Transformation in uranium-0.45% molybdenum alloy in relation to cooling rates and overheating temperature

JOURNAL

OF NUCLEAR

LETTERS

MATERIALS

TO THE

TRANSFORMATION

31 (1060)

107-110.

EDITORS

-

IN URANIUM-0.45%

COOLING

RATES

AND

A. MIHAJLOVIC, Boris

Kick%

Insrtitute

OVERHEATING

alloy

PUBLISHING

AUX

MOLYBDENUM

RELATION

TO

TEMPERATURE

Yugoelavia

1968

-

holding at these temperatures utes, - cooling at different rates. The specimens were examined larised light method.

for 10 min-

by the po-

Results: The data provided by the cooling curves were used to construct C-C diagrams (temperature of the beginning of B -+ LYtransformation as a function of the time interval until the beginning of transformation). Five curves were obtained for five different cooling (overheating) temperatures: 720, 700, 690, 680 and 670 “C (fig. 1). The change of the microstructure with the

(from 2 “C/set to 80 “C/set) from in the ,!? and the (/? + y) fields. was

made

from

uranium

with

imnslofm SfWllsW Fig.

1.

TT diagram of U-0.46%

ent overheating tempemtures Weight

IN

and P. TEPAVAC

24 September

350 ppm total metallic impurities. Before treatment, the specimens were homogenized by annealing in vacuum for 2 h at 850 “C and furnace-cooled. The detailed description of the device for continuous cooling was given in an earlier report 6). The specimens were treated as follows: - heating at a rate of approx. 10 “C/set to the temperatures from which they were cooled (overheating temperature), *

CO., AMSTERDAM

REDACTEURS

ALLOY

for Nuclear Sciences, Beograd,

The influence of thermal treatment before continuous cooling on the kinetics of transformation in uranium-molybdenum alloys has not yet been the subject of an exhaustive research. The problem of “thermal history” was examined by Donze i), Beaudier 2) and Townsend and Burke 3) by means of isothermal treatment of uranium alloys with molybdenum, manganese and chromium. Some of our earlier investigations 495) also dealt with the effect of the degree of overheating in the ,3 and (/?+r) phase fields on the kinetics of the B --+ a transformation in uranium low-molybdenum alloys. The investigation reported in this paper covers the kinetics of transformation in uranium at different alloy with 0.45% * molybdenum

The

LETTRES

M. KOSTIC

Received

cooling-rates temperatures

0 NORTH-HOLLAND

percent.

for different final structures 107

MO alloy, for differ-

and cooling rates. Fields are indicated.

108

A.

MIRAJLOVId

ET

AL.

d)

t) 690 “C, 3 “C/see

b)

c) 690 ‘C,

Fig.

2.

Influence

e)

690 “C, 20 “C/set

of

40 ‘C/see

overheating

temperature

f)

and

cooling

670 “C, 3 ‘C/set

670 “C, 20 “C/see

same as e) after

rate on microstructure

24 h.

of U-0.45%

MO alloy.

TRANSFORMATION

change in overheating

temperature

and cooling

IN

URANIUM

cannot

be explained

rate are shown in fig. 2. A change in the mechanism of transformation as a function of the cooling-rate, a well-known

influence

phenomenon,

already

may be observed

in all sets of

specimens. But it seems that the overheating temperature has an effect on /? -+ 01 transformation

which is at least equally significant.

For a constant cooling-rate of 3 ‘C/see, overheating at 690 “C and 670 “C retains the ol-grains (fig. 2a, 2d). On increasing the cooling rate (20 ‘C/see), overheating at 690 “C gives very fine a-grains (fig. 2b) but lowering the overheating temperature to 670 “C for the same cooling-rate of 20 “C/set retains the pphase (fig. 2e). The same specimen, with metastable p-phase, after 24 h begins to transform to 0~’ (fig. 2f). Further increasing the cooling-rate to 40 ‘C/set, overheating at 690 “C produces very irregular a-grains (fig. 2c), but does not stabilize the p-phase. For that overheating temperature, cooling-rates over 60 “C/set suppress the p -+ LYtransformation.

kinetics of that

109

ALLOY

of

the

by the theory 192) of the

“nucleus

memory”

on

the

of transformation. The untenability theory in such conditions has been

demonstrated 738). Burke and his associates believe that the effect of the temperature of overheating is related to the LYphase nucleation depending on the shape and size of the ,!? grain, since the nucleation of the 01 phase

at higher temperatures, close to the equilibrium transformation temperature, occurs at the boundaries of the B grains. But it seems that this does not help towards an interpretation of the effect of cooling temperature in specimens with the same grain size. Both of these theories are founded on data suggesting that a lower temperature of overheating accelerates the transformation and shifts it towards higher temperatures. However, this has not been substantiated by the results we obtained by means of continuous cooling. It should be borne in mind, however, that there are two substantial differences between the experiments conduated here and those of The lowering of the overheating temperature, Townsend and Burke. They quenched the at a constant cooling rate, lowers the @ + 01 specimens from the y field, with a shorter or transformation temperature, producing in the longer retention in the /3 region, so that it was same time change in its mechanism, in a similar possible to obtain after very short holding manner as by increasing the cooling rate at a times a direct y + 0~’ reaction of an athermal martensitic type. Our specimens were treated constant overheating temperature. This conclusion is supported by analysing of cooling by continuous cooling from the /l and the curves for different overheating temperatures (b+ y) field, so that no athermal martensitic at constant cooling rate. At a cooling rate y --f LY’reaction could set in. At high cooling10 “C/set from 700 “C, the p + 01 change is rates only the metastable /? phase was retained, diffusion-controlled resulting in equiaxed OL- which was transformed (isothermally) into 01’ grains ; cooling from 690 “C gives finer a-grains; martensite after being kept at room temperature cooling from 680 “C, the j3 + OLtransformation for a certain period. Shear transformation octemperature falls to the temperatures 320 curred only at intermediary cooling rates, 340 “C (fig. 2), where a bainitic type of reaction where the /l --+ oc transformation has a bainitic proceeds, producing irregular Ix-grains (similar character (fig. 2), if it is accepted that that to fig. 2~); for 670 “C, the @ -+ 01 change is reaction comparises components of shear as suppressed and the metastable b-phase retained. well as of diffusion. This leads to the inference Townsend and Burke 3) noted that the temthat the observations made by the aboveperature within the p region at which the mentioned authors cannot be used to explain specimens were held for a time during cooling why the degree of overheating affects the from the y region, affected the temperature of critical cooling rate needed to retain the the B + LY transformation. This phenomenon metastable p-phase; this is all the more evident

110

A.

MIHAJLOVI6

as in the case under consideration the size of the fi grain obtained by cooling does not depend to temperature

any substantial of overheating.

It has been found

extent

that metastable

on

the

B-phase

ET

AL.

local

depleting

in MO atoms.

ordered state requires rate to retain p-phase,

Such

a less-

higher critical cooling and at the same time

slows down the /l+ 01 transformation. This view is supported by the fact that the bainitic

retained by cooling from lower /l-temperatures transforms to LX’ faster than /?-phase from

range becomes

narrower

higher B-temperatures. Recent experiments s) on the kinetics of the /3 + 01’ transformation

in molybdenum transformation,

showed that p-phase retained by quenching the O-O.45 “,& MO alloys from a higher ptemperature to transform to 0~’takes 50% more time than from the lower /&temperatures. The influence of overheating temperatures in the p-range on the kinetics of /? -+ LYtransformation could be explained if one assumes that between the upper and the lower b-range there exists a difference either in atomic arrangement or local inhomogeneity in molybdenum distribution, i.e. some sort of clustering. In the lower /?-region the system has a regular /l arrangement, (more ordered state) resulting in low critical cooling rate to retain the metastable B-phase. Increase in temperature to the upper #?-region leads to the rearrangement of the atoms to an irregular p-lattice and/or

higher cooling rates. The inactivation of these centers requires high cooling-rates, and hence the bainitic range is wider at higher overheating temperature.

heating temperature

with decreasing

over-

(fig. 1). The areas depleted

can be the centers of shear similary to pure uranium at

References G. Donze,

i;

Mat.

3)

R. D. Townsend A. Mihajlovid A.

J. Nucl.

end J. Burke, J. Nucl. Mat.

17

Report Report

Inst.

(1964) and

Dj.

IBK-282

Drobnjak,

IBK-26

Boris

KidriE

Boris

Kidrii:

(1965)

6) A. Mihrtjlovi6 and M. Guozdenovi6, Inst.

150

and M. KostiO, Boris KidriE

IBK-155

Mihajlovid

Inst.

5 (1962)

and G. Cabane,

215

Report

5)

Mat.

G. Donze

4 (1961)

(1965)

4)

J. Nucl.

J. Beaudier,

(1963)

7)

J. Burke and P. H. Dixon, J. Nucl. Mat. 7 (1962) 38

9

To be published.