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

tel-00751814, version 1 - 14 Nov 2012
Chapter 3. Oxide Dispersion Strengthened Steels
Figure 3.7. In the first case (case 1, on Figure 3.7), if the field at the tip apex is high enough,
all ions have the same evaporation probability at (V0+Vp). This may be achieved by applying
faster pulse repetition rates, lower temperature of analyse as well as lower standing field and
higher pulse fraction. In second case, if the field at the tip apex (V0+Vp) is too low, then A
atoms could stay at the surface while B elements will be evaporated (case 2, on the Figure
3.7).
A
B
Figure 3.7. Schematic diagram of the field evaporation rate as a function of specimen
temperature for A-B binary alloy, after [8]. Constant temperature is considered. In case 1,
conditions of analyse allowing to get equal field evaporation probability at (V0+Vp) for both
elements are shown. In case 2, the increase of electric field (V0+Vp) is too low to induce
evaporation of element A (that is more difficult to field evaporate in comparison to B).
The latter effect could occur in the case of ODS model alloy, when Y-Ti-O-rich phases
reach the surface of the sample. The affinity between Ti, Y, Fe and Cr with O is high. As it is
shown in Table 3.5, the enthalpy of formation of oxide is higher for Y, Ti and slightly lower
for Cr and Fe. As a consequence, their dissociation is difficult and they appear as molecular
ions. The values of the field of evaporation are only known for Fe, Cr, Ti, Y ions embedded in
pure Fe matrix [8]. There is no data for molecular ions (TiO, YO, CrO, ...). From the
observation of TiOx, CrOx and FeO behaviour in our experiments, it is suggested that the field
required to evaporate TiOx as well as FeO and CrOx is higher in comparison to YO molecular
ions. Such a difference may be the reason of the shifts described previously.
The previously described effects are taken into account for chemical composition
measurements of Y-Ti-O –rich phases. Indeed, if care is not taken Ti as well as O atoms may
be lost, resulting in their underestimation. The methods used for calculation of chemical
composition are detailed below.
Vo
Vp
1 2
10 6
10 2
10 -2
10 -2
10 2
10 6
Field evaporation
rate (atom layers/s)
98