Ductile-brittle transition in alpha brass

DUCTILE-BRITTLE

TRANSITION

H. NICHOLS

IN ALPHA

BRASS*

and W. ROSTOKERt

It is demonstrated that embrittlement of alpha brass by liquid mercury is temperature dependent and that the functional relationship is a transition from ductile to brittle behavior in the same manner as experienced by iron and other body-centered cubic metals tested in their normal recrystallized state. The transition temperature is governed by the recrystallized grain size. The significance of this dependency is discussed in terms of current dislocation theory of the transition effect. TRANSITION DE RUPTURE DUCTILE-FRAGILE DANS LES LAITONS ALPHA Les auteurs d&nontrent que la fragiliti: d’un laiton cc sous l’action du mercure est fonction de la tempbrature. 11s indiquent ensuite que la relation fondamentale montre une transition de 1’8tat ductile iL l’&at fragile, semblable B celle observbe dans le cas du fer et d’autres metaux B structure cubique cent&e 6tudi& dans leur &at normal de cristallisation. La tempbrature & laquelle s’op&re cette transition et fonction de la dimension des grains cristalli&. 11s discutent cette relation en fonction dela thborie courante des dislocations SUI l’effet de transition. DER UBERGANG VOM DUKTILEN ZUM SPRODEN BRUCH IN ALPHA-MESSING Es wird gezeigt, dal3 die VeTersprBdungvan Alpha-Messing durch fliissiges Quecksilber van der Temperatur abhiingt und da0 der funktionale Zusammenhang einen Ubergang vom duktilen zum spriiden Verhalten in derselben Art darstellt, wie 81‘bei Eisen und anderen kubisch raumzentrierten Metallen in ihrem normalen rekristallisierten Zustand auftritt. Die tfbergangstemperatur wird bestimmt durch die KorngrGDe des rekristallisierten Materials. Die Bedeutung dieser Abhiingigkeit wird an Hand der neueren versetzungstheoretischen Deutung des iibergangseffektes diskutiert.

It has been known

for many

years that residual

stresses in 70130 brass cause cracking on immersion in aqueous mercurous nitrate solution. The cracking can occur instantaneously always

associated

liquid

mercury

or after a lapse of time, but is

with

the

on the

deposition

surface

aqueous solution is only a convenience oxide

films and for supply

chemical displacement.

of

of the

The

for removal of

of metallic

Identical

metallic

brass. mercury

by

cracking effects can

diffuse or penetrate into brass before the initiation fracture. With increasing temperature

of

of testing, the brittle-

ness engendered by wetting with liquid mercury undergoes a sharp reversion to the ductile state. The transition from ductile to brittle behavior of structural materials

wetted

teristic behavior ples, high-strength

be brought about by any other means by which liquid

mercury,

mercury

indium

by liquid metals is a charac-

of many systems.

By way of exam-

aluminum alloys wetted by molten and tin exhibit

this.(l)

Rozhanski

or its amalgams

can be made to wet the

et uZ.(~) have shown that single cryst$als of zinc wetted

surface of brass sustaining

residual or applied tensile

with mercury also experience

temperature-dependent

stresses. The correlations

of fracture with stress and wetting

by a liquid metal are more conveniently simple uniaxial, continuous-loading these circumstances, liquid

mercury

annealed

or mercury

35 Average

studied in the

tensile test. Under

70/30 brass wetted by

amalgams

area in the gauge section will break at or slightly above

fracture

is invariably

examination

intergranular.

Microstructural

reveals no plastic distortion

of grains on

either side of the crack. Time of prior exposure to liquid mercury is not a factor in cracking under continuous loading. Except at elevated temperatures, as will be pointed

out later, mercury

E '= 15z 6 ii IO-

5-

does not visibly

0.003

.. J,,, .

.

t

50

100

150

Temperature,

FIG.

mm

l

0 0

* Received January 6, 1960; revised April 14, 1960. t Metals Division, Armour Research Foundation, Chicago, Illinois. 848 ACTA METALLURGICA, VOL. 8, DECEMBER 1960

Diameter,

142%

over a small

the engineering yield point. The fracture always occurs in the wetted zone. In recrystallized metal the

Grain

200

250

300

3

D

OC

1. Tensile elongation as a function of temperature for 70/30 brass wetted with mercury, at one grain size.

NICHOLS

45

ROSTOKER:

AND

DUCTILE-BRITTLE

40

2 3 4 5 6

35

IN

steel,c4) molybdenum(6)

Groin Diameter - 0.003mm 0.007 mm - 0.017mm mm - 0.029 mm - 0.055 0.265mm -

Average

I

TRANSITION

this.

205OC 4

262°C

f

Recently,

Petch@)

ALPHA

BRASS

849

and niobiumt7) amply verify in analysing the dislocation

model for initiation

of a brittle cra*ck has postulated

that the interrelation

between transition temperature,

T,, and average grain diameter,

d, should be of the

form :

i

T, cc In d.

29WC

The transitions 142’C

from brittle to ductile behavior

of

70130 brass of various annealed grain sizes, all wetted with liquid mercury, are shown in Fig. 2. The transitions are all quite sharp. and so the transition perature

can be estimated

100

FIG. 2. Curves behaviour

of

150 200 Temperature,

250 OC

300

350

are plotted

of the mean grain

against the logarithm

peratures. obstacle

ple of the transition

from brittle to ductile behavior of

70130 brass wetted

with liquid

mercury

as evidence

by the establishment

as occur in body-centered

An exam-

that

linear

brittle

corre-

fracture

of a liquid

metal-

cubic metals at low tem-

One must wonder about the nature of the

to dislocation

liquid-solid

pile-up which can exist at the

interface

and,

of course,

one can only

speculate at this point. The simple picture wherein a slip band emerges to the free surface or interface

is shown in

Fig. 1. The present work has undertaken

is taken

an excellent

solid metal interface is governed by the same processes

of transition from brittle to ductile 70/30 brass wetted with mercury et various grain sizes.

transition from brittle to ductile behavior.

demonstrating

produced

40

In

for five grain sizes

lation. This 50

tem-

accuracy.

Fig. 3, the transition temperatures diameter,

0

with good

to provide

cannot provide the proper conditions.

In their study

evidence that the transition in 70130 brass is governed

of dislocation

by the same model

coarse grained silicon ferrite, Low and Guardcs) showed

and mechanisms

posed for body-centered

currently

pro-

cubic metals.

distribution

that these assumed

by etch pit patterns

the appearance

in

of the pile-ups

According to various models proposed by Strohc3), Petchc4) and Cottrell(5), a stable crack nucleus can

against

result from

the shear stress operative

held up by grain boundaries and slip bands which had

dislocations

in a slip plane piled up against a barrier.

intersection

with another

slip band.

Micrographs

on a line of

This barrier can be either a grain boundary

grain

forced

or the

From

From this, the

develop

boundaries showed

continued

postulated

etch-pit

propagation

this one can deduce

by

identified into

Stroh(n.

slip

adjacent

bands grains.

that slip directions

not only at various inclinations

can

with respect

grain size is deduced to be a primary factor governing

to the surface

the temperature

at which a transition from ductile to

When the free surface becomes a liquicl-solid

brittle behavior

occurs.

it must be those slip systems with slip directions in the

Experimental

work on mild

I

IOr) .OOl

I Averclge

FIG. 3. Correlation

and

average

I

I1111

Groin

I .OlO

Diameter,

between

grain diameter for mercury.

but also in the surface

I

III

mm

transition temperature 70/30 brass wetted with

.070

plane itself. interface,

ACTA

850

plane

of

the

interface

nuclei. The dislocation

which

METALLURGICA,

generate

the

crack

models

for the transition

in duc-

a balance between yield stress and

effective surface energy associated with crack formation such that a high surface energy and a low flow stress are conductive to ductile behavior and a low surface energy and a high flow stress are conductive to In a forthcoming

behavior.

paper it will be

shown that the creation of a liquid-solid significantly remains

reduce the effective

therefore

to apply

interface can

surface energy.

the temperature

It

depen-

dence of the flow stress of 70130 brass to the model to produce the condition where a critically low surface energy and a critically high flow stress combine to produce

brittle

fracture.

The brass represented

in

Fig. 1 showed an almost linear yield point dependence rising from 38,000 lb/in2 at 250°C to 56,000 lb/in2 at 100°C. It will be noted ductility sively

8, 1960

Metallographic

examination

to the visible occurrence

tility(4,5) construct

brittle

VOL.

in Fig. 2 that the resurgence

of

in the coarser grain sized brasses is progres-

more restricted

with increasing

temperature.

brass.

shows that this is related

of mercury diffusion into the

This clearly demonstrates

liquid metal embrittlement accentuated

that two species of

can occur--one

by decreasing temperature

is accentuated

which is

and one which

by increasing temperature. ACKNOWLEDGMENT

This work was made possible by the financial support of the Pitman-Dunn

Laboratory

Arsenal of the Army Ordnance

of Frankford

Corps.

REFERENCES 1. W. ROSTOKER, to be published. 2. V. N. ROZHANSKI, N. N. PERTSOV, E. D. SHCHUKIN md P. A. REBINDER, Dokl. Akad. Nauk SSSR 116, 769, (1954). Roy. Sot. A223, 404, (1954). 3. A. N. STROH,Proc. 4. N. J. PETCH in Proc. Conf. ow Fracture, Sumnpscott, Mass., April 1959. Technology Press of Massmhusetts Institute of Technology and Wiley, New York. Also, Chapman and Hall, London. A. H. COTTRELL, Trans. Amer. Inst. Min. (Metall.) Engrs. 212, 192, (1958). J. H. BECHTOLD, Trans. Amer. Inst. Min. (MetaZZ.) Engw. 197,1469, (1958). E. T. WESSELL end D. D. LAWTHER, Technology of Columbium p. 66. Wiley, New York (1958). J. R. Low and R. GUARD, Acta Met. 7, 171, (1959).