4.01.1.4.1 Crystalline to amorphous transformation of Zr-(Fe, Cr, Ni) intermetallic precipitates

In addition to point-defect cluster formation, irra­diation of metals can affect the precipitation state as well as the solid solution. In the case of zirco­nium alloys, while investigating the effect of irradi­ation on corrosion, TEM observations revealed that for Zircaloy, irradiated at temperatures typical for commercial light water reactors (lower than 600 K), Zr(Fe, Cr)2 precipitates began to become amorphous after a fluence of about 3 x 1025 nm~2. Interestingly, the other common precipitate in Zy-2, Zr2(Fe, Ni), remained crystalline up to higher irra­diation doses.77 The instability of these precipitates under irradiation is of great importance since the secondary-phase precipitate plays a major role on

(b)

(d)

Figure 8 Examples of radiation-induced cavities in zirconium alloys. (a) Annealed crystal-bar zirconium, prism foil, 673K, 1.2x1025n/m2; (b) annealed zircaloy-2, prism foil, 673K, 1.2x1025n/m2; (c) annealed Zr-2.5 wt% Nb, basal foil, 923K, 0.7x1025n/m2; (d) typical cavity attached to inclusion on a grain boundary, material (c). Adapted from Gilbert, R. W.; Farrell, K.; Coleman, C. E. J. Nucl. Mater. 1979, 84(1-2), 137-148.

the corrosion resistance of Zircaloy (see Chapter 5.03, Corrosion of Zirconium Alloys).

The effect of temperature on the crystalline to amorphous transformation has been studied by vari­ous authors.7 ’ 3 It is shown that at low tempera­

tures (353 K), under neutron irradiation, both Zr(Fe, Cr)2 and Zr2(Fe, Ni) undergo a rapid and complete crystalline to amorphous transformation. As the irra­diation temperature increases, a higher dose is required for amorphization. It is indeed seen that, at 570 K, Zr(Fe, Cr)2 precipitates undergo only a partial amorphous transformation and Zr2(Fe, Ni) particles remain crystalline (Figure 9).

It is also observed that the crystalline to amor­phous transformation starts at the periphery of par­ticles, and then the amorphous rim moves inward until the whole precipitate becomes fully amor­phous. The chemical concentration profile within the precipitates also exhibits two distinct zones corresponding to the two different states: the crystal­line core and the amorphous periphery. It is observed that the amorphous layer exhibits a much lower iron

500
400
FeK,
300
CrK.
O. О
200
100
0 350
450 550 650 750 850 Energy (eV)x10-1
500
CrK,
‘a
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350 450 550 650 750 850

Energy (eV)x10-1

Figure 10 Crystalline to amorphous transformations of Zr(Cr, Fe)2 particle in Zy-4 irradiated at 560 K at 3.5 x 1025 n m~2. EDX spectrum shows that the amorphous volume is coincident with a depletion of Fe. Adapted from Griffiths, M.;

Gilbert, R. W.; Carpenter, G. J. C. J. Nucl. Mater. 1987, 150(1), 53-66.

content than the precipitate, the iron profile showing a local drop from the standard value of 45 at.% to below 10 at.% (Figure 10).

At higher temperatures (T> 640 K), amorphiza — tion was not detected and the precipitates remain crystalline, but some authors79 have nevertheless observed loss of iron and even total dissolution of Zr2(Fe, Ni) and Zr(Fe, Cr)2 precipitates and redistri­bution of alloying elements.

The crystalline to amorphous transformation is eas­ily understood in terms of ballistic radiation-induced disordering at a temperature where recombination ofpoint defects or recrystallization within the interme­tallic precipitate is too slow to compensate for the rate of atomic displacement (at 350 K).79 The dissolution of alloying elements remains limited at this low tempera­ture and the amorphization is mainly due to sputtering, that is, transfer of material from the particle because of atomic displacements by neutrons. When the point — defect concentration becomes too high and/or when the chemical disordering is too high, the crystalline structure is destabilized and undergoes a transforma­tion to an amorphous phase.75,79

The fact that the Zr2(Fe, Ni) phase remains crys­talline at intermediate temperatures (520-600 K) is presumably due to a more rapid reordering than the disordering in this structure (Zintl phase structure).

Concerning the Zr(Fe, Cr)2 (Laves phase structure), it is seen that the amorphization starts at the precipi­tate-matrix interface forming a front that gradually moves into the precipitate. The amorphization is believed to happen by a deviation from stoichiometry due to a ballistic interchange of iron and zirconium atoms across the precipitate-matrix interface. It also agrees with the observed kinetics of amorphization, predicting an amorphous thickness proportional to fluence and the absence of an incubation period for the transformation to start.84

The reason for the depletion of iron from the precipitates is not clearly understood yet, according to Griffiths et al.79 It is suggested that iron may be in some form of irradiation-induced interstitial state in irradiated Zr-alloys and may then diffuse intersti­tially out of the intermetallic particles.

At high temperatures (640-710 K), corresponding to 0.3 Tm the thermal activation is sufficient to induce dynamic recrystallization impeding the amorphiza — tion of the precipitates. However, depletion and some precipitate dissolution would still occur, but the level of damage necessary for amorphization would not be reached due to the absence of cascade damage.8 Because of the high mobility of Fe and Cr, redistribution of solute can occur, leading to secondary-precipitate formation.