Plastic deformation of FeN single crystals in the temperature range between 4.2 and 300 K

Scripta

MI!TALhUR(;ZCA

V o l . 14, p p . 3 2 1 - 3 2 4 , 1980 Printed in the U.S.\.

P e r g a m o n PYess I~td. AI] r i g h t s reseYved.

PLASTIC DEFORMATION OF Fe-N SINGLE CRYSTALS IN THE TEMPERATURE RANGE BETWEEN 4.2 AND 300 K

Y. Aono, K. Kitajima and E. Kuramoto Research Institute for Applied Mechanics, Kyushu University Hakozaki 6 - lO - l, Higashi-ku, Fukuoka 812 Japan (Received November ]S, ]979) (Revised January 28, 1980) Introduction Many investigations on the solid solution hardening and softening in BCC metals caused by interstitial solute atoms, i.e. carbon, nitrogen, oxygen, etc., have so far been carried out (l 8), but the detailed mechanism has not yet been clarified, partly because of lack of data at very low temperatures and partly because of the presence of the interaction between dopant solute atoms and residual impurity atoms in the host metals (the scavenging effect (7)). Plastic deformation of BCC alloys at temperatures below 77 K, especially around 4.2 K, is usually impossible because of the occurrence of twinning and cleavage fracture. Even in the recent papers on Fe-C alloys ( Cottu et al. (5)) and Nb-N alloys (Bowen and Taylor (I0)) no experimental data was obtained below 35 and 60 K, respectively. In order to reach a comprehensive understanding on the strength on BCC alloys, low temperature data, however, must be obtained by using strictly purified specimens. As reported in previous works (ll), the authors succeeded in plastically deforming high purity iron thin crystals at 4.2 K and in the work reported here this method was applied to Fe-N alloy thin crystals prepared from high purity host metal. Complete results obtained between 4.2 and 300 K and discussions are presented in the followings. Experimental

Procedure

The starting material for the present investigation was MRC pure iron (MARZ grade). The details of zone-refining and preparation of single crystals were described elsewhere (If). The specimens with diameter of 0.2 mm and RRR. = 4000 were doped with nitrogen by annealing them at o • H 550 C ~n a flow of dry hydrogen gas which had passed through liquid ammonia. To disperse nitrogen homogeneously, specimens were quenched into the mixed solution of ice and water from 550°C and then stored in liquid nitrogen until tests. The content of nitrogen ranged from 156 to 1288 appm, which were determined by the electrical resistivity measurements using the value of 7.0 ~ c m / at.%N at 78 K obtained by Wagenblast and Arajs (12). Specimens have lengths of 20 mm with a gauge length of IO mm and orientation of ~ = 38 ~ l° and X = 21 ± l°, where ~ is the angle between the tensile axis and [Ill] and X the angle between the maximum shear stress plane containing [lll] direction and (lOl). Tensile tests were carried out at a nominal strain-rate of 1.7 x lO-4sec -I in the temperature range from 4.2 to 300 K. The specimens were fixed to chucks and set to the testing machine without raising the temperature higher than that of liquid nitrogen. Experimental

Results

Figs. l(a) and l(b) show the behaviours of stress-strain curves at 4.2 and 77 K for Fe-N alloys compared with pure iron specimems. All the pure specimens and alloy specimens of 156 and 318 appm nitrogen could be plastically deformed at 4.2 and 77 K without prestraining. Specimens of 779 and 1288 appm nitrogen, however, showed plastic elongation at 4.2 K only when a prestrain of I ~ 2 % was given at 200 K before the test; otherwise twinning or cleavage fracture occurred before plastic yield. The latter specimens did deform plastically at 77 K without a prestrain. After prestraining, both re-yield micro strain and homogeneity in deformation increased, but the value of yield stress, which is defined as the stress at 0.5 % tensile plastic strain, was unchanged. Figs. 2(a) and 2(b) show the temperature of the yield stress for pure iron and.Fe-N alloy specimens, where the yield stress is resolved on the maximum shear stress plane and in the direction of Jill]. Following the addition of nitrogen atoms, the yield stress increase in the temperature range below 140 K, particularly at 4.2 K, but decreased in the intermediate temperature range from 140 to 250 K, removing a hump (concave down part) observed in pure iron (13), and again increased

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above 250 K. In Fig. 3 the value of yield stress are plotted against nitrogen content. The yield stress monotonically increases with increasing nitrogen content at very low and high temperatures (the hardening region), while at intermediate temperatures the yield stress has a minimum at nitrogen content of about 600 appm, resulting in softening around 200 K (the softening region). Discussion As mentioned above, it was clearly shown that both hardening and softening occur in the plastic deformation of Fe-N alloy single crystals. Similar softening in single crystals was observed by Nakada and Keh (1), Maeda and Sakamoto (3) and Diehl et ai. (4) in the Fe-N system and by Quesnel et al. (9) and the present authors (13) in the Fe-C system. However, the hardening below 77 K was first observed in this paper. There is no unified theory which can completely explain the effect of alloying (interstitial type) on the mechanical behaviours of BCC metals. Softening in the systems containing a small amount of dispersed interstitial-type alloying elements with comparatively large misfit strain, e.g. 0.35 for nitrogen in iron, was explained by Sato and Meshii in terms of the elastic interaction between solute atoms and screw dislocations surmounting the Peierls potential hill (14). Large hardening at 4.2 K in Fe-N alloys, however, is considered not to be explained by their theory. Recently, a new trial has been made in order to explain the low temperature interstitial solute hardening on the basis of the in-situ experiments in a transmission microscope by Kubin and Louchet (15). The main point of discussion is the decrement of dislocation segment lengths because of pinning by interstitial solute atoms, which results in the decrement of double kink formation events, and hence the increase in the yield stress. But this interpretation cannot explain the hardening near absolute zero temperature, i.e., at 4.2 K. Then, the observed hardening at 4.2 K must be understood to be the result of the interaction between a dislocation and interstitial solute atoms. This argument has, however, the weak point that each interstitial solute atom should have an interaction force of about ~b 2 to produce the total observed strength at 4.2 K (m400 MPa = ~/200), because the interaction force and the Peierls force cannot be additive. This value is too large compared with the usually accepted value of O.l ~ 0.2 ~b 2. An alternative explanation is jog formation at the site of a pinning point, where the probability of cross slip into a different slip plane is quite high because of the elastic interaction between the dislocation segment and a solute atom. Jogs formed at the pinning points act as dragging points to the motion of dislocation lines and the maximum resistant force should be about ~b 2, which can explain the hardening at 4.2 K. Recently, the experimental evidence for the jog dragging at very low temperatures has been obtained in our experiments by transmission electron microscopy and will be described elsewhere. It seems difficult to explain the steep temperature dependence of the yield stress in the low temperature range, but thermally activated forward motion of the bowing dislocation segment might force some dragging jog points to move sideways, resulting in the decrement of the number of dragging point, namely, the decrease in the yield stress with increasing temperature. Acknowledgements The authors would like to present their cordial thanks to Messrs. Y. Miyamoto and T. Fujiwara for their assistances in preparation of pure iron and iron alloys and performing tensile tests. References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13 14. 15.

Y. Nakada and A. S. Keh, Acta Met., 166, 903 (1968). B.W. Christ, R. P. Gamble and G. V. Smith, Scrlpta Met., 3, 521 (1969). K. Maeda and K. Sakamoto, Scripta Met., lO, 147 (1976). J. Diehl. M. Schreiner, S. Staiger and S.---Zwlesele, Scripta Met., 10, 949 (1976). J . P . Cottu, J. P. Peyrade, P. Chomel and P. Groh, Acta Met., 26, ]-]-79 (1978). T . E . Mitchell and R. L. Smlalek, 2nd Intern. Conf. on Strength of Metals and Alloys, Asilomar, 73 (1970). R. Gibala and T. E. M i t c h e l l , Scripta Met., 7, 1143 (1973). K . V . Ravi and R. Gibaia, Acta Met., 18, 623--(1970). D . J . Quesnel, A. Sato and M. M e s h i i , ~ a t . Sci. Eng., 18, 199 (1975). g. K. Bowen and G. Taylor, Acta Met., 25, 417 (1977). K. Kitajima, Y. Aono, H. Abe and E. Kuramoto, 5th Intern. Conf. on the Strength of Metals and Alloys, Aachen, p 965 (I-79). H. Wagenblast and S. Arajs, phys. star. sol., 26, 409 (1968). E. Kuramoto, Y. Aono and K. Kitajima, Scripta M~t., 13, 1039 (1979). A. Sato and M. Meshii, Acta Met., 2~, 753 (1973). L. P. Kubinand F. Louchet, Phil. Mag., 3-8, 205 (1978).

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PLASTIC D E F O R M A T I O N OF Fe-N SINGLE CRYSTALS

151X

= I 7 X 10-4Secq T=4.2K

779 v IOOC

,,~/q--

~ ~

/

,,,

/ 1 5 */. I~ prestrainedat 20OK

~

5OC'

~ ~

no prestrain

7 /

7

i

J

/

/ i

/

J

/' Cross

Fig. l(a) Stress-strain

head displacement

curves of iron and Fe-N alloys at 4.2 K.

e:l 7 X10~%ec 1

800

T=?TK no prestrain

~-

1288

6o0 pure Fe

o

/

400

/

200

/ /'

/

/

I "I* l

/ /

/ Cross head displacement

Fig.

l(b) Stress-strain

curves of iron and Fe-N alloys at 77 K.

323

324

500

PLASTIC D E F O R M A T I O N OF Fe-N SINGLE CRYSTALS

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,0o[

f

F~ 5001~

Fe - N allOyS

Fe - N attoys

: 1.7 X TO-4sec-I op

o pure iron 156 (at.ppmN}

l

.&

v

.......

779(at.ppm N)

400

I

30C

20C

200

IOC

100

10o

2 T(K)

300

400

~\L

~

0

100

~

200

Fig. 2(a) Temperature dependence of the yield stress of iron and Fe-N alloys (156, 318 appm).

400

Fig, 2(b) Temperature dependence of the yield stress of iron and Fe-N a]loys (779, 1288 appm).

T =4.2K 5OO o o-30K

300

77K

2O0 ° ~ ° ~ ° ~

~

)00(

I

500 J

10100

~500

N content ( at.ppm )

Fig. 3

300

T (K)

Relation between the y i e l d stress and nitrogen content.