Hydrogen embrittlement of α titanium: In situ tem studies

A micromechanics approach to the study of hydrogen transport and embrittlement

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.

Hydrides and delayed hydrogen cracking in zirconium and its alloys

This chapter focuses on two kinds of environmental fracture, stress corrosion cracking (SCC) and hydrogen embrittlement, and discusses the role in such failures of a number of metallurgical variables. These include chemical composition; microstructural components such as precipitate type and structure, and grain size and shape; crystallographic texture; heat treatment and its effect on the foregoing variables; and processing, particularly the thermomechanical treatments (TMT), which are attracting increased attention for property optimization. These variables are expected to be of great importance in the development of new engineering materials to meet demanding service conditions.

The Role of Metallurgical Variables in Hydrogen-Assisted Environmental Fracture

The precipitation of γ-hydride plates 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

The velocity of hydride induced subcritical crack growth in Zr-2.5 pct Nb has been determined using the potential drop method for measuring crack extension. A revised picture of the two-stage, crack velocity-stress intensity relationship has been obtained with a threshold stress intensity of 6 MPa·m1/2, independent of temperature. A consistent temperature dependence of the crack velocity has been determined for hydrided material but the velocity measurements in as-received material are unexpectedly high. A previous theoretical model has been improved. The improved model has provided a useful basis for explaining some of the present data which could not be rationalized in terms of the previous model. Criteria for the stepwise crack propagation behavior are discussed.

L. A. Simpson

Papers

Mechanisms of hydrogen induced delayed cracking in hydride forming materials

The Effects of Stress, Temperature and Hydrogen Content on Hydride-Induced Crack Growth in Zr-2.5 Pct Nb