Time-of-flight atom probe

Frequently a material fails in service, and it is necessary to analyze the failed component to determine the causes of failure. While the metallographic and analytical methods described here should be undertaken, the failed surface itself can often yield valuable information. Failure frequently occurs by the gradual opening of a crack for some time before catastrophic failure actually occurs. Often a crack may be detected in the component during routine nondestructive 

The oxide thickness can he used to determine the age of different parts of the crack and hence the rate of grow th. A freshly exposed metal surface w-ill, in general, oxidize in such a way that the thickness of the oxide increases as the square root ol the time of exposure at temperature [54- 551. namely 

Frequently the oxide may be many microns thick as in the example of a ferritic steel operating at several hundred degrees Celsius, in which a crack had opened over a period of months or years before final failure. In such cases the oxide thickness may be readily determined from optical examination of a met-allurgically prepared specimen. The thickness may then be used with the time-temperature history to build up a picture of the crack history. 

Lejcek and S. Hofmann, Crit. Rev. Solid State Mate. Sci. 20, 1 (1995). 

Nellle.ship and R. K. Wild.. Surf Interface Anal. 16. 552 (1990). 

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By combining with a time-of-flight atom-probe method, FIM is capable of providing information about the chemical identity and even the isotope number of the surface atoms, when it is evaporated from the surface (Tsong, 1990). In this application, FIM is unique. 

Fig. 1.4 Diagram showing the principle of operation of a time-of-flight atom-probe. The tip is mounted on either an internal or an external gimbal system. The tip orientation is adjusted so that atoms of one s choice for chemical analysis will have their images falling into the small probe-hole at the screen assembly. By pulse field evaporating surface atoms, these atoms, in the form of ions, will pass through the probe hole into the flight tube, and be detected by the ion detector. From their times of flight, their mass-to-charge ratios are calculated, and thus their chemical species identified.

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Fig. 3.10 Linear type high voltage pulse time-of-flight atom-probe with an...

Flight-time-focused time-of-flight atom-probe... 

Let us discuss some of the advantages and disadvantages of the pulsed-laser time-of-flight atom-probe as compared with the HV pulse atom-probe. Some of the advantages are as follows ... 

In a time-of-flight atom-probe, if the vacuum condition is not good enough, many hydride ions of the tip material can be found in the mass... 

By device Time-of-flight atom-probe field ion microscope None... 

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