Surface film softening in iron single crystals

Scripta METALLURGICA

Vol. i0, pp. 5 4 7 - 3 5 2 , 1976 P r i n t e d in the U n i t e d S t a t e s

Pergamon

Press,

Inc

SURFACE FILM SOFTENING IN IRON SINGLE CRYSTALS

K. Kojima*, S.Kobayashi and M. Meshii Department of Materials Science and Engineering Northwestern University, Evanston, IL 60201, U. S. A. {Received

February

12,

1976)

Introduction It is well known that the plastic deformation behavior of crystals can be markedly affected by the surface films. In fcc metals, surface films result in a strengthening effect such as an increase in flow stress (1,2). Recently it was reported that oxide films on Nb and Ta single crystals decrease the flow stress at low temperature (3). This surface film softening in bcc metals is very interesting in contrast with the surface film strengthening in fcc metals. A similar contrast can be pointed out with regard to the effects of alloying, irradiation, and prestraining. In this paper the effects of Ni filmson the low temperature deformation of iron single crystals have been investigated in order to characterize the surface film softening phenomenon in iron. Experimental Procedure Large (001) [II03 sheet single crystals~ were grown from vacuum melted electrolytic iron using the strain annealing technique (4). The main impurities of as-grown crystals were reported as follows: C: 0.002, Si: 0.001, Mn: 0.002~ P: 0.001, S: 0.002, Ni: 0.005, Co: 0.01, Cu: 0.001~ AI: 0.001 (wt%). Tensile specimens were cut by a wheel cutter and a spark erosion machine. Two tensile axis orientations are included here~ namely, the [I00] direction (A orientation) and I~ from the [II03 direction (B orientation) as shown in Fig. I. These specimens had a gage length of 4. S mm and a cross section of approximately 1.2 x 0.7 mm. After polishing in an H202 + S~ HF solution, all specimens were purified in a ZrH 2 system at 8S~ C for 48 hrs. Thin Ni films were deposited by electreplating in a Watts bath. The bath composition was NiSO 4 • 6H20 21.5 g/lO0 cc, NiCI 9 • 6H20 3.0 g/lO0 cc and a few drops of H3BO 3. The 5ath temperature was 3~C. The electroplating current decreased rapidly to a steady state within A B the first minute. This steady state value was used to calculate the plating current density. The tensile tests were performed on an Instron machine at 7 4 K with a crosshead speed of 0.002 in/min which corresponds to a nominal strain rate of 2 x 10-4/sec. FIG. 1 Orientation of specimens.

*

On leave from Department of Physics, Yokohama City University, Yokohama 236, Japan.

%

These single crystals were kindly provided by Dr. T. Takeuchi of the National Research Institute for Metals, Tokyo, Japan.

547

348

SURFACE FILM SOFTENING IN IRON

Vol.

10,

No.

4

Results Since the deformation behavior of iron single crystals at 7 ~ K is strongly orientation dependent, the effect of the Ni film on the deformation behavior was also orientation dependent. The stress-strain curves for the [I00] tensile axis are shown in Fig. 2. In the A orientation there are four [i12} slip systems with a Sehmid factor greater than 0.45. Without the Ni film, the control specimens fractured without detectable plastic deformation on the stress-strain curves. On the other hand, the Ni-plated specimens exhibited eonsiderable plastic deformation before fracture. The deformation was usually initiated by twinning. The plastic deA|77°K) formation occurred by the SO slip mechanism throughout the gage length of the specContro! imens except in cases where a few twins occurred. It 4o should be noted that the W E E slip deformation started substantially below the fracture stress of the con2OO trol specimens. The flow Z Ju stress increased rapidly due j..,. 5% to work-hardening. The enI 0 hancement of the plastic de(%) TENSILE STRAIN formation depended on the plating condition (Fig 8). The uniform elongation, measured at the maximum stress, increased with the plating current density and reached the maximum. A FIG. 2 further increase in the Stress-strain curves for orientation A. plating current density decreased the elongation.

,oo

I

u

u

A Control

~6

u

A (770K)

• NI-Platod

Z 0

~4 Z 0

~z •

%

0.5 PLATING

1.0 CURRENT

1.5

2.0

DENSITY(Aldm 2)

FIG. 3 Effect of current density on elongation in orientation A.

In orientation B, there are two [I10]
Vol.

i0, No.

SURFACE

4

IN IRON

B(77eK)

Control

~E 6 ° °

W

FILM SOFTENING

60

40O

Ild

=,, ,.I

349

E

Ni~oted (l-~A/dma)

200

_ ~ _

_ _

~O

Z I--

TENSILE

STRAIN

(%)

FIG. 4 Stress-strain curves for orientation B.

I

I

I

1

B177°K)

• Control

* Nl-Platod (No Serration) 20

o I~-I~otod (Serrotion)

Z

o

Z O -J

eJ

4~%

-,'

IO

I O

I

I 2

I 3

I 4

PLATING CURRENT DENSITY(A/din2)

FIG. 5 Effect of current density on elongation in orientation B.

The softening effect due to Ni-plating is as large as 98 MPa (20% of the yield stress of the control specimen). The maximum surface film softening is accompanied by the appearance of serrations, suggesting their interrelation in the softening mechanism. Many distinct lines were formed during the plastic deformation of the plated specimens. An optical micrograph of the deformed specimen (orientation B) is shown in Fig. 7. Two types of lines can be seen: coarse lines and fine lines. These lines tended to form perpendicular to the tensile direction. The direction of deviation, if it occurred, was towards the trace of the primary slip direction. The coarse lines were formed during serration and their formation occasionally propagated from one end of a specimen to another somewhat like a LUders band. The fine lines appeared in the regions between the coarse lines in the later stage of deformation and their number increased with strain. SEM observation indicated that these lines were cracks in the surface film as can be clearly seen in Fig. 8. The serration was apparently caused by the intermittent movement of slip disloca-

SURFACE FILM SOFTENING IN IRON

350

;;

....zE

!

Ul Ul I&J

..

550

500

B(77°K)

Control

0

NI-Plated (No Serration)

0

NI-Plated (Serration)

Ul

450. - - -

,, , ,,

--0,

;J 0

... U.

50 ;; E

a:

I-

55

I

400

#-

'0'

,

,,

,

....E

,,

LJ

'0

,0',

45 ~

I

2

3

tions. The optical microscopic observation of the specimen surface after removal of the Ni film, and the direct observation of the iron specimens by the transmission electron microscope, did not reveal microtwins, the formation of which could account for the observed serration.

40

350 0

Vol. 10, No.4

4

PLATING CURRENT DENSITY (A/dm 2 )

FIG. 6

Variation of 1% flow stress with current density in orientation B.

Discussion Most of the surface film effects reported previously are the hardening effects (1,2). In these studies the substrates were typically soft fcc metals. The present observation of surface film softening, therefore, may be regarded as opposite to these observations. In this regard, it is interesting to point

Tensile Direction

FIG. 7

An optical micrograph of Ni-plated surface of a B orientat~on specimen deformed 22% at 7~K. (current density 1.5 A/dm )

•

Vol, I0, No. 4

SURFACE FILM SOFTENING IN IRON

351

Tensile Direction

d

FIG. 8 A scanning electron micrograph of Ni-plated surface of a B orientation specimen deformed 22% at 74 K. (current density 1.5 A/dm 2)

out that other treatments, which harden fcc metals, can cause a softening effect in bcc metals, particularly in the low temperature region where the strength of these metals increases rapidly. Such examples are: solid solution effect (5), irradiation effect (6), prestraining effect at a higher temperature (7,8), and prior pressurization (9,10). Various mechanisms have been proposed for these softening phenomena and can be classified in two~ groups; one is based on the enhanced mobility of dislocations (due to reduction in intrinsic resistance, elimination or reduction of extrinsic resistance, or dislocation motion assisted by the local stress field etc.) and the other is based on an increase in mobile dislocation density, particularly nonscrew dislocations which are thought to be more mobile than screw dislocations. The surface film softening clearly belongs to this category of hardening-softening phenomena. A common mechanism may not exist for all the phenomena. However, there is a common feature; that is, the causes for hardening in soft fee metals reduce the low temperature yield stress of bcc metals which exhibit a high value due to a large temperature dependence. Some of the mechanisms mentioned above are not important in the surface film softening. The structural change in the substrate and the influence due to the surface film, would be primarily confined to the vicinity of the interface. The presence of a surface film would change the image force exerted on dislocations near the surface of a substrate. A substantial internal stress is reported to be present in an electroplated film (II); therefore the corresponding internal stress exists in the substrate. The force on the dislocations resulting from these effects may assist the generation and the motion of dislocations near the interface. Another important effect is the accomodation strain which must be generated when an elastoplastically heterogeneous specimen is subjected to deformation. The plastic strain is induced in the substrate by the presence of the surface film and is increased by the formation of cracks in the film (12). If this plastic strain generates a substantial density of non-screw dislocations, a reduction in yield stress is expected to occur (8). The reduction in yield stress of I0 ~ 20% appears reasonable with respect to such a mechanism. This magnitude is also comparable to the surface film softening observed in Nb and Ta (3). On the other hand, a significantly larger softening effect induced by the combination of a surface film and prestraining in Nb (13) was not reproduced in the iron specimen used in the present study. Clearly, more experimental work is needed to characterize the surface film effects on the low temperature plastic deformation of bcc metals. Acknowledgements The authors would like to express their appreciation to Dr. Tomoyuki Takeuchi of the National Research Institute for Metals, Tokyo, for providing the iron single crystals. The research on which this paper is based is supported by the United States Energy Research and Development Administration.

5S2

SURFACE FILM SOFTENING IN IRON

Vol. I0, No, 4

References

2__33,I I I I

I.

T. Takasugi and O. Izumi, Acta Met.

(1975).

2.

J. Pridans and J. C. B i l e l l o , Acta Met. 20, 1339 (1972).

3.

V. K. Sethi and R. Gibala, Scripta Met. 2, 527 (1975).

4.

T. Takeuchi and S. Ikeda, Trans. Iron and Steel Inst. of Japan 2, 483 (1969).

5.

J. W. Christian, Proc. 2nd Int. Conf. on Strength of Metals and Alloys, ASM p. 31 (1970).

6.

A. Sato and M. Meshii, Scripta Met. 8, 851 (1974).

7.

W. A. Spitzig and A. S. Keh, Acta Met. 18, 611 (1970); Can. J. Phys. 45, II01 (1967).

8.

A. Sato and M. Meshii, phys. star. sol. (a) 28, 561 (1975).

9.

M. Yajima and M. Ishii, Acta Met. 15, 651 (1967).

A. S. Keh and Y. Nakada,

10.

D. Aichbhaumik and W. L. Haworth, Scripta Met. 2, 1351 (1975).

II.

W. H. Safranek, The Properties of Electrodeposited Metals and Alloys, American Elsevier Pub. Co., New York (1974).

12.

R. M. Johnson and R. J. Block, Acta Met. 16, 831 (1968).

13.

V. K. Sethi and R. Gibala, NATO Advanced Study Institute on Surface Effects in Crystal Plasticity (1975).