Knight shift and calorimetric measurements in liquid sodium alloys

LETTERS

by the moving boundary.

An alternative

TO

mechanism,

THE

EDITOR

63

The present mechanism

envisages the general grain

in terms only of what goes on within the boundary,

boundary

is here suggested.7 Consider a general

only a few lattice spacings being sufficient for the boundary to be regarded as a “phase” functioning

high-angle

grain

boundary

as having some thickness,

a magnitude

of

between two grains, one having a larger free energy

simply as the medium for transfer of atoms from one

than the other because of a higher dislocation

grain to another.

Assume

a homogeneous

distribution

at a very low concentration, grains.

In obedience

therm,

the

grain

concentration, temperature

of

boundary.

will

adsorption have

As the temperature

is attained

detachment

of solute atoms

c, in the bulk of the two

to the Gibbs

boundary

c’.

atoms

density.

that from

a different is raised,

is high both

iso-

enough

grains

into

a for

the

There will then be transfer of atoms from

the grain of higher free energy to the other, through the boundary will move.

the vanishing disturbed

layer or “phase”,

However,

and the boundary

for very thick grain boundaries.

a twin boundary,

and

or a boundary

in special crystallographic no boundary

Inman

and

Tiplerc3)

find

However,

between two grains

relationship,

probably

has

“phase”,

boundary

and the motion of such a should be very little affected by dissolved

impurities.

The suggested mechanism for the advance

of a general grain boundary offers the possibility of understanding the orientation dependence of boundary since the nature of the growth sites of the

advancing

edge of the growing

from the vanishing

easier than the attachment

grain should

be

of atoms to the growing

grain will depend on

the orientational relationship between and the lattice of the growing grain.

the boundary

The writer is grateful to Karl T. Aust for bringing the problem to his attention, and for helpful conversations.

of atoms into the lattice at the advancing

between the growing grain and the boundary

phase.

R. A.

ORIaN

General Electric Research Laboratory Schenectady, New York

It is now suggested

that the strong effect of dis-

solved impurities upon grain-boundary migration is due to an “adsorption” of the solute atoms at the advancing accretion

edge of the growing of solvent

of advance

grain.

The rate of

atoms, which is to say the rate

of the boundary,

atoms occupying

is impeded by the solute

sites which the solvent atoms must

in order to permit

the grain to grow.

grain can grow only if solvent up such “adsorbed”

atoms forcibly

case,

the grain-boundary

migration

References 1. K. L~~CKEand K. DETERT, Acta Met. 5, 628 (1957). 2. J. W. CAHN and J. E. HILLIARD, J. Chem. Phys. 28, 258 (1958). 3. M. C. IKMAN and H. R. TIPLER, Acto Met. 6, 73 (1958). 4. P. A. BECK, Metal Inte@zces,p. 208. Amer. Sot. Metals (1952). * Received

May 12, 1958.

The cover

solute atoms, or if the tempera-

ture is high enough so that the “adsorbed” atoms may be shaken off into the boundary. latter

boundary,

evidence

motion,(4)

grain. Hence, the assumption will be made that the rate-controlling step in boundary migration is the

occupy

a grain

grain is larger and because of the more

ment of atoms

contact

of Cahn and

both because the free energy of

nature of the lattice of that grain, detach-

organization

The considerations

Hilliardt2) support the concept of a finite thickness of

solute In the will

be

Knight shift and calorimetric measurements in liquid sodium alloys* Measurements magnetic

of the Knight

resonance frequency

shift of the nuclear of a nuclear species in

analogous to the process of zone melting, in that the grain boundary will increase in solute concentration

a metal or alloy may lead to information about the conduction electrons. We have measured the sodium

to some other steady-state

Knight shift indilute liquid alloys Na(Hg) and Na(Au).

new regions temperatures, necessary

value as it sweeps into

of the vanishing grain. At higher higher solute concentrations will be

to slow

down

grain-boundary

migration

The alloys were prepared under vacuum dispersed ultrasonically at 130°C in mineral oil. particle

size in the resonance

and The

samples was less than

than at lower temperatures. In direct opposition to the hypothesis of Liicke and Detert,o) the present model entails either an unchanged solute concentra-

15,~. Resonance measurements were made at 130°C and 6.75 k oersteds using a Varian 6 in. magnet and a Numar spectrometer. The Na line width was approxi-

tion, c’, in the boundary, higher temperatures.

mately 500 c/s, determined by magnetic field inhomogeneities. The sodium resonance frequencies in the

or an enrichment

therein at

alloys 7 The writer became awrtre after these thoughts had been expressed that & similar view w&s being independently developed by G. W. Sertrs of this Laboratory.

were compared

dispersions of temperature.

pure

to measurements metallic

sodium

on similar

at the

same

ACTA

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METALLURCICA, AT.%

VOL.

7,

1959

TABLE 1. Heats of formation of liquid Na-Hg and

SOLUTE __

N+Au .-.-

solutions .._from the liquid components, 130°C I

Alloy

-33 -230 ---560 -.- 835 -1030 -1007

Na.,,,Hg.,,,

H

Na,,,,Hg.,,, Na.,,,Hg.,,, Ea.,,,Hg.,,, ~,.,,,~.@a ,967 .053

\

-6

t -6

AH

(d/g. atom)

/ _-___.

-____.-

-~~ -_ .-_ t This is calculated from the heat of mixing of liquid Na

-I

with solid Au by the assumption that the heat of fusion of Au is independent of temperature.

AU i"' Fig. 1.

The results for Na(Hg) and Na(Au) alloys are plotted in Fig. 1. The indicated uncertainties are estimated from the reproducibility of many measurements. The Na(Au) data represent earlier work with larger ~ce~ainties. The observed changes in the Knight shift on alloying are not in accord, even as to sign, with either the rigid band(l) or the Friedel modeW of terminal solid solutions. Clearly valence differences alone cannot explain the results and undoubtedly, a more elaborate treatment, giving electronic wave functions in some detail, is required. The enthalpy of formation of the liquid alloys was measured in a simple calorimeter consisting of the reaction vessel, calibration heater, stirrer, and thermocouple immersed in a volume of mineral oil contained in a dewar. The heat of mixing of the two components could be calculated from the recorded thermocouple e.m.f.-time curve and the calibration factor of the calorimeter as measured electrically. The reaction vessel was a device that permitted holding the two separated components in vacuum at 130°C within the calorimeter before the mixing. The mixing was ac~omplish~ by electroma~etie~ly lifting a stainless steel valve from its seat, thereby

permitting the liquid sodium to drop onto the weighed amount of either liquid mercury or solid gold. Table I presents the results obtained; the accuracy is estimated at -&5 per cent. The present values for Na(Hg) solutions are about 30 per cent larger negative numbers than those that would be obtained by extrapolation of the values derived by Kubaschewski and Cattorall.c3) To round out the thermodynamic information and to aid in future understanding of the Knight shift data, it is pointed out that liquid Na-Hg solutions are characterized by considerable negative volumes of formation.f4) Aek~owledgment

The authors are happy to acknowledge the experimental aid rendered bv John Buiake and E.-McCliment. R. A. ORLANI M. B. Wxsa General Electric Research Laboratory Schenectady, New York References 1. H. &XiES, Proc. Roy. &c. Al& 255 (1934). 2. J. FRIEDEL, Adv. in,Physics 3,446 (1954); J. Ph,ys. RacXum 16, 444 (1955). 3. 0. KUBASCHEW~KI and J. A. CATTERALL, ThemochemicaZ Data of AEEoys. Pergamon Press, London (1956). 4. E. VANSTONE, !l'mm. ~~~a~~oc. 7, 42 (1911). *Received May 21, 1958.