of slip bands through the entire remaining thickness of the sample. This material removal also made it possible to calculate the irradiation hardening in the region with peak hardness – thus enabling the separation of hardening effects in the high and low damage regions. © 2015 Elsevier Ltd.