Some observation on grain boundary melting

LETTERS

TO

Some Observations on Grain Boundary Melting * Investigations so far

failed

melting

of to

visible

grain

boundary

indicate at grain

boundaries

property of the grain boundary solute segregation. During

a study

lead and dilute boundary

of grain

alloys,

melting

the

or is the result of

agreement wit’h the recent Teghtsoonia n(l).

681

beads of liquid

molten

boundary

at the

boundaries

could

be extruded

from

region and fissures would of

similar conditions,

the

doped

samples.

the grain boundaries

the

develop Under

of the zone-

refined lead samples would also show some melting, i.e. when a force was applied. When directional melting occurred under a tempera-

in zone-refined

observat,ions

were made

have

preferential

is an intrinsic

growth

some

EDITOR

tongs,

melting

whether

THE

which

work

on grain

are in general

of Weinberg

and

ture gradient, a pure sample exhibited no anomalies at the liquid-solid interface. However, when a doped sample was melted under a temperature gradient,

t’he grain boundary

to melt in advance

region

could

be seen

of the general interface.

Fig. 1

dilute alloys of tin, silver or gold in lead were deformed

is representative of the phenomenon. The lower field, which is out of focus, is the once liquid region;

to a coin shape by severe deformation

the upper portion,

Twenty-gram

hammer.

buttons

of

After deformation,

approximately were given

3 mm thick an annealing

zone-refined

lea,d

or

under a drop

the samples which were and 30 mm in diameter

treatment

at 10°C below

the melting point, and then quenched

in water.

The

several

boundary. interface

time of annealing was adjusted to obtain various grain sizes. The samples were then placed on an open hot plat,e at, 5°C below the melting

0.02”C.

point. plate

After

was slowly

some

was turned

melting

raised had

and

occurred,

the

hot

off and a jet of air was directed

t’owards the sa,mple to promote Different

point,

events

could

solidification.

be seen,

depending

upon

of

This effect doped

with

non-directional it proceeded

melting melting

in zone-refined

was applied,

occurred

gradient. in doped

from grain boundaries

This preferential obvious

a temperature

of the boundary lead.

samples,

into the grains. was not

If an external

such as a slight squeeze

force

by laboratory

point

interface

was readily

at the grain

of

seen in zone-refined

concentrations

of tin.

0.005 per cent,

not sufficiently

These

The

depression

approximately

0.05

and

When

to a melting

pronounced

observations

twin

liquid-solid

boundary.

below

containing

was on the side of the hot plate,

twin

above

comparable

experienced

coherent the

preferentially

at the

general

certain

one that

concentrations,

or whether

the sample

not

and seen

concentrations,

of the hot

thus

but the

was in the centre

the sample

be

has advanced

plate, and thus had a uniform temperature,

whether

can

corresponds

ahead

to the melting

which was always solid, exhibits

boundaries

It

boundaries, advance

the temperature

grain

per cent,

and

at

high

the effect

was

to be visible. were made

lead

At low

Somewhat

on specimens

small amounts of gold or silver. observations

emphasize

the difference

be-

tween the grain boundaries

of zone-refined

lead and

those of doped

It is interesting

to note

specimens.

that the coherent of

preferential

observations,

twin boundary melting.

shows no evidence

From

it may be concluded

these

qualitative

that the presence of

a small amount of a second element added to a pure metal

enhances

or even

promotes

grain

boundary

melting.

G. F. BOLLING

Dept. of Metallurgical Engineering

W. C.

TVINEGARD

University of Toronto Reference 1.

WEINBERG published. * Received

F. and TEGHTSOONIAN E. May

Acta

Met.

To be

15, 1957.

Evidence for the occurrence of surface diffusion during the experiments on grain-boundary segregation of Thomas and Chalmers In their autoradiographic tion in lead-bismuth FIQ. 1. Photograph x 150 showing an advancing liquidsolid interface in a sample containing 0.05 per cent Sn. The lower field shows the “once liquid” portion.

attributed

investigation

alloys, Thomas

the whole

at the grain boundaries

of the

of segrega-

and Chalmers(l)

additional

blackening

to the segregation

of 210Po