The form and initiation of serrated yielding in an AlMg2 Si alloy

Scripta METALLURGICA

Vol. 4, pp. 221-224, 1970 Printed in the United States

THE FORM AND INITIATION

OF SERRATED

YIELDING

Pergamon

IN AN AI-M~2Si

Press,

Inc.

ALLOY

P.G. McCORMICK,

School of Metallurgy, University of New South Wales, KENSINGTON. N.S.W. AUSTRALIA (Received Recent tional alloys temperatures

Locking

by two distinct

serrations,

of the curve•

followed

Unlocking

high temperature

serrations

strain for the onset of serrated model.

temperature

At high temperatures

behaviour

is often found

strain rate dependence A1-Mg2Si

ized by the negative Tensile quenching

the serrations (Russell

initiation specimen

1

strain

with Cottrell's

original

show that unlocking

serrations

are character

of e ° at low strain rates.

(0.04 mm) specimens,

load-elongation

are the periodic

locking

type,

(1,2).

of the

in a commercial

of nominal

were carried out at room temperature

Type A) and others

of a deformation

the opposite

note, observations

after

curves for high and low strain

For strain rates greater

•

at

the critical

with increasing

and the form of serrations

Typical

curve as abrupt

in flow stress

however,

In the present

tests on fine grained

from 500°C.

drops

serrations

E , increases o in agreement

strain rate dependence

rates are shown in Fig. (4),

flow,

and low strain rates,

0.7%Mg and 0.4%Si,

locking

drops back to or below the level

For locking

(1-4,8)

o alloy are reported, which

composition

appear on a stress-strain occur as repeated

(2,5,8,9).

of e

in substitu-

types of serration,

by discontinuous

and low strain rates•

rate and decreasing

yielding

which are found at low and intermediate

in the serration range,

in flow stress

19, 1970)

(1-7) have shown that serrated

is characterized

and unlocking. rises

studies

January

than 2 5 x 10 -3 min -i •

similar

•

,

to that found by Russell

Each serration

band, which then traverses

corresponds

to the

the gauge length of the

(4,10). For strain rates

less than 2.5 x 10 -3 min. -I, unlocking

occur at the start of serrated amplitude

and are associated

With increasing serrations

flow.

The serrations

with discontinuous

strain a gradual

transition

is observed.

221

serrations

have a relatively

deformation

from unlocking

constant

band propagation(6) to periodic

locking

222

SERRATED YIELDING

IN AN AI-Mg2Si ALLOY

@l

w

I[LOI,IGATION

FIG.

i

Typical load-elongation curves exhibiting locking and unlocking serrations.

Io"

Uld LOCK I MG ~.__~.LOC t~ IJG S ERIIA'TIOkI$ I lie lilt ATlOkl~il

16 2

I Io 3

I I0 ~

I 16'

(m,n-') FIG.

2

Strain rate dependence

of e 0

I I

Vol., No. 3

Vol.

4, No.

3

SERRATED

YIELDING

IN AN AI-Mg2Si

ALLOY

The effect of strain rate on e rates

greater

is shown in Fig. 2. o than 2.5 x 10 -3 min. -I, where locking serrations

225

For strain occur,

e O

increases

with increasing

where unlocking

serrations

strain rate. occur,

The correspondence strain rate ized.

(or temperature)

Locking

subsequsnt

serrations

breakaway

large strain rates, dynamic

locking.

decrease

a negative

between

strain mate dependence

the form of serrated

dependence

result

yielding

of e ° has not previously

from the dynamic

mobile

the solute mobility

is initially

insufficient

However,

the increase

velocity

in vacancy

accompanying

E

and the

been emphas-

dislocations.

concentration

deformation

to be met at c . Cottrell o at constant temperature

is found•

locking or ageing and

of initially

locking

have shown that,

for e < 2.5 x 10 -3 min. -I,

and multiplication

in dislocation

for dynamic

However,

(11,12)

allows

At

to cause and the the condition

and Ham and Jaffrey

(13)

= Ac I/(m+8)

(i)

o where A is constant strain relation, From Fig.

and m and 8 are parameters

Cv a m , and dislocation

In the region is initially

with dislocations. yielding

low enough

Therefore

but rather,

the unlocking

stress

relation,

(7) for solute

the critical

v a e -8

is reached.

to be dragged

existing

atmospheres,

Thus a rise in flow stress

stress

strain-rate

along

dependence of e O

once

is not observed

In this region the (14).

is required for breakaway, dependence

alloys.

for the onset of serrated

only the yield drop occurs.

a greater

in the negative

(4)

as is the case for locking

from already

strain-rate

2.2

the dislocation

atmospheres

condition

of an atmosphere,

size also has a negative

lower strain rates,

and 2.14 (i) in gold-indium

strain rate dependence

the breakaway

prior to the yield drop; atmosphere

alloys,

of negative

is not the acquisition

serrations,

reflected

velocity-strain

2, m + 8 = 2.7, which may be compared with the (m + 8) values

and 2.8 - 3.1 (3) in copper-tin

velocity

in the vacancy concentration-

Thus at

which is

224

SERRATED YIELDING IN AN AI-Mg2Si ALLOY

Vol.

4, No.

REFERENCES i.

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2

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I0.

P.J. Worthington and B.J. Brindley, Phil. Mag.

Ii.

A.H. Cottrell, Phil. Mag.

12.

A.H. Cottrell, Relation of Properties to Micros tructure, p. 131, A.S.M.

13, 436 (1965).

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4_~4, 839 (1953).

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R.K. Ham and D. Jaffrey, Phil. Mag.

14.

J.P. Hirth and J. Lothe, Theory of Dislocations, McGraw-Hill, New York (1968).

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3