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Compressive creep of polycrystalline ice containing a liquid phase
saipta Metallutgica dMatair&
Vol. 33. No. 3. pp. 447450.1995
Pergamon o!m-71fix(95po2073
COMPRESSIVE CREEP OF POLYCRYSTALLINE ICE CONTAINING A LIQUID PHASE S. de La Chapelle*, P. Duval* and B. Baudelet** *Laboratoire de Glaciologie et de G&physique de 1’Environnement BP96,38402 St. Martin d’Heres Cedex, France **Genie Physique et Mecanique des Mat&iaux, U.A. du CNRS, I.N.P.G. ENSPG, BP46,38402 St. Martin d’H&resCedex, France (Received December 19,1994) (Revised March 14,1995) Introduction
Deformation in crystalline materials comaining a liquid phase is of a great interest in metals, ceramics and geological materials. The creep behavior of these materials depends on the liquid phase content and, particularly, on the localization of this phase (l-3). Liquid enhanced creep is generally observed when grain boundaries are sutllciently wetted. The main creep mechanism is grain boundary sliding ; the liquid phase providing a lubricatingtlhn_The necessary accommodation processes are diffusion or cavitation (4-6). When there is sohrbililyofthe solid in the fluid, dilXrsionalflow involving dissolution, diflirsion and redeposition can be an efficient deformation process (4,s). In these deformation processes, strain rate is grain-size dependent and varies linearly with stress. Tempemte ice is defined as ice at its melting point and is in thermal equilibrium with liquid water (7). Ice in Alpine glaciers and in other glaciers in similar climates is temperate. On the Vallee Blanche (French Alps) the water content is typically around 1% ; it increases to a value of 1.8% at a depth of 18On~(8). This value seems to be an upper bound for glacier ice. Temperate ice is also found in some sites near the bottom of the Antamtic and Cireenland ice sheets. A third power creep law is generally adopted for temperate and cold ice for stresses higher than 0.1 MPa. (9). The creep Irateeahatmment due to water was analyzed by Duval(l0) through creep tests on glacier ice samples at a .temperature close to 0°C. The role of water was discussed in relation to the occurrence of dynamic recrystalhzation and other accommodation processes for basal glide. A non-linear flow law with a stressexponemn=4.15wasfoundbyPharraudGodaverti(11)onsalineicetestedat-8”Cforstresseshigher than 0.3 MPa. Due to the high liquid content in the samples (about 24%), some loss of brine was observed during the mechanical test Experimental studies of the creep of pure polycrystalline ice, which did not contain a liquid phase, have shown that at k)w stresses i.e. for conditions prevailing in glaciers and polar ice sheets the stress exponent is lower than 2 [ 121. Diffusional creep cannot be invoked. Indeed, it yields a viscosity much larger than that deduced from both laboratory and in situ measurem ents and cannot be at the origin of the development of lattice prefarred orientations [ 121. The objective of this paper was to assess the role of a liquid phase in the creep behavior of polycrystallme ice. A particular attention was focused to the low stress creep behavior.
447
448
COMPRESSIVE
CREEP OF POLYCRYSTALLTNE
Emerimental
ICE
Vol. 33, No. 3
Methods
Spedmen Premration
Isotropic polycrystalline ice having a grain size between 2 and 5 mm. were mechanically tested. The method
ofspecimen pmpamtion described by Duval and Le Gac( 131was followed. Sieved pure ice grains, made from de-ionized water, were packed into a cylindrical mould. The mould was de-aerated and filled with air-free water at 0°C. Samples were frozen at -20°C for 24 hours. Saline ice was obtained by using brine instead of pure water ; but, sieved grains were always prepared Tom pure water. Thin sections were made to determine crystal size and the localizationof the liquid phase. Equiaxcd grains were observed and the brine was located at the triple junctions. Grain size was between 1 and 3.5 mm. Mechanical Tests
Creep compressive tests were performed with the machine described by Duval(l0) at two temperatures (-24.0 and -13.OiO.l “C). At -24.O”C, the NaCK!H,O eutectic is completely solid since the eutectic temperature of this salt Te is -2 1.2”C. At - 13.O”C,the liquid content of samples was determined by using NaCK?H,O phase diagram (14). Liquid content variation was obtained by changing the salinity of the brine during the sample preparation Exmvimental
Results
Figure shows the results obtained with saline ice containing 3 and 7 wt. % of liquid and with pure polycrystalhne ice. The strain rates correspond to the steady creep obtained for a strain of about 1%. The tests were stopped before the outset of reciystalhzation, i.e., for strains below 1.5%. In the same figure, results obtained elsewhere for monocrystalhne ice (15) are also shown. The inset shows data obtained for pure ice and for completely solid saline ice at a temperature of -24&O. 1“C. The liquid phase exerts a considerable influence on the mechanical behavior of polycrystalline ice. The strain rate of ice contaiuing 7% of water is more than an order of magnitude greater than that of pure ice. The solid ice/salt eutectic which segregates on grain boundaries signitlcantly sotlens the ice (inset). These results indicate two m&s of defbrmation behavior for pure and saline polyc@alline ice , i.e., one at low stresses forwhich the coe#icient of strain rate sensitivity, n, is equal to l.W.2 and the other for which n is in the neighMood of 3. It should be noted the first mode was not observed in solid saline ice since this modeshouldoccurattoolowstrainrates.Themodeinwhichn-2istobecomparedtothatofmonocrystalline ice since this does not show the n-3 behavior. The transition between these two modes seems to depend on the liquid phase content. As the liquid phase content increases, the behavior mode with n-2 moves towards high stresses. This last point will not be discussed any further.
The large etlbct of the liquid intergranular phase on the compressive creep behavior of polycry&lhne saline ice cannot be related to the wetting characteristics of the liquid. The liquid phase is only located along the triple junctions of grain boundaries aud the absence of a regime with n =l rules out the preponderance of diffusional or solution-deposition creep. For the same reason grain boundary sliding accommodated by the liquid phase cannot longer be invoked. The mechanical behavior of pure monocrystalline and polycrystalline ice was extensively studied. For the n=3 mode, Duval et al (15) invoked either dislocation climb occurring in series with basal glide or slip on
Vol. 33, No. 3
COMPRESSIVECREEP OF POLYCRYSTALLINEICE
449
J
0.1
0.2
Axial stress (M Pa)
’
Figrael.~~rateasafUnctionoftheappliedstre9s,forpureandsalinepolycrystallineiceat-l3”C;dataforbasalglidein singlegystalsaregivenforcomparisonTbepointwithanarrowindicatesthatitisanupperlimitforthe~rate.Inset:strainrate ~a~afthe~at-24’C.forsalineandpureice;datawithinthehatchedzonecorrespondtosarnpleswill,asalinitybetween 0.5 and 1.5%. Te is the esiectic temperature ofNaCLXH,O. or pyrmidal planes in order to sati@ the requirement of the presence of 4 independent deformation systems (16). Recently, non-basal slip was observed (17,18) ; edge dislocations on non-basal planes glide with velocities one order of magnitude higher than dislocations on the basal plane but the density of mobile dislocations on non-basalplanes is too low to significantly contribute to the deformation of ice single crystals. The large difference between the flow stress on the basal and the non-basal planes develops a non uniform intemal stress field within the polycrystal(15). A comparison of the mechanical behavior of monocrystalline and polycrystalline ice, in figure, shows the iufluence of this ditikrence. Higher internal stresses are expected when grain boundaries form obstacles to basal slip. The observed behavior at low stresses with n=2 may be due to the occtnrence of grain boundary migration driven by strain energy ; this partly relieves the deformation incompatibility between grains and therefore reduces the internal stresses. Besides, a newtoniau viscosity is expected in polar ice at very low skesses (
This study was supported by CNRS, D&artement des Sciences Pour l’lngenieur. The authors thank M. Creseveur for ihe support in the study of the te2ctureof ice.
COMPRESSIVE CRFiEP OF POLYCRYSTALLINE ICE
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
E.H. Rut& Phil. Trans. R Sot. Lad. A 293,203 (1976) G.M.Phm,P.S.GodavertiandB.L.Vaandrager,J.Mat.Sci.24,784(1989) B.L Vaaadrager and G.M. Phm, kta Met. 37,1057 (1989) RL. Tsai and R Raj, Acta Met 30,1043 (1982) G.M. Phm and M.F. Ashby, Acta Met. 31,129 (1983) B. Baudelet, M.C. Daag and F. Bordeaux, Scripta Met. et Mat. 26,573 (1992) L. Lliboutry, J. ofGlac. 10,lS (1971) M. Vallon, J.R Petit and B. Fabre, J. of Glac. 17,13 (1976) L. Llibo&y and P. Duval, Amal. Geophysicae, 3,207 (1985) P. Duval, IAHS Publ. 118,29 (1977) GM Pharr and P.S. Godaverti, Cold Regions Sciences and Tech, 14,273 (1987) P.Pimknta and P. Duval, Journal de Physique, C1,48,243 (1987) P. Duval and H. Le Gac, J. of Glac. 25,15 l(l980) W.F. Weeks and S.F. Ackley, CRREL Monograph 82-1,1(1982) P. Duval, M.F. Ashby and I. And J. ofPhys. Chem. 87,4066 (1983) W. Hutch&m, Met Tram. 8A, 1465 (1977) S. Alnnad and RW. Whitworth, Phil. Msg. A 57,749 (1988) T. Hondoh, H. Iwamatsu and S. Mae, Phil. h4ag. A 62,89 (1990) RB.Alley, J. of Glac. 38,245 (1992) P. Pimien& P. Duval and V. Ya Lipenkov, IAI-IS Publ. 170,57 (1987)
Vol. 33, No. 3
Vol. 33. No. 3. pp. 447450.1995
Pergamon o!m-71fix(95po2073
COMPRESSIVE CREEP OF POLYCRYSTALLINE ICE CONTAINING A LIQUID PHASE S. de La Chapelle*, P. Duval* and B. Baudelet** *Laboratoire de Glaciologie et de G&physique de 1’Environnement BP96,38402 St. Martin d’Heres Cedex, France **Genie Physique et Mecanique des Mat&iaux, U.A. du CNRS, I.N.P.G. ENSPG, BP46,38402 St. Martin d’H&resCedex, France (Received December 19,1994) (Revised March 14,1995) Introduction
Deformation in crystalline materials comaining a liquid phase is of a great interest in metals, ceramics and geological materials. The creep behavior of these materials depends on the liquid phase content and, particularly, on the localization of this phase (l-3). Liquid enhanced creep is generally observed when grain boundaries are sutllciently wetted. The main creep mechanism is grain boundary sliding ; the liquid phase providing a lubricatingtlhn_The necessary accommodation processes are diffusion or cavitation (4-6). When there is sohrbililyofthe solid in the fluid, dilXrsionalflow involving dissolution, diflirsion and redeposition can be an efficient deformation process (4,s). In these deformation processes, strain rate is grain-size dependent and varies linearly with stress. Tempemte ice is defined as ice at its melting point and is in thermal equilibrium with liquid water (7). Ice in Alpine glaciers and in other glaciers in similar climates is temperate. On the Vallee Blanche (French Alps) the water content is typically around 1% ; it increases to a value of 1.8% at a depth of 18On~(8). This value seems to be an upper bound for glacier ice. Temperate ice is also found in some sites near the bottom of the Antamtic and Cireenland ice sheets. A third power creep law is generally adopted for temperate and cold ice for stresses higher than 0.1 MPa. (9). The creep Irateeahatmment due to water was analyzed by Duval(l0) through creep tests on glacier ice samples at a .temperature close to 0°C. The role of water was discussed in relation to the occurrence of dynamic recrystalhzation and other accommodation processes for basal glide. A non-linear flow law with a stressexponemn=4.15wasfoundbyPharraudGodaverti(11)onsalineicetestedat-8”Cforstresseshigher than 0.3 MPa. Due to the high liquid content in the samples (about 24%), some loss of brine was observed during the mechanical test Experimental studies of the creep of pure polycrystalline ice, which did not contain a liquid phase, have shown that at k)w stresses i.e. for conditions prevailing in glaciers and polar ice sheets the stress exponent is lower than 2 [ 121. Diffusional creep cannot be invoked. Indeed, it yields a viscosity much larger than that deduced from both laboratory and in situ measurem ents and cannot be at the origin of the development of lattice prefarred orientations [ 121. The objective of this paper was to assess the role of a liquid phase in the creep behavior of polycrystallme ice. A particular attention was focused to the low stress creep behavior.
447
448
COMPRESSIVE
CREEP OF POLYCRYSTALLTNE
Emerimental
ICE
Vol. 33, No. 3
Methods
Spedmen Premration
Isotropic polycrystalline ice having a grain size between 2 and 5 mm. were mechanically tested. The method
ofspecimen pmpamtion described by Duval and Le Gac( 131was followed. Sieved pure ice grains, made from de-ionized water, were packed into a cylindrical mould. The mould was de-aerated and filled with air-free water at 0°C. Samples were frozen at -20°C for 24 hours. Saline ice was obtained by using brine instead of pure water ; but, sieved grains were always prepared Tom pure water. Thin sections were made to determine crystal size and the localizationof the liquid phase. Equiaxcd grains were observed and the brine was located at the triple junctions. Grain size was between 1 and 3.5 mm. Mechanical Tests
Creep compressive tests were performed with the machine described by Duval(l0) at two temperatures (-24.0 and -13.OiO.l “C). At -24.O”C, the NaCK!H,O eutectic is completely solid since the eutectic temperature of this salt Te is -2 1.2”C. At - 13.O”C,the liquid content of samples was determined by using NaCK?H,O phase diagram (14). Liquid content variation was obtained by changing the salinity of the brine during the sample preparation Exmvimental
Results
Figure shows the results obtained with saline ice containing 3 and 7 wt. % of liquid and with pure polycrystalhne ice. The strain rates correspond to the steady creep obtained for a strain of about 1%. The tests were stopped before the outset of reciystalhzation, i.e., for strains below 1.5%. In the same figure, results obtained elsewhere for monocrystalhne ice (15) are also shown. The inset shows data obtained for pure ice and for completely solid saline ice at a temperature of -24&O. 1“C. The liquid phase exerts a considerable influence on the mechanical behavior of polycrystalline ice. The strain rate of ice contaiuing 7% of water is more than an order of magnitude greater than that of pure ice. The solid ice/salt eutectic which segregates on grain boundaries signitlcantly sotlens the ice (inset). These results indicate two m&s of defbrmation behavior for pure and saline polyc@alline ice , i.e., one at low stresses forwhich the coe#icient of strain rate sensitivity, n, is equal to l.W.2 and the other for which n is in the neighMood of 3. It should be noted the first mode was not observed in solid saline ice since this modeshouldoccurattoolowstrainrates.Themodeinwhichn-2istobecomparedtothatofmonocrystalline ice since this does not show the n-3 behavior. The transition between these two modes seems to depend on the liquid phase content. As the liquid phase content increases, the behavior mode with n-2 moves towards high stresses. This last point will not be discussed any further.
The large etlbct of the liquid intergranular phase on the compressive creep behavior of polycry&lhne saline ice cannot be related to the wetting characteristics of the liquid. The liquid phase is only located along the triple junctions of grain boundaries aud the absence of a regime with n =l rules out the preponderance of diffusional or solution-deposition creep. For the same reason grain boundary sliding accommodated by the liquid phase cannot longer be invoked. The mechanical behavior of pure monocrystalline and polycrystalline ice was extensively studied. For the n=3 mode, Duval et al (15) invoked either dislocation climb occurring in series with basal glide or slip on
Vol. 33, No. 3
COMPRESSIVECREEP OF POLYCRYSTALLINEICE
449
J
0.1
0.2
Axial stress (M Pa)
’
Figrael.~~rateasafUnctionoftheappliedstre9s,forpureandsalinepolycrystallineiceat-l3”C;dataforbasalglidein singlegystalsaregivenforcomparisonTbepointwithanarrowindicatesthatitisanupperlimitforthe~rate.Inset:strainrate ~a~afthe~at-24’C.forsalineandpureice;datawithinthehatchedzonecorrespondtosarnpleswill,asalinitybetween 0.5 and 1.5%. Te is the esiectic temperature ofNaCLXH,O. or pyrmidal planes in order to sati@ the requirement of the presence of 4 independent deformation systems (16). Recently, non-basal slip was observed (17,18) ; edge dislocations on non-basal planes glide with velocities one order of magnitude higher than dislocations on the basal plane but the density of mobile dislocations on non-basalplanes is too low to significantly contribute to the deformation of ice single crystals. The large difference between the flow stress on the basal and the non-basal planes develops a non uniform intemal stress field within the polycrystal(15). A comparison of the mechanical behavior of monocrystalline and polycrystalline ice, in figure, shows the iufluence of this ditikrence. Higher internal stresses are expected when grain boundaries form obstacles to basal slip. The observed behavior at low stresses with n=2 may be due to the occtnrence of grain boundary migration driven by strain energy ; this partly relieves the deformation incompatibility between grains and therefore reduces the internal stresses. Besides, a newtoniau viscosity is expected in polar ice at very low skesses (
This study was supported by CNRS, D&artement des Sciences Pour l’lngenieur. The authors thank M. Creseveur for ihe support in the study of the te2ctureof ice.
COMPRESSIVE CRFiEP OF POLYCRYSTALLINE ICE
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
E.H. Rut& Phil. Trans. R Sot. Lad. A 293,203 (1976) G.M.Phm,P.S.GodavertiandB.L.Vaandrager,J.Mat.Sci.24,784(1989) B.L Vaaadrager and G.M. Phm, kta Met. 37,1057 (1989) RL. Tsai and R Raj, Acta Met 30,1043 (1982) G.M. Phm and M.F. Ashby, Acta Met. 31,129 (1983) B. Baudelet, M.C. Daag and F. Bordeaux, Scripta Met. et Mat. 26,573 (1992) L. Lliboutry, J. ofGlac. 10,lS (1971) M. Vallon, J.R Petit and B. Fabre, J. of Glac. 17,13 (1976) L. Llibo&y and P. Duval, Amal. Geophysicae, 3,207 (1985) P. Duval, IAHS Publ. 118,29 (1977) GM Pharr and P.S. Godaverti, Cold Regions Sciences and Tech, 14,273 (1987) P.Pimknta and P. Duval, Journal de Physique, C1,48,243 (1987) P. Duval and H. Le Gac, J. of Glac. 25,15 l(l980) W.F. Weeks and S.F. Ackley, CRREL Monograph 82-1,1(1982) P. Duval, M.F. Ashby and I. And J. ofPhys. Chem. 87,4066 (1983) W. Hutch&m, Met Tram. 8A, 1465 (1977) S. Alnnad and RW. Whitworth, Phil. Msg. A 57,749 (1988) T. Hondoh, H. Iwamatsu and S. Mae, Phil. h4ag. A 62,89 (1990) RB.Alley, J. of Glac. 38,245 (1992) P. Pimien& P. Duval and V. Ya Lipenkov, IAI-IS Publ. 170,57 (1987)
Vol. 33, No. 3