Specific features of structure-phase transformation in materials under irradiation

Full text: Using as an example nickel-chromium alloys, austenitic and ferritic-martensitic stainless steels as well as vanadium alloys the paper shows that the effects of in-pile (neutron) irradiation (strengthening, embrittlement, swelling) are directly related to the processes of decomposition of a non-equilibrium solid solution that is characteristic all commercial steels and alloys, in other words, to the metastability of structure-phase state. It is demonstrated that it is the non-equilibrium metastable nature of the initial structure that predetermines the degree of changes in the material properties. This proposition is well illustrated by the results of investigation of nickel-chromium alloys having various compositions. As quenched and tempered structure of alloys having various chromium contents reveals the following: a) quasiperiodic arrangement of domains upon the long-range ordering of alloys containing ∼33 % mass Cr; b) stage of transition from quasiperiodic to modulated structure in alloy containing ∼39 % mass Cr; c) modulated structure (tweed contrast) of alloys containing 39-41 % mass Cr; d) discontinuous decomposition of alloys containing >41 % mass Cr. The above mentioned structure-phase features effected by different chromium contents in nickel alloys under neutron irradiation at ≥350 deg. C lead to evolution of different structures of irradiation induced defects. The alloys having the non-equilibrium metastable modulated structure (tweed contrast short-range ordering) evolve a 'stable' irradiation induced structure i.e. the structure in which traps for point defects are formed. As a result of this, the mobility of point defects decreases and their recombination degree increases which provides for their high mechanical properties and the unavailability of pores. One of the most important factors that control the aftereffects of the irradiation damages in steels and alloys, particularly, the time range of the incubation period upon irradiation induced swelling is the stability (to be more exact to say the instability) of their structure-phase state under neutron irradiation. The processes of metastable phase precipitation, short-range ordering, solid solution lamination might be managed by both alloying also including impurity elements-modifiers (B-type) and by heat treatments. The conclusion is drawn that no pores are observable in the material subjected to the temperature ranges within which the structure-phase transformations of the short-range ordering type, lamination, G-P zones formations etc proceed. In the temperature range in which in the end the stable structure is formed with time we observe porosity and property degradation