A review of microstructure evolution in zirconium alloys during irradiation

The dilatational misfit of zirconium hydrides precipitated in zirconium

Mechanisms which have been formulated to describe delayed hydrogen cracking in hydride-forming metals are reviewed and discussed. Particular emphasis is placed on the commercial alloy Zr-2.5 pct Nb (Cb) which is extensively used in nuclear reactor core components. A quantitative model for hydrogen cracking in this material is presented and compared with available experimental data. The kinetics of crack propagation are controlled by the growth of hydrides at the stressed crack tip by the diffusive ingress of hydrogen into this region. The driving force for the diffusion flux is provided by the local stress gradient which interacts with both hydrogen atoms in solution and hydrogen atoms being dissolved and reprecipitated at the crack tip. The model is developed using concepts of elastoplastic fracture mechanics. Stage I crack growth is controlled by hydrides growing in the elastic stress gradient, while Stage II is controlled by hydride growth in the plastic zone at the crack tip. Recent experimental observations are presented which indicate that the process occurs in an intermittent fashion; hydride clusters accumulate at the crack tip followed by unstable crack advance and subsequent crack arrest in repeated cycles.

Mechanisms of hydrogen induced delayed cracking in hydride forming materials

Cavity (void) formation and swelling in non-fissile materials during neutron irradiation and charged particle bombardments are reviewed. Helium is the most important inert gas and is primarily active as a cavity nucleant. It also enhances formation of dislocation structure. Preimplantation of helium overstimulates cavity nucleation and gives a different temperature response of swelling than when helium is coimplanted during the damage process. Helium affects, and is affected by, radiation-induced phase instability. Many of these effects are explainable in terms of cavity nucleation on submicroscopic critical size gas bubbles, and on the influence of the neutral sink strength of such bubbles. Titanium and zirconium resist cavity formation when vacancy loops are present.

Experimental effects of helium on cavity formation during irradiation—a review

A critical review is presented of published data on hydrogen, hydrides, and delayed hydrogen cracking (or hydrogen-induced delayed cracking, HIDC) in zirconium and its alloys. It is shown that factors such as solution temperature, cooling rate, and applied or residual stresses greatly affect the hydride morphology and orientation. The most detrimental hydride morphology is shown to be that when the hydride is oriented normal to the applied stress and has the morphology of a continuous sheet. Although knowledge of the mechanism(s) of delayed hydrogen cracking has improved greatly in the past five years, further research is needed to provide a better understanding of the process. The review concludes with a discussion of delayed hydrogen cracking in the practical case of nuclear-reactor pressure tubes, and presents possible remedies to ensure no further reoccurrence of the phenomenon in commercial reactors.

R.W. Gilbert

Papers

Dislocations generated by zirconium hydride precipitates in zirconium and some of its alloys

Damage structure in zirconium alloys neutron irradiated at 573 to 923 k

Amorphous intermetallics in neutron irradiated zircaloys after high fluences

Dislocation loop generation and irradiation growth in a zirconium single crystal

Tempered structures of a Zr-2.5 wt % Nb alloy