The internal friction of cold-worked niobium and tantalum containing oxygen and nitrogen

ACTA

1008

METALLURGICA,

a larger bond energy than tin because measurements

on self-diffusion

activation tin.

distorted

half the

in tin and that zinc has a higher zinc in the crystalline

h.c.p. structure,

distance of 2.907 AU.

If the bond

6 ligates tribution

wormmg

up

larger

7.975 x lop3

and

energy

atoms with an equilibrium at a relatively

contribute

M

Besides the force constants(lO) is approximated

do)2, where d is the distance

by K(d -

Fz 0.46 c/s

Measured

having 6 ligates at a small

71.78 x 1O-3

respectively

Mdyn cm-l.

-Cold-worked

than

form has a

distance of 2.659 A and other 6 at a relatively are

1966

zinc@) and

energy (5600 versus 2600 Cal/g-atom)

Furthermore,

14,

the reported

in liquid

liquid tinc3s5) show zinc to diffuse at about rate of self-diffusion

VOL.

separation

between

two

of do, then the

larger distance

of 2.907 A

to the lattice energy only 0.8 of the confrom each ligate at a distance

The effective

number

of bonds

of 2.659 8.

are thus only

6.8.

With a lattice energy of 31.24 for zinc, the effective contribution

per bond is 55.3 kcal/g-atom

of bonds,

giving Qi equal to 3270 Cal/g-atom for diffusion of zinc

1 (OCI

FIG. 1. The internal friction of niobium versus temperature. Note the change of scale in the ordinate.

in liquid tin. Support

for

Energy Agency

this

research

is gratefully

by

the

U.S.

Atomic

acknowledged.

for niobium,

Y. P. GUPTA School of Mineral

LY Metallurgical

Engineering

apply to tantalum as well.

of temperature a diameter

1. Y. P. GUPTA, Acta Met. 14, 1006 (1966). 2. L. PAULING, Nature of the Chemical Bond, Third Edition. Cornell, Ithaca (1960). 3. C. H. MA and R. A. SWALIN, J. Chem. Phys. 36, 3014 (1962). 4. C. H. MA and R. A. SWALIN, Acta Met. 8, 388 (1960). 5. G. CARERI, A. PAOLETTI and M. VICENTINI, ANuovo Cim. 10, 1088 (1958). 6. R. HTJLTCREN, R. L. ORR, P. D. ANDERSON and K. K. KELEY, Selected Values of Thermodynamic Properties of Metals and Alloys. John Wiley (1963). I. K. G. DAVIS and P. FRYZNK, Trans. Am. Inst. Min. Metall. Engrs 233, 1662 (1965). R. A. SWALIN, Acta Met. 7, 736 (1960). :: W. LANGE, W. PIPPEL and F. BENDEL, 2. Phyls. Chem. 212, 238 (1959). 10 J. WASER and L. PAULING, J. Chem. Phys. 18, 747 (1950). 11. G. CA~~ERIand A. PAOLETTI, Nuowo Cim. 14, 1373 (1959). December

was measured

friction of cold-worked niobium containing oxygen and nitrogen* on the Snoek

(CWP)

which are

caused in niobium and tantalum by oxygen nitrogen. An oxygen-nitrogen substitution

and by mecha-

as a function

A torsion

pendulum

in a vacuum

furnace

( 1O-5 torr). The specimens were heated and cooled at a rate of about S”C/min.

The maximum

strain ampli-

tude was 1 x 10-5. Niobium wires were soft-annealed for 3 hr at lOOO”C, after being worked from rod into wire.

As the dislocation

cold-work

density

peaks (CWP) are absent.

is very

low,

the

Only the ox.ygen

and nitrogen Snoek peaks (SP) are found (Fig. l(a)). The internal friction of the same wire after subsequent drawing

at

room

temperature

(Al/l := 34%)

measured

with increasing

Fig. l(b).

The oxygen SP has decreased,

has appeared

increased.

peak

literature(3s4) on

on wires about 20 cm long and with

of 0.8 or 0.7 mm.

oxygen SP.

In this letter we present measurements

earlier(l)

and is found to

needs rediscussion in

at 420°C.

temperature

and

is shown

in

a new peak

We ident,ify this new peak

as a CWP due to the segregation

20, 1965.

peak (SP) and the cold-work

friction

was used that was mounted

dislocations.

The internal and tantalum

Previous

the internal friction of tantalum The internal

Minnesota References

* Received

as was proposed

has now been confirmed

terms of this mechanism.

University of Minnesota Minneapolis,

nism at the dislocations,

of 0 atoms t(o the

This agrees with the lowering

of the

The nitrogen SP, on the other hand, has Apparently

the cold-work

has released

N atoms that previously were not contributing to the SP. There are several ways in which N atoms can be present in the metal without giving a contribution to the SP:

segregated

at the crystal

boundaries,

in ordered domain@ and in (precipitated) nitrides. A more trivial explanation is the presence of a texture that is altered by cold-work.

LETTERS

The internal friction with decreasing

after cold-work,

temperature,

TO

THE

EDITOR

Further results on the nucleation of precipitates in the Al-Zn-Mg system*

as measured

is given in Fig.

1009

l(c).

The differences with Fig. 1 (b) can be explained straightforwardly

by the fact that N atoms

for the 0 atoms at the dislocations. was proposed earlier(l) on account of only the oxygen niobium. oxygen

This substitution

of the behaviour and nitrogen SP in cold-worked

It; explains (a) the disappearance CWP and the simultaneous recovery

oxygen SP a.nd (b) the appearance

of the of the

of a nitrogen CWP

at 500°C and the si~nult’aneous decrease of the nitrogen SP.

The ten~perature of the nitrogen CWP agrees well

with the work of Boone and Wert.@) The internal friction profile of curve c is stable with respect

to temperature,

i.e. no further

changes

found on a second run of the temperature and

back

t,o 20°C.

restores the profile the

segregated

Renewed

cold-work,

back

are

up to 700°C however,

of curve b, i.e. cold-work

N atoms

In a recent paper (l) it was shown that the precipitsin the alloy system Al-Zn-Mg could

are substituted

randomly

sends

into

the

tion behaviour

be largely explained in terms of a new concept whereby the

nucleation

dependent

similar,

and

the temperature

540°C

Sehoeck

for

The results were entirely of the CWP now being 470°C

oxygen

and

and Mondino(3)

found

a peak at the temperature

doubted

this peak

on cold-worked

CWP;

and Wert

Ta

SP that

had been applied.

as an oxygen

by de Lamotte

respectively.

of the nitrogen

was absent before cold-work labelled

nitrogen

They

this was

although

their

findings

were the same. c4) The experimental results of S. and M.,(3) however- can be explained unambiguously by our substitution that the peak observed work is not oxygen behaviour and

mechanism.

by S. and M.m after cold-

CWP,

but a nitrogen

of this peak upon annealing

the accompanying

This implies

changes

between between

the vacancies

(Fig.

model of the interaction

and the precipitates

nucleus

the stability

size of

model

(where

transport

the the

vacancies

for solute

the

atoms

decomposition nuclei).

of the nucleating

and reduced

precipitates) act

and

of

the

the critical

and

a “kinetic”

as a means

increased solid

of

the rate

solution

Recent experiments

into

by Holi(a) and

by ourselves strongly suggest that kinetic effects play a dominant

role and it is the purpose of this letter to

report on our results and the consequent of the nucleation

model outlined

alloy (Al containing techniques

the

quenching

ageing

concern

i.e. quenching

temperature

the procedure

the alloy

without

period at room t,emperature.

any

on

dislocations

4) (Ref. 3)

the

ageing

results

of precipita~s

as shown

temperature

in the microstructure

mainly

l(a).

in Fig.

results

in little

until temperatures

follow directly from our model in much the same way as for niobium.

quenched

to 135°C shows very fine homogeneously

nucleated

precipitates

transition

precipitates moderately Clearly

are

are reached. (Fig.

An alloy

in

the range

narrow

150-16O’C

to

intermediate

18O”C,this heat treatment

ture is high, say

of

direct

If the ageing tempera-

in a very coarse distributio~l

change

The

used were the same as in the previous

paper. The first set of results direct

development

previously.(ll

5.9% Zn and 2.9% Mg) and the

Reducing

SP

(where the vacan-

cies, acting as a chemcial constituent site, increased

nucleated

of the oxygen

of

At the time it was not possible to distinguisll

The

SP.

critically

a ~‘thermodynami~”

precipitate

for Nb were repeated on Ta.

was

and distribution

Other results which support have been obtained independently.(2)

this suggestion

of

above

precipitates

alloy after quenching.

the 0 atoms to return to the dislocations. as described

the

vacant lattice sites which existed in the supersaturated

lattice (increase of nitrogen SP), while it causes part of The same series of experiments

of

on the concentration

direct

l(c)),

while there is a

range at about

155°C where the

homogeneously

fine dispersion

155°C is a critical

nucleated

is produced temperature

and

(Fig.

a

l(b)).

in this alloy

since below this tenlperat,ure nuclei are able to form References 1. D. J.

VAX OOIJEN and A. S. VAN DER GOOT, Philips Res. Rep. 19, 505 (1964). 2. D. H. BOONE and C. A. WERT, J. Php. Xoc. Japan 18, suppl. I, 141 (1963). 3. G. SCROECI~and M. MONDINO, J. Phys. Sot. Japan 18, suppl. I, 149 (1963). 4. E. DE LAIIOTTE and C. A. WERT, J. Phys. Sot. Jqan 19, 1560 (1964).

* Received November 16, 1965.

easily

and these

subsequently

produce

a fine dis-

persion of precipitates, while above this temperature nucleation is extremely difficult and dislocations form

the main

sites for precipitation.

of this critical temperature of the precipitates

The effect is to change the dispersion

by a factor

temperature

range of 10°C.

temperature

as the

of about

1000 in a

We shall identify

metastable

phase

this

boundary(*)