An internal friction peak due to slow moving dislocations in iron-nitrogen alloys

AN

INTERNAL

FRICTION

PEAK

DUE

TO

SLOW

IN IRON-NITROGEN R. RAWLINGS

MOVING

DISLOCATIONS

ALLOYS*

and P. M. ROBINSONT

9 small peak in the internal friction-temp~:rature curve has been observed a fen- tIegrew abow tlw Snoek peak (due to stress induced ordering) in iron-nitrogen alloys. The peak is produwd whew sprv*i~nons are quenched from above the alpha~gamma transformation temperature or whrn slwrimrws quenched from below the transformation ternperaturc are lightly strained. It is absent in the spwi~wns quenched from below the transformation temperature and not strained. Eoidrnw is given for t lw belief that, this ne\v peak is due to slow moving dislocations. LE PIG D’AJIORTTSSEMEXT

INTERXE DU _%c:x DAXS LES .ALLIAGES

DISLOC4TIOSS FE&.A%OTI<:

h DEPL_~-\(‘~:JlEST

I,I<:YT

Dam des alliagrs fer-nzot,e, les auteurs ont, observ6 un Ibger pit d’amortissenrentj Irrtzc‘rrlc~ lwur uw t.ernp&xture de quelques degrbs sup6rieure iLcelle du pit dr Snoek. Le pit se rnarque lorsque les Bchantillons sent trernp& & partir d’unr trmpbrature supPriewe A c~llc tit. la transformation m-7 ou pour des 6chantillons t,rempPs 2 p”rtir d’unr t~omp6ratwr inf&irurr pllis It:g&rement dPformPs. 11 n’existe pas dam CPSderniers avant la dt!forrnation. Lrs auteurs attribuent, ce nouveau pit aux dislocations qui SP d6placmt lrntjement.

UN

DCJRCH

LAXGSAME BEWEGUNG DEK INNERES KEIBCNG

VON VEHYETYXJNGES VEKL-MACH IN EISEX-STICKSTOFF-LEGIERCSGES

TES

MASIMI’M

Einigc Grad oberhalb des Snoek-Maximums (&as duwh sl’“,nnungsbedingte O~dnung-;einstcllung verursaoht ist) wurde bei Eisen-Stickstoff-Legierungen ein kleines Maximum der inneren R&bung in Abhtingigkeit) van der Temperatur beobachtet. Das Maximurn trit,t auf, wenn Proben VOILTompemt~urcw oberhalb des rA-~,-UI11wandlungspunktes abgeschrockt werden ocler >venn Proben, die van Tempera tuwn unterhalb dieses TTmwnndlungspunktes abgeschreckt wurdm, leicht verformt werden. 1’:s frhlt twi Proben, die nach Abschrecken van Ternperaturen unterhalb des CTrrlRandlllngspurllit,rs rricht verforlnt wurden. Es wrrdm Argumente dafiir anget’iihrt. dass dies M:rxirnum auf drr lang+mrn I%rwrgung van Vwsetmmgcm brruht.

1. INTRODUCTION Rlow moving

slowly

dislocations,

because

solute

atoms,

micro-creep(1) The evidence

of the

“drag”

friction

effects.(2-3)

for the former is much stronger,

friction

being

than for the latter.

The

of an aluminium-copper K@ to slow moving dislocations

by

at 1 c/s and at a temperature

of 100°C.

Apart, however, from t’hc fact that the peak occurred only in the cold worked alloy, hhe explanation

seems

t’o be surmise only. The investigation

arose when it was found

that

iron-silicon-nitrogen alloys when quenched from the gamma state possessed five internal friction peaks (all

of

which

were

quenched).

silicon-nitrogen alpha

stat,c

induced

binary and ternary) are due

ordering

state only

absent

in

iron-silicon

alloys

When. however, the same iron-

alloys

were

three internal

quenched friction

from

the

peaks M’ere

found. Further investigation revealed that ironnitrogen alloys quenched from the gamma stab had two peaks. but only one when quenched from the alpha state. It is believed that the peaks found for *

l~ewivt~d October 24, 1’3.58. t Drpartlnmt of Mrta.llurgy,

University

College,

Cardiff.

t,o the

quenched

from the

wrll-l~nown(4)

of the nitrogen

will call Snack peaks.

internal

nature,

similarly

(both

iron nitrogen

of

peak@)

occurred

accompanying

alloys alpha

and

of a quanhitative ascribed

from

moving

have been suggest’ed as the source of

internal alloy

i.e. dislocations

atoms.

st.ress

These w’c

The a,dditionaI peak found in

alloys quenched

from the gamma state

is the subject of this work.

lt is hoped to discuss the

more complicated

sgst,rnt in a later paper.

ternary

2. EXPERIMENTAL

The

iron

used

in this

investigation

was kindl)

supplied l)y the British Iron & Steel Research Association and was of high purity.: The material was hot rolled to rod 0.25 in. dia. and cold drawn to 0.027 in. dia. wire.

The \virt specimens

were a,nnealed for 3 hr at 900°C in wet hydrogen point room temperature) nit’rogen. This treatment

to remove was found

(de\\

c:arhon anal sufficient, to

remove the peaks due to carbon and nit,rogcn in the interna,l

friction-temperature

friction being pracbically

curve.

the

internal

constant1 (0.002) at tempc,ra-

tures between 18°C and 100°C. Nitriding was carried out, in an atmosphere of ‘cracked’ ammonia. The ammonia. supplied from a : Anal,vsis of’ B.I.S.R.A. iron A.H.L. (analysis by B.1.S. &A.) 0.0034~;, C’. o.ownq; Si, O.OO.i”,, Mn. 0.00ii”b S, 0.001 I’(, P, 0.0000,5”~ H. O.‘OlO~ Si. O.OOlO,, <‘I,. O.OO(i”,, (lu. 0.001 “,] .-\I. 0.0017”, 0. 0.0035”, s.

cylinder:

was passed through

heated to a temperature

a steel ‘cracker’

tube

between 500” and 700°C and

thence through two silica tubes in parallel containing the specimens heated to the appropriate The ammonia

was partially

t*ube. the degree temperature Thus

of the

at, any

‘cracked’

of dissociation ‘cracker’

particular

the temperature

t,he gas flow. The ‘cracker’

controlled

details are given with the results.

the

using chromel-alumel

3. RESIJLTS

The

wa,s constant

friction

pendulum

apparatus

similar

friction-temperature

wires quenched

from

time of relaxation,

approsimatcly

I.F. where

by a, single

O,T

c

=

(7 is a constant,

(1)

(m)2

1f

a

used by K8@).

for

i.e.

to

of

curve

the alpha range

is shown in Fig. 1. In each case. the experimental

(2 in.).

consisted

to that

internal

curve could be described

The tempera-

furnace

A typical iron nitrogen

temperature

over the length of the specimen

internal

t’orsional

and

furnace were

thermocouples.

ture zone in the nitriding within &3’C

of the ‘cracker’

at, constant,

peak

of gas.

temperature

furnace and the nitriding

or not the second

a series of experiments involving straining and agring was carried out, For the sake of continlmy the

The average gas flow was 100 ml/min.

automatically

to retain some of

vacancies.

on the

nitrogen cont’ent of the specimens could be controlled by controlling

be sufficient

was due iu some way to the presence of dislocations,

in the ‘cracker’

and the flow

nitriding

speed would

In order to test whether

temperature.

depending

cooling

the high temperature

o, = 2n;f’. and 7 (= 7” rxp

The temperat,ure of the specimen was varied by using

(H/ET))

a small eureka xvire-wound furnace.

of vibration,

H t,he activation

energy of the process

causing

internal

and

A specimen only

2 in. in length was used in order to avoid temperature Measurements

gradients.

were taken at a frequency

of vibrabion

of 1.03 c/s.

varied

1 cm to 4 cm at 3 m, corresponding

from

The amplitude

a shear strain of from All readings

at amplitudes

range in which the internal friction The actual

int,ernal friction The tensile

could

within

is independent

measurements

be reproduced

stress on the specimen

to

0.025)~ friction

to

nitrogen

introduce into

the

of the quenched

over the temperature

between specimen.

of the

specimens

Results.

range.

0.015?;,

and

internal

the

temperature. gamma

For

range,

temperatures

friction

specimens

however.

the

,f is the frequent) 2’ th(L absolutrX

quenched

from

internal

friction

peak \vas greater than that expected (1) (see Fig. 2). resolved

into

smaller

one

occurring

The expcrimcnhal

one large peak at

WC.

range, is obviously

from equation curve

could

large

different

peak.

from temperatures

identifial)le

as t,he normal nitrogen

and denitrided

in curves

specimens

quenched

within the ga,tuma range>.

Other specifrom, tempera-

Details are given under

determined

after

quenching

from

curve

was

1’

1050°C and sub-

\

ageing at 23°C. liO”C, 80°C. and 115”C, for about 0.03:/, and O.O_Rq;, cont(aining

‘\\\,\

specimens

nit#rogcn Other experiments

\

EXPERIMENTAL \ \\

were carried

out to elucidate

burizcd and denitrided) air cooled from 900°C and 1050°C’. Air cooling was used instead of quenching in an effort to avoid quenching strains which ~oultl complicate the interpretation of the results. With the wires used (0.027 in. dia.) it was hoped that the

\ \’ \

.CALCULATEDt

the nature of the second peak. Electrical resistivity mcasurcments were carried out on iron wires (decar-

I 33

from

quenched from t,hr alpha

The smaller peak was not observed

for decarburized

bc

although

tetnperaturc

was determined

temperature

at

at 21°C and a much

The

at a slightly

t’hat in curves for specimens peak.

the

higher than that, of t,he normal nitrogen

Tests were also carried out on decarburised

and dmitrided specimens. The internal frict’ion sequent

of

conditions

The

range I&1OO’C.

mens were nitrided at, and quenched tures in the gamma

of

&0.0002.

Specimens were nitrided for 3 hr at temperatures adjusted

t’he

was 346 lb/u?.

OOO”, 700“: 800’ and OOO’C, the nitriding being

to

1.125 x 10d5 to 4.5 x 10m5.

u-erc taken

strain amplitude.

of vibration

is the time of relaxation.

30 TEMPERATURE,

\

‘K

40

50 “C

-

.

20

30

40

TEMPERATURE,

xl

I

‘c

in the latter The height of the smaller peak vary

(P,)was found

linearly with t,he concentration

soWion,

of nitrogen

to

case, the decrease

only 2 hr. No explanation

of the latter

(see Fig. 3).

The existence

These were made in order to check the idea t,hat small

peak

require the presence of nitrogen in solution, associated

into

w&h a quench from the gamma to alpha phase.

alpha phase transformations.

peak

was thus conclusively

shown

t’o

the

the curve 600°C. internal sgeing

friction-temperature

half t,he originai reciprocal

curves

times were resolved

relaxat,ion peaks. tkc

after

into single time

The plot of the log time to reach peak is shown,

of the absolute

in Fig. 4, against ageing

temperature.

was the

obtained

lation with a maximum

Fig. 5.

the

A

for

region and for those cooled

t,est hut

peak t>o decrease

i.e. as in t,he curves

makes its appearance.

using

a maximum

shear

5.7 x 1W.4 caused t’he normal

further

and

the second

peak

to

quickly quickly

--

was

from the gamma from the alpha

The decrease was t,he same in both eases but

in the former dccressc

the gamma,

o~014r7------:

at room temperature

found for the wires cooled region.

from

an oscil-

solutions.

~~*~~~t~~.~t?~ ~,~~~~~,~~~~?l,t~

decrease in resistivity

quenched

shear strain of approximately

A second peak at W(Y,

strain of approximately

in iron-nitrogen

to

and then tested, with the result, shown iu curve (I)),

Repeating

precipitatinn

gamma

2.8 x 10-4: aged for one hour a,t room ternperatl~r~~

similar to that of the normal

energy is similar t.o t*hat, obtCained by Dijkstratj)

dislocations

the

Curve (a) Fig. 5. shows

for a specimen

In each CRSCthe rste of decrease of the small peak is The activation

with

during

Aft,er t~his test the wire was given

from

pea,k.

associated lattice

quenched

3.2.

after

in

introduced

of the small

The

was observed

is offered for these rcwIt.s.

using the height of the normal peak (PI) as

a meawre

carioux

DWKSTRA

case the resistance

did not begin to

until after 20 hr at, room temperature,

while

8 s”

0‘004 L ._ 20

FIG. 3. \‘arialion

in height of t,he anomulous nitrogen content.

30

-I’-

TEMPERATURE,

P, xro3

peak (I’?) with

40

50 “C

FIG. 5. The effect of strain in producing the ru~~rnnlons seoond peak in nl~ccimelts quenched front t,ltc alpha region.

room temperature the

caused a decrease in the height of

first peak and the reappearance

peak (See Fig. 6.

of t,he second

Note that curvca (c) in Fig. 5 is

repeated in Fig. 6). The decrease in the level of both peaks on ageing can be explained

if the nitrogen

or has formed condensed as well

as forming

atmospheres

only

has formed a nitride

atmospheres

at dislocations,

dilut,rl atjrnosphcrcs,

If dilute one 11ould exl)ect a

were formed

much larger second peak. nitride

or a condensed

ageing

was decided

l’htl question of \I-h&her R atmosphere

11.a~ f’ormcd ou

by rr4raining

the sl)ecimen

7

per cent in tension. The height of the first f)eak then increased almost to tJhc height it had I)ef’orcbagcing. This indicates t’hat nitride ~vas not formed on ageing. A specimen TEMPERP,TURE,

quenched from the gamma rc$on.

having two peaks. was strained approximately

‘C

cent and r&e&cd.

It

cm

tw

thus i pi

from Fig. 7 that, t,hc

swn

FIG. 6. The effect of increasing amounts of strain and subsrquent aping and st,raining on the second pea.k.

7 per cent) st,rain has brought about, the disappearance

increase.

obtained

of the second This is taken to mean that the second peak

is associated

with the presence of dislocations

in the

peak--that

is, the same

results

as for the second peak produced

straining a specimen

arc

1)~ slight13

from the alpha r+on.

quenched

lattice, and that the height of the peak depends upon the concentration The following by Kunz’“) men,

now

strained disappears

of dislocations

present.

4. DISCUSSION

further test was inspired by a paper

dealing with a similar problem. showing

two

peaks

(on The

7 per cent, in tension.

The speci-

This suggests

that the dislocations

curve for iron nitrogen

analysis),

was

alloys

are quenched

second

peak

when

alloys

and the height of the first peak increases.

away from the atmospheres,

The second peak in the internal friction-temperature

have

been torn

the nitrogen atoms being

by very

evidence

then

ante

associated

Ageing

at,

small

indicabes

of the

second

(I ) when the

t,he gamma

from

the

range or (2)

alpha

range

are

amount~s ( 10-4-10~ 3).

Thtl

that) the presence

nitrogen and dislocations

ordering

rise to the Snoek peak.

quenched

strained

now mobile and free to take part in stress induced giving

alloys appears

from

of both

is nchcessarq-for the apprar-

peak.

The

with the dislocations

nitrogtan must

1~

as an atmosphere

(if

it is in solution at normal sites it’ merely incrrascs the height of the first peak). that the second movement

peak

of dislocations

their atmospheres the explanation

Cottrell”).

This is analogous

for a low frequency,

peak in an Al-Cu

alloy given t)y KP). dislocations

The peak cannot

as this movement, iron specimens

1)s to

low tcmperaturc A theoretical is discussed t)y

I)(>due to movcmcnt

atoms in st.raintd sites around

peak obtained

postulated

whose speed is reduced

of nitrogen.

treat,ment of slow moving nitrogen

It, is therefore

is due t)o the sbrcss induced

of

dislocations

is bclicaverl to I)c the cause of the

at about _‘OO’(! on testing cold worked (the plastic st’rain being Itlu(‘h grrat’er

than in t,he present case).

t , TEMPERATURE,

“C

FIG. 7. A specimen quenched from the gamma range and then given a 7 pw rent, strain (showing the disn,ppearance of the second peak).

5. CONCLUSIONS

(1) A second interna,l friction peak att temperature not far above the Snoek peak is introduced into iron nitrogen alloys either when (a) specimens arc quenched from t,he gamma st,at,eor I\-hen (t)) specimens quenched

IiATVLISGS

ANL) ROBIK-SOS:

AN

ISTERS.AI>

FKIC‘TIOS

l’E:.-tK

from the alpha state are given a strain of 2.8 x 1O-4

work.

One of the authors

-5.1

ported

by a grant from the Department’

x 10-4.

(2) The second peak disappears

if the specimen

is

and Industrial

strained 7 per cent and the Snoek peak increases. (3) Both peaks decrease on ageing. (4) Tha second peak is believed to be due to energy losses associated

with slow moving

may

locations.

be obtained

due

2.

dislocations.

(5) In t,he case of iron-silicon-nitrogen peaks

Do slow

alloys, two moving

1.

dis-

3: r,,

These will be dealt wit’h later. 6. ‘i.

ACKNOWLEDGMENTS

Thanks

are due to Professor

his cncowagement

W. R. D. Jones for

and for providing

facilities for this

Research.

HW

(P. M. Robinson)

is sup-

of Scientific