To the first-order most researchers concentrate on the cold-work level as the primary way to delay void swelling, although it is known that increasing cold work beyond a certain level specific to each alloy yields diminishing returns, with the optimum level usually chosen to be 20-25% for austenitic alloys. Larger levels are often counter-productive in that the additional stored energy at higher cold-work levels sometimes induces recrystallization during irradia­tion, often resulting in higher swelling.1

Additionally, in some alloys and metals it is diffi­cult to nucleate voids under some combinations of temperature and dpa rate due to the difficulty to establish a stable dislocation network. Cold working in some cases can actually shorten the transient by providing a stable glissile dislocation network and thereby accelerate swelling, as observed in model Fe-Cr-Ni alloys and simple metals such as nickel

132-134

and iron.

The starting thermal-mechanical condition ofthe alloy plays an important role in determining the transient duration via its influence on the starting dislocation density, but more importantly in deter­mining the distribution or chemical activity of the active elements. For instance, aging of an alloy at intermediate temperatures that encourage carbide precipitation, for instance, is the most effective way to produce the shortest transient and the highest swelling.1

There are many other examples. For instance, the chemical activity of an element like phosphorus is very sensitive to the inter-pass annealing tempera­ture range employed in producing cold-worked tub­ing by multiple drawings. It is speculated that

phosphorus can be either in solution or existing as small invisible precipitates oflesser chemical activity depending on the inter-pass annealing temperature or tube feed rate through the furnace.1,135

As carbon plays a role in both carbide and inter­metallic phase evolution, and its chemical activity can be strongly affected by thermal and mechanical his­tory, it exerts a strong and often complex effect on the transient duration. One aspect of this complexity is the often-observed two-peak swelling behavior ver­sus temperature that strongly varies with thermal — mechanical treatment.1 This effect is so strong that the swelling valley between the two peaks often occurs at the peak flux position. Cold-working tends to suppress the low temperature peak more than the high temperature peak due to its effect to delay and homogenize carbide formation. Removing almost all carbon into precipitates by aging erases the double peak behavior and usually produces the largest amount of swelling, as shown in Figure 52.