The γ → γ' ordering reaction during isothermal transformation of γ in a U-1.6 wt% Mo alloy

THE ;’ i

y ORDERING

R,EACTION DURING

ISOTHERMAL

TR~ANSFORMATION

OF ;’

IN A U-l .6 WT 7’0 MO ALLOY G. OSTBERG

and B. LEHTINEN

Aktiebolayet Atomenery%, LZljeholmaviigen

lteceived

In a previous formation

studyl)

of the isothermal

by X-ray

trans-

examination

Stockholm,

technique.

scope

confining

raction t’o an area expected

and

bright field observation

in an electron microscope.

The conclusions

were limited

detect

drawn

difficulties

in the

techniques

by some inused.

rium

is a substantial

amount

proportion

at the completion

and therefore a considerable tion had occurred electron

microscope

before.

of the reaction,

mentioned

be possible

to

one should

arrangement

be

of y’.

y0 MO a’lloy was used as

specimens

were thinned

elec-

in a bath of phosphoric

being ejected from two nozzles

and t,he current density

about 0.2 A/cm2.

Using the (002), (004) and (006) reflexions y’, this phase was detected

an attempt

has now been made to refine the electron

wt

by immersion

acid, the electrolyte

of interest to study the detail,

by diff-

against both sides of the foil. The voltage was 35 V

no signs of the phase change

in further

Transformed

trolytically

degree of transforma-

could be found by bright field observation. features

by dark field technique

The same U-l.6

the equilib-

that

to be richer in y’ than it might

able to study the microstructural

before it was ever seen. In the

Since it was considered

of the alloy,

firstly,

area electron

this phase at an earlier stage than before.

Secondly,

The

amount of y’ required for the detection of this phase by X-rays

It was thought’,

the beam for selected

the average

herent

Sweden

4 May 1964

of y in a. U-l.6 y0 MO alloy, the y + y’

react’ion was followed

32,

from

in former y areas (t’he

elect,ron beam covering about 0.7 pm2) of specimens

micro-

(a)

Fig. 1.

LJ-l.ci(j,;

Mo isothwnmlly

(a) Dark field, (002);

(h) bright

transfornled

at 550 “C to 2

1 :J’ -I- ;‘. Thill foil electron

field of part, of the same area, showing partially transformed 123

to y’.

dark etching

portion

tnicrograph, :.’ 57000: of a former ;I Ianlrlla,

(:. OSTBERC:

124

held at’ 550 “C for 0.5 h. A comparison TTT-diagram

ANl’1) H. LEHTINISN

with t,he

of t,hc previous paper shows that this

discrete part’icles in t,he y matrix. To t,he authors’ knowledge

is a short’er time than t’hat at’ which y’ was first’

the product

detectSed by X-rays.

previously

diffraction

Thus the selectSed area electron

technique

than the X-ray

proved

to be more sensitive

such a microst,ructural of a disorder-order been described.

microstructure t,ransformation

met’hod.

For the dark field observat,ion of the microstructure t’he (002) reflexion from y’ was employed.

Fig. 1

nucleation

arrangement1 of rea&ion

has not

The appearance

of t,he

suggests that the mechanism

of this

does not differ from

and growth

processes

t,lic common

in phase trans-

format,ions.

shows t,he structure in light and dark fields respectively.

The bright areas of y’ in t,he latter photo-

micrograph

correspond

Ln conformity previous Virgiliev’)

to dark areas in the former.

with the suggestions

paper and the hypothesis it is thus concluded

References ‘)

made in the of Ivanov

and

t’hat y’ is formed as

CT.Ostberg, M. Mijller and B. Schkning-Christiansson, J. Kucl. Mat.

‘)

0.

S. lvanov

(1962) 199

10 (19fi3) 329 and 171u. S. Virgiliev,

J. Xucl.

Mat. 6