Metallographic and Fractographic Observations of Hydrides during Delayed Hydride Cracking in Zr-2.5% Nb Alloy

Review of the thermodynamic basis for models of delayed hydride cracking rate in zirconium alloys

“…Direct confirmation of this step-wise sequence of events characterizing DHC was obtained in a limited number of TEM experiments in different hydride-forming metals [14][15][16][17], while indirect confirmation was obtained from metallographic and fractographic observations of hydrides at the crack tip and striation spacings, respectively [18]. In some favourable cases these observations could be correlated with corresponding step-wise intervals of acoustic emission and electrical resistivity signals [19][20][21] corroborating the metallographic and fractographic evidence of step-wise growth and fracture of the hydrided region. The key observation, relevant to the subsequent discussion of Kim's model, which could be inferred from these observations, is that, during each crack propagation step, the first requirement for DHC is that a new, radially-oriented hydride must be able to form at the flaw after each propagation step.…”

“…(20) was V hydride . This is incorrect as the volume change on transforming the matrix to hydride that is given by the transformation strains is measured from the volume of the matrix phase, which is given by V Zr .…”

Stage I and II behaviors of delayed hydride cracking velocity in zirconium alloys

“…Since DHC is a discontinuous process of crack growth, the critical hydride length, l c should correspond to the spacing of the striation lines that can be determined from the fracture surfaces of the Zr-2.5Nb CT specimens [12,20,21]. To identify a correlation of l c and K I , we determined the striation spacing on the fracture surfaces with the striation line sequences.…”

Delayed Hydride Cracking: Theory and Experiment

“…Direct confirmation of the stepwise sequence of events characterizing DHC was obtained in a limited number of TEM in situ experiments in different hydride forming metals [6,36,41,73], while indirect confirmation was obtained from metallographic and fractographic observations of hydrides at crack tips and from striation spacing length measurements, respectively [79]. In some favorable cases these observations could be correlated with corresponding stepwise intervals of acoustic emission and electrical resistivity signals [27,69,94] corroborating the metallographic and fractographic evidence of stepwise growth and fracture of the hydrided region. The rate of growth of the DHC crack suggests that the process is diffusion driven.…”