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Effect of quenching on the low stress creep behaviour of alpha-titanium
Scripta M E T A L L U R G I C A
EFFECT OF QUENCHIHG
Department
Vol. 15, pp. 1083-1086, 1981 Printed in the U.S.A.
OH 'THE LOt J STRESS CREEP BE}~VIOUR
Pergamon Press Ltd. All rights reserved
OF ALPILA-TIT;~IIUM
G . M a l a k o n d a i a h and P.Rama Rao of M e t a l l u r g i c a l Engineering, Banaras }{indu University, Varanasi - 221 005, India (Received June 8, 1981)
Introduction Substantial experimental evidence (1-7) gathered in recent years through work on p o l y c r y s t a l l i n e metals, ceramics and alloys has e s t a b l i s h e d that at low stresses ( < I 0 - 5 G where G is the shear modulus) and e l e v a t e d temperatures (• 0.4T m where T m is the m e l t i n g temperature (Kelvin)) viscous creep of one of three kinds, namely N a b a r r o - H e r r i n g (N-H) (8,9), Coble (i0) or H a r p e r - D o r n (H-D) (ii) may prevail. A microstructural v a r i a b l e p r e d o m i n a n t l y i n v e s t i g a t e d in all of these studies is the grain size since N-H and Coble theories of diffusional creep p r e d i c t creep rates to be inversely proportional to square and cube of the grain size while H-D creep observed in coarse grained m a t e r i a l s has been found to be independent of grain size (11,12). A recent study on a l p h a - t i t a n i u m by the p r e s e n t authors (6) was aimed at c o m p r e h e n s i v e l y c h a r a c t e r i s i n g N-H, Coble and H-D creep in this metal. In v i e w of the fact that there are hardly any reports on the influence of prior t r e a t m e n t on the low stress creep of metals, we e x a m i n e d the influence of solid state quenching, not involving any phase change, on creep b e h a v l o u r of a l p h a - t l t a n i u m at low stresses and d]is b r i e f report p r e s e n t s the interesting observations. Exp@rimental Titanium, p r o c u r e d from Titanium Metal and Alloys Limited, London in the form of wires of length 2 m and d i a m e t e r 1500 pm, contained, in ppm, <25 AI, 200 C, < I0 Co, < 5 0 Cr, 47 Cu, 480 Fe, 140 Mg, < i0 Mn, 81 N, <25 Ni, 2600 O, < 25 Pb, < 25 Sb, 48 Si, 400 Sn, < 25 V and < I 0 0 0 Zr. To m e a s u r e low strain rates ( < 10 -9 sec -I) at I000 K and stresses less than 2.0 M N / m 2 a high sensitive spring specimen geometry (2,13) has been employed. The e x p e r i m e n t a l set-up as well as the c o n c e r n e d formulae for the calculation of stresses and strain rates from the m e a s u r e d average loads and coil deflection rates have been d e s c r i b e d in detail p r e v i o u s l y (14). Specimens of grain sizes ( L, length of mean linear intercept ) 72 and 240 ~ m were used. A grain size of 72 ~ was obtained through p r e a n n e a l i n g (I000 K/~2h), intermediate reduction (25~ reduction in cross-sectional area by wire drawing) and a final heating at 1123 K for 1/3 h. On the other hand, a grain size of 240 ~m resulted when the a s - r e c e i v e d material was heated to 1123 K for I h. In both the cases, spring specimens on a mandrel were subjected to oil q u e n c h i n g from 1123 K, after the specified soaking time, using anAdamel Lhomarciy %~-02 v a c u u m (better than 10 -3 Pa) q u e n c h i n g furnace.
1083 0036/9748/81/101083-04502.00/0 Copyright (c) 1981 Pergamon Press Ltd.
1084
CREEP OF ALPHA-TITANIUM
Vol,
15, No. i0
TO reveal the microstructure, mechanically polished sections were etched in a solution containing 10% HF (48%), 5% ~IO 3 (70%) and 85% H20 (distilled) for 3 to 6 seconds. Specimens quenched from 1123 K were found to retain equiaxed grain structure. The grain size was uniform over the entire crosssection and was measured by the linear intercept method. Spring specimens were crept at I000 K under their own weight in a manner described elsewhere (6,14). Grain size measurement both before and after the test revealed no grain coarsening during the course of testing. Results a~d Discussion Strain rate ( ~ ) ve~s~s stress ( ~ ) data for the test at I000 ]" using the oil quenched specimen of grain size 72 pm are presented in Fig.l. T o bring out the effect of quenching, included in Fig.l are strain rate versus stress data for a furnace cooled sample of nearly identical grain size ( L = 69 ~m ) and tested at I000 K, taken from our earlier work (6). Fig.2 presents the strain rate versus stress data for the coarse grained oll quenched sample ( L = 240 ~m ) tested at I000 K. Once again, a comparison is made in Fig.2 with the data reported earlier (6) for a furnace cooled sa_mple of grain size 195 ~ m tested at I000 K. A t coarse grain sizes ( L , Ii0 ~m ) H-D creep was f~Ind (6,12) to be the dominant creep mechanism for alpha-titanium tested at i000 K. Since the creep rate under H-D creep conditions is independent of grain size, a comparison can be made of the data obtained for slightly different grain sizes (Fig.2). It is evident from the data in Table I and }'igs.l and 2 that solid state quenching does influence significantly the low stress creep of titanium. The stress at which an upward deviation from linearity of strain rate-stress curve occurs was shown (6) to be the stress of transition from viscous to power-law creep domination in annealed titanium. A t stresses below the transition stress, creep rate is directly proportional to stress in the furnace cooled condition whereas for the oil quenched condition (Figs.l and 2) creep rate is directly proportional to stress in excess of a threshold value (the stress below which no detectable creep occurs). In other words, quenching has modified the creep behaviour of titanium, at stresses below the transition stress, to Bingham flow which was otherwise IJewtonian viscous with no threshold stress. Threshold stress of 0.i and 0.14 5 ~ / m 2 (Table I), introduced in alpha-titanlum as a result of quenching, is of significant magnitude when compared to the transition stress of 1.0 and 1.2 ~ / m 2 (Figs.l and 2). It is also seen from Figs.l and 2 that under the concerned stresstemperature-grain size conditions respectively favourable for N-H and H-D creep to dominate, as established previously by us (6), oil quenching has nearly halved the creep rate at a given stress as compared to the furnace cooled condition. This is not insignificant from the viewpoint of the material's resistance to creep flow. From the data above the transition stress in the as-quenched condition (Figs.l and 2) it may be noted that oil quenching strengthens the material against climb creep, too. A satisfactory model is yet to be developed which exPlains the occurrence of a threshold stress for low stress creep of metals (2,15). However, the role of grain boundary dislocations is central to any mechanism concerned with the emission and absorption of vacancies by grain boundaries in diffusion creep ( 1 6 - 1 8 ) . In the same way, climb of lattice dislocations is of significance for power-law (19) and li-D creep (20) processes. The present results showing differences in the low stress cree~ behaviour of quenched and furnace cooled samples suggest that impurity-dislocation interactions in the two treatments need to be clearly understood. Although an adequate explanation is not offered, the importance of the present results lles in the fact that significant effects on low stress creep arise due to quenching even when no phase change occurs. Further experimental work will certainly help not only in clarifying the effect but also in understanding basic mechanisms such as underlying the threshold stress.
Vol.
15, No. i0
CREEP OF ALPHA-TITANIUM
1085
TABLE I A Comparison of Creep Behaviour at I000 K of Samples of Alpha-Titanlum Oil Quenched (OQ) and Furnace Cooled (FC) from 1123 K
Prior Treatment
i ~m
d~/dv (below the transition stress)
Threshold stress*
m 2 ( ~ ) - I s -I
MN/m 2
I000 K / !h2 ÷ 25% RA + 1123 K / ~h, OQ
72
1.0 x 10 -9
0.I0 ± 0.07
I000 K / !h2 + 25% RA ÷ 1123 K/ ~h, FC
69
1.5 x 10 -9
negligible
1123 K / lh, 0(3
240
7.7 x I0 -I0
0.14 +_ 0.13
1123 K / lh, FC
195
1.2 x 10 -9
negligible
* 90% confidence limits RA
Reduction in cross-sectional area by wire drawing
•
~ :
72 pm
OIL QUENCHED o
T = IO00K
•
"[ = 2 4 0 p r n
o
-L=195
OIL QUENCHED
"[ = 69 pm FURNACE COOLED
T =IO00K
pm
FURNACE COOLED 2
%
%
1
0 O
10
05 0"-
,
IS
20
MN/m 2
FIG. I Strain rate versus stress data at I000 K for the oil quenched (closed circles) and furnace cooled (open circles) alphatitanium of grain size 69-72Dm. Note the significant threshold stress and a decrease in the creed rate at a given stress caused by solid state quenching.
0
05
~5
1.o
O-
MN / ,m2
FIG. 2 Strain rate ve.r~n4s stress data at I000 K for the oil quenched (@rain size 240~m and closed circles) and furnace cooled (grain size 195Dm and open circles) alpha-titanium. Note the significant threshold stress and a decrease in the creep rate at a given stress resulting from solid state quenching.
1086
CREEP OF ALPHA-TITANIUM
Vol, 15, No. i0
Acknowledqemen~ Financial support received from the University Grants Commission and the Aeronautics Research and Development Board, Union ~linistry of Defence is grateful ly acknowl edged. ReferenGes i. B.Burton, Diffusional Creep of Polycrystalline Materials, Trans.Tech. Publications, Aedermannsdorf, Switzerland (1977). 2. D.J.Towle and H.Jones, Acta metall. 24, 399 (1976). 3. T.Sritharan and H.Jones, Acta metall. 27, 1293 (1979). 4. B.Burton, I.G.Crossland and G.W.Greenwood, Metal Sci. 14, 134 (1980). 5. T.Sritharan and H.Jones, Acta metall. 28, 1633 (1980). 6. G.Malakondaiah and P.Rama Rao, Acta metall. 29, 1263 (1981). 7. G.Malakondaiah and P.R~na Rao, Mater. Sci. Eng. (in press). 8. P.R.N.Nabarro, Rep. Conf. on Strength of Solids, The Physical Society, London p.75 (1948). 9. C.Herring, J. Appl. Phys. 21, 437 (1950). I0. R.L.Coble, J. Appl. Phys. 34, 1679 (1963). Ii. J.G.Harper and J.E.Dorn, Acta metall. 5, 654 (1957). 12. G.Malakondaiah and P.R~na Rao, Scripta metall. 13, 1187 (1979). 13. I.G.Crossland, R.B.Jones and G.W.Lewthwaite, J. Phys. D: Appl. Phys. 6, 1040 (1973). 14. G.Malakondaiah and P.Rama Rao, Trans. Indian Institute of Metals 31, 361 (1978). 15. I.G.Crossland and R.B.Jones, Metal Sci. II, 504 (1977). 16. G.W.Greenwood, Scrlpta metall. 4, 171 (1970). 17. M.F.Ashby, Scripta metall. 3, 837 (1969). 18. B.Burton, Mater. Sci. Eng. I0, 9 (1972). 19. A.K.Mukherjee, J.E.Bird and J.E.Dorn, Trans. ASM 62, 155 (1969). 20. F.A.Mohamed, K.L.Murthy and J.W.Morris, Jr., Rate Processes in Plastic Deformation (edited by J.C.M.Li and A.K.MukherJee) p.459, ASM (1975).
EFFECT OF QUENCHIHG
Department
Vol. 15, pp. 1083-1086, 1981 Printed in the U.S.A.
OH 'THE LOt J STRESS CREEP BE}~VIOUR
Pergamon Press Ltd. All rights reserved
OF ALPILA-TIT;~IIUM
G . M a l a k o n d a i a h and P.Rama Rao of M e t a l l u r g i c a l Engineering, Banaras }{indu University, Varanasi - 221 005, India (Received June 8, 1981)
Introduction Substantial experimental evidence (1-7) gathered in recent years through work on p o l y c r y s t a l l i n e metals, ceramics and alloys has e s t a b l i s h e d that at low stresses ( < I 0 - 5 G where G is the shear modulus) and e l e v a t e d temperatures (• 0.4T m where T m is the m e l t i n g temperature (Kelvin)) viscous creep of one of three kinds, namely N a b a r r o - H e r r i n g (N-H) (8,9), Coble (i0) or H a r p e r - D o r n (H-D) (ii) may prevail. A microstructural v a r i a b l e p r e d o m i n a n t l y i n v e s t i g a t e d in all of these studies is the grain size since N-H and Coble theories of diffusional creep p r e d i c t creep rates to be inversely proportional to square and cube of the grain size while H-D creep observed in coarse grained m a t e r i a l s has been found to be independent of grain size (11,12). A recent study on a l p h a - t i t a n i u m by the p r e s e n t authors (6) was aimed at c o m p r e h e n s i v e l y c h a r a c t e r i s i n g N-H, Coble and H-D creep in this metal. In v i e w of the fact that there are hardly any reports on the influence of prior t r e a t m e n t on the low stress creep of metals, we e x a m i n e d the influence of solid state quenching, not involving any phase change, on creep b e h a v l o u r of a l p h a - t l t a n i u m at low stresses and d]is b r i e f report p r e s e n t s the interesting observations. Exp@rimental Titanium, p r o c u r e d from Titanium Metal and Alloys Limited, London in the form of wires of length 2 m and d i a m e t e r 1500 pm, contained, in ppm, <25 AI, 200 C, < I0 Co, < 5 0 Cr, 47 Cu, 480 Fe, 140 Mg, < i0 Mn, 81 N, <25 Ni, 2600 O, < 25 Pb, < 25 Sb, 48 Si, 400 Sn, < 25 V and < I 0 0 0 Zr. To m e a s u r e low strain rates ( < 10 -9 sec -I) at I000 K and stresses less than 2.0 M N / m 2 a high sensitive spring specimen geometry (2,13) has been employed. The e x p e r i m e n t a l set-up as well as the c o n c e r n e d formulae for the calculation of stresses and strain rates from the m e a s u r e d average loads and coil deflection rates have been d e s c r i b e d in detail p r e v i o u s l y (14). Specimens of grain sizes ( L, length of mean linear intercept ) 72 and 240 ~ m were used. A grain size of 72 ~ was obtained through p r e a n n e a l i n g (I000 K/~2h), intermediate reduction (25~ reduction in cross-sectional area by wire drawing) and a final heating at 1123 K for 1/3 h. On the other hand, a grain size of 240 ~m resulted when the a s - r e c e i v e d material was heated to 1123 K for I h. In both the cases, spring specimens on a mandrel were subjected to oil q u e n c h i n g from 1123 K, after the specified soaking time, using anAdamel Lhomarciy %~-02 v a c u u m (better than 10 -3 Pa) q u e n c h i n g furnace.
1083 0036/9748/81/101083-04502.00/0 Copyright (c) 1981 Pergamon Press Ltd.
1084
CREEP OF ALPHA-TITANIUM
Vol,
15, No. i0
TO reveal the microstructure, mechanically polished sections were etched in a solution containing 10% HF (48%), 5% ~IO 3 (70%) and 85% H20 (distilled) for 3 to 6 seconds. Specimens quenched from 1123 K were found to retain equiaxed grain structure. The grain size was uniform over the entire crosssection and was measured by the linear intercept method. Spring specimens were crept at I000 K under their own weight in a manner described elsewhere (6,14). Grain size measurement both before and after the test revealed no grain coarsening during the course of testing. Results a~d Discussion Strain rate ( ~ ) ve~s~s stress ( ~ ) data for the test at I000 ]" using the oil quenched specimen of grain size 72 pm are presented in Fig.l. T o bring out the effect of quenching, included in Fig.l are strain rate versus stress data for a furnace cooled sample of nearly identical grain size ( L = 69 ~m ) and tested at I000 K, taken from our earlier work (6). Fig.2 presents the strain rate versus stress data for the coarse grained oll quenched sample ( L = 240 ~m ) tested at I000 K. Once again, a comparison is made in Fig.2 with the data reported earlier (6) for a furnace cooled sa_mple of grain size 195 ~ m tested at I000 K. A t coarse grain sizes ( L , Ii0 ~m ) H-D creep was f~Ind (6,12) to be the dominant creep mechanism for alpha-titanium tested at i000 K. Since the creep rate under H-D creep conditions is independent of grain size, a comparison can be made of the data obtained for slightly different grain sizes (Fig.2). It is evident from the data in Table I and }'igs.l and 2 that solid state quenching does influence significantly the low stress creep of titanium. The stress at which an upward deviation from linearity of strain rate-stress curve occurs was shown (6) to be the stress of transition from viscous to power-law creep domination in annealed titanium. A t stresses below the transition stress, creep rate is directly proportional to stress in the furnace cooled condition whereas for the oil quenched condition (Figs.l and 2) creep rate is directly proportional to stress in excess of a threshold value (the stress below which no detectable creep occurs). In other words, quenching has modified the creep behaviour of titanium, at stresses below the transition stress, to Bingham flow which was otherwise IJewtonian viscous with no threshold stress. Threshold stress of 0.i and 0.14 5 ~ / m 2 (Table I), introduced in alpha-titanlum as a result of quenching, is of significant magnitude when compared to the transition stress of 1.0 and 1.2 ~ / m 2 (Figs.l and 2). It is also seen from Figs.l and 2 that under the concerned stresstemperature-grain size conditions respectively favourable for N-H and H-D creep to dominate, as established previously by us (6), oil quenching has nearly halved the creep rate at a given stress as compared to the furnace cooled condition. This is not insignificant from the viewpoint of the material's resistance to creep flow. From the data above the transition stress in the as-quenched condition (Figs.l and 2) it may be noted that oil quenching strengthens the material against climb creep, too. A satisfactory model is yet to be developed which exPlains the occurrence of a threshold stress for low stress creep of metals (2,15). However, the role of grain boundary dislocations is central to any mechanism concerned with the emission and absorption of vacancies by grain boundaries in diffusion creep ( 1 6 - 1 8 ) . In the same way, climb of lattice dislocations is of significance for power-law (19) and li-D creep (20) processes. The present results showing differences in the low stress cree~ behaviour of quenched and furnace cooled samples suggest that impurity-dislocation interactions in the two treatments need to be clearly understood. Although an adequate explanation is not offered, the importance of the present results lles in the fact that significant effects on low stress creep arise due to quenching even when no phase change occurs. Further experimental work will certainly help not only in clarifying the effect but also in understanding basic mechanisms such as underlying the threshold stress.
Vol.
15, No. i0
CREEP OF ALPHA-TITANIUM
1085
TABLE I A Comparison of Creep Behaviour at I000 K of Samples of Alpha-Titanlum Oil Quenched (OQ) and Furnace Cooled (FC) from 1123 K
Prior Treatment
i ~m
d~/dv (below the transition stress)
Threshold stress*
m 2 ( ~ ) - I s -I
MN/m 2
I000 K / !h2 ÷ 25% RA + 1123 K / ~h, OQ
72
1.0 x 10 -9
0.I0 ± 0.07
I000 K / !h2 + 25% RA ÷ 1123 K/ ~h, FC
69
1.5 x 10 -9
negligible
1123 K / lh, 0(3
240
7.7 x I0 -I0
0.14 +_ 0.13
1123 K / lh, FC
195
1.2 x 10 -9
negligible
* 90% confidence limits RA
Reduction in cross-sectional area by wire drawing
•
~ :
72 pm
OIL QUENCHED o
T = IO00K
•
"[ = 2 4 0 p r n
o
-L=195
OIL QUENCHED
"[ = 69 pm FURNACE COOLED
T =IO00K
pm
FURNACE COOLED 2
%
%
1
0 O
10
05 0"-
,
IS
20
MN/m 2
FIG. I Strain rate versus stress data at I000 K for the oil quenched (closed circles) and furnace cooled (open circles) alphatitanium of grain size 69-72Dm. Note the significant threshold stress and a decrease in the creed rate at a given stress caused by solid state quenching.
0
05
~5
1.o
O-
MN / ,m2
FIG. 2 Strain rate ve.r~n4s stress data at I000 K for the oil quenched (@rain size 240~m and closed circles) and furnace cooled (grain size 195Dm and open circles) alpha-titanium. Note the significant threshold stress and a decrease in the creep rate at a given stress resulting from solid state quenching.
1086
CREEP OF ALPHA-TITANIUM
Vol, 15, No. i0
Acknowledqemen~ Financial support received from the University Grants Commission and the Aeronautics Research and Development Board, Union ~linistry of Defence is grateful ly acknowl edged. ReferenGes i. B.Burton, Diffusional Creep of Polycrystalline Materials, Trans.Tech. Publications, Aedermannsdorf, Switzerland (1977). 2. D.J.Towle and H.Jones, Acta metall. 24, 399 (1976). 3. T.Sritharan and H.Jones, Acta metall. 27, 1293 (1979). 4. B.Burton, I.G.Crossland and G.W.Greenwood, Metal Sci. 14, 134 (1980). 5. T.Sritharan and H.Jones, Acta metall. 28, 1633 (1980). 6. G.Malakondaiah and P.Rama Rao, Acta metall. 29, 1263 (1981). 7. G.Malakondaiah and P.R~na Rao, Mater. Sci. Eng. (in press). 8. P.R.N.Nabarro, Rep. Conf. on Strength of Solids, The Physical Society, London p.75 (1948). 9. C.Herring, J. Appl. Phys. 21, 437 (1950). I0. R.L.Coble, J. Appl. Phys. 34, 1679 (1963). Ii. J.G.Harper and J.E.Dorn, Acta metall. 5, 654 (1957). 12. G.Malakondaiah and P.R~na Rao, Scripta metall. 13, 1187 (1979). 13. I.G.Crossland, R.B.Jones and G.W.Lewthwaite, J. Phys. D: Appl. Phys. 6, 1040 (1973). 14. G.Malakondaiah and P.Rama Rao, Trans. Indian Institute of Metals 31, 361 (1978). 15. I.G.Crossland and R.B.Jones, Metal Sci. II, 504 (1977). 16. G.W.Greenwood, Scrlpta metall. 4, 171 (1970). 17. M.F.Ashby, Scripta metall. 3, 837 (1969). 18. B.Burton, Mater. Sci. Eng. I0, 9 (1972). 19. A.K.Mukherjee, J.E.Bird and J.E.Dorn, Trans. ASM 62, 155 (1969). 20. F.A.Mohamed, K.L.Murthy and J.W.Morris, Jr., Rate Processes in Plastic Deformation (edited by J.C.M.Li and A.K.MukherJee) p.459, ASM (1975).