surface martensite in iron-nickel

6
SURFACE MARTENSITE IN IRON-NICKEL* J. A. KLOSTERMANNt and W. G. BURGERS: On single crystals of the composition 30,2% Ni-0,04% C-balance Fe, surface martensite was found in the form of needles lying parallel to the surface. The only condition controlling the orientation of these needles appears to be that they must lie in {112), planes. MARTENSITE DE SURFACE DANS LE FER-NICKEL Experimentant sur des monocristaux comportant 30,2 % Ni et 0,04 % C (le reste &ant du fer), les auteurs ont observe une martensite superficielle, presentant la forme d’aiguilles parall&les iL la surface. La seule condition regissant l’orientation de ces aiguilles semble irtre qu’elles doivent se trouver dans des plans (112},. OBERFLBCHENMARTENSIT IN EISEN-NICKEL An Einkristallen der Zusammensetzung 30,2 ‘A Ni-0,04 y0 C-Rest Fe wurde Oberfliichenmartensit in der Form van Nadeln gefunden, die parallel zur Oberfl(iche lagen. Eine Lage der Sadcln in {ll_“),- Ebenen scheint die einzige Bedingung fiir deren Orientierung zu sein. INTRODUCTION Recent investigations have revealed that during electrolytical thinning of metastable &brass(l) and of Fe-307; nicke1(2,3) martensite formed spon- taneously at a temperature above the Ms-tempera- ture of bulk material. Warlimont(3) relates this phenomenon to the reduction of the strain-energy term which is introduced in calculating the critical embryo size. This would result in reducing the Ms-tempera- ture. Bastien and Stora(*) observed in stainless steel (17,5% Cr-7,551/, Ni-0,05 C-balance Fe) a surface martensite that grew with a directly observable speed (0,05 mm/‘sec and higher). This martensite was formed after removing a thin layer electrolytically. They attribute the initiation of this phenomenon to the energy liberated when the removal of material disturbs the balance of microstrains in the surface. Surface martensite in stainless steel of the type 18-8 was also report.ed by R. Margerandc5). By a simple reasoning one can understand that in a thin layer or on an undisturbed surface of a specimen martensite can form above the MS-temperature which holds for “bulk martensite”. It is unnecessary to resort to an explanation on the basis of microstresses or a specific nucleation theory. The cc’ phase has a greater specific volume than the y phase; consequently a martensite crystal which has formed within a block of y-material will be subject to a hydrostatic pressure, which depresses the MS-temperature. For Fe-30% Ni Kaufman et .Z.t6) observed a decrease in Ms-tempera- ture of about 5°C per kilo atmosphere. On the surface a hydrostatic pressure cannot be built up, and thus martensite may form at a higher temperature. To the authors’ knowledge surface martensite in iron- nickel has not previously been observed. In the present work this form of martensite was found and investigated. EXPERIMENTAL Single crystals of the composition 30,2% Ni-0,04% G69,76v/b Fe were prepared by the method of Czoch- ralsky and electrolytically polished in the as-grown condition. On the polished surface at room tempera- ture martensite formed spontaneously, often in the form of very narrow, long needles which extended over the whole width of the specimen (10 mm) as shown in Fig. 1. Single crystal slices of known orientation were obtained by the method described by B0gers.c’) After a mechanical preparation wit’h emery paper, the * Received August 13, 1963. 7 Dutch Foundation for Fundamental Research (F.O.M.). $ Labomtory for Physical Chemistry of the Solid State, Technological University, Delft. ACTA METALLURGICB, VOL. 12, APRIL 1964 355 Frc~. 1. Surface martensite in the form of narrow long needles. The needles extend over the full breadth of t,he specimen (10 mm) ~40 X .

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Page 1: Surface martensite in iron-nickel

SURFACE MARTENSITE IN IRON-NICKEL*

J. A. KLOSTERMANNt and W. G. BURGERS:

On single crystals of the composition 30,2% Ni-0,04% C-balance Fe, surface martensite was found in the form of needles lying parallel to the surface. The only condition controlling the orientation of these needles appears to be that they must lie in {112), planes.

MARTENSITE DE SURFACE DANS LE FER-NICKEL Experimentant sur des monocristaux comportant 30,2 % Ni et 0,04 % C (le reste &ant du fer), les

auteurs ont observe une martensite superficielle, presentant la forme d’aiguilles parall&les iL la surface. La seule condition regissant l’orientation de ces aiguilles semble irtre qu’elles doivent se trouver dans des plans (112},.

OBERFLBCHENMARTENSIT IN EISEN-NICKEL An Einkristallen der Zusammensetzung 30,2 ‘A Ni-0,04 y0 C-Rest Fe wurde Oberfliichenmartensit in

der Form van Nadeln gefunden, die parallel zur Oberfl(iche lagen. Eine Lage der Sadcln in {ll_“),- Ebenen scheint die einzige Bedingung fiir deren Orientierung zu sein.

INTRODUCTION

Recent investigations have revealed that during

electrolytical thinning of metastable &brass(l)

and of Fe-307; nicke1(2,3) martensite formed spon-

taneously at a temperature above the Ms-tempera-

ture of bulk material. Warlimont(3) relates this

phenomenon to the reduction of the strain-energy term

which is introduced in calculating the critical embryo

size. This would result in reducing the Ms-tempera-

ture.

Bastien and Stora(*) observed in stainless steel

(17,5% Cr-7,551/, Ni-0,05 C-balance Fe) a surface

martensite that grew with a directly observable speed

(0,05 mm/‘sec and higher). This martensite was

formed after removing a thin layer electrolytically.

They attribute the initiation of this phenomenon to

the energy liberated when the removal of material

disturbs the balance of microstrains in the surface.

Surface martensite in stainless steel of the type 18-8

was also report.ed by R. Margerandc5). By a simple

reasoning one can understand that in a thin layer or

on an undisturbed surface of a specimen martensite

can form above the MS-temperature which holds for

“bulk martensite”. It is unnecessary to resort to an

explanation on the basis of microstresses or a specific

nucleation theory. The cc’ phase has a greater specific

volume than the y phase; consequently a martensite

crystal which has formed within a block of y-material

will be subject to a hydrostatic pressure, which

depresses the MS-temperature. For Fe-30% Ni

Kaufman et .Z.t6) observed a decrease in Ms-tempera-

ture of about 5°C per kilo atmosphere. On the surface

a hydrostatic pressure cannot be built up, and thus

martensite may form at a higher temperature. To

the authors’ knowledge surface martensite in iron-

nickel has not previously been observed. In the

present work this form of martensite was found and

investigated.

EXPERIMENTAL

Single crystals of the composition 30,2% Ni-0,04%

G69,76v/b Fe were prepared by the method of Czoch-

ralsky and electrolytically polished in the as-grown

condition. On the polished surface at room tempera-

ture martensite formed spontaneously, often in the

form of very narrow, long needles which extended over

the whole width of the specimen (10 mm) as shown in

Fig. 1. Single crystal slices of known orientation were

obtained by the method described by B0gers.c’) After

a mechanical preparation wit’h emery paper, the

* Received August 13, 1963. 7 Dutch Foundation for Fundamental Research (F.O.M.). $ Labomtory for Physical Chemistry of the Solid State,

Technological University, Delft.

ACTA METALLURGICB, VOL. 12, APRIL 1964 355

Frc~. 1. Surface martensite in the form of narrow long needles. The needles extend over the full breadth of t,he

specimen (10 mm) ~40 X .

Page 2: Surface martensite in iron-nickel

356 ACTA METALLURGICA, VOL. 12, 1964

FIG. 2. Klectron diffraction reflection pattern of a speci- men surface on which surface martensite has not yet formed. The specimen surface coincides with the (llO), plane; the electron beam is /I (1 lo), and 11 [l 111,. Value 2Li of the apparatus: 31, 0 x lo-’ mm2, magnification

in reproduction 1,6 x .

polishing was performed on a Disa-electropol appara- tus (electrolyte A2, voltage 36). By removing material electrolytically under conditions such that the speci- men was warmed up by the high current, it was possible to take away a layer of the material without the possibility of forming new martensite on the sur- face. By thus removing layers of different thickness it was established that the surface martensite ex- tended to a depth of 5 to 30 1~. If a relatively thick layer (for inst,ance 0,OS mm) was removed from a

single crystal, surface martensite often formed only after a few hours. Figure 2 shows an electron diffrac- tion pattern of a specimen prepared in this way which was made when the surface was completely austenitic. In the pattern only austenite spots appear. Figure 3 gives a reflection pattern of the specimen in the same orientation after surface martensite had formed. Besides the y spots there are now also u’ spots. Although the electron diffraction method was not accurate enough for a precise determination of the orientation relation, analysis of Fig. 3 leads to the following relation

(llO),ll(OOl),~ [iii],ll[iio],, I

This orientation relation agrees with that found by Nishiyama for Fe-30% Ni.@) We conclude from this result that the actual relationship between the orien- tation of the surface martensite and that of the matrix cannot be very different from that found by Nishiyama for bulk martensite@‘.

To determine the habit plane of the surface marten- site, a number of single-crystal specimens were pre- pared with electrolytically polished surfaces lying in

various crystallographic planes. Surface martensite formed spontaneously at room temperature. Figure 4 shows this phase on a surface lying in the (100) plane. The angles between the direction of the marten- site needles and the reference line A-B were measured. The position of the austenite lattice relative to the specimen surface and the reference line A-B was found with a Laue-pattern and is represented in a stereographic projection (Fig. 5). In Fig. 5 the nor- mals to the spurs indicated in Fig. 4 are drawn. These normals appear to go through the (112}, poles.

Figure 6 gives a print of a Laue back-reflection photograph of a specimen with a (ill), orientation. This photograph was obtained with the X-ray beam incident perpendicular to the specimen surface, which was electrolytically polished and in the austenitic state. Figure 7(a) gives a photomicrograph of the surface after surface martensite has formed. The black reference line C-D (a scratch) on Fig. 7(a) corresponds with the line C-D on Fig. 6. Figure 7(b) is a photomicrograph taken at the same place on the specimen but with higher magnification. For this specimen the directions (spurs) of the martensite needles relative to C-D were measured directly with a microscope provided with a cross thread. Figure 8 shows in stereographic projection the orientation of the y-lattice (found with the help of Fig. 6) relative to the line C-D. In addition the normals to the spurs of the martensite needles are drawn in Fig. 8. Also in this case the normals appear to go through the {112}, poles.

In the same manner surface martensite spurs were measured on specimen surfaces lying in respectively

FIG. 3. Electron diffraction reflection pattern of the same specimen surface in the same position as in Fig. 2 but after surface martensite has formed. In addition to the y reflection now also a’ reflections occur.

Value 2L1 of the apparatus: 31,O x IO-’ mm2, mag- nification in reproduction 1,6x .

Page 3: Surface martensite in iron-nickel

KLOSTERMANN AND BURGERS: SURFACE MARTENSITE IN FeNi 357

FIG. 4. Direction of the martensite needles with reference to the line A-B. Specimen surface {lOO}r, 50x, dark field illumination.

FIG. 5. Position of the normals of the martensite spurs relative to the {112}, poles of the y lattice.

Fro. 6. Laue-back reflection photograph of a specimen with a surface in the austenitic state lying in { 111 }r. The line C-D corresponds to the line C-D in Figs. 7 and 8.

Direction of the X-ray beam (11 I),,.

Page 4: Surface martensite in iron-nickel

FIQ. 7(a). Surface martensite on an electropolished speoi- men surface lying in {ill},. The line C-D is scratched in the specimen and corresponds to the line C-D in Fig. 6,

40X.

FIU. 9. Surf~e mtmtensite with. straight boundaries. This photomicrograph was taken 3 months after forma- tion of the initklly narrow needles. Owing to isothermal

growth the needles have become broad, -100 x .

Fra. 7(b). Detail of Fig. 7(a), 300x.

martensite spurs

FIG. 8. Position of the norm& to the surface mar- tensite needles formed on a surface lying in & {l l~l}, plane,

relative to the (I 121, poles of the austenite lattice.

358 ACTA METALLURGICA, VOL. 12, 1964

FIG. 10(a). “Butterfly” martensite. Preparation of the specimen: grinding (grit 400600): cooling to - 10°C; electro-polishing. After the polishing, surface m&ens&e needles grew out of the wings of the

“butterflies”, 80 x .

FIG. IO(b). Detail of Fig. 10, 500 x.

Page 5: Surface martensite in iron-nickel

KLOSTEHMANN AND BURGERS:

a (112), plane and a {llO), plane. Onoe again for surface ma&en&e on these specimens the normals to the rna~~~~~ SImrS a to go through the Cl f2)y poles. Thus, although the surface martens&e forms in needles, these needles do not orient in a particular crystallogrraphic direction but instead lie in a crystallo- graphic plane of the form f 1 X2&.*

It is not impossible that the needles are actually lath-shaped, but in any case their length is often f@M times as large as their breadth and their thickness.

The above mentioned specimens were all slightly deformed by machining. Ib seems probable that the habit plane of surface martens&e will conform to the form f112f, within very narrow limits if it forms on a perfect undeformed specimen. In such ease one should expect a deviation far less than that found in the ex- periments described above. On the other hand it has already been known for a long tirne(l**“l) that on cooling irannickel alloys with a&m% 30% Ni, ma.- tensite forms in the form of plates in the whole speci- men (“bulk martensite”) and that these plates lie in planes not far from (3,10,15},. Thus it is note- worthy that in the same material two different forms can appear for the “habit plane”, namely one for the rna~~~~ that forms as plates through the whole body of a specimen: $3,XO,E),; and one for the martensite tlmt formed on the surface as needles lying in {112},, planes.

On studying surface martens&e the following characteristics were found :

(1)

(21

(3)

Surface martensite oan form spontaneously after tbe eleetro-polis~~u~ of single crystals in the as-crow ~ond~t~o~ which are not worked in any way and in which no ~l~ti~all~ deformed domains occur and presumably ouly smaI1 elastic &rains. The needles can be nucleated by pla,stic deformation (scratching with ai needle). Surface martens&e forms at 5 to 30°C (or more) above the us-~rn~e~ature which applies for bulk martens&e. It forms more easily and in greater quantity as the temperature nes,rs the _M.s-temperature for “bulk martensite”.

* This is &us just the opposite behaviaur as found for stainless ste,d by Kelly and N?MngCgl for marten&e formed in bulk m&erial and thinned for transmission &at~ranmioro- soopy. For this case they faund that the len@;th direction of the needles aoinoides with rliTOlv. The nom&s to the

SURFACE MARTENSITE IN Fo-~Ni 959

(4) The martensite needles extend to a depth of 5to30&

(5) The rn~~si~~ needles grow isothermally with a dire&y observable speed. The growth in *he longitudinal direction of a needle is much fastir than the growth in the transverse direction. The speed of growth varies greatly. A needle may attain virtually full growth in a small fraction of a semnd. In other cases needles grow as very narrow lines in fongitudin~f direction with a speed varying from <O,Ol mm/ set to > 1 mm/see. This is followed by growth in the transverse direction. After 3 to 1 hr the transverse growth has become very small, but continues, however, for a long time; there is still growth in the breadth after days or weeks: see Fig. 9. Surface martensite is presumably bounded by such ~~tallo~a~hi~ planes as satisfy the condi- tion of a minimum boundary energy, The boundary planes presumabbbly strive to approach a set of cry&allographic planes that satisfy the requirement of a minimum boundary energy, Figure 9 gives an example of such broadened needles, thFeemo~~thsaftertheir~~ta~~~~~ee.

“EUTTERFLY &%ARTENSITE”

On specimens cooled just above the Ms.temperature for bulk martensite besides surface martensite in needle form, martensite with a different habit also appeared. This rnar~ns~~~ of which Fig_ IQ(a) gives an example, has been na.med by us buttertly marten- site. “‘Butterfly martensite” extends to a greater depth. It seems to be an intermediate form between the surface marten&e of needle habit and the bulk rna~~~s~te studied by Greninger and Troiano. The reb~t~r~ie~i’ oeeu~ in most cases in rows as shown by Fig. 10(a). Figure 10(b) gives a detail. It shows t&at thFt “body”’ of the ‘“butterfly” is formed by a kind of a, midWrib. The “wings” seem to be composed of fringes which are parallel to the surface martensite needles (dark lines on Figs. IQ(a) and IO(b)). The but~r~~es often appear joined together in formations ~sernbli~~ those found for bulk martensite. Figure 11 (a) gives a photomicrograph of bulk martensite formed by cooling a specimen to - 12°C. To remove the surface marten- site the specimen was mechanically and then electra- lytieaily I&shed and immediately pho~gra~hed~ The bulk martensite plates are often broken up and composed of formations which cut the surface in the form of a “butterfly’“. Figure 1 l(b) gives a detail of Fig* 11(a). In Fig. II(b) one can see, clearly the mid- rib of the bulk marten&e formations,

Page 6: Surface martensite in iron-nickel

360 ACTA METALLURGICA, VOL. 12, 1964

Fig. 11(a). Bulk martensite, formed by cooling to - 12°C. On some places the bulk martensite formations

am built up out of “butterflies”, ~40 x .

(1)

(2)

(3)

(4)

CONCLUSIONS

On single crystals of the composition 30,2x Ni-0,04°~C-balance Fe, surface martensite was observed in the form of needles lying parallel to the surface. The only condition controlling the orientation of these needles appears to be that they must lie in { 1121, planes. The needles form at room temperature and grow isothermally with directly observable speed. Surface martensite can be nucleated by plastic deformation (scratching with a needle), but can also form spontaneously after electro-polishing of as-grown single crystals, which are not worked in any way and therefore free from plastically deformed domains and in which presumably only small elastic strains occur. A martensite form was found which appeared on the surface and possesses a butterfly appearance. This seems to be a form intermediate between surface martensite and bulk martensite.

Fm. 11(b). Detail of Fig. 11. The mid-ribs of the bulk martensite formations are clearly visible, 300 x .

ACKNOWLEDGMENT

This work is part of the research programme of the Metals Research Group “F.O.M.-T.N.O.” sponsored by Z.W.O.

REFERENCES

1. D. HULL, Phil. &fag. 7, 537 (1962). 2. J. GAQ~ERO, D. HULL, Acta Met 10, 995 (1962). 3. H. WARZIMONT, Trans. Amer. Inst. Min. (Metdl.) Engra. 991, 1270 (1961).

4. P. BASTIEN and G. STORA, C. R. Acad. Sci. Paris 244, 2613 (1957).

5. R. MARQERAND, Metaux CWTOS. Industr. 37, 85, 154, 204 (1962).

6. A. KAUFMAN, A. LEYENAAR and 5. S. HARVEY, A& Met. 8, 270 (1960).

7. A. J. BOQERS, Thesis, Delft (1962). 8. Z. NISHIYAMA, Sci. Rep. Tohoku Univ. 23, 638 (1934-

1935). 9. P. M. KELLY and J. NUTTING, J. Iron St. Inst. 197, 199

(1961). 10. A. B. GRENIN~ER and A. R. TROIANO, Trans. Amer.

In&. Min. (Met&.) Engrs. 149, 307 (1940). 11. J. F. BREEDIS and C. M. WAYMAN, Trans. Amer. Inst.

&fin. (MetalE.) Engr.9. 224, 1128 (1962).