Electron microscope observations on the precipitation of zirconium hydride in zirconium

ELECTRON

MICROSCOPE

OBSERVATIONS

ZIRCONIUM

HYDRIDE

ON THE

PRECIPITATION

OF

IN ZIRCONIUM*

J. E. BAILEY? A study has been made of the precipitation of zirconium hydride in zirconium at hydrogen concent,rat,ions in the range 100-1000 p.p.m. using the transmission electron microscopy technique. In rapidly cooled or quenched specimens the precipitates appear needle-like with axes lying along (1120) directions In slowly cooled specimens the and are probably platelets parallel to {lOiO} planes of the matxix. Electron and X-ray diffraction experiments precipitates appear comparatively large in two dimensions. Identical orientation relationships show that all precipitates have a face centred tetragonal structure. also appear to hold for all precipitates. The needle-like precipitates grow rapidly along their (1120) type axes apparently remaining coherent; growth in the direction normal to their axes is accompanied by the formation of dislocations and the precipitates become incoherent on the appropriate faces. Beam heating experiments show that nucleation of the hydride does not necessarily require the presence of dislocations but that preferential precipitation will occur on them and on grain boundaries when present. OBSERVATIONS

EN MICROSCOPIE ELECTRONIQUE L’HYDRURE DE ZIRCONIUM DANS

SUR LA PRECIPITATION LE ZIRCONIUM

DE

L’auteur a &udi8 la pr&ipitation de I’hydrure de zirconium dans le zirconium, pour des teneurs en hydrogQne comprises dans la gamme de 100 8. 1000 p.p.m., & I’aide de la microscopic Bectronique par transmission. Dans les Bchantillons refroidis rapidement ou trempbs, les pr&ipit& apparaissent sous la forme d’aiguilles ax&es suivant les directions (1 lZO), et sent vraisemblablement des plaquettes paralleles Dans les Bchantillons refroidis lentement, les precipit& apparaissent aux plans {lOiO}; de la matrice. relrttivement d&elopp& suivant deux dimensions. Les essais de diffraction Blectronique et. de diffraction des rayons X mont,rent que tous les p&cipit& ant une structure t&ragonale 8,faces cent&es. 11semble

bien que tous les pr&cipit& observes prbsentent des relations d’orientation identiques. Les pr&ipit& aciculaires croissent rapidement le long de lenrs axes (1120) et restent apparemment coh8rents; la croissance dans la direction normale B leur axe s’accompagne de la formation de dislocations et les pr&ipit& deviennent incoh&ents sur les nouvelles faces ainsi form6es. Des essais de chauffage par le faisceau montrent que la germination de l’hydrure ne demande pas nBcessairement la pr&ence de dislocations, mais qu’on observe une pr6cipitation pr&f&entielle sur celles-ci ainsi que sur les limites de grains quand il s’en rencontre dans le champ observ& ELEKTRONENMIKROSKOPISCHE UNTERSUCHUNG DER ZIRKONHYDRID IN ZIRKON

AUSSCHEIDUNG

VON

Die Ausscheidung van Zirkonhydrid in Zirkon wnrde bei Wasserstoffkonzentrationen van 100 bis 1000 p.p.m. mittels Durchstrahlung im Elektronenmikroskop untersucht. Bei schnell abgekiihlten oder abgeschreckten Proben zeigen sich nedel&hnliche Ausscheidungen, deren Achae parallel zu (1120)Richtungen liegt; wahrscheinlich handelt es sich urn Plllttchen parallel zu {lOiO}-Ebenen der Matrix. In langsam abgekiihlten Proben sind die Ausscheidungen in zwei Dimensionen relativ ausgedehnt. Wie Experimente mit Elektronen- und RBntgenstreuung zeigen, besitsen alle Ausscheidungen tetragonal flllchenzentrierte Strnktur; such scheinen die Orientierungsverhiiltnissebei allen Ausscheidungen gleich zu sein. Die nadelftirmigen Ausscheidungen wachsen schnell und kohiirent entlang ihrer (1120)-Achse, wlihrend beim Wachstum senkrecht dazu Versetzungen entstehen und die Ausscheidungen nur teilweise inkoh&rent sind. Aufheizungsversuche mit Elektronenstrahlen zeigen da13 zur Keimbildung des Hydrids nicht unbedingt Versetzungen erforderlich sind, die Ausscheidungen jedoch vorzugsweise an vorhandenen Versetzungen und Korngrenzen entstehen. 1. INTRODUCTION

Extensive

metallographic

lying in (lOi5)

studies have been made

of the zirconium hydride precipitates decomposition

of the supersaturated

produced

associated

by the

near

the

the

high

hydride-metal

planes which are probably hydrogen

concentrations

interface.

These authors suggest that Kunz and Bibb were observing surface

solid solution.

Langeron and Lehr(l) established that at low hydrogen

twins produced

concentrations

pletely

(~100

and {lOil}

with

p.p.m.) the hydride precipitates

removed

during grinding which were not comby

chemical

polishing

and

that

in platelets lying parallel to {IOiO} planes of the zirconium matrix. Kunz and Bibbc2) reported however

subsequent

that the hydride precipitates as platelets on the twinning planes, i.e. (lOi2) (1121) and (1122). A further

X-ray studies of the zirconium hydrogen phase diagram by Gulbransen and Andrewc4) and Whitwamf5) appear to lead to the conclusion that two stable hydride phases exist, a face centred cubic and

ACTA METALLURGICA VOL.

11;

APRIL

1963

may have occurred

in these twins.

study by Westlake and Fisherc3) supported the work of Langeron and Lehr (l) by showing that the hydride platelets precipitated predominantly on {lOiO} planes together with some platelets of secondary importance * Received April 28, 1962. t Berkeley Nuclear Laboratories, Central Generating Board, Berkeley, Glos., England.

hydride precipitation

a face centred tetragonal phase designated 6 and E respectively, together with a minor metastable phase having a face centred tetragonal structure and desig-

Electricity 267

nated yl. The metastable y1 phase was found only in association with the 6 phase. However, Vaughan

ACTA

268

METALLURGICA,

and Bridgec6) on the basis of their high temperature X-ray

studies

exists whose position

concluded tetragonitl

that

one

structure

and temperature.

hydride

varies

The phase

phase

with

com-

diagram

ac-

cording to Vaughan and Bridge@) and Ellis and McQuillan(7) suggests that the hydride platelets formed from solutions

containing

tents may be the 6 phase. obtained electron diffraction

low hydrogen

con-

Schwartz and MalletW evidence which suggests

VOL.

11,

1963

Studies were also made of zirconium quenched

an inert atmosphere. The zirconium foils after being given the appropriate treatment, were thinned by electropolishing and examined in a Siemens electron microscope 100 kV.

of perchloric

acid to 90 parts glacial acetic acid.

bath

was operated

which

stirred and kept cool.

X-ray studies have been unable to detect the structure

men was removed

of these precipitates at low hydrogen concentration. The precipitates are of considerable importance since hydrogen

of zirconium

that occurs

concentrations

appears

present

the

to

be

with them. In

the

3.1

Zirconium

After work

transmission

electron

microscopy techniques and supplementary X-ray experiments have been used to study the precipitation

at The

at 20 V was vigorously

When suitably

thin the speci-

and washed in distilled water.

3. ELECTRON MICROSCOPE OBSERVATIONS

at low

associated

operating

The polishing bath used consisted of 10 parts

that these platelets were a tetragonrtl phase. To date,

the embrittlement

foils that were

into iced brine after annealing at 800°C in

foils-vacuum

this treatment

contain hydrogen

annealed

the foils

were expected

in concentrations

An electron micrograph

of ~10

to

p.p.m.(*)

taken from a typical

area of

of hydride in zirconium at low hydrogen concentrations

one of these foils is shown in Fig. 1 (a) ; a high density of small clusters is observed which must result from

The

the presence of some impurity.

technique

has clearly

hydride precipitates both

the

crystal

revealed

the

zirconium

and provided evidence concerning structure

of the precipitates

and

their stability. 2. EXPERIMENTAL

The

majority

of the

obser-

vations have been carried out on commercial

and re-

These clusters were The clusters could

be made to disappear in the same temperature range as hydrogen was found to precipitate from solution (these experiments

microscope

actor grade zirconium

electron

observed in all the foils examined.

are

described

below).

clusters that form preferentially

Furthermore,

on both

the grain

supplied by Murex Ltd., in the

form of foil of thickness 0.010 in.

Both grades had a

hardness in the annealed condition of ~200 VHN; no distinction will be made between these grades since their

behaviour

was

experiments. Some observations

identical

in

the

following

have been made on argon arc

melted crystal bar zirconium,

which is purer than the

grades described above and has a hardness number of ~100 VHN when fully annealed. The few experiments in which

this material

was used are clearly

identified below. Foils of suitable size were annealed at temperatures of about 800°C for several hours in a vacuum of & 10P5 mm of Hg to remove any residual hydrogen. The analyses carried out by Schwartz and Mallet@) show that this treatment reduces the hydrogen content to below 10 p.p.m. The vacuum apparatus

incorporated

a palladium

leak through which the required amounts of hydrogen were passed into the system. Hydriding was generally carried out by heating the zirconium foil to 800°C in a known pressure of hydrogen and allowing the specimens to furnace cool. In order to obtain a faster cooling rate some foils were heated electrically quenched by switching off the current.

and

FIG. l(a).

Small clusters observed on zirconium foils ( x 26,000).

electropolished

BAILEY:

PRECIPITATION

OF ZIRCONIUM

FIG. l(b). Clusters formed preferentially on grain boundaries lie on the surface of the foil ( x 26,000).

boundaries

(Fig. l(b))

and on dislocations

(Fig. l(c))

appear to be either on the top or bottom the foil.

surfaces of

From the above evidence it is concluded

these clusters are hydride

that

clusters which form on the

surface of the foil during electropolishing. The density of such clusters seems to depend sensitively on the polishing conditions.

Some specimens show relatively

few clusters and there is often a wide variation

in the

269

HYDRIDE

FIG. l(c). Clusters formed preferentially on the points of emergence of dislocations from the foil ( x 26,000). needle-like vations.

will be used here to describe These

precipitates

can

seen in Fig. 2(c).

Fig. 2(c) also shows that some pref-

erential precipitation

occurs on grain boundaries.

Fig. 2(b) shows the same area as Fig. 2(a) after tilting the specimen to produce different diffracting conditions.

A

semi-circular

“halo”

now appears

same specimen.

strong Bragg reflections are operating the double

clusters when referred to below. 3.2

Zirconium

These observations been hydrided

area.

foils-hydrided to contain

~500

on foils that had

p.p.m.

of hydrogen.

Figs. 2 and 3 show examples of the hydride precipitates observed in the furnace

cooled specimens.

It is clear

from Fig. 2(a) that the hydride precipitates assume a needle-like appearance at a very early stage of growth, and electron diffraction patterns show that these “needles” have axes lying along (1120) directions. When the plane of the foil is accurately parallel to the basal plane of the zirconium the hydride “needles” appear very thin as shown in Fig. 2(c). This evidence suggests that these “needles” are really platelets lying parallel to {IOiOo) planes which is in agreement with the metallographic work,(1,3) however, the term

dislocation

contrast Two

as indicated

images observed

by

in the same

that this contrast is due to the

strains induced

the precipitate

type

on one side of the precipitates.

It is considered

coherency

were made

distin-

guished from the surface clusters which can also be

density of clusters observed from different areas of the These artefacts will be called surface

the obser-

be clearly

in the matrix

surrounding

and is a similar effect to that observed

around G.P. [2] precipitates (Nicholson and Nutting@)). The larger needle-like

precipitates,

an example

of

which is shown in Fig. 2(d), result from the continued growth of the small precipitates; this is borne out by the electron beam heating experiments described in $4. As the precipitates

increase in size arrays of dis-

locations are observed to be associated with them. At this stage the precipitates must be incoherent on certain faces. Electron diffraction patterns taken from areas including these precipitates have so far failed to reveal diffraction spots that cannot be attributed to the zirconium matrix. These needle-like precipitates all have axes lying along the (1120)

ACTA

270

METALLURGICA,

VOL.

11,

1963

FIG. 2(a). Smell hydride precipitates in zirconium hydrided to contain &500 p.p.m. of hydrogen ( x 40,000).

FIG. 2(b). Same area as shown in Fig. 2(a) after tilting the foil to produce different diffracting conditions ( x 40,000).

directions

of the zirconium

matrix

has

obtained

suggests

been

orientations. Hydride precipitates

which

and no evidence other

possible

that were generally relatively

large in two dimensions compared with those described above were also found in these specimens ; examples are shown in Fig. 3(a) and (b). Diffraction patterns obtained from these precipitates suggest that they are the “so called” metastable y1 phase (see $5).

These large precipitates also have arrays of dislocations associated

with them.

Attempts

to find precipitates

that could be clearly interpreted as the 6 hydride phase have not been successful. Zirconium specimens that were electrically heated and quenched in the hydrogen atmosphere by switching off the current showed the presence of both long needle-like precipitates (Fig. 4(a) and (b)) and precipitates which appear wide in a direction normal to

BAILEY:

FIG. 2(c).

PRECIPITATION

Preferential precipitation of hydride boundaries ( x 11,000).

on grain

(a)

ZIRCONIUM

FIG. 2(d).

211

HYDRIDE

Needle-like

hydride precipitate

( x 26,000).

(b)

FIG. 3(a) and Fig. 3(b). Examples tain

OF

-500

of the large hydride p.p.m. of hydrogen

Pre:cipitates found is furnace cooled specimen and x 13,000 respectively). (x 26,000

hydrided

to con-

ACTA

252

(a)

METALLURGICA,

VOL.

11,

1963

(b)

FIG. 4(a) and FIG. 4(b). Examples of needle-like precipitates in rapidly cooled hydrided zirconium foils showing clearly the associated dislocation structures ( x 22,000 and x 26,000 respectively).

Fm.. 4(c) Hydride precipitate large in two dimensions also observed in rapidly cooled hydrided specimens ( ~26,000).

BAILEY:

(1120)

axes

(Fig.

4(c)).

PRECIPITATION

The precipitates

OF

that

are

ZIRCONIUM

periments

273

HYDRIDE

in which localized

regions of the electron

similar in habit to those shown in Fig. 3(a) and (b).

specimens have been heated by increasing the intensity of the electron beam. The electron beam

The majority

is reduced to a minimum

large in two dimensions

ofthe precipitates

specimens were however, dislocation arrangement precipitates

is clearly

The dislocations the precipitate

Zirconium

in these rapidly cooled of the needle-like type. The surrounding the needle-like

revealed

in Fig. 4(a) and (b).

are generally attached or appear

closed elongated the precipitate 3.3

e.g. Fig. 4(c), would appear

to the ends of

as completely

or partially

loops running parallel to the axis of

into iced brine

of the

quenching

needle-like

zirconium

argon atmosphere diffraction

foils annealed

by

at 800°C in an Electron

taken from areas including phase and X-ray

below confirmed this interpretation

These quenched

specimens

tain any of the “two discussed

high

these

contained spots which could be interpreted

in terms of the y1 hydride described

precipitates

into iced brine (Fig. 5(a)).

patterns

precipitates

hydride

of zirconium

dimensional”

studies (see $5).

did not con-

type precipitates

above. BEAM

HEATING-STAGE

Considerable

information

in order to increase the

of the beam ; the specimen

heated by increasing the beam current. cedure a hot zone is produced is surrounded Micrographs

(~5

is then

By this pro-

,u diameter) which

by a sharp radial temperature

gradient.

are taken after reducing the beam current

is illustrated

in such an ex-

in Fig. 6. An area of vacuum

annealed crystal bar zirconium is shown in Fig. 6(a) in which several needle-like hydride precipitates are present. beam

On heating with a suitably

(15 ,uA) for several seconds

disappear.

On rapidly

new precipitates

reducing

intense electron

these precipitates the electron

nucleated and grew;

beam

these are shown

in Fig. 6(b).

Repeating

intense

(20 ,uA), i.e. heating to a higher tem-

beam

this procedure

perature,

reduces

specimen

since no precipitation

Fig. 6(c).

4.’ ELECTRON

and the condenser I aperture

removed

general intensity

periment

It was found possible to produce a relatively density

of the microscope

rapidly and replacing the aperture. The type of behaviour observed

(Fig. 4(a)). foils-quenched

microscope

the amount

with a more

of hydrogen occurred

in the

on cooling,

Further beam heating at a lower intensity

(10 ,uA) again caused the nucleation

and growth

EXPERIMENTS

relatively

It is concluded

has been gained from ex-

from these experiments

HEATING

AND

FIG. 5(a). Needle-like hydride precipitates in zirconium foils quenched into iced brine from 800°C ( x 26,000).

large precipitates

FIG. 5(b).

(Fig. 6(d)).

of

that there must be a residual

Removal of the precipitates shown in Fig. S(a) by election beam heating ( x 26,000).

ACTA

274

(cl FIG. 6. Sequence of micrographs

METALLURGICA,

VOL.

11,

1963

(4 showing the effect of electron beam heating on hydride precipitation.

(See text)

(x

13,000).

BAILEY:

pressure of hydrogen

inside the electron

from which hydrogen specimens.

PRECIPITATION

is absorbed

The above experiments since they demonstrate precipitates directions

shrink

from

microscope

by the zirconium

The source is considered

present in the microscope tamination originates.

OF

to be the oils

which

carbon

are of considerable clearly

and grow

interest

that the needle-like rapidly

these beam heating experiments cipitates

disappeared

temperature

range as the normal hydride precipitates

do

not

imply

that

another

form

of

does not

for the “punching precipitates

these experiments.

In Fig.

e.g.

either with some precipitated

present.

The removal

with

cipitates

by electron

panied

beam heating

by the complete

surrounding

them;

removal

accom-

of the dislocations

for example the precipitates

and B, Fig. 5(a), were observed appear together

is often

at A

to shrink and dis-

with their associated

dislocations

as

shown in Fig. 5(b). Experiments

been carried

out using a cali-

brated heating stage in order to determine perature

at

removed.

which

the

hydride

has a relatively

being of the furnace

at temperatures

are

type.

of

heating experiment be explained.

by

ential cooling

events

the impurity

described

beam

above (Fig. 6) can now

Since the precipitates

become unstable

The loops

At

300°C

(Schwartz

the

terminal

and MallettP)

< C,

at 300°C in this case.

solubility

is ~100

and Gulbransen

p.p.m.

and An-

drew(lO)) and therefore pH is < 10-5. Since the terminal solubility decreases with decreasing temperature, on rapidly

cooling

(Fig. 6(b)).

the

Reheating

specimen

precipitation

occurs

the specimen to a higher tem-

perature reduces the value of C, sufficiently to prevent precipitation on subsequently quenching (Fig. 6(c)). Further heating of the specimen to a temperature below 300°C again raises the value of C, to a value above C and precipitation It was also observed

occurs directly,

Fig. 6(d)).

that on heating a specimen to a

temperature at which the precipitates in the hot zone disappear, precipitates were appearing in the region surrounding the hot zone, this situation is similar to that observed in Fig. 6(d), since in this region the temperature is such that C, > C. Occasionally circular type precipitates surrounded by dense tangles of dislocations

were observed during

by the differset up

and impurity

during particle.

arrangements

loops

and more

which

resemble

been produced

by a hydride

this procedure

removed

the hydride

at B (Fig. 8(b)) and has nucleated Numerous

new

loops are now observable

in

the field of view and these must be associated with the hydride

appear

precipitates. (1120)

precipitates.

to be “thrown

In

off”

these

loops

the sides of the

and since they must glide in this

the Burger’s type.

vectors

are expected

to be of

In the case of these hydride pre-

cipitates, ‘loops and dislocations growth

general

from

All arrays of loops have axes parallel to

directions,

the l/3(1120)

C, in the specimen such that C,

or

This has been confirmed by beam heating

the specimen;

H, in the microscope

C being the terminal solubility

out”

has produced

at B have

precipitate.

direction

concentration

particle

dislocation

at 300°C it would appear that the residual pressure of pH gives rise to an equilibrium

hydride

case of impurity

stresses

the matrix

are always

closely the similar effects observed inmagnesium.(11-13)

precipitates.

in the

In the

contraction

between

these

An example of this effect can be seen at A in Fig. S(a) ;

new

observed

C;

the loops are “punched

These experiare removed

B and

particles.

thermal

precipitate

of about 300°C.

sequence

particles

large thermal

ments show that the hydride precipitates The

the tem-

precipitates

The heating stage which was supplied

Siemens and Halske capacity,

impurity

complex

have

at A,

from

S(a) arrays of loops are

observed,

pre-

is

out” of complete loops

associated

hydride

hydride

has also been obtained

observed however that hydride precipitates do nucleate preferentially on dislocations when they are of the zirconium

These pre-

in the same

precipitating. Evidence

It is

(Fig. 7).

and reappeared

and it is considered that they may result from the growth of the surface clusters described in $3.1, and

by hydride

require the presence of dislocations.

275

HYDRIDE

(1120)

along

of the matrix and that nucleation

necessarily

con-

ZIRCONIUM

and it is difficult

are produced

to distinguish

during

additional

loops that may be produced during cooling. Some preliminary cooling stage experiments do suggest however that loops can be produced

on cooling.

5. CRYSTALLOGRAPHIC ANALYSIS HYDRIDE PRECIPITATES

OF THE

5.1 Structure of the hydride precipitates

Electron diffraction patterns were easily obtainable from the large precipitates (Fig. 3) observed in slowly cooled

zirconium

specimens.

from the precipitates

The pattern

obtained

in Fig. 3(a) is shown in Fig. 9.

These patterns could be accurately interpreted in terms of the tetragonal y1 phase using lattice parameters given by Whitwam.c5) Although the yl phase may be treated as a slight distortion of the S f.c.c. phase, the distortion is sufficient to be detected on the electron diffraction pattern, e.g. the angle between (ill) and (202) for the y1 phase is 93” as was observed on the diffraction pattern taken from the precipitate

276

ACTA

METALLURGICA,

VOL.

11,

1963

FIG. 7. Circular type hydride precipitates sometimes observed during electron beam heating experiments ( x 20,000).

FIG. 8(a). Examples of “punched out” dislocation loops produced by hydride precipitates and impurity particles.

BAILEY:

PRECIPITATION

OF

ZIRCONIUM

HYDRIDE

FIG. 8(b). Same area as Fig. 8(a) after the production of hydride precipitates by electron beam heating (x 20,000).

FIQ. 9. Selected area electron diffraction pattern taken from the large precipitate shown in Fig. 3(a).

shown in Fig. 3(b);

this angle should be 90” for the

the electron

diffraction

the precipitates

8 phase. Electron

diffraction

patterns from the thin needle-

like precipitates have been more difficult to obtain. Patterns were occasiona,lly obtained from the

For the large precipitates the orientation

Previous

X-ray studies have been unable to detect

with certainty the needle-like hydride precipitates found in zirconium at low hydrogen concentrations. In t’he present York it was found

that a large con-

centration of these precipitates could be produced by quenching zirconium foils from 800°C int,o iced brine ($3.3) and therefore an attempt was made to determine their crystal structure by X-ray studies. X-ray diffraction photographs were taken with a Guiner focussing camera permitting the simultaneous examination of the quenched and unhydrided foils: using monochromated MO K, and Cu K, radiat’ion. The quenched foils clearly showed the presence of additional

reflections

from

the hydride

precipitates

which corresponded closely to t’hose observed by Whitwamc5) from the phase designated y’. These reflections

could be indexed

the strong prominent

on a trtragonal

cell and

lines were :

d Z”2 = 1.A75 *

These

values

A further

r = 1.HX8 A

orientation

wit’h those

of

out on slowly

yl phase, and not to the face centred cubic that

available,

is identical

{ll:O},

and (lOlO),,

{llzO),

is parallel to {llO},,1 and

(IOTO}, is almost parallel to {131);,1 (ml” inclination). It is assumed here that the habit planes are the planes perpendicular to the foil. By similar arguments the habit planes of the needle-like precipitates platelets

are {lOTO), parallel to (11 l}+

if these are The change

in habit during growth from platelets to massive precipitates is not easily explained since it would not result from a straightforward thickening of the platelets as suggested by the form of the precipitate in Fig. 4(c);

such a thickening

would

suggest

(liOO),

parallel to (Ill),,1 as a habit plane. It may be of significance that the (131},1 type planes appear to be habit planes. The spacing of (lOlO},

planes c&,,

= = to

6. DISCUSSION

to the

6 phase.

the hydride

at these low hydrogen

and electron

“two

and rapidly

dimensional”

diffraction

studies estab-

precipitates

observed

cooled zirconium

precipitates

in

foils and the

observed

in the rela-

cooled foils both have a face

centred

tetragonal structure corresponding closely to the y1 st,ructure previously detected in zirconium cont,aining

c = 4.960 P,

therefore

evidence

wit’h (1). From the shape of the two large precipitates (Fig. 3(a) and (b)) the habit planes would appear to be

quenched

hydride were again found to clearly correspond

observed

the

t’ively slowly

was then carried

It is concluded

From

lished that the needle-like

cooled zirconium foils containing ~1000 p.p.m. of hydrogen and the additional reflections due to the

cipitates

the

relationship for the needle-like precipitates

The X-ray

arc t’o be compared

study

(1)

(1 l20), parallel to (I lO)+ 1

and

for the y1 phase :

IL = 1.610 A

(TI1)71

these planes.

(202) reflrctions are :

Whitwam

can be expressed as:

2d (131j; the two lattices fit therefore nearly perfectly (misfit 0.07 per cent) in the direction normal to

0.005 a

as calculated from the (Ill)

a = 4.617 B

relationship

them.

2.798 and the spacing of the {131},1 planes do,,, 1.396, and hence d,,,I,, is almost exactly equal

dzoo= 2.28 -f 0.01 a The cell dimensions

from both

shown in Fig. 3(a) and (b)

(IlOO), parallel to

“quenched in” precipitates discussed in $3.2. These patterns were also interpretable HAdue to a tetragonal phase.

patt’erns obtained

and the matrix surrounding

prc-

concentra-

the face centred cubic hydride phase (Gulbransen Andrew(d) and Whitwam(5)).

Furthermore,

and

the X-ray

work showed that the y1 phase was present in slowly cooled zirconium foils containing ~1000 p.p.m. of hydrogen; no evidence for t)he presence of the f.c.c. B phase was obtained.

It is concluded

the hydride phase in zirconium

therefore that

at low hydrogen

con-

tions have the tetragonal y1 structure for which c/a > 1

centrations

and lattice parameters of Q = 4.617 A and c = 4.888 1%arcording to the present X-ra.;v study.

to be expected according to the current phase diagram.

5.2 Orientation

These results confirm the previous work of Schwartz and Mallett(*) who concluded that the needle-like

and eirconium

rp,latio?lshipsbetlc~epn

h!/rlrid~preci~itcxths

matrix

The orientation

relationships

( < 10 at. ‘A,) is a face centred tetragonal

phase and not the face centred cubic 8 phase which is

hydride precipitates wclre dct,crmined from

were a tetragonal phase analogous

to that found in the H,-H

spst,em.

BAILEY:

PRECIPITATION

The needle-like zirconium hydride precipitates axes lying along (1120) directions it is clear

therefore

that

OF

have

of the matrix,

if these

precipitates

and are

platelets they must lie parallel to planes of the form (101x); the evidence suggests that they are parallel to (1010) planes, which is in agreement with the observations

of Langeron

and Lehro)

and Westlake

No evidence for precipitation on and Fisher.c3) twinning planes has been obtained. These precipitates which appear coherent when small ($3.2) grow easily along the direction

of their axes and the observations

show that as they grow in thickness dislocations

associated

the number

of

with them increases, e.g. com-

pare the thin precipitates

in Fig. 5(a) with those in

ZIRCONIUM

279

HYDRIDE

occasionally,

e.g. around some precipitates

and 8. One dislocation 30 (loio),

spacings

in Figs. 4

will be required roughly every in order

to

accommodate

the

strain ; thus for the large precipitate in Fig. 4(b) which is ~1500 A thick about 20 dislocation loops would be expected. This figure is close to the number of segments observed. In cases where complete loops appear to be given off, e.g. at B in Fig. 8(a) their axes appear to lie along (1120) directions suggesting that the Burger’s vectors of the loops are the normal i (1120)

type.

dislocation cipitate

It is clear that the long segments

of

and sides of the loops parallel to the pre-

axis must

glide

away

on the basal plane.

These results therefore provide evidence for slip on the

Figs. 4(b) and 2(d). It is considered therefore that some interfaces of the precipitates become incoherent

basal plane in zirconium under appropriate conditions. Hydrogen forms an interstitial solution in zirconium

during the thickening

and can migrate rapidly at relatively low temperatures

process.

Consider precipitates

with axes [1210], then the spacing of the planes of the matrix

and precipitate

and (ill),,,,

normal to [lOiO],

i.e. (lOiO),

are 2.798 A and 2.706 A respectively.

If

( <300°C) to form hydride precipitates. At these low temperatures appreciable concentrations of point defects produced

thermally

are not expected

and it is

the strain resulting from growth in the [lOiO], direction is accommodated by dislocations then ideally

considered unlikely that a large proportion of the strain associated with the growth of the precipitates

prismatic

can be accommodated

dislocation

loops would be punched out in planes parallel to (lOTO),. These loops will be vacancy loops since the matrix spacing is larger than the precipitate spacing. However, the precipitates do not necessarily thicken uniformly in the [lOTO], direction and complete

loops may not form;

instead the seg-

7. CONCLUSIONS

A study

has been made

p.p.m.

of hydrogen

illustrated in Fig. 10. This will, in general, be the case -since the precipitates grow preferentially in the [1210],

scopy.

Electron

direction precipitate

will end on the precipitates,

and will therefore

taper at their ends; the Evidence

at A in Fig. 7 is a good example.

for the punching

out of complete

loops can be found

of the morphology

and

crystallography of the zirconium hydride precipitates formed in zirconium specimens containing
as

ments of dislocation

by such defects.

by transmission

and X-ray

electron

diffraction

micro-

evidence

has

shown that all of these precipitates have a tetragonal structure similar to the y1 phase, (Gulbransen and Andrewc4)

and Whitwamc5)).

In rapidly

cooled

or

quenched specimens the precipitates appear needlelike with axes lying along (1120) directions and may be interpreted as platelets parallel to {IOiO} planes of the matrix. In slowly cooled specimens the precipitates are comparatively large in two dimensions. Identical orientation relationships between hydride and matrix hold for both types of precipitate. It is concluded ticular

from

nucleation

the

from the observations beam

heating

of the hydride precipitates

sarily require

the presence

preferential precipitation

liner

FIG. 10. Illustration of the formation of dislocation segments during the growth of the precipitates in the [lOTO] directions. 4

that

does not neces-

of dislocations

although

will occur if they are present,

as well as on grain boundaries.

Wslocation

and in par-

experiments

These experiments

showed that under the conditions operating in the microscope the hydride precipitates become unstable and were completely removed at temperatures of -3oo”c. The needle-like precipitates which are considered to be responsible for the hydrogen embrittlement of zirconium grow rapidly along the direction of their

280

ACTA

METALLURGICA,

axes and apparently remain coherent; growth in the direction normal to their axes is accompanied by the formation of dislocations and the precipitates become incoherent on the appropriate faces. ACKNOWLEDGMENTS

The author is indebted to Dr. G. K. Williamson for his critical discussion of the content of this paper. He particularly wishes to thank Mr. W. T. Eeles for his advice and help in carrying out the X-ray experiments. He is also grateful to Mr. R. Redford who constructed the hydriding apparatus and Mr. G. Rickards for assistance in electron microscopy. This paper is published by permission of the Director of the Berkeley Nuclear Laboratories, C.E.G.B., Berkeley, Glos.

VOL.

11,

1963 REFERENCES

;: 3. 4. :. 7: 8. 9. 10. 11. 12. 13.

J. P. LANQERON and P. LEHR, Rev. M&all. 60, 901 (1958). F. W. KUNZ and A. E. BIBB, Trans. Amer. Inst. Min. (Metall.) Engm. 218, 133 (1960). D. G. WESTLAKE and E. S. FISHER. Trans Amer. Inst. Min. (Metall.) Engrs. 224, 254 (1962): E. A. GULBRANSEN and K. F. ANDREW, J. Electrochem. sot. 101,474 (1954). D. WHITWAM, Mem. Sci. Rev. MetalE. LVII, 1 (1960). D. A. VAUQH~N and J. R. BRIDLE, J. Metals 8,528 (i956). C. E. ELLS and A. D. MCQUILLAN, J. Inst. Met. 85, 89 11956). C. M. SCHWARTZand M. W. MALLETT, Trans. Amer. Sot. Metals 46, 640 (1954). R. B. NICHOLSON and J. NUTTING, Phil. Mag. 2, 531 (1958). E. A. GULBRANSEN and K. F. ANDREW. J. Metals 7. 136 (1955). A. FOURDEUX and A. BERQHEZAN,C. R. Acud. Sci. Paris 252, 1462 (1961). S. LALLY and P. G. PARTRIDOE. To be published. R. I. HUTCHINSON. Private communication. \----I-