The precipitation of γ-zirconium hydride in zirconium

Abstract A mechanism for the precipitation of γ-zirconium hydride in zirconium is presented which does not require the diffusion of zirconium. The transformation is completed by shears caused by 1 3 〈1010〉 Shockley partial dislocations on alternate zirconium basal planes, either by homogeneous nucleation or at lattice imperfections. Homogeneous nucleation is considered least likely in view of the large nucleation barrier involved. Hydrides may form at dislocations by the generation of partials by means of either a pole or ratchet mechanism. The former requires dislocations with a component of Burgers vector along the c-axis, but contrast experiments show that these are not normally observed in annealed zirconium. It is therefore most likely that intragranular hydrides form at the regular built1 3 〈1120〉 dislocations, possibly by means of a ratchet mechanism. Contrast experiments in the electron microscope show that the precipitates have a shear character consistent with the mechanism suggested. The possibility that the shear dislocations associated with the hydrides are emissary dislocations is considered and a model suggested in which this function is satisfied together with the partial relief of misfit stresses. The large shear strains associated with the precipitation mechanism may play an important role in the preferential orientation of hydrides under stress.

[1]  G. Lorimer,et al.  The precipitation of zirconium hydride in zirconium and zircaloy-2 , 1972 .

[2]  M. Ashby,et al.  On the generation of dislocations at misfitting particles in a ductile matrix , 1969 .

[3]  C. Ells.  Hydride precipitates in zirconium alloys (A review) , 1968 .

[4]  G. Carpenter.  The dilatational misfit of zirconium hydrides precipitated in zirconium , 1973 .

[5]  J. E. Bailey.  Electron microscope observations on the precipitation of zirconium hydride in zirconium , 1963 .

[6]  J. Venables.  Deformation twinning in face-centred cubic metals , 1961 .

[7]  J. Christian,et al.  The martensitic transformation in cobalt , 1953 .

[8]  A. Akhtar,et al.  Plastic deformation of zirconium single crystals , 1971 .

[9]  D. Westlake.  Disposition of hydrogen in quenched zirconium-hydrogen alloys☆ , 1965 .

[10]  A. Cottrell,et al.  LX. A mechanism for the growth of deformation twins in crystals , 1951 .

[11]  G. Lorimer,et al.  Effect of hydrogen concentration and cooling rate on hydride precipitation in α-zirconium , 1975 .

[12]  J. F. Watters,et al.  Vacancy precipitation in zirconium alloys , 1973 .

[13]  Jens Lothe John Price Hirth,et al.  Theory of Dislocations , 1968 .

[14]  A. Sleeswyk.  Emissary dislocations: Theory and experiments on the propagation of deformation twins in α-iron , 1962 .

[15]  J. A. Jensen,et al.  DEFORMATION AND FRACTURE OF ALPHA ZIRCONIUM ALLOYS. , 1972 .

[16]  B. Bilby.  On the mutual transformation of lattices , 1953 .

[17]  A. Akhtar.  Compression of zirconium single crystals parallel to the c-axis , 1973 .

[18]  C. J. Simpson,et al.  Delayed hydrogen embrittlement in Zr-2.5 wt % Nb , 1974 .

[19]  J. F. Watters,et al.  Dislocations generated by zirconium hydride precipitates in zirconium and some of its alloys , 1973 .

[20]  G. Lorimer,et al.  The relationship between gamma and delta hydrides in zirconium-hydrogen alloys of low hydrogen concentration , 1974 .

[21]  E. Fisher,et al.  ADIABATIC ELASTIC MODULI OF SINGLE CRYSTAL ALPHA ZIRCONIUM , 1961 .

[22]  M. Ashby,et al.  Diffraction contrast from spherically symmetrical coherency strains , 1963 .