Pulsed Field Evaporation Research Articles

Direct observation of the vacancy structure of a (220) platelet in an ion-irradiated platinum-4·0 at. % gold alloy

A detailed field-ion microscope (FIM) study has been made of the vacancy structure of a (220) platelet created by a single 30 keV W/sup +/ ion in a platinum-4.0 at. percent gold alloy; the specimen was maintained at 40/sup 0/K (below substage II/sub B/) during the in-situ irradiation at approximately 2 x 10/sup -9/ torr. Prior to the pulsed field-evaporation dissection of the specimen at 40/sup 0/K it was warmed isochronally to 100/sup 0/K (above substage II/sub c/). The (220) platelet was found to consist of 31 vacant lattice sites, lying in four (220) planes, and clustered in a disc-shaped region which is approximately 20 A in diameter. If only first nearest-neighbor lattice sites are considered then the distribution of cluster sizes is as follows: (1) two monovacancies; and (2) one jumbo vacancy cluster containing 29 vacancies. The range of the vacancy concentration within the (220) vacancy platelet is approximately 35 to 44 at. percent. Employing the modified Kinchin-Pease equation it was calculated that the displacement efficiency (kappa) for this platelet is 0.12. It is suggested that the prismatic dislocation loops lying on (220) type planes, observed by transmission electron microscopy, in ion or fast-neutron irradiated platinum can form asmore » a result of the direct collapse of (220) type vacancy platelets.« less

The point-defect structure in stage II of ion or electron-irradiated tungsten as studied by field-ion microscopy

Abstract A search had been performed for the thermally converted self-interstitial atom (SIA), which is required in a two-interstitial model, in both ion- or electron-irradiated tungsten employing the field-ion microscope (FIM) technique. FIM specimens were prepared from four-pass zone-refined tungsten [resistivity ratio R(R = ρ 273 K/ρ 4.2 K) of 4 × 104 to 5 × 104 where ρ represents the resistivity un corrected for the specimen size effect] were irradiated in-situ at ∼10−9 Torr with 20 or 30 keV W+ ions to doses varying from 1.2 × 1012 to 1.1 × 1013 ion cm−2 at temperatures in the range 273 to 558 K (Stage II to bottom of Stage III). A total of ∼ 105 atomic sites, in high index planes, were examined for SIAs at 20 K by the pulsed field-evaporation technique and no definite contrast patterns for isolated SIAs were detected. In addition, four-pass zone-refined single-crystals [R = (4 to 5) × 104] and Materials Research Corporation VP-grade (R = 15) polycrystalline specimens were irradiated simultaneously, ...

Field desorption of helium and neon

Employing our energy-focused atom-probe FIM, providing high resolution and freedom from artifacts, we have obtained complete crystallographic distribution of helium and neon field adsorbed on W, Mo, Ta, Rh, Ir, Pt, and Re. In addition we have determined the abundance of metal–helium and metal–neon molecular ions of these metals as obtained by field evaporation. The dependence of the relative abundance of metal–helium molecule ions upon the substrate temperature and the crystallographic surface area is established. Whereas the triply charged W, Ta, Mo, and Re helides and doubly charged helides of other metals are predominant molecular ion species in pulsed field evaporation, metal–neïdes are formed with a probability of less than 10−3. We were unable to confirm with our precise instrument a high probability of IrHe+++ and IrNe++ as indirectly deduced by Panitz. We are convinced that the high abundance of IrNe++ and IrNe+++ reported by Waugh from observations with a time-gated imaging field desorption microscope must also be artifacts.

Field ion microscope studies of solute atoms in dilute Pt alloys—I. Contrast due to Au, Ni and W atoms

Platinum based alloys containing dilute concentrations of either gold, nickel or tungsten atoms were examined in the field ion microscope during pulsed field evaporation. The contrast patterns produced by the solute atoms were established by comparing the images of the alloy specimens, which contained known concentrations of solute atoms, and the images of pure platinum control specimens. The gold and nickel atoms appeared as α-dark spots, i.e. single dark atomic sites lying within net planes which were visible during all stages of the pulsed field evaporation. In many cases platinum atoms near solute atoms were preferentially field evaporated producing β-dark spots, i.e. single dark atomic sites which appeared suddenly within a well resolved net plane during observation and remained there until all the atoms of that plane evaporated. The tungsten atom contrast was more complex and generally consisted of a cluster of contrast effects which often contained multiple a-dark spots, β-dark spots and also bright spots.

Field ion microscope studies of solute atoms in dilute Pt alloys—II. Distributions and interactions of Au and Ni atoms

Previous work (Part 1) has established that the position of each Au and Ni solute atom in dilute solution in Pt can be identified in the field ion microscope (FIM) as an α-dark spot. In the present work the exact positions of large numbers of solute atoms in the lattice were first determined from measurements of α-dark spot positions on ciné films which were taken during the pulsed field evaporation of several dilute binary alloys in the FIM. From these data radial distribution functions were generated, and solute-solute interaction energies up to 9th nearest-neighbors were calculated using the Clapp-Moss model. Heats of mixing were then calculated from the interaction energies. The research was aided by specially developed computer techniques which included displaying the solute atom distributions in a rotatable coordinate system. The results indicate clustering of Au atoms and ordering (anti-clustering) of Ni atoms as expected generally from previous thermodynamic data. The calculated heats of mixing were generally consistent with measured values. It was concluded that the FIM can be used, under appropriate conditions, as an effective tool for studying solute atom distributions.

Field ion microscope observations of the three-fold slmmetric dissociation of [formula omitted] screl dislocations in mollbdenum

A field ion microscope (FIM) studl has been performed of the dislocation structure lhich exists in the {222} planes and environs of mollbdenum specimens. The dislocation structure allals consisted of three-fold slmmetric dissociated 1 2 a〈111〉 screl dislocations lhich emerged normal to the {222} planes. Emploling the pulsed field evaporation technilue it las sholn that the dissociated dislocation structure consisted of a ka〈111〉 tlpe partial screl dislocation (lhere k is a constant) lhich emerged normal to the {222} planes and three stacking faults on the {110} planes that intersected one another along a 〈111〉 tlpe direction. The three stacking faults exhibited three-fold slmmetrl about the 〈111〉 tlpe vector. This represents the first experimental observation of a three-fold slmmetric dissociated 1 2 a〈111〉 screl dislocation in anl bodl-centered cubic metal. The dislocation structure las produced in situ in the FIM specimen at a specimen temperature of less than 30 K as a result of the mollbdenum specimen lielding to electric field induced shear stresses. It is pointed out that the observations are so far tlpical to the FIM and that the exact role plaled bl the electric field induced stresses, surface image effect and the small tip sile on dislocation geometrl remain to be determined.

Energy deficits in pulsed field evaporation and deficit compensated atom-probe designs

Energy deficits of field evaporated ions which limit the mass resolution of the straight TOF atom-probe are measured by incorporating a 90° deflecting energy discriminator. The observed dependence on the mass-to-charge ratio shows premature evaporation during the less than perfect subnanosecond pulse front to be the cause of the energy spread. For narrow beam apertures the mass resolution may be improved by tilting the detector plane to provide a compensating shorter path for the slower ions. For wider apertures an energy focusing combination of straight path sections and a 163° toroidal deflector, as conceived by Poschenrieder, has been adapted to our atom-probe. Offering a mass resolution of better than 1/1000 and complete rejection of artifacts, the compensated atom-probe FIM is now a microanalytical tool of ultimate sensitivity and high reliability.

A magnetic sector atom-probe FIM

The 60 ° magnetic sector atom-probe field ion microscope displays a section of the momentum spectrum on a stack of microchannel plates and a proximity-focused phosphor screen. A mass resolution ΔM / M ∼ 1/2000 is achieved, and “spectral lines” made up of a few individual ions may be used for the detection and identification of products of field evaporation of metals and their compounds with adsorbate gases. The capability of the new instrument is complementary to the established ToF atom probe as it employs very low evaporation rates in slow pulse or dc field evaporation, and displays the energy distribution of field ions. As an example of the difference, a palladium–neon ion compound PdNe+ is demonstrated to be very abundant, while it is a rare species in the ToF atom probe.

On the interpretation of ledge ‘bright spot’ contrast effects in field ion microscope images

Abstract A detailed pulse field evaporation study was made of the field evaporation characteristics of atoms from the first five ledges of (011) planes of high purity (⩽1.5 × 10−6 at. fr. impurity level) tungsten of well-annealed specimens and specimens irradiated with 35 or 40 keV W+ ions at room temperature (middle of Stage II) in vacua of ∼ 10−6 torr. The detailed examination of > 1.8 × 10−4 frames of cine film of the well-annealed control specimens and > 2.6 × 10−4 frames of film of the irradiated specimens showed that the ‘bright spots’ identified by Buswell as vacancies decorated by surface contamination, were in reality normal tungsten atoms associated with the ledges of (011) planes. The identification of these ‘bright spots’ as tungsten ledge atoms was achieved by employing a field evaporation increment of ∼2.5 × 10−3 of an (011) plane per evaporation pulse. The concentrations of ledge ‘bright, spots’ in the control and in the irradiated specimens were almost identical. The vacancy concentrations...

A quantitative study of vacancy defects in quenched platinum by field ion microscopy and electrical resistivity—I. Experimental results

Platinum wires were quenched from 1700°C and the 4.2°K resistivity of the quenched-in vacancy defects was measured. Field ion microscope (FIM) specimens were then prepared from these same wires and the vacancy defects were imaged in a FIM as each specimen was dissected by pulse field evaporation. The image was recorded on ciné film after each pulse and every atomic site in a large number of consecutive layers was recorded, A total of 157 monovacancies and 9 divacancies were found in 593,800 sites in 3 as-quenched specimens and 76 monovacancies and 1 divacancy were found in 321,300 sites in 5 specimens which were partially annealed after quenching. To correct for the presence of surface artifact vacancy defects, 319,600 sites were scanned in well-annealed specimens under identical conditions and only 11 artifact monovacancies were found. These observations constitute direct evidence that the predominant equilibrium defect at elevated temperatures is the monovacancy. The value of the monovacancy concentration quenched-in from 1700°C is (2.64 ± 0.14) · 10 −4 at.fr. which is less than the equilibrium value because of quenching losses. The combination of the measured concentrations and resistivity increments yielded a monovacancy resistivity of (5.75 ± 0.30) · 10 −4 Ωcm (at.fr.) −1 . An observed perferential loss of divacancies from the 5 partially annealed specimens provided direct evidence that the divacancy is more mobile than the monovacancy. The measured ratio of the divacancy concentration to the monovacancy concentration in the as-quenched specimens was (0.06 ± 0.02). The relationship of this value to the divacancy binding free energy is analyzed in Part II.

The direct observation of point defects in irradiated or quenched metals by quantitative field ion microscopy

The progress made at Cornell University over the past few years in applying the field ion microscopy (FIM) technique to the study of point defects in irradiated or quenched metals is reviewed. The techniques involve essentially the atom by atom dissection of specimens by pulse field evaporation and the corresponding recording on film of the extensive number of FIM images produced at each stage of the process. The application of these techniques to the following problems are discussed: (i) the mobility of self interstitial atoms (SIA) in substage IE of irradiated tungsten and platinum, (ii) the geometric configuration of the SIA, (iii) the point defect structure of depleted zones in tungsten and (iv) the properties of monovacancies and divacancies in quenched platinum.

The defect structure of depleted zones in irradiated tungsten

A quantitative field ion microscope study was made of the defect structure of two depleted zones detected in high purity (≤ 1.5 · 10 −6 at.fr. impurity level) tungsten specimens irradiated in situ under ultra-high vacuum conditions at a specimen temperature of 18°K with 20 keV W + ions to a dose of ∼ 1.10 12 W + ions cm −2. The irradiated specimens were examined by the pulse field evaporation technique at 18°K, hence their structure is characteristic of this temperature. The depleted zone detected in the (111) plane had a local vacant site concentration of ∼ 8.7 at.% and a self-interstitial atom concentration (SIA) of ∼ 0.92 at.%, while the depleted zone detected in the (141) plane of a second specimen had values of ∼ 10 and ∼ 0.5 at.% for these same quantities. The SIAs found locally were on the peripheral surfaces of the depleted zones. In addition to this large local imbalance in point defect concentrations the depleted zones were elongated along 〈011〉 directions. It was shown that each of these depleted zones was created by a single incident ion, and a comparison of the number of displaced atoms in each zone with the value predicted by the Kinchin-Pease model showed that this model strongly overestimates the number of displacements created per incident ion. For the depleted zone detected in the (111) plane it was demonstrated that 23 of 25 SIAs found in the lattice around this zone could have been propagated away from it along 〈011〉 and 〈111〉 directions as focused replacement sequences. The measured distances of these SIAs away from the depleted zone along 〈011〉 and 〈111〉 directions were between 45–150 and 45–85Å, respectively. The upper and lower values of these distances were determined by a geometric sampling problem and not by the intrinsic range of focused replacement sequences.

An in situ field ion microscope study of irradiated tungsten

Abstract The mobility of single self-interstitial atoms was studied in a series of in situ field ion microscope (FIM) experiments. High purity (≲ 1.5 × 10−6 at. fr. impurity level) W specimens were irradiated with 20 kev W+ ions under ultra-high vacuum (∼ 10−10 torr) conditions at temperatures between 8°K. The specimens were examined directly at the irradiation temperature employing the pulse field evaporation technique. The initial state of damage at 15°K consisted of depleted zones within 100 A of the irradiated surface, and a distribution of immobile self-interstitial atoms which were distributed throughout the bulk of the specimen, and which originated at the depleted zones. Direct observation of the FIM specimens during subsequent annealing experiments between 15°K and 120°K showed that these self-interstitial atoms underwent long-range migration (i.e. distances of ∼ 200 A) in Stage I, and were mobile at temperatures as low as ∼ 28°K. Warming these same specimens to 298°K (∼ middle of Stage II) follo...

Field evaporation rates of tungsten

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Pulsed field evaporation mass spectrometry

Pulsed field evaporation mass spectrometry

The Atom-Probe Field Ion Microscope

A serious limitation of the field ion microscope has been its inability to identify the chemical nature of the individually imaged atoms. The newly conceived atom-probe FIM is a combination probe-hole FIM and mass spectrometer having single particle sensitivity. During observation, the observer selects an atomic site of interest by placing it over a probe hole in the image screen. Pulsed field evaporation sends the chosen particle through the hole and into the spectrometer section. Preliminary results show that field evaporation of tungsten under poor vacuum conditions occurs as triply or quadruply charged WO, WN, WO2, and WN2 ions, while under better conditions doubly and possibly triply charged tungsten can be observed. Mo–Re alloys always produced doubly charged molybdenum and rhenium ions when examined in the atom-probe. Wide applications for the study of short range order in alloys, the chemical nature of precipitates and impurity atoms, and information regarding the imaging properties of various atom species, of both the substrate and adsorbed material, are foreseen.