The deformation of polycrystalline niobium at room temperature

JOURNAL OF THE LESS-COMMON METALS

THE

DEFORMATION AT

OF

POLYCRYSTALLINE

Carbide

NIOBIUM

ROOM TEMPERATURE

A. WRONSKI linimt

205

Ewopean

AND 22. FOURDEUX

Research

(Received

Associates.

February

Brussels

(Relgiun~)

z rst, 1964)

SUMMARY A series of experiments on polycrystalline “flash-annealed” niobium are described, which were designed to relate the room temperature yielding and work-hardening properties to purity and microstructure, as observed by transmission electron microscopy. It was found that, for the same low strain rate, whereas the shape of the loadPelongation curve appears to depend markedly on purity, the dislocation density and structure does not and that dipoles, tangles and eventually cells are formed for all purities of the material studied. Qualitatively and quantitatively the electron microscope results are very similar to some reported earlier for less pure niobium, for iron and even for face-centred metals, such as copper. The mechanical properties of the purest material resemble those recently found for zone-melted niobium single crystals, namely yield stress of N 3.5 kg/mm’ and a region of rapid work hardening. Results are discussed in terms of a model in which yielding is governed by the dislocation multiplication and velocity characteristics in the material. A new and suprising feature of the results is the observation that the strain rate, which apparently does not influence the dislocation density, has a pronounced effect on the dislocation arrangement.

The effects

of grain size, purity,

testing

speed and dislocation

structure

on the de-

formation properties of niobium have been recently extensively investigated. It is generally agreed1 that grain size has a very small, if at all detectable, effect on the value of the yield stress, or, but controversy dislocation structure and the barriers to

exists about the roles of impurities, the dislocation motion. VAN TORNE AXI)

postulate that cry depends solely upon the total impurity the purest niobium - which exhibits very little work-hardening -

THOMAS”

are uniformly distributed, that dislocation material and that “solute atom clusters” motion.

content, that in the dislocations

cell structures are formed only in impure are the strongest barriers to dislocation

It has been shown earlier, however, that the effect on or of nitrogen1 is about five times greater than that of oxygena.4 and that the effects of carbon5 and hydrogen6 are very small by comparison, and more recently that “flash-annealed” polycrystalline7 and zone-refined monocrystallines materials, for both of which the lowest CJY is about half of that of VAN TORNE AND THOMAS, exhibit high work-hardening rates at room temperature. In view of these discrepancies it was decided to reinvestigate the problem. Both conventionally and flash-annealed foils were used for the investigation; the experimental procedures have been reported previously799. Variations of the flashJ.

Less-Commorl

Metals,

7 (1964) LOS-Z~ 1

206

A. WRONSKI, A. FOURDEUX

annealing technique resulted in different preferred orientations, grain sizes and values of GY. Representative data are presented in Fig. I for a strain rate of N 1% min-l. In an attempt to correlate or with interstitial impurity contents, specimens were sent to two different laboratories for chemical analyses. The results of these analyses are

I 5

OO

Fig.

I. Load-elongation

curves

d

405

e

247

(111)

;

298

(100)

9

176

h

198

I

U,niforZ? elongation’?%) of singly “flash-annealed” orientations.

kcl0)and(111)

II)and (100 W)

20 specimens

of (100)

and/or

{III>

not consistent, which is not surprising as similar difficultieshave already been encountered with molybdenum10 and tungstenrr. A correlation exists, however, between (TY and the “degree of purity” as determined by measurement of the superconductivity transition temperature and the resistivity ratio (Table I). TABLE RATIOS

OF

RESISTIVITY

CONVENTIONALLY

Specimen treatment

AT AND

I

77.4” AND IO0 TO 295’K FLASH-ANNEALED

FOR

NIOBIUM

Ratio of resistiuity at 77.4”K to 295°K (x10-3)

Ratio of resistivity at IO’K to 295°K (x IO-“)

Conventionally annealed

N I4

214.0 213.9 249.8 235.3

52.65 51.63 92.85 75.02

“Flash-annealed”

(4

177.’ ‘75.6 183.9 178.6

9.88 8.52 16.69 II.01

Examination in the electron microscope of the undeformed structure of the flashannealed foils covering the whole range of values of UYdid not reveal any significant J. Less-Common Metals, 7 (1964) 205-211

D~~OR~ATIO~

Fig. 2. Dislocation

OF POLY~RYSTA~,I_IN~

SIORIUM

structure in (f) just before rupture -

2oy

18~5 elongation.

Fig. 3. Dislocation structure in (a) just before rupture - 22% elongation. Selected area diffraction pattern across cell wall AB showed that the nlisor~entati~)n was less than z*.

A. WRONSKI,A.FOURDEUX

208

differences. All the material was, in general, free of precipitates and dislocations, but some local scattered precipitates and dislocations generated by prismatic punching12 along were sometimes observed. The precipitates were not numerous or large enough for identification by electron diffraction. A detailed qualitative comparison was made between the deformation structure of specimens (a) and (f) of Fig. I. Dipoles, tangles and eventually a cellular (Fig. 2) or band-like (Fig. 3) structure with the walls along and, less frequently, directions were found to be formed in both (a) and (f). These results are similar to those for a few monocrystineall oils and conventionally annealed material (OY M 15 kg/mmQ) now being studied and also to previous published data of FOURDEUX AND BERGHEZAN~~ and GREGORY et a1.14. Dislocations were never found to be distributed uniformly, but appreciable unloading effects (which were studied by calculating the gradients of the relaxation curves) show that dislocation rearrangements may take place. Traction experiments performed inside the electron microscope yielded results similar to those for less pure material13 and easy cross-slip was observed. No evidence of “solute atom clusters” was ever found. Study of the dislocation density, e, in the body-centred cubic transition metals is hampered by the general inability to determine specimen thickness, t2.15. For e< N 109 cm-Q, however, it is possible to determine16 e by counting the number of intersections of dislocations with both the foil surfaces. Therefore by applying the more general method17 to the same electron micrographs, one can estimate t for lightly deformed specimens. For quantitative study, therefore, a mean value of t, T, was always determined from at least ten electron micrographs for each foil and this was taken to be the thickness of the specimens from the same foil for which t could not be measured. Both the effect of purity and speed of testing, i, on the dislocation density, as a function of strain, E, were investigated (Table II). It is evident that, TABLE THE DEPENDENCE OF DISLOCATION Foil

8

(O/o min-‘)

I 2

I

3 4 5 5 I 2 5 5 I 2 3 3

0.5 I

0.5 500 I I 0.5 500 I

0.5 500

DENSITY OY

(kglmm2) 4.2 9.0 3.4 5.6 4.0 11.6 4.2 9.0 4.1 11.6

4.2 9.0 3.5 II.1

II ON PURITY,

(kl 0.2

STRAIN

STRAIN

RATE

e

(Io-9cm-2)

0.2

o.9zto.4 1.oho.4

I.0

1.0*0.2

I.5 2.5 2.5 3.5 3.5 5.0 5.0 7.0 7.0 8.5 8.5

AND

1.3fo.1 1.g*o.6

fO.7 1.9fo.5 2.1 io.7 2.4hI.O 2.1

5300f1200 4800&I 300 zSgo& 820 2580& 560 3Igof 760 272O~t 590

2.9k1.2 3.7fI.3 3.8f1.3 3.8fI.3 3.OiI.4

within the (substantial) experimental scatter, e is a function of E alone for the purity and strain rate ranges investigated (three orders of magnitude). For each E (2 1%) all the e results were averaged and plotted on a logarithmic scale (Fig. 4). The plot J.

Less-Common

Metals.

7 (1964)

205-21

I

DEFORMATION

OF POLYCRYSTALLINE

209

NIOBIUM

comes out to be roughly linear, which means that @ = c E(%)”

(1)

C and a evaluate to N ro9cm-2 %-I and -0.6, respectively. These results are consistent with HAHN'S model of yielding and deformationl*zr9 based on the dislocation multiplication and velocity characteristics in the material. According to HAHN~~,

where (~0is the stress corresponding to unit velocity, @ an orientation factor (0.5 for tensile strain), b the Burgers vector and Ly (m 0.1 p) the density of mobile dislocations at the lower yield point. The parameter 1z was evaluated to be - 6 by measuring the strain rate dependence of the lower yield stress7. (The relaxation methElongation, E (7~) I-7

Fig. 4. The variation of the dislocation density with strain in “flash-annealed”

niobium,

od”O, however, yields values of] N r7,at CY increasing to about double this value at 7% strain). Taking .LY and n to be 5 . 107 cm-z and 6, respectively, ~0 comes out to be - 9.5 kg/mm2 and the stress needed to produce a dislocation velocity of 10-7 cm SK-1 (taken to be that at which dislocations move and tentatively identified as the elastic limit) evaluates to - 0.9 kg/mm2. This agrees well with our observations that relaxation effects can be detected for stresses in the region of I kg/n-m-~. A new feature of the dislocation configurations in deformed niobium found in this study is the observation that, for the same purity (specimens from the same foil), although Q does not appear to depend on d, the dislocation arrangement does. Figure 5(a) shows the larger density of loops and the tendency to form dense, short walls

210

A. WRONSKI. A. FOURDEUX

(4

(b) Fig. 5. Dislocation

structure resulting from deformation at a strain rate of (a) 500 and (b) 0.5 min-1 after 8.5% elongation. J. Less-Common

Metals,

7 (1964) 205-21 I

DEFORMATION OF POLYCRYSTALLINE at C: = min-1

5oo”/9 min-1 (Fig.

compared

with

the structure

NIOBIUM

at the lower

211 strain rate of 0.5%

5(b)). ACKNOWLEDGEMENTS

This workformspart the general C.E.N.,

direction

of a programme of Dr.

Mol, for carrying

The technical

assistance

of study of the refractory

A. BERGHEZAN.

out the resistivity rendered

by Mrs.

We

metals carriedout

are indebted

to Dr.

and superconductivity

G. HECKMUS,

Mrs.

L.

under

J. NIHOUL

of

measurements. GLAUDE

and

Mr.

J. I
1 P. 2 L.

3 C. -L M. s R.

R. \-. EVANS, J. Less-Common

Metals, 4 (1962) 78. I. VAX TORNE AND G. THOMaS, Acta Met., II (1903) 881. R. TOTTLE, ,J. Inst. Metals, 85 (1956) 375. J. LEADBETTER AND B. B. ARGENT, 1. Less-Common Metals, 3 (1961) 19. T. BEGLEY AND A. I. LEWIS, Columbium Metallurgy, A.L.M.E. Symp., Interscience,Ncm

York, 1961,~. 53. 6 B. A. WILCOX AND K. A. HUGGINS, ,J. Less-Common Metals, z (1960) 292. 7 :\. FOURDEUX AXD A. WRONSKI, J. Less-Common Metals, 6 (1964) II. 8 T. E. ~IITCHELL, R. A. FOXALL AND I'.B. HIRSCH, Phil. Mug.,8 (1903) 1895. 9 A. FOURDEUX AND A. WRONSKI, Acta Met., II (1963) 1270. 1" A. S. WRONSKI AND A. A. JOHNSON, Phil. Msg.; 7 (i962) 213. 11 A. \VRONSKI AND A. FOURDEUX, J. Less-Common Metals, 6 (1964) 413 12 A. FOURDEUX AND A. BERGHEZAN, Compt. Rend., 252 (1961) 1462. 1:~ X. FOURDEUX AND A. BERGHEZAX, 68me Colloque de Me’talluv&e, C.E.N., Saclay, 1962. p. 91; see also Symposium on the Role of Substructure in the &hanical Behaviour ‘of >i&ls, .4.SD-TDR-63-32#~ p. 437. 14 I).P. GREGORY, A. N. STROH AND G. I-1.ROWE, l‘ralzs A.I.M.E.. 227 (1963) 678. 15 ,I. S. KEH, Direct Observation of Tmpevfectzons in Crystals. A.1.ZI.E. Symp., Interscience, 1’; K. Ii. HAM AND N. G. SHARPE, Phil.

Mag., 6 (1961) 1193. 17 R. Ii.HAM, Phil.Afag.,6 (1961) 1183. 18 G. T. HAHN, .4&a Met., IO (1962) 727. 1~ .-\. H. GOTTRELL, Relation betwetw Structure and Strength sf Metals axd .4lloys, Office,Iondon, 1963, p. 596. 21)G. T. HAHN AND A. GILBERT, .~SD-TDR-~~-IO~J, 1962. ,J. Less-Common

Metals,

II. X

Stationery

7 (1964)

205-2

II