ATOMIC ENERGY OF CANADA LIMITED

HYDROGEN-INDUCED DELAYED CRACKING:

2. EFFECT OF STRESS ON NUCLEATIONGROWTH AND COARSENING OF ZIRCONIUM

HYDRIDE PRECIPITATES

by

M.P. Pula

Whiteshell Nuclear Research EstablishmentPinawa, Manitoba ROE 1LO

1984 DecemberAECL-8381

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HYDROGEN-INDUCED DELAYED CRACKING:

2. EFFECT OF STRESS ON NUCLEATION,

GROWTH AND COARSENING OF ZIRCONIUM

HYDRIDE PRECIPITATES

by

M.P. Pula

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1. INTRODUCTION

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TABLE 1

SUMMARY OF TSS DATA FOR HYDROGEN IN ZIRCONIUM AND ZIRCALOY-2

USED TO DETERMINE THE CHEMICAL FREE ENERGY FOR NUCLEATION

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TABLE 2

VALUES OF UNDERCOOLING FROM THE CONSTRAINED SOLVUS DERIVED FROM

THE TSS DATA (SUMMARIZED IN TABLE 1) FOR ZIRCONIUM AND ZIRCALOY-2

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TABLE 3

CRITICAL HOMOGENEOUS NUCLEATION ENERGIES

FOR AND HYDRIDES IN ZIRCALOY-2 AND ZIRCONIUM

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TABLE 4

VALUES OF SOME PHYSICAL CONSTANTS AND CRITICAL NUCLEI DIMENSIONS

THAT GIVE ACCEPTABLE NUCLEATION RATES AT ZERO EXTERNAL STRESS

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TABLE 5

EFFECT OF A UNIAXIAL EXTERNAL TENSILE STRESS ON TEH ORIENTING TENDENCY

OF v- AND HYDRIDES DURING NUCLEATION, USING THE HYDRIDE MISFIT STRAINS

DETERMINED BY CARPENTER [18]

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TABLE 6

TIME, At, TO INCREASE THE INITIAL MEAN HYDRIDE RADIUS

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APPENDIX A

EXPRESSIONS FOR THE CRITICAL NUCLEATION ENERGY

FOR DELTA AND GAMMA HYDRIDE PRECIPITATES

A. Delta Hydrides

We assume that the hydrides precipitate as disca on the basal

plane of. the hop zirconium lattice. For the purpose of calculating the

strain energy. the plate is approximated by an oblate spheroid with major

axis a and minor axis c. To simplify the calculation for the total inter-

facial energy, it is convenient to consider the plate as a disc of radius

'a' and thickness '2c'. It in then assumed that the face of the disc is

coherent with the matrix, with a specific interfacial energy y, whereas

the sides are incoherent, with a specific interfacial energy The total

free energy for nucleation, AG. for the oblate spheroid having volume

Vplate 4/3 * a c Is given by

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where Ag is the absolute value of the Gibbs free energy of transform-

ation per unit volume, excluding strain energy contributions (the chemical

free energy), and Ag and Ag are the self-strain and interaction

energies per unit volume, respectively. In the notation of I,

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