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

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

tel-00751814, version 1 - 14 Nov 2012

Chapter 1.Bibliography

elements as Nb and Mo are not desirable. They are replaced by W (and Ta in some cases).

Alloying elements or impurities such as Ni, Cu, Co, Ag or Al also should be kept as low as

possible. The content of C should be kept as low as possible in the case of ferritic ODS steels.

A small amount of C (300 ppm 1 ) may result in austenite formation at high temperatures [65].

The most commonly used alloying elements in ODS and their effects are summarised in Table

1. 1.

Table 1. 1. Effect of alloying additions (the most commonly used elements are shown)

Cr Ferrite stabilizer, corrosion resistance

Ti Grain refinement, participation in nanoclusters and solution strengthening

W Solid solution strengthening [68], ferrite stabilizer [69].

Mo

Solid solution strengthening, ferrite stabilizer [69] less used due to high activation

by neutrons

Ni, Mn Austenite stabilizers [69]. Increase toughness [69,70].

C Austenite stabilizer [65], enhance Ni effect [71].

b) Dispersion of oxides particles

In order to obtain a fine dispersion of nanometer scale oxides particles in a consolidated

ODS material, 0.25-0.5 wt.% of Y2O3 [25,72–76,66,77] dispersoid is usually added to Fe-Cr

matrix during MA. Yttria is known as one of the most stable oxides. As an example, Y2O3 has

the highest heat of formation which is 1906.7 kJ.mol -1 , in comparison to other oxides such as

Al2O3, SiO2, ThO2 TiO2 with heats of formation respectively of 1678.2, 910.9, 1227.6, 944.1

kjmol -1 at 25°C [78]. In addition, iron and yttrium are immiscible, so it also contributes to the

stability of Y2O3 oxide in iron chromium alloys [71].

Ukai et al. [23] have studied the effects of yttria content (up to 0.56 wt.%) on strength

properties and ductility of hot extruded Fe-13Cr-3W-0.5Ti ODS steel. The tensile strength

along extrusion direction as well as creep rupture strength increases with increase in Y2O3

content, whereas total elongation decreases. So the amount of Y2O3 (0-0.56 wt.%) should be a

1 Ppm – one part per million - is a unit to describe small values of miscellaneous

dimensionless quantities. It denotes one part in 10 6 or 1/1.000.000 × 100% = 0.0001%

16

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