Creep tests and stress levels in determining mechanical behavior of polycrystalline nonmetallic materials at elevated temperature

Burke, P.

Sherby, O. D.

English

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NASA RESEARCH GRANT SC-NGR-05-020-084 
MECHANICAL BEHAVIOR OF POLYCRYSTALLINE 
NON-METALLICS AT IZLEVATED TEMPERATURE 
for the period 
October 1, 1965 through March 31, 1966 
submitted to 
Office of Grants and Research Contracts 
Attention: Code SC 
National Aeronautics and Space Administration 
Washington, D. C. 20546 
submitted by 
Department of Materials Science 
Stanford University 
Stanford, California 
Professor Oleg D. Sherby, Principal Investigator 
Mr. Peter Burke, Research Assistant 
m (THRUI 
I UCCESSION NUMBER) 
16 I 
E (PAOESI 
2 ; Cp-75-64 
(NASA OR OR TMX bR AD NUMBER) 
https://ntrs.nasa.gov/search.jsp?R=19660018661 2020-03-16T19:35:36+00:00Z
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MECHANICAL BEHAVIOR OF POLYCRYSTALLINE 
AT ELEVATED TEMPERATURES 
(SC-NGR-05-020-084) 
NON-METALLICS 
Summary of Research f o r  t h e  period October 1, 1965 Through March 31.  1966 
During t h e  l a s t  s i x  months, r e sea rch  e f f o r t s  h a v e b e e n  concent ra ted  
p r imar i ly  on c reep  of sodiuc? ch lor ide .  P o l y c r y s t a l l i n e  samples have been 
made by ex t ruding  s i n g l e  c r y s t a l s .  These samples have been t e s t e d  i n  
compression c reep  a t  temperatures of 512 and 740 C and stress l e v e l s  vary- 
i n g  from 38 t o  850 p s i .  The r e s u l t s  of t h e s e  tests i n d i c a t e  t h a t  t he  
s t eady  s ta te  creep  r a t e ,  i ,  can b e  expressed by a n  equat ion  which has  
been used t o  desc r ibe  d i s l o c a t i o n  climb and g l i d e  c o n t r o l l e d  creep i n  
me ta l s ,  namely, 
0 
-Qc/RT n d = A e  CT 
where Qc - 73 Kcal/mole = 3.2eV = t h e  a c t i v a t i o n  energy f o r  c r e e p ,  
n 7 = t h e  s t r e s s  exponent, 
R = gas cons tan t ,  
T = abso lu te  temperature 
and a = creep  stress.  
For  metals,  n =  5 ,  and Qc equals  t h e  a c t i v a t i o n  energy f o r  d i f f u s i o n ,  
Qd, p l u s  f a c t o r s  dependent upon s t ack ing  f a u l t  energy and e l a s t i c  modulus 
changes wi th  temperature  according t o  t h e  fol lowing equat ion  (7)  ; 
d In  E d In Y 
Qd = Qc - nR d 1/T - 15R d 1 / T  
where E = e l a s t i c  modulus 
and y = s t ack ing  f a u l t  energy 
A value  f o r  Qd f o r  sodium c h l o r i d e  can  be  c a l c u l a t e d  from t h e  above 
equat ion  by us ing  e l a s t i c  modulus va lues  ex t r apo la t ed  from a v a i l a b l e  
d a t a  (9) and assuming t h a t  the change i n  s t ack ing  f a u l t  energy wi th  
temperature  i s  n e g l i g i b l e .  The c a l c u l a t e d  va lue  of Q d e 6 1  Kcal/mole 
ag rees  f a i r l y  w e l l  wi th  observed a c t i v a t i o n  ene rg ie s  f o r  c h l o r i n e  
d i f f u s i o n  i n  sodium c h l o r i d e ,  which range from 45 t o  54 Kcal/mole (1,2). 
The a c t i v a t i o n  energy f o r  sodium d i f f u s i o n  i n  sodium c h l o r i d e  i s  37 
Kcal/mole (1). The d i f f e rence  between t h e  observed and c a l c u l a t e d  
va lues  of Qd could be  a real  d i f f e r e n c e  due t o  d i f f e r e n c e s  i n  t h e  
compositions and h i s t o r i e s  of t h e  c reep  and d i f f u s i o n  samples, o r  i t  
1. 
. 
. 
could be due t o  experimental  e r r o r ,  e r r o r  i n  t h e  ex t r apo la t ed  modulus 
va lues ,  changes i n  s t ack ing  f a u l t  energy wi th  temperature ,  o r  e f f e c t s  
due t o  g r a i n  growth and g r a i n  s i z e  d i f f e r e n c e s  between the creep  samples. 
Micros t ruc tures  developing dur ing  c reep  of sodium c h l o r i d e  w e r e  
found t o  be  s imilar  t o  those  developing i n  metals undergoing creep  
deformation. I n  both cases  sabgrains  form during c reep ,  and t h e  
subgra in  s i z e  i s  i n v e r s e l y  p ropor t iona l  t o  t h e  c reep  stress. Grain 
growth occurred,  bu t  f o r  metals it has  been found t h a t  t h e  g r a i n  s i z e  
has l i t t l e  e f f e c t  on e i t h e r  the subgra in  s i z e  o r  t h e  c reep  r a t e  (10). 
From t h e  above r e s u l t s  i t  seems l i k e l y  t h a t  c reep  i n  sodium 
c h l o r i d e  i s  s i m i l a r  t o  c reep  i n  pure  meta ls  from both a k i n e t i c  and 
a m i c r o s t r u c t u r a l  po in t  of view. 
Creep t e s t i n g  of aluminum oxide has  no t  begun, b u t  it i s  hoped 
t h a t  the equipment f o r  t h e s e  t e s t s  w i l l  be  ope ra t ive  i n  about t h r e e  
months. 
( > 10-3sec-1> creep  t e s t i n g  of sodium ch lo r ide .  
The equipment w i l l  a l s o  be used f o r  h igh  s t r a i n  ra te  
The p r o j e c t  now has  a labora tory  of i t s  own t h a t  has  become 
a v a i l a b l e  a s  t h e  materials sc ience  f a c i l i t i e s  of S tanford  have grown. 
Most of t h e  equipment p e r t i n e n t  t o  the p r o j e c t  h a s  been moved i n t o  i t .  
The l abora to ry  i s  w e l l  equipped wi th  power and water, inc luding  a 
30 KVA t ransformer ,  which w i l l  b e  necessary  f o r  h e a t i n g  a s i l i c o n  
c a r b i d e  furnace  t o  be  used f o r  aluminum oxide t e s t i n g .  
A r e p o r t  i s  c u r r e n t l y  under p repa ra t ion  by Oleg D. Sherby and 
P e t e r  M. Burke on t h e  e leva ted  temperature  behavior  of p o l y c r y s t a l l i n e  
m e t a l s  and non-metal l ics .  This r e p o r t  w i l l  c r i t i c a l l y  review t h e  
f a c t o r s  i n f luenc ing  creep  behavior of c r y s t a l l i n e  s o l i d s  a t  e l eva ted  
temperature .  Spec ia l  emphasis w i l l  be  placed on a comparison of t h e  
d i s s i m i l a r i t i e s  and s i m i l a r i t i e s  of c reep  f o r  metal l ics  and non- 
m e t a l l i c s .  
and should be completed by the end of June. 
C r e e D  Tes t ing  of Sodium Chloride 
Sample P repa ra t ion  
The paper w i l l  be submit ted as a t e c h n i c a l  r e p o r t  t o  NASA 
P o l y c r y s t a l l i n e  samples  of 0.003 nun g r a i n  s i z e  have been made by 
ex t ruding  a s i n g l e  c r y s t a l  grown from reagent  grade sodium ch lo r ide .  
2 .  
I . 
The extrusion dies are made of hardened steel. The reduction ratio is 
16:1, and the sample diameter is 1/4 inch. 
ranged from 250 to 35OoC, and extrusion forces ranged from 17,000 to 
35,000 pounds. 
Extrusion temperatures 
The extruded samples were glued to a piece of lucite for support 
and sawed approximately to length with a jeweler's saw. The glue and 
plastic were dissolved in acetone. 
glued to a metal jig and polished so that the ends were flat and 
parallel and normal to the sample cylindrical axis. 
The rough cut samples were then 
Density was determined by measuring the sample dimensions and 
weight and calculating the density. 
of theoretical density. 
CreeD Testing 
Densities measured were 98 to 99% 
Creep tests were conducted using a dead load compression creep 
unit. The sample length was measured by measuring the displacement 
of the load ram with two dial gages and correcting for the bending of 
the apparatus caused by the applied load. 
constant stress on the sample, the load was adjusted at 1% intervals 
of engineering strain by adding predetermined load increments. 
In order to maintain a 
Many of the tests were conducted at two stress levels, a technique 
first used in determing the stress dependence of creep in pure aluminum 
(8). 
determined. Then the stress was changed and maintained until the 
steady state creep rate was determined for the second stress. A 
representative creep curve is shown in figure 1. The tests were 
conducted in an atmosphere of dry helium. 
Microstructure Analysis 
The stress was applied until the steady state creep rate was 
The sodium chloride samples were polished by rubbing them first 
on wet velvet or silk and then on tracing paper smeared with one micron 
diamond paste. The samples were etched with glacial acetic acid plus 
ferric chloride. A sample showing both grains and subgrains is shown 
in figure 2. 
3. 
Grain sizes were determined by a straight line intercept technique 
through the microscope. Subgrain sizes were determined with a closed 
ring intercept technique applied to a photograph of each sample. 
Summary of Test Results 
Stress, temperature, creep rates, grain sizes and subgrain sizes -_ 
are presented in Table I. 
log stress is shown in figure 3.  
determined from this plot to be 73.5 Kcal/mole at 160 psi. 
stress exponent determined from this plot was 7.0, while that determined 
from individual stress-change tests was 6.9. 
A plot of log steady state creep rate versus 
The activation energy for creep was 
The average 
Large amounts of grain growth occurred during the creep test 
0 
procedure. 
and 1-3 mm for the 740 C tests. 
curves, however, implies that the bulk of the grain growth had occurred 
while the samples were coming to temperature and before the actual creep 
testing was started. No abrupt changes in creep rate, such as those 
connected with abnormal grain growth during creep, were observed. 
seems possible that grain size has a small effect on creep rate in this 
case. In metals creeping by dislocation climb mechanism, the creep 
rate is related to the subgrain size, rather than the grain size. 
Subgrain size as a function of stress for NaCl and several metals is 
shown in figure 4. 
The final grain sizes were about 0 . 3  mm for 512 C tests, 
0 The regular behavior of the creep 
It 
4. 
I 
c 
. .  . 
TABLE I. Steady s t a t e  creep r a t e ,  stress exponent,  g r a i n  s i z e  and subgra in  
s i z e  as a func t ion  of stress and temperature.  
Test  
No. 
4 
6 
7 
10 
11 
Sample Temp. 
No. 0 
2.1.2 512 
2.1.6 512 
2.1.1 512 
2.1.3 740 
2.1.7 740 
S t r e s s  Creep Rate Stress F ina l  Grain Subgrain 
( p s i )  (sec-1) Exponent, n S i z e ,  p S i z e ,  ~1 
365 8.1 x loe6 7.3 
305 2.2 x 10-6 300 76 
1200-+800 -2.5 x 
(850 avgJ 
700 20 
7.6 470 1.8 
200 2.8 x 500 61 
2000 138 
4.8 140 1.9 70 7.0 x 
-8  
3000 8.0 
38.5 2 . 2 . x  10 
133 4.8 10-4 - 
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80 160 240 320 400 480 560 648 
TIME, min. 
Figure 1. Representative creep curve for polycrystalline 
sodium chloride tested at 512OC. 
6 .  
I -  
- -  . 
0 Figure  2. Micros t ruc ture  of sodium c h l o r i d e  t e s t e d  a t  740 C 
and 70 p s i  f i n a l  stress. 
7. 
I . -  
* .  
" 
/ 
51OOC I 
i = 7.0 
-X- POLYCRYSTALLINE 
SINTERED CUHWT, 
Sop GRAIN SIZE, 
0. I %AI203 AOOEO 
i / COMp_REsSloN TESTS 
SINGLE CRYSTALS 
I BEND TESTS 
( ) NUMBERS REFEa l3 
I I I I REFERENCES I 
40 roo 200 400 Kx)O 
COMPRESSIVE STRESS NORM& TO SAMPLE CROSS SECTION, psi 
Figure 3. Plot of log steady state creep rate versus log stress 
€or single and polycrystalline sodium chloride. 
8 .  
. -  . . 
I o3 
I o2 
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IO 
lo" 
IO' 
IO IO2 
0 99.9(95%Al 
99.95% AI (+ .OS%Fe) 
A NICKEL 
El a - F e  
V COPPER 
X NaCl 
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(T, CREEP STRESS, psi 
Figure 4. Influence of creep stress on the subgrain size 
developed during steady state creep €or several 
pure polycrystalline metals (10, 11, 12, 13) and 
sodium chloride. 
9 .  
I 0' 
REFERENCES 
1. J. F. Laurent  and J. Benard, J. Phys. Chem. So l ids  2, 218 (1958). 
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624 (1965). 
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- 5, 1900 (1963); t r a n s l a t i o n  i n  Sov. Phys.-Sol. S t a t e  5 1387 (1964). 
6. R. W. Chr i s ty ,  Acta M e t  4, 443 (1956). 
7. C. R. B a r r e t t  and 0. D. Sherby, Trans.  AIME 23, 1116 (1965). 
8. a )  0. D. Sherby, et a l ,  Trans. AIME 200, 275 (1954). 
b) 0. D. Sherby, T. A. Trozera and J. E. Dorn, Proc. ASTM56, (1956). 
c)  H. Laks e t  a l ,  AaSoc .  Mech. Eng. Trans. S e r i e s  E 79 (J. Appl. 
Mech. 241, 207 (1957). 
9. S ,  V. Subrahmanyam, Acustica 12, 37 (1962). 
10. C. R. Barrett ,  J, L. Lytton and 0. D. Sherby, Trans. AIME, t o  be 
published.  
11. A. G. Goldberg, J. I r o n  and S t e e l  I n s t .  204, 268 (1966). 
12. R. W. Guard i n  C r e e D  and Recbvery, pp. 251-254. h e r .  SOC. f o r  
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Mechanical Behavior of Polycrystalline Non- Metallics at Elevated Temperature Progress Report, 1 Oct. 1965 - 31 Mar. 1966

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