Etude par Sonde Atomique Tomographique de la formation de nano ...

tel-00751814, version 1 - 14 Nov 2012
Chapter 2. Materials, experimental and simulation techniques
Measured concentration Ti, Y, O (at,%)
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14
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2
0
Figure 2.7. Comparison of chemical composition estimated for nanoparticles in ODS
steel by Laser-Assisted Tomographic Atom Probe (LATAP) and Wide Angle Laser-Assisted
Tomographic Atom Probe (LAWATAP).
II.4. Particular case of ODS and NDS materials
a) Determination of experimental parameters
In order to get reproducible and reliable results during APT analysis, it is important to set
correctly experimental conditions. Temperature, Laser wavelength and pulse fraction are
important parameters influencing results.
LATAP
LAWATAP
core TAP
core LAWATAP
Ti Y O
Laser wavelength. Two different Laser wavelengths were available on LATAP and
LAWATAP at the beginning of this work: λ=1030 nm (infrared) and λ=515 nm (green). It is
clearly established that for ferritic materials, green Laser pulses give largely better mass
resolution than infrared. Thus, 515 nm wavelengths were used in all experiments.
Temperature effect. The temperature of the sample during analysis may have different
consequences. First, the risk of sample breaking increases when the temperature decreases.
However, by increasing the temperature, mass resolution and spatial resolution are affected
and additional effect such as preferential evaporation 2 may bias the chemical composition
2 This effect happens when elements have different field of evaporation. If the standing voltage of the
specimen is too high, elements with lower field of evaporation will preferentially evaporate during the
time between the pulses. This effect can be avoided by increasing the pulse fraction (which results in a
decrease of standing voltage) or decreasing the temperature (differences in evaporation fields are
normally decreases with decreasing of the temperature).
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