228 Tomas L. Martin et al.
geometry and an updated hit detection algorithm to better identify multiple hit events. The LEAP 5000 design replaces the cylindrical channels in the MCP found in the LEAP 3000 and 4000 with a newer design where the openings of each channel are widened into a conical shape, reducing the deadspace between channels where impacting ions do not create a signal. Consequently, this increases the detection efficiency of the instrument to a reported ~52% for the reflectron-equipped model and ~81% for the straight flight path instrument (Prosa et al., 2014). In early 2016, the University of Oxford acquired a new
LEAP 5000 XR, to complement the LEAP 3000X HR used at the university since 2007. The presence of bothmachines in the same research lab offers the opportunity tomake comparisons between the two instruments on a wide variety of material systems. There have been a number of studies comparing the effect of the laser wavelength on atom probe measurements (Amouyal&Seidman, 2012; Santhanagopalan et al., 2015), and so this study will not explore the effect of laser energy other than to attempt to use equivalent laser powers when compar- ing results in lasermode between the two instruments. Instead, this study will use a variety of materials with small-scale chemical segregation toexplore theeffect of thechangein detector geometry and hit detection on the size and chemistry of small clusters and surface segregation. There has been work in the literature to quantify the
effect of dead-time on the efficiency of microchannel and delay line detectors, and Jagutzki et al. (2002) showed that the use of a third delay line results in better readout of multihit events, both by reducing ambiguity for simulta- neous events, but also by reducing the size of the dead region on the detector after a hit, increasing the chance that a subsequent hit will be detected. Similarly, the work of Meisenkothen et al. explored the effects of detector dead- time on boron, one of the elements most prone to field evaporating preferentially in multiples. They showed that the dead-time surrounding the first hit in a pulse contributes most to the under-reporting of boron (Meisenkothen et al., 2015). In this study, however, the focus was on the practical effects of the change in detector on specimens for materials science projects rather than model systems, to allow users of LEAP instruments to be aware of the effects switching between the instruments might have on their data. The materials chosen for this project present a variety of
different microstructures and chemistries in order to identify the influence of the change in detector design between the two instruments. First, to characterize the impact of detector efficiency on cluster detection and analysis, two steels
containing nanometer-scale clustering were analyzed. The first steel was an Rolls Royce SG model alloy steel with small-scale clustering containing only metallic species. The second was an oxide-dispersion-strengthened (ODS) steel, where the clusters contained large quantities of oxygen, to determine whether oxide species behave differently than metallic clusters with a change in detection efficiency. The third steel contained carbide precipitates, as the composition observed from carbon-containing materials
prone to evaporation in multiple ions is known to be affected by changes in detection efficiency (Cerezo et al., 1984; Rolander & Andrén, 1989; Kinno et al., 2012; Thuvander et al., 2013). Finally, a silicon sample containing small quantities of implanted phosphorus was studied, to compare the effects of the change in detector on the observation of low levels of nonclustered impurities.
MATERIALS ANDMETHODS
Four materials with microstructural heterogeneities on a scale appropriate to be sampled by APT analysis were used in this study: three steels, incorporating metallic, oxide and carbide precipitates, respectively, and a phosphorus- implanted silicon material with low concentrations of the implanted dopants close to the surface. The first steel sample was a Rolls Royce SG model alloy
steel used in nuclear reactor pressure vessels, with a compo- sition of 1.52%Mn, 0.81% Si, 0.43% Cu, 0.29% Ni, 0.28% Mo, and 0.24% C (all wt%). Before aging, all specimen blocks underwent a standard post weld heat treatment of annealing at 920±20°C for 6h followed by a water quench, then tempering at 600±15°C and stress relief at 650±15°C for 42 and 6h, respectively, followed by slow cooling (≤50°C/h) after each treatment stage. Samples were aged in sealed quartz tubes under Ar atmosphere or vacuum to limit the effects of oxidation and were water quenched upon removal from the furnace. The second material was an ODS steel with nominal
composition Fe–0.3wt% Y2O3, as well as trace amounts of W, Cr, and Ti. The material was produced by mechanical alloying followed by hot-isostatic pressing, as described by Robertson et al. (2012). The third material and final alloy sample was an M50
bearing steel prepared at the Cambridge SKF University Technology Centre, containing 4.38% Cr, 3.84% C, 2.40% Mo, 1.05% V and lower levels of Mn, Si, and Al (all at%). A vacuum arc remelting–vacuum induction melting casting process was followed by hot-rolling and soft-annealing for 2 h at 880°C. The ingot was then cooled to 600°C over 11 h. A 5-min austenisation in a vacuum furnace at 1105°C was followed by quenching to room temperature. Three tempering cycles were performed at 545°C for 2 h each with 2 h cryogenic treatments between the tempering cycles. For each of the three steels, the bulk material was cut
into 25× 0.5×0.5mm matchsticks using a Buehler Isomet 5000 diamond wafering saw and needle specimens of each steel were produced using a two-stage electropolishing process, using standard preparation methods (Gault et al., 2012). Each matchstick was first electropolished using 25% perchloric acid (60%) in acetic acid, using voltages between 5 and 16V direct current (DC) to control etching speed, until the matchstick split into two needle-shaped specimens. These specimens were then more finely polished under a microscope using 2% perchloric acid (60%) in 2-butoxy-ethanol and voltages of 8–15V DC, to produce needles with an end radius of ~100nm.
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