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Stress-relaxation and creep behaviour of some rapidly solidified magnesium alloys
Materials Science and Engineering, A134 (1991) 1197-1200
1197
Stress-relaxation and creep behaviour of some rapidly solidified magnesium alloys H. Gjestland Norsk Hydro a.s., Magnesium Material Technology, Porsgrunn (Norway)
G. Nussbaum and G. Regazzoni Pechiney, Centre de Recherches de Voreppe, Voreppe (France)
O. Lohne and I~. Bauger Foundation of Scientific and Industrial Research, Trondheim (Norway)
Abstract Application of rapid solidification to magnesium alloys generally increases the mechanical properties by a factor of 2-2.5 compared with conventional wrought magnesium alloys. This is mainly due to a smaller grain size. What is the creep behaviour of these high strength magnesium alloys? Planar flow cast magnesium alloys have been cast and extruded. The mechanical properties in tension and compression, the stress-relaxation in compression and the tensile creep properties have been tested at temperatures from RT to 150 °C. Traditionally one would expect poor creep resistance in a small grained material. In some rapidly solidified aluminium alloys, however, the creep resistance is good, due to fine particles pinning grain boundaries and impeding dislocation movements. In the magnesium alloys tested there is a big difference in the creep resistance depending on the microstructure of the alloys. The on-going study focuses on the optimal microstructure for good mechanical properties and creep resistance.
1. Introduction Norsk Hydro a.s. and Pechiney Electrometallurgie have, during the last 3 years, collaborated in a program sponsored by The Commission of the European Communities (EURAM program) and The Royal Norwegian Council for Scientific and Industrial Research (NTNF). The objective of the program has been to study the potential of magnesium alloys, emphasizing the mechanical properties and corrosion behaviour when they are produced by rapid solidification (RSP). The study has revealed that application of RSP to magnesium results in a large increase in the tensile and compressive strength [1]. This presentation focuses on the creep behaviour at RT and 150 °C.
typical dimensions of 100 p m thickness and a width of 12-15 mm. A conventional magnesium alloy AZ91 (nominal composition 9 wt.% AI, 0.7 wt.% Zn, 0.15 wt.% Mn) was used as base alloy during the alloy development program. The main compositions for the investigated alloys are listed in Table 1. 2.2. Consolidation The rapidly solidified materials were compacted directly into the extrusion container (75 mm diameter) and extruded with a container TABLE 1 Chemical composition and grain size of the extruded materials
2. Experimental details
Alloy
A1
Zn
2.1. RS-production The alloys have been produced by the planar flow casting method. The RS materials have
AZ91 conv. AZ91 AZ91 +Ca
8.50 8.50 8.40
0.50 0.50 0.60
0921-5093/91/$3.50
Ca
Mg
d (/~m)
2.30
bal. bal. bal.
11.6 1.5 0.6
© Elsevier Sequoia/Printed in The Netherlands
1198 TABLE 2 Container temperature and temperature on the surface of the profiles at the outlet of the die Alloy
Container temperature
Exit temperature
AZ91 cony. AZ91 AZ91 + C a
300 300 300
377 338 371
(oc)
x 36 ram. Owing to limitations in the measuring equipment only the secondary creep strains are shown. A line fit of the results is presented where the slope of the lines expresses the creep rate.
(oc)
temperature of 300 °C and a speed of 1.2 m min t, into 12 mm diameter bars. The profile temperature was measured directly on the extrusion surface at the outlet of the die (Table 2).
2.3. Testing Tensile properties have been measured according to ISO 6892 Standard. The compression tests have been carried out on specimens 12 mm dia.× 24 mm long. The stress relaxation testing has been carried out in compression on specimens of dimension 12 mm dia.× 5 mm long. In tests at elevated temperature an initial stress of 80 MPa has been applied to all specimens. In tests at room temperature an initial stress equal to two-thirds of the tensile yield strength of the alloy has been used. The creep testing has been a traditional tensile creep test with constant applied load. The specimens have a parallel testing area of 6.4 mm
AZ91 cony.
Fig. 1. Micrographs showing the "as extruded" structures.
3. Results
Micrographs from the metallographic investigations are shown in Fig. 1. The grain refining effect of the RSP can be seen by comparing the grain size of the extruded conventional and RSAZ91 (Table 1). The main Precipitates are identified by X-ray diffraction to be Mg17Al12 in the AZ91 alloys and A1ECa in the R S - A Z 9 1 + C a alloy. The results from the mechanical testing are shown in Fig. 2. A considerable increase in both yield strengths and tensile strength are observed in the RSP materials compared with the conventional AZ91 alloy. The increase is mainly a grain size effect. The tensile yield strengths are plotted in Fig. 3 with the grain size giving a traditional Hall-Petch relation. It should be noted that the compressive yield strengths in the RSP materials are equal to or higher than the tensile yield strengths. In wrought coarse grained magnesium alloys the compressive yield strength can be as low as 60-70% of the tensile yield strength. The compressive strength is greatly increased with reduced grain size. During
AZ91 RS
AZ91 +Ca
1199
~ ]
TYS RT TYS 150"C
100~
CYS RT
~ AZgl cony. RT
ii
960 80 70
0
.
~
. .
. .
. .
IAZgl i AZtH~RS CaRT RSRT
. .
~ AZgl~ca RS ~50'C
TS RT
TS 1500C
~E E
500
450
40 30 20
AZgl (onv. 150 'C
l0
400
0
350
AZ91
0
~
10
~ 20 30
~
40
~
50
60
--770 80
RSISW(
'
"
911
100
Time (h)
300
Fig. 4. S t r e s s - r e l a x a t i o n at R T a n d 150 °C.
, ~ 250 200 150
5O 0
, AZ91 RS ~dd~ ]-.5~m ! ~=5.1xl0~s , I
4.0 3.5
100
-
3.0
AZ9I conv.
AZ91 RS
AZ91 c o n y . d
= 2.5
AZ91+Ca RS
I 1 1
11.6 I~m
[ ~ ~ 5.9x10%~ =
.-
Fig. 2. T e n s i l e a n d c o m p r e s s i v e s t r e n g t h s at R T a n d 150 °C.
,0
500
o
4so
/y
. . . . . . .
o.o
g 400
0
•= 350
10
20
AZ91 RS
g 3oo
30
40
50 60 Time (h)
70
80
~----~ 90 100
Fig. 5. S e c o n d a r y c r e e p strain at 150 ° C / 5 0 MPa.
"~ 250 "~ '~. 200
¢~ 150,
TABLE 3
y. too
S e c o n d a r y c r e e p rates at R T / t w o - t h i r d s of TYS a n d 151) °C/ 50 M P a
50
o 0.0
I r 0.5 1.0 1/[SQR (grain size)] [I/SQR (~m)]
Fig. 3. T e n s i l e y i e l d H a l l - Peteh relation.
strength
vs. g r a i n
size
1.5
showing
Alloy
S e c o n d a r y c r e e p rate is ~) 150 ° C / 5 0 M P a
R T / t w o - ~ h i r d s TYS
58.9 x 1 (1 '~ 5060.0×10 '~ 1 1 . 2 × 1 0 '~
0 . 8 0 × 10 " 3.34×10 " 1 . 8 ( 1 × 1 0 '~
a
processing of the material it is therefore important to reduce grain growth to obtain high strengths both in tension and compression. This is effectively done in the calcium containing alloy where the small stable A12Ca particles are pinning the grain boundaries making the alloy less sensitive for exposure to elevated temperatures. This effect can be seen by comparing the grain sizes from Table 1 and the exit temperatures of the extrusions, Table 2. The tensile properties have also been tested at 150 °C (Fig. 2). A dramatic drop in the strength, especially in the RS-AZ91 alloy, is observed. The pinning effect of the Al2Ca particles are, however, still active at 150 °C giving the RS-AZ91 + C a alloy a relatively high strength in a conventional tensile test.
A Z 9 1 con,,'. A Z 9 1 RS A Z 9 1 + C a RS
The materials were tested in stress-relaxation and tensile creep at 150 °C. The results are shown in Fig. 4 and Fig. 5 and Table 3. From Fig. 4 it is seen that the conventional alloy retains approx. 40% of the initial applied stress after a 100 h stress-relaxation test, while the rapidly solidified AZ91 alloy loses most of the stress within 1-2 h. The same poor creep strength can be seen in Fig. 5 and Table 3 where the secondary creep strains are plotted vs. time. At 15{) "C and a stress of 50 MPa the secondary creep rate of the RS-AZ91 alloy is almost 100 times greater than the creep rate of the conventional AZ91. As the metallographic investigations show that the RS material has about 10 times smaller grain size d, the difference in creep properties is as expected
1200 0.14 0.12 0.10
"-~ 0.08 0.06
LJ 0.04 0.02 0.00 0
10
20
30
40
50 60 Time (h)
70
80
90
100
Fig. 6. Secondary creep strain at RT/two-thirds TYS.
from the well-known g - d -2 relationship [2]. The particles in the two AZ91 alloys are coarse ( - 0 . 5 /am) and have most likely little effect on the creep behaviour. The addition of 2.3 wt.% Ca has a great effect on the secondary creep rate. Although the alloy RS-AZ91 + Ca has a smaller grain size and hence from the above could be expected to show poor creep properties, the results show a secondary creep rate being 400 times smaller than the creep rate of RS-AZ91 and 5 times smaller than the conventional AZ91. Pole figures of the extruded materials do not look very different. Therefore the change in the creep rate by adding calcium is not due to another texture. X-ray diffraction and TEM micrographs (Fig. 1) show, however, that in RSAZ91 + Ca there is an even distribution of fine A12Ca particles, about 10 u particles mm -3, with a mean diameter of about 0.05 pm. These particles are very stable at 150 °C and pin the grain boundaries and impede dislocation motion in the interior of the grains [3].
Stress-relaxation and creep tests were also carried out at RT. In both these tests a stress equal to two-thirds of the tensile yield strength has been used (Figs. 4 and 6). The results indicate that there is only a minor difference in the relaxation and creep behaviour of the alloys at RT. The extreme change in creep behaviour at elevated temperatures is shown by comparing the results from RT and 150 °C confirm that the RS-AZ91 alloy may be very sensitive to exposure to temperatures above RT. 4. Conclusions
(1) Application of RSP to the AZ91 alloy gives a considerable increase in the tensile and compressive strengths of the alloy compared with conventionally extruded material. This is due to a refinement of the grain structure. (2) Refinement of the grain structure and the absence of stable dispersoids result in poor creep resistance of the alloy at elevated temperatures. (3) By the addition of calcium a high density of small stable dispersoids may be formed. The dispersoids pin the grain boundaries and impede the dislocation movement and increase the creep resistance. References 1 G. Nussbaum, H. Gjestland and G. Regazzoni, Rapid Solidification of Magnesium Alloys, 45th Int. Magnesium Association Annual Meeting, Washington, 1988. 2 R. W. Evans and B. Wilshire, Creep of Metals andAlloys, The Institute of Metals, London, 1985, p. 14. 3 J. E. Harris, Z Metals Sci., 7(1973) 1.
1197
Stress-relaxation and creep behaviour of some rapidly solidified magnesium alloys H. Gjestland Norsk Hydro a.s., Magnesium Material Technology, Porsgrunn (Norway)
G. Nussbaum and G. Regazzoni Pechiney, Centre de Recherches de Voreppe, Voreppe (France)
O. Lohne and I~. Bauger Foundation of Scientific and Industrial Research, Trondheim (Norway)
Abstract Application of rapid solidification to magnesium alloys generally increases the mechanical properties by a factor of 2-2.5 compared with conventional wrought magnesium alloys. This is mainly due to a smaller grain size. What is the creep behaviour of these high strength magnesium alloys? Planar flow cast magnesium alloys have been cast and extruded. The mechanical properties in tension and compression, the stress-relaxation in compression and the tensile creep properties have been tested at temperatures from RT to 150 °C. Traditionally one would expect poor creep resistance in a small grained material. In some rapidly solidified aluminium alloys, however, the creep resistance is good, due to fine particles pinning grain boundaries and impeding dislocation movements. In the magnesium alloys tested there is a big difference in the creep resistance depending on the microstructure of the alloys. The on-going study focuses on the optimal microstructure for good mechanical properties and creep resistance.
1. Introduction Norsk Hydro a.s. and Pechiney Electrometallurgie have, during the last 3 years, collaborated in a program sponsored by The Commission of the European Communities (EURAM program) and The Royal Norwegian Council for Scientific and Industrial Research (NTNF). The objective of the program has been to study the potential of magnesium alloys, emphasizing the mechanical properties and corrosion behaviour when they are produced by rapid solidification (RSP). The study has revealed that application of RSP to magnesium results in a large increase in the tensile and compressive strength [1]. This presentation focuses on the creep behaviour at RT and 150 °C.
typical dimensions of 100 p m thickness and a width of 12-15 mm. A conventional magnesium alloy AZ91 (nominal composition 9 wt.% AI, 0.7 wt.% Zn, 0.15 wt.% Mn) was used as base alloy during the alloy development program. The main compositions for the investigated alloys are listed in Table 1. 2.2. Consolidation The rapidly solidified materials were compacted directly into the extrusion container (75 mm diameter) and extruded with a container TABLE 1 Chemical composition and grain size of the extruded materials
2. Experimental details
Alloy
A1
Zn
2.1. RS-production The alloys have been produced by the planar flow casting method. The RS materials have
AZ91 conv. AZ91 AZ91 +Ca
8.50 8.50 8.40
0.50 0.50 0.60
0921-5093/91/$3.50
Ca
Mg
d (/~m)
2.30
bal. bal. bal.
11.6 1.5 0.6
© Elsevier Sequoia/Printed in The Netherlands
1198 TABLE 2 Container temperature and temperature on the surface of the profiles at the outlet of the die Alloy
Container temperature
Exit temperature
AZ91 cony. AZ91 AZ91 + C a
300 300 300
377 338 371
(oc)
x 36 ram. Owing to limitations in the measuring equipment only the secondary creep strains are shown. A line fit of the results is presented where the slope of the lines expresses the creep rate.
(oc)
temperature of 300 °C and a speed of 1.2 m min t, into 12 mm diameter bars. The profile temperature was measured directly on the extrusion surface at the outlet of the die (Table 2).
2.3. Testing Tensile properties have been measured according to ISO 6892 Standard. The compression tests have been carried out on specimens 12 mm dia.× 24 mm long. The stress relaxation testing has been carried out in compression on specimens of dimension 12 mm dia.× 5 mm long. In tests at elevated temperature an initial stress of 80 MPa has been applied to all specimens. In tests at room temperature an initial stress equal to two-thirds of the tensile yield strength of the alloy has been used. The creep testing has been a traditional tensile creep test with constant applied load. The specimens have a parallel testing area of 6.4 mm
AZ91 cony.
Fig. 1. Micrographs showing the "as extruded" structures.
3. Results
Micrographs from the metallographic investigations are shown in Fig. 1. The grain refining effect of the RSP can be seen by comparing the grain size of the extruded conventional and RSAZ91 (Table 1). The main Precipitates are identified by X-ray diffraction to be Mg17Al12 in the AZ91 alloys and A1ECa in the R S - A Z 9 1 + C a alloy. The results from the mechanical testing are shown in Fig. 2. A considerable increase in both yield strengths and tensile strength are observed in the RSP materials compared with the conventional AZ91 alloy. The increase is mainly a grain size effect. The tensile yield strengths are plotted in Fig. 3 with the grain size giving a traditional Hall-Petch relation. It should be noted that the compressive yield strengths in the RSP materials are equal to or higher than the tensile yield strengths. In wrought coarse grained magnesium alloys the compressive yield strength can be as low as 60-70% of the tensile yield strength. The compressive strength is greatly increased with reduced grain size. During
AZ91 RS
AZ91 +Ca
1199
~ ]
TYS RT TYS 150"C
100~
CYS RT
~ AZgl cony. RT
ii
960 80 70
0
.
~
. .
. .
. .
IAZgl i AZtH~RS CaRT RSRT
. .
~ AZgl~ca RS ~50'C
TS RT
TS 1500C
~E E
500
450
40 30 20
AZgl (onv. 150 'C
l0
400
0
350
AZ91
0
~
10
~ 20 30
~
40
~
50
60
--770 80
RSISW(
'
"
911
100
Time (h)
300
Fig. 4. S t r e s s - r e l a x a t i o n at R T a n d 150 °C.
, ~ 250 200 150
5O 0
, AZ91 RS ~dd~ ]-.5~m ! ~=5.1xl0~s , I
4.0 3.5
100
-
3.0
AZ9I conv.
AZ91 RS
AZ91 c o n y . d
= 2.5
AZ91+Ca RS
I 1 1
11.6 I~m
[ ~ ~ 5.9x10%~ =
.-
Fig. 2. T e n s i l e a n d c o m p r e s s i v e s t r e n g t h s at R T a n d 150 °C.
,0
500
o
4so
/y
. . . . . . .
o.o
g 400
0
•= 350
10
20
AZ91 RS
g 3oo
30
40
50 60 Time (h)
70
80
~----~ 90 100
Fig. 5. S e c o n d a r y c r e e p strain at 150 ° C / 5 0 MPa.
"~ 250 "~ '~. 200
¢~ 150,
TABLE 3
y. too
S e c o n d a r y c r e e p rates at R T / t w o - t h i r d s of TYS a n d 151) °C/ 50 M P a
50
o 0.0
I r 0.5 1.0 1/[SQR (grain size)] [I/SQR (~m)]
Fig. 3. T e n s i l e y i e l d H a l l - Peteh relation.
strength
vs. g r a i n
size
1.5
showing
Alloy
S e c o n d a r y c r e e p rate is ~) 150 ° C / 5 0 M P a
R T / t w o - ~ h i r d s TYS
58.9 x 1 (1 '~ 5060.0×10 '~ 1 1 . 2 × 1 0 '~
0 . 8 0 × 10 " 3.34×10 " 1 . 8 ( 1 × 1 0 '~
a
processing of the material it is therefore important to reduce grain growth to obtain high strengths both in tension and compression. This is effectively done in the calcium containing alloy where the small stable A12Ca particles are pinning the grain boundaries making the alloy less sensitive for exposure to elevated temperatures. This effect can be seen by comparing the grain sizes from Table 1 and the exit temperatures of the extrusions, Table 2. The tensile properties have also been tested at 150 °C (Fig. 2). A dramatic drop in the strength, especially in the RS-AZ91 alloy, is observed. The pinning effect of the Al2Ca particles are, however, still active at 150 °C giving the RS-AZ91 + C a alloy a relatively high strength in a conventional tensile test.
A Z 9 1 con,,'. A Z 9 1 RS A Z 9 1 + C a RS
The materials were tested in stress-relaxation and tensile creep at 150 °C. The results are shown in Fig. 4 and Fig. 5 and Table 3. From Fig. 4 it is seen that the conventional alloy retains approx. 40% of the initial applied stress after a 100 h stress-relaxation test, while the rapidly solidified AZ91 alloy loses most of the stress within 1-2 h. The same poor creep strength can be seen in Fig. 5 and Table 3 where the secondary creep strains are plotted vs. time. At 15{) "C and a stress of 50 MPa the secondary creep rate of the RS-AZ91 alloy is almost 100 times greater than the creep rate of the conventional AZ91. As the metallographic investigations show that the RS material has about 10 times smaller grain size d, the difference in creep properties is as expected
1200 0.14 0.12 0.10
"-~ 0.08 0.06
LJ 0.04 0.02 0.00 0
10
20
30
40
50 60 Time (h)
70
80
90
100
Fig. 6. Secondary creep strain at RT/two-thirds TYS.
from the well-known g - d -2 relationship [2]. The particles in the two AZ91 alloys are coarse ( - 0 . 5 /am) and have most likely little effect on the creep behaviour. The addition of 2.3 wt.% Ca has a great effect on the secondary creep rate. Although the alloy RS-AZ91 + Ca has a smaller grain size and hence from the above could be expected to show poor creep properties, the results show a secondary creep rate being 400 times smaller than the creep rate of RS-AZ91 and 5 times smaller than the conventional AZ91. Pole figures of the extruded materials do not look very different. Therefore the change in the creep rate by adding calcium is not due to another texture. X-ray diffraction and TEM micrographs (Fig. 1) show, however, that in RSAZ91 + Ca there is an even distribution of fine A12Ca particles, about 10 u particles mm -3, with a mean diameter of about 0.05 pm. These particles are very stable at 150 °C and pin the grain boundaries and impede dislocation motion in the interior of the grains [3].
Stress-relaxation and creep tests were also carried out at RT. In both these tests a stress equal to two-thirds of the tensile yield strength has been used (Figs. 4 and 6). The results indicate that there is only a minor difference in the relaxation and creep behaviour of the alloys at RT. The extreme change in creep behaviour at elevated temperatures is shown by comparing the results from RT and 150 °C confirm that the RS-AZ91 alloy may be very sensitive to exposure to temperatures above RT. 4. Conclusions
(1) Application of RSP to the AZ91 alloy gives a considerable increase in the tensile and compressive strengths of the alloy compared with conventionally extruded material. This is due to a refinement of the grain structure. (2) Refinement of the grain structure and the absence of stable dispersoids result in poor creep resistance of the alloy at elevated temperatures. (3) By the addition of calcium a high density of small stable dispersoids may be formed. The dispersoids pin the grain boundaries and impede the dislocation movement and increase the creep resistance. References 1 G. Nussbaum, H. Gjestland and G. Regazzoni, Rapid Solidification of Magnesium Alloys, 45th Int. Magnesium Association Annual Meeting, Washington, 1988. 2 R. W. Evans and B. Wilshire, Creep of Metals andAlloys, The Institute of Metals, London, 1985, p. 14. 3 J. E. Harris, Z Metals Sci., 7(1973) 1.