NBSIR 76-1161

High Voltage Microscopy of

the Reduction of Hematite to

Magnetite

P. R. Swann and N. J. Tighe

Department of Metallurgy and Materials Science

Imperial College of Science and Technology

London, SW7 2BP, England

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

December 1976

Interim report for Period 1971 to 1973

Prepared for EOAR*

223/231

Marylebone Road

London, NW1

England

NBSIR 76-1161

HIGH VOLTAGE MICROSCOPY OF

THE REDUCTION OF HEMATITE TO

MAGNETITE

P. R. Swann and N. J. Tighe

Department of Metallurgy and Materials Science

Imperial College of Science and Technology

London, SW7 2BP, England

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

December 1976

Interim report for period 1971 to 1973

Submitted to Metallurgical Transactions

Prepared for EOAR

223/231

Marylebone Rd.

London, NW1

England

v-'i-v "•f.g 0*

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director

HIGH VOLTAGE -MICROSCOPY OF THE REDUCTION

OF HEMATITE TO MAGNETITE v

By

P. R. Swann^ and N. J. Tighe^

Abstract

The microstructural changes associated with the formation of magnetite

in hematite have been studied in specimens which have been partially

reduced outside the microscope, thinned until electron transparent, and

then examined in the normal way. Three types of structure have been

observed in varying proportions which depend on the reduction temperature.

At low temperatures, magnetite grows by the propagation of a cellular

interface, the gas phase being transported to the cell boundary by a

network of tunnels. At intermediate temperatures, magnetite plates are

formed, whereas at high temperatures, both plate magnetite and blocky

magnetite appear. It is proposed that the factor controlling the

morphology which develops is the ratio of the cell boundary diffusivity

to volume diffusivity of ferrous ions. It is noted that the decomposition

morphologies of hematite and austenite have many similarities. The

basic reason for this similarity is that both transformations involve

substantial redistribution of elements in the solid state and the microstructures which develop are those that perform this redistribution the

most efficiently at the temperatures involved.

^ Department of Metallurgy and Materials Science, Imperial College of

Science and Technology, London, SW7 2BP, England

2 Inorganic Materials Division, National Bureau of Standards,

Washington, D. C. 20234

This paper is based in part on a thesis submitted by N. J. Tighe in partial

fulfillment for the degree Doctor of Philosophy at the University of London.

Introduction

The kinetics of the reduction of hematite to a porous magnetite are

often discussed in terms of three rate-controlling processes, namely:

( 1-4)

(1) chemical, (2) diffusional, and (3) mixed control . Under chemical

control, most of the total driving force available for the reaction is

consumed by the gas-solid reaction occurring at the hematite/magnetite

interface. In other words, the resistance to diffusion of the reaction

gases through the porous magnetite layer to the reaction interface is

negligible. If this were not the case, the reaction would proceed either

under mixed control in which the chemical and diffusion steps involve

approximately equal fractions of the overall driving force or under pure

diffusion control in which the interfacial reaction proceeds with negligible

resistance. Surprisingly, very little emphasis is given in the literature

to the role of solid state diffusion processes in the hematite phase and to

the nucleation event at the reaction interface. Furthermore, although the

porosity of the product magnetite phase is an important feature of most

theories of hematite reduction, relatively few attempts have been made to

explain how this porosity develops.

In this work we have tried to gain some insight into the reduction

mechanism by studying the microstructure of the unreacted hematite, the

reaction interface and the magnetite product. Specimens partially reacted

outside the microscope were examined after ion thinning suitable sections.

Many of the electron microscope observations were correlated directly with

further structures seen in the light microscope and scanning electron

microscope
Experimental Procedures

Specimens used in the reduction experiments were made from:

(1) micaceous and other natural hematites, (2) pressed powder discs

sintered at 1200 °C, and (3) platelets grown in a borax-flux. The

micaceous hematites cleaved readily into electron transparent plates

with faces parallel to the basal plane of the hexagonal hematite structure.

The non-cleavable specimens were ground to form wafers 40 pm thick,

ultrasonically cut into discs 3 mm in diameter and thinned for electron

microscopy by ion bombardment.

A furnace was built for partially reducing bulk specimens of hematite

outside the electron microscope at gas pressures up to one atmosphere.

These specimens were supported in a platinum boat and could be heated in

the temperature range 400 °C to 1100 °C in hydrogen atmospheres and in a

wide range of CO/C^ mixtures. In addition, a differentially pumped

environmental cell was designed and constructed in order to carry out

reduction experiments in the AEI EM7 million volt microscope. The cell

spans the gap between the pole piece of the objective lens and provides

a 500 ml reservoir for the reducing gas around the specimen. More

detailed descriptions of the cell and its performance have been given

1 u (5,6) elsewhere.

Experimental Results

Low Temperature Reduction

Single crystal plates of hematite 40 pm thick were fully reduced to

magnetite at 400 °C and then prepared for electron microscope examination

by ion thinning. Figure 1 shows a typical example of the microstructure
3

Fig. 1 Magnetite produced by reducing a single crystal of hematite in one atmosphere (101 kNm-2

) CO at 400°C. The electron micrograph is a cross section through the tunnel structure of several reduction colonies.

The changes in orientation of the magnetite across the boundaries of the

individual colonies produce the sharp changes of contrast indicated by the arrows in the micrograph.

Fig. 2 High magnification view along the axes of reduction tunnels in

magnetite. Note the uniformity of the tunnel spacing and diameter.
4

produced by this low temperature reduction. It is seen that the magnetite

product is composed of cells or colonies containing a dense network of

tunnels with an average spacing of about 27 nm and a mean diameter of

about 8 nm. The tunnel or sponge- like structure is extremely fragile

and colonies often fracture under the stresses generated by the reduction

reaction. Figure 2 is a higher magnification view of the porous magnetite

normal to the axes of the reduction tunnels and clearly shows the uniformity

of the tunnel spacing and diameter. A light micrograph of a hematite

specimen partially reduced at 400 °C is shown in Fig. 3a. Two impinging

cells of magnetite have grown from the left of the field of view and the

magnetite/hematite interface is roughly perpendicular to the plane of

the micrograph. The specimen has been thinned until electron transparent

so that the structure of the interface could be examined in the high

voltage microscope. Figure 3b shows such a view and it can be seen that

the tunnels extend right up to the reaction interface and are roughly

perpendicular to it. The diameter and spacing of the tunnels was found

to vary from one specimen to another and are thought to depend on the

reducing power of the gas and the reaction temperature. At present, no

consistent trends have been established and further experiments are

planned to determine the exact relationship.

High Temperature Reduction

A typical electron micrograph of a hematite specimen partially

reduced to magnetite at 1000 °C is shown in Fig. 4. The magnetite is

present in the form of single crystal plates with habits approximately

parallel to the (0001) planes. The orientation relationship is (0001) |

| (111)
5

Fig. 3 (a) A light micrograph of two adjoining reduction colonies of

magnetite in a partially reduced single crystal specimen of hematite.

The reduction was carried out in CO at atmospheric pressure for 15

minutes at 400°C. (b) Transmission electron micrograph of region near

the arrow in figure 3a showing that the fibrous tunnel structure extends

right up to the reaction interface at the boundaries of the colonies.

Fig. 4 A typical electron micrograph of the "high" temperature reduction morphology in a hematite crystal partially reduced to magnetite in

CO/CO 2 = 0.125 for one minute at 1000°C. The magnetite crystals are in

the form of lenticular plates and do not contain the tunnel structure

observed at lower temperatures.
and [1010] |

| [ Oil ] . The hematite matrix shows evidence of elastic distortion

around the magnetite plates and contains a recovered dislocation substructure which was not present before the reduction. Dislocations are

also present at the hematite/magnetite interface. The magnetite plates

do not contain the tunnel structure observed at the lower reduction

temperature and must have formed, therefore, by a different mechanism.

The dislocation density of the magnetite plates depends on their

rate of growth. Crystals growing rapidly under highly reducing conditions

at 1000 °C usually contain a high density of dislocations as can be seen

from the example in Fig. 5. It appears that accommodation stresses are

significantly relaxed by diffusion and dislocation climb at the lower

reaction rates used to produce the low dislocation density magnetite

shown in Fig. 4.

The inset low magnification light micrograph in Fig. 5 includes the

area of the adjacent electron micrograph and shows the blocky grains of

magnetite which form in addition to the plates. Some of the blocky

magnetite grains can be seen in the electron micrograph, but a clear

example revealing their internal structure is shown in Fig. 6. It is

seen that they are polycrystalline; and, from diffraction analysis, it

was found that many of the individual grains are twin related.

It is interesting to note that the thickening of magnetite plates

occurs predominantly by the sidewise growth of only one of the magnetite/

hematite interfaces. This can be seen in Fig. 7, which is a light

micrograph of the top and bottom surfaces of a fractured, partially
7

Fig. 5 A transmission electron micrograph of magnetite plates and nodules

formed at the surface of a hematite crystal reduced in CO/CO 2 = 0.06 at

986°C for one minute. The magnetite plates contain a high density of

dislocations which anneal out rapidly during growth. The outlined area in

the inset light micrograph corresponds to the area of the electron micrograph.

Fig. 6 A high magnification view of a single magnetite nodule showing its twinned internal substructure.
8

reacted, 30 ym thick single crystal slab of hematite. In this figure,

the hematite/magnetite interface contrast of one side of the magnetite

plates is much darker than the other, e.g., at X. The reason for this

difference in contrast can be understood from the scanning electron

micrograph in Fig. 8 which shows that one of the magnetite plate interfaces

contains rows of micropits which although not resolved in the light

microscope would scatter light more strongly and appear in dark contrast.

The opposite interface of the plates are free from pits and they appear

in weak contrast even in the scanning image. The fissures and pits

associated with the dark interface indicate that it is sessile and,

thus, magnetite growth is accomplished by migration of the opposite

interface. It is interesting to note from a comparison of the micrographs

of the top and bottom surfaces of the broken single crystal slab in Fig.

7 that the mobile interface on one surface corresponds to the sessile on

the other surface and vice versa. In other words, it appears that the

mobility of a magnetite interface is different when it intersects (hkil)

and (hkil) hematite surfaces.

Figures 5 through 8 all show examples of magnetite plates and

blocky grains, which have nucleated at the hematite surface. However,

subsurface nucleation and growth also occur during high temperature

reduction and an example is shown in Fig. 9. This is a light micrograph

of a section through a sintered disc of hematite powder partially reduced

at 1020 °C in a CO/C^ mixture at one atmosphere. The dark network of

lines on the micrograph are the interfaces between sintered grains which

have fractured during the reduction process and the grey areas are
Fig. 7 A broken 30 micron thick rhombohedral-f aced crystal of partially

reduced hematite showing magnetite plates in cross section (i.e., in the

fracture face) and matching views on opposite surfaces of the slice. The

micrograph shows that the plates have an (0001) habit. Specimen reduced

in CO/CO 2 = 0.03 for one minute at 1000°C.

Fig. 8 A scanning electron micrograph showing the surface morphology associated with the magnetite plates seen by light microscopy in fig. 7. A

surface groove or porous fracture is present where one face of the magnetite

plate intersects the surface of the crystal. This morphology is responsible

for the dark contrast along one edge of the plates in figure 7.
10

sections through magnetite plates. Most of the magnetite plates extend

to the boundaries of the individual grains and plates lying totally within

the grains are presumed to intersect the boundary at some point outside

the plane of the section. However, in addition to the plates there are

several small particles of magnetite, e.g. , at X which appear to have

nucleated totally within the grains. These particles could only have

formed as a result of matrix diffusion through the hematite.

The above description of the high temperature reaction product is

based on preliminary observations and is certainly a simplification of

the actual microstructure. In reality, the structure is constantly

changing during the reaction and freshly formed magnetite in the reaction

zone has a different microstructure from the magnetite formed at the

beginning of the reaction. For example, after some time at the reaction

temperature, dislocations in the magnetite plates relax their long range

stress fields by rearranging to form networks, as shown in Fig. 10.

In many cases, the elastic stresses generated during reduction can lead

to fracture of the magnetite before other elastic recovery processes

occur. In addition, the magnetite can also recrystallize.

Similar changes also occur during aging of the low temperature

reaction product. However, in this case the high surface energy associated

with the tunnel structure can cause an additional discontinuous sintering

reaction to take place which has a pronounced effect on the interconnectivity of pores and the pore size. It is clear that the pore size

measured at the end of a complete reduction to magnetite is not a true

reflection of the pore size existing at the reaction interface.
11

Fig. 9 A light micrograph of an area near the surface of a hematite polycrystal partially reduced to magnetite in CO/CO 2 = 0.03 for 10 minutes at

1020°C. Fractures are seen along the hematite grain boundaries and many

magnetite plates extend from the boundaries into these grains. The micrograph shows many examples of subsurface nucleated magnetite precipitates,

e.g. , at X.

Fig. 10 Network of dislocations within a specimen fully reduced to magnetite at 1000°C. These networks form by a recovery process at temperatures after the reduction has occurred.
Reduction at Intermediate Temperatures

At intermediate temperatures (600-800 °C)

,

magnetite can form by

both plate and cellular growth. The light micrograph in Fig. 11a is a

typical example and shows plate nuclei at X and roughly hemispherical

cells possessing the tunnel structure at Y. The magnetite plates also

appear to act as nuclei for the cellular structure; two examples are

indicated by arrows in Fig. 11a. It is interesting to note that initially

the cellular structure grows preferentially from only one side of the

magnetite plates in any one grain.

Figure lib is an electron micrograph of the plate-nucleated cellular

structure in the same specimen used to obtain Fig. 11a. The original

traces of the magnetite plate nuclei can be recognized by the long

cracks running diagonally across the micrograph. These cracks or grooves

are responsible for the lines of dark contrast seen in plates viewed in

the light microscope. There is also a second system of shorter, light

contrast lines steeply inclined to the main grooves. These lines appear

to be enlarged tunnels in the magnetite.

Discussion

The electron microscope observations have revealed that the reduction

of hematite to magnetite can occur by at least two different mechanisms,

one of which predominates at temperatures below about 650 °C and the

other at temperatures above 800 °C. In the temperature range 650 °C to

800 °C, both mechanisms operate in varying proportions. The experimental

observations also indicate that solid state diffusional processes play a

much wider role in the reduction mechanism than has been realized in
Fig. 11 (a) A light micrograph showing the typical reduction morphology of

specimens partially transformed at "intermediate" temperatures. Note that

the specimen contains both magnetite plates (at x) and reduction tunnel

colonies (at y) . The magnetite plates appear to nucleate the tunnel

reaction; e.g., at points indicated by arrows. (b) A transmission electron

micrograph of a colony such as y showing the central systems of fractures

associated with the plate nucleus.
14

earlier publications. Furthermore, the microstructural changes which

occur bear many similarities to well-known solid state transformations

in alloys as will be discussed below.

Reduction at Low Temperatures

(a) Reduction Morphology

The most striking feature of the low temperature (400 °C-650 °C)

reduction reaction is the formation of magnetite colonies having a

highly branched tunnel structure extending like an arterial system from

the reaction interface through the magnetite to the original hematite

surface. This morphology, which is drawn schematically in Fig. 12,

't- 6nm

Fig. 12 Schematic

diagram illustrating

the mechanism of

tunnel formation

during the initial

reduction of

hematite

.

bears a marked similarity to that developed during the cellular decomposition of a supersaturated solid solution; e.g., the pearlite reaction

in steels. In such discontinuous reactions, the incoherent boundary

forming the reaction interface plays a key role by providing a rapid

diffusion path for partitioning the alloy components between the two

phases . In view of the microstructural similarity, it is possible

that a similar transport mechanism may also operate during the low

temperature reduction of hematite. In this case, the two components,
15

iron and oxygen, must be partitioned between the magnetite phase and the

gas phase according to the reaction,

6 Fe

3+

0

2 " + 4 Fe

3+

Fe

2+

0

2_ + 0

Z j Z 4 Z

However, in contrast to the purely solid state discontinuous reactions,

the reaction interface (as shown in Fig. 1) is only partially composed

of incoherent boundary, the rest being the free surface between the

hematite and the gas phase. Nevertheless, this mixed interface is

ideally suited for the rapid transport of the constituents involved,

since the oxygen diffusion distance in the solid state can be negligible

and excess ferrous ions, produced when oxygen enters the gas phase, can

diffuse rapidly over the surface at the bases of the tunnels and along

the hematite/magnetite boundary to their correct sites in the growing

magnetite lattice.

The scale of the tunnel structure will depend on the way in which

the overall energy available for reduction is distributed. Clearly, the

smaller the tunnel spacing, the shorter will be the diffusion distance

between tunnels, but the greater will be the energy consumed in creating

/ o \ the tunnel surfaces. Furthermore, as pointed out by Cahn , the

magnitude of the interfacial mobility will exert a profound influence on

the tunnel spacing by controlling the amount of boundary diffusion

required to drive the interface forward. The scale of the tunnel structure

is also expected to depend on temperature. With increasing temperature,

the diffusion of ferrous ions into the matrix will become more important

than their transport along the boundary. Thus, more of the free energy

available is consumed in driving the interface forward and less remains

for the creation of new tunnel surface. The tunnel spacing should,

therefore, increase with rising reduction temperature.
16

(b) The Spacing to Diameter Ratio of the Reduction Tunnels

A primitive unit cell of the reaction product is defined in figure

13 and has edge dimensions at the reaction interface of d (i.e., the

tunnel spacing) and a reduction tunnel diameter of 2r. The reduction

tunnels are distributed in a hexagonal closest-packed array normal to

the reaction interface (see Fig. 2) and, thus, the area of a primitive

2 unit cell at this interface is d cos30°. Since this area is intersected

2 -1

by just one tunnel, the tunnel density is (d cos30°) . In unit thickness

of Fe^O^, the number of iron ions per unit cell of the reaction interface

2 is Nd cos30°, where N is the number of iron ions per unit volume of

hematite. The number of oxygen ions in this initial volume before

2 reaction is 1.50 Nd cos 30° and, thus, the number which must be removed

2 2 to achieve the composition Fe^O^ is (1.50-1.33) Nd cos30° = 0.147 Nd .

It was proposed above that a reduction tunnel is formed by the

removal of oxygen into the reducing environment at its base and that the

excess ferrous ion concentration so created diffuses away from the

tunnel base and parallel to the reaction interface to form the surrounding

magnetite. In the simplest case, it can be assumed that the cell boundary

diffusivity of ferrous ions is so rapid that the flux of oxygen ions

into the volume which forms the reduction tunnels is negligible. Consequently,

all the oxygen ions which are removed to accomplish the change in composition

2 from Fe20^ to Fe^O^ are those in the tunnel volume, i.e., 1.5 Nirr . In

0.147 Nd

2 1.5 Nut

2

this case
17

giving - = 5.66

r

If there is a flux of oxygen ions into the reduction tunnel volume from

the matrix, the ratio d/r will be correspondingly larger than that given

above. Taking x as the fraction of all oxygen ions removed during the

reaction that originates from the matrix surrounding the tunnels, then

the spacing to radius ratio of the tunnels becomes

Measurements taken from micrographs such as that in Fig. 2 show that

d - 27 nm and r - 4 nm giving d/r = 6.75 and x = 0.30. This crude

calculation, which ignores the movement of any vacancies generated

during the reduction reaction, indicates that about 30% of the oxygen

removed originates from the matrix between the reduction tunnels.

(c) Volume Change During Low Temperature Reduction

The number of iron ions in the reacted layer is unaltered by the

reaction and since the volumes of magnetite and hematite per atom of

iron are almost equal, any expansion resulting from the reduction must

be accounted for almost entirely by the volume of the tunnels and fissures

created by fracture of the fragile reaction product. Any sintering of

the reaction product during and after the reaction would tend to reduce

the magnitude of the expansion. The contribution to the volume increase

from the formation of tunnels will now be calculated with reference to
Fig. 13. The diffusion of iron from the tunnel region during reduction

causes the original hematite layer to increase in thickness from 1 to

1 + y. If all the expansion occurs parallel to the direction of motion

of the reaction front, then the primitive unit cell dimensions, d, at

the reaction interface are unchanged and the volume change is simply

Outer Surface

Fig. 13 Schematic

diagram of a 'unit cell'

of a magnetite colony

formed at low reduction

temperature. The

reduction tunnel has a radius (r) and intertunnel

distance (d)

.

100y%. The value of y can be obtained by equating the volume of a

2 tunnel, tt r , to the volume of the extra surface layer created by the

2 2 reaction yd cos30° = nr and using the value for d/r given above. If

the flux of oxygen into the tunnel from the matrix is negligible, i.e.,

x = 0, the volume increase is a maximum of 12.5%. However, if x is not

negligible, the volume increase is less than 12.5% and is given by

100 (1-x) / (8+x) % . For the particular example shown in figure 2 in which

x was found to be 0.3, the volume increase would be 8.4%. This agrees

well with magnitude of volume changes recently measured by Bradshaw and

Matyas , who obtained values ranging from 8.6-11.7% at 600 °C and 6.1

to 7.0% at 700 °C. These values are all substantially less than 12.5%,

indicating that the flux of oxygen into the reduction tunnel region from

the surrounding matrix is not negligible, and also that significant

amounts of sintering of the tunnels may occur during the reduction

process

.
(d) The Low Temperature Reaction Rate

The driving force for the boundary migration of a porous magnetite

colony is related to the deviation from stoichiometry maintained at the

hematite/gas interface at the bases of the reduction tunnels. If this

does not change during the course of the reaction (e.g., gaseous diffusion

along the tunnels is not rate controlling), then the driving force and,

hence, the velocity of the interface would be constant. In this case,

many factors at the reaction interface can control the growth rate,

e.g., the rate of diffusion of ferrous ions along the magnetite/hematite

interface, the rate of removal of oxygen ions from the base of the

tunnels, the intrinsic mobility of the magnetite-hematite interface,

boundary drag due to interaction of the interface with foreign ions.

However, whichever of these factors is rate controlling, the overall

reaction rate, R, would be expected to obey some form of the JohnsonMehl^^ equation which takes impingement of neighboring colonies into

account ; e.g.,

R = 1-exp (-kIG^t

n

)

where I is the nucleation rate of the magnetite colonies, G is their

interface growth rate, n is the time exponent which takes on a range of

values according to the conditions given by Cahn^^ in his treatment of

grain boundary nucleated eutectoid reactions.

Reduction at High Temperatures

The magnetite produced at reduction temperatures above 800 °C is

predominantly in the form of lenticular plates or blocky grains. The

morphologies bear many similarities to the Widmanstatten plates and

chunky ferrite allotriomorphs found in steels. The development of the

plate morphology is indicative of growth controlled by long range diffusion

The small radius of curvature at the tip of a growing plate produces

steeper concentration gradients of the diffusing species there and leads
20

to a higher diffusion flux of ferrous and oxygen ions to and from the tip

This would account for the enhanced tip velocity necessary to maintain

the plate morphology during continued lengthwise growth. The sidewise

growth rate of the plates appears to be controlled more by the anisotropy

of interface mobility than by diffusion. The microscopic observations

revealed a strong tendency for plates to lie parallel to the basal plane

of the hematite and it was noted that the interface on one side of a

growing plate was more mobile than that on the other side. The tendency

for a groove to form where the less mobile interface intersected the

hematite surface suggests that boundary diffusion plays a role in the

transport of ions to and from the surface. However, it is evident from

Fig. 7, which shows copious amounts of sub-surface magnetite nucleation,

that the growth of plates does not depend on this type boundary diffusion.

The formation of blocky grains results from the cooperative nucleation

of several grains of internally twinned magnetite. In these clusters of

grains, the (0001) | | (111) orientation relationship is not observed and

the special low mobility-basal plane interface associated with plate

growth cannot develop. As a consequence, the growth proceeds at similar

rates in all directions.

Reduction at Intermediate Temperatures

At temperatures in the range 650 °C to 800 °C, cellular and plate

growth occur simultaneously. The microscopic observations revealed

that, in general, the magnetite nucleates in the form of plates, but

subsequent growth occurs by the cellular mechanism. This results in the formation of plate-like colonies containing a dense tunnel structure radiating from
21

the mid rib of the plate nucleus. The transition from cellular to plate

growth is believed to depend on the relative importance of cell boundary

and matrix diffusion fluxes. Thus, a high cell boundary migration rate

requires a steep concentration gradient across the magnetite/hematite

( 12 ) interface and this is best achieved by cell boundary diffusion

With increasing temperature, matrix diffusion becomes increasingly

important and the concentration gradient normal to the interface should

become flatter. Under these conditions, the lengthwise growth of a

coherent interface may exceed that of a cellular interface in which

case, a plate morphology develops. Presumably, the plates nucleate with

a (0001) hematite habit plane because this has an unusually low interfacial

energy. However, because of the low mobility of this interface, it

appears that sidewise growth at these intermediate temperatures is

greater by the cellular mechanism. This leads to the interesting situation

in which lengthwise and sidewise growth occur by different mechanisms to

produce a morphology analogous to bainite structures in steels. It is

interesting to note from Fig. 11 that the cellular reaction tends to

proceed first from only one of the two parallel interfaces of a plate

nucleus, indicating the important role played by the crystallography in

controlling interface mobility.

Conclusions

1. At temperatures below about 650 °C, the reduction of hematite to

magnetite proceeds by the migration of a cellular interface along

which oxygen and ferrous ions are partitioned by boundary diffusion.
22

2. A system of tunnels extends from the reaction interface to the outer

surface of the hematite and provides an arterial system for the

rapid transport of the gas involved in the reaction.

3. At temperatures above about 800 °C, a solid plate-like magnetite

product is formed. The lengthwise and sidewise growth of these plates

is thought to be controlled by matrix diffusion and interface mobility,

respectively.

4. Blocky grains of magnetite are also formed above 800 °C. These have

a different orientation relationship from the plates and do not exhibit

their pronounced growth anisotropy.

5. At intermediate temperatures (650 - 800 °C) , magnetite forms by

both cellular and plate growth mechanisms. The main factor responsible

for the transition between the two growth modes is thought to be the

relative proportion of cell boundary and matrix diffusion fluxes.

Acknowledgement

The authors gratefully acknowledge the encouragement given by

Professor J. S. Ball and the provision of the High Voltage Microscope

Facilities by the Science Research Council. The work was sponsored by

the U.S. Air Force Office of Scientific Research under Grant EOOAR-69-0060

.
References

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J. Szekely and J. W. Evans, Blast Furnace Technology , 35-75,

Marcel Dekker, New York (1972)

E. T. Turkdogan and J. V. Vinters, Met . Trans . , 1971, 2 , 3175-3188.

F. Habashi, Principles of Extractive Metallurgy , Vol. 1,

General Principles, Gordon and Breach, London 1969.

P. R. Swann and N. J. Tighe, Jernkont . Ann . , 1971, 155 , 497-501.

P. R. Swann, in Electron Microscopy and Structure of Materials , 878-903, U. of Calif. Press, Berkeley, 1972.

D. Turnbull, Acta. Met . , 1955, _3, 55-63.

J. w. Cahn, Acta. Met. , 1959, 7, 18-28.

A. V. Bradshaw and A. G,, Matyas, Met. Trans. B, 1976, 81-87.

W. A. Johnson and R. F. Mehl, Trans. AIME, 1939, 135, 416-442

J. W. Cahn, Acta. Met. , 1956, 449-459.

M. Hillert, Mechanisms and Phase Transformations in Crystalline

Solids , Institute of Metals, London, 1969.
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 76-11612. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

High Voltage Microscopy of the Reduction of Hematite to

Magnetite

5. Publication Date December 1976

6. Performing Organization Code

7. AUTHOR(S)

P. R. Swann, N. J. Tighe

8 . Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3130151

11. Contract/Grant No. EOAR 69-0067

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

EOAR

223/231

Marylebone Rd

.

London, NW1

Engl and

13. Type of Report & Period Covered

Interim 1971-1973

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The microstructural changes associated with the formation of magnetite in hematite

have been studied in specimens which have been partially reduced outside the microscope,

thinned until electron transparent, and then examined in the normal way. Three types

of structure have been observed in varying proportions which depend on the reduction

temperature. At low temperatures, magnetite grows by the propagation of a cellular

interface, the gas phase being transported to the cell boundary by a network of tunnels

At intermediate temperatures, magnetite plates are formed, whereas at high temperatures,

both plate magnetite and blocky magnetite appear. It is proposed that the factor

controlling the morphology which develops is the ratio of the cell boundary diffusivity

to volume diffusivity of ferrous ions. It is noted that the decomposition morphologies

of hematite and austenite have many similarities. The basic reason for this similarity

is that both transformations involve substantial redistribution of elements in the

solid state and the microstructures which develop are those that perform this redistribution the most efficiently at the temperatures involved.

17. KEY WORDS (six to twelve entries; alphabetical order, capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

High voltage electron microscopy, hematite Fe^O^, magnetite, Fe^O^, gaseous reduction,

iron oxide. *

18. AVAILABILITY |X~| Uni imited

For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. Cl 3 § Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

26

22. Price

$4.00

USCOMM-DC 29042-P74