Cryogenic EBSD on ice: preserving a stable surface in a low pressure SEM

Abstract

SummaryNaturally deformed ice contains subgrains with characteristic geometries that have recently been identified in etched surfaces using high-resolution light microscopy (LM). The probable slip systems responsible for these subgrain boundary types can be determined using electron backscattered diffraction (EBSD), providing the etch features imaged with reflected LM can be retained during EBSD data acquisition in a scanning electron microscope (SEM). Retention of the etch features requires that the ice surface is stable. Depending on the pressure and temperature, sublimation of ice can occur. The equilibrium temperature for a low pressure SEM operating at 1 × 10 −6 hPa is about −112 • C and operating at higher temperatures causes sublimation. Although charging of uncoated ice samples is reduced by sublimation, important information contained in the etch features are removed as the surface sublimes. We developed a method for collecting EBSD data on stable ice surfaces in a low pressure SEM. We found that operating at temperatures of <-112 • C reduced sublimation so that the original etch surface features were retained. Charging, which occurred at low pressures (<1.5 × 10 −6 to 2.8 × 10 −5 hPa) was reduced by defocusing the beam. At very low pressures (<1.5 × 10 −6 hPa) the spatial resolution with a defocused beam at 10 kV was about 3 µm in the x-direction at −150 • C and 0.5 µm at −120 • C, because at higher temperature charging was less and only a small defocus was needed to compensate it. Angular resolution was better than 0.7 • after orientation averaging. Excellent agreement was obtained between LM etch features and EBSD mapped

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

“…Sample transfer to the SEM involved a further short sublimation step of a few minutes under vacuum (Weikusat et al, 2010). Typical measurement conditions for EBSD mapping were a working distance of 6-8 mm, −123 to −150 • C of sample stage temperature (−145 to −170 • C of cold trap temperature), a 5 × 10 hPa chamber pressure, 10 or 20 kV of accelerating voltage, and an 8.4 nA beam current.…”

“…The typical mapping rates was 0.15 s pixel −1 with an indexing rate of 90 %. EBSD data processing was performed using standard noise reduction and orientation filtering as described by Weikusat et al (2010). The angular resolution was about 0.5 • after orientation averaging.…”

“…Neighbour pixel misorientation angles between 0.5 and 5 • were plotted in an inverse pole figure (IPF) for each subset to determine the rotation axes of the SGBs. The trace of the SGB was determined from the LM image and compared to the EBSD orientation information to avoid any artefacts caused by charging or poor sample alignment with respect to the tilt axis (Weikusat et al, 2010). …”

“…These arrangements are shown in Fig. 5, which is an extended version of that given by Weikusat et al (2010Weikusat et al ( , 2011. A SGB boundary plane that is perpendicular to the basal plane (N-type) is shown in Fig.…”

“…Maintaining a stable ice surface during EBSD assessment Prior et al, 2015) is essential for these studies to allow for correlation between LM-etched surfaces and to differentiate between in situ deformation occurring in the ice sheet and any relaxation effects that occur during the various sample preparation procedures (Weikusat et al, 2010). When a subgrain boundary is formed by the polygonization of geometrically necessary dislocations (Humphreys and Hatherly, 2004;Weertman and Weertman, 1992;Hirth and Lothe, 1982), the crystallography (boundary plane and rotation axis) of subgrain boundaries can be interpreted in terms of the active dislocation slip systems.…”