Stainless steels undergo an evolution of phase struc­ture at reactor-relevant temperatures, even in the absence of radiation. These changes involve the for­mation of various carbides, later followed by various intermetallic phases.1,108 This evolution is accompa­nied by net changes in average lattice parameter arising from differences in partial molar volume of elements when passing from one phase to another.

By-92

52 dpa 29.8% U-796

34 dpa max 14% swelling

Figure 42 Severe embrittlement and failure in three BOR — 60 reflector assembly ducts. The ducts were made of annealed X18H10T, the Russian equivalent of 321 steel. Reproduced from Neustroev, V. S.; Ostrovsky, Z. E.; Teykovtsev, A. A.; Shamardin, V. K.; Yakolev, V. V. In Proceedings of 6th Russian Conference on Reactor Materials Science; 11-15 September 2000, Dimitrovgrad, Russia, in Russian. The maximum swelling values (from left to right) were 27.8, 29.8, and 14%. Failure was the result of high withdrawal loads arising from both swelling and bending, the latter a consequence of radial dpa gradients in the reflector.

The resulting macroscopic strains are sometimes very counterintuitive, however, especially with re­spect to their sign.

For example, formation of the less dense carbide phases leads to macroscopic densification of the alloy and shrinkage of volume,109 while the formation of denser intermetallic phases (Chi, Sigma, Laves) usu­ally leads to an increase in volume, a form of nonvoid swelling.110,111 This counterintuitive behavior is the result of the different partial molar volumes of criti­cal elements (C and Mo primarily) between the new

precipitates and the alloy matrix in which they form. Both the carbide and intermetallic phase evolution appear to be accelerated and sometimes altered under irradiation.

Other radiation-produced phases (W, G-phase) also appear to induce changes in lattice parameter

but these have not been well characterized, primarily because these phases develop concurrently with void swelling that masks their contribution.1

Garner1 provides a review of precipitation-induced strains. For the current purpose it is sufficient to note that carbide-induced densification increases with carbon content and with increasing irradiation temperature. Such volume changes for the most
common carbon levels range from 0.1% to 0.4% decrease in volume. The resulting strains may or may not be isotropically distributed, depending on whether there is a pronounced starting dislocation texture on which the carbides nucleate. This process is most pro­nounced for titanium carbides in Ti-stabilized steels. Carbide-induced strains usually develop quickly enough to be measurable before swelling strains become dominant and therefore are relatively easy to identify compared to those of slower forming phases.

The formation of intermetallic phases can gener­ate strains in the order of 1-3%. There is insufficient evidence to support anisotropy of resulting strains, but there exists some evidence that tensile stress states may accelerate the formation of these phases.110

Additionally, there is a decrease in density and a concurrent increase in volume when ferrite is formed from austenite as a result of radiation-induced segre­gation of nickel. Formation of ferrite from austenite can lead to volume increases as large as 3%, but there are no available data on potential anisotropy or stress dependence. As opposed to carbide-induced strains that develop relatively quickly, ferrite and interme­tallic strains develop rather slowly, and therefore are usually unrecognized, especially when other strain contributions arising from swelling and creep are present.

Such precipitation-induced strains are important in that while they usually saturate in magnitude, they can be a significant portion of the total net strain at low dpa levels, thereby complicating the analysis and extrapolation of void swelling and irradiation creep data. Such strains can also affect the stress distribu­tion and level in a structural component. For instance, a preloaded tie-rod or bolt will initially increase in load as a result ofcarbide-induced shrink­age even while irradiation creep proceeds to relax the load.

It should be noted that radiation-induced segre­gation can lead to overall changes in average lattice parameter without actually culminating in observable precipitation. Although there is no convincing evi­dence that segregation to void and grain boundaries produces measurable strains, it has been shown that radiation-induced spinodal-like decomposition in Fe-35Ni and Fe-Cr-35Ni alloys produces periodic oscillations in composition that are accompanied by densification in the order of ^1%.112,113 Oscillations in nickel level are almost exactly offset by out-of­phase oscillations in chromium. This demonstrates that in a single phase system the lattice parameter of a given element is not constant but is influenced by its local concentration and its association with other elements.