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Review of creep in Magnesium matrix composites

Seminar title:. Review of creep in Magnesium matrix composites. By: Farid Labib. Supervised by: Prof. R. Mahmudi Prof. H. R. Ghasemi. Stage I : Primary Creep Strain rate decreases as strain increases. Resistance to plastic deformation: strain hardening.

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Review of creep in Magnesium matrix composites

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  1. Seminar title: Review of creep in Magnesium matrix composites By: Farid Labib Supervised by: Prof. R. Mahmudi Prof. H. R. Ghasemi

  2. Stage I: Primary Creep Strain rate decreases as strain increases. Resistance to plastic deformation: strain hardening Stage II: Secondary (steady-state) Creep (used as design tool) Strain rate minimum and constant Balance between recovery = strain hardening. Fracture will not occur. Stage III: Tertiary Creep (failure-rupture) Strain rate increases. reduction in cross-sectional area due to voids, necking reduce Main Mechanisms of Creep in metals: • Dislocation slip and climb • Grain boundary sliding • Diffusional flow M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  3. Dislocation slip and climb: Dislocation climb is the mechanism responsible to achieve the desired balance between recovery and strain hardening during secondary creep. dislocations must escape from tangled regions by a process called dislocation climb. At low temperatures, dislocation slip dominates. (high temp. diffusion is dominates) M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  4. Dislocation climb requires diffusion vacancies or self-diffusion.So the rate is control by atomic diffusion As temperature increases atoms have more thermalenergy (proportional to RT) and the equilibrium concentration of vacancies in ametal increases exponentially Q is the activation energy for vacancy creation and movement, (or the activation energy for self-diffusion). At intermediate and higher temperatures (at high stresses), dislocation climb dominates Climb is, however, still the basic controlling mechanism in steady-state creep of solid solution alloys M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  5. Grain boundary sliding: the grains in polycrystalline metals are able to move relative to each other. Occurs due to increasing the temperature and stress and/or decreasing the strain rate. Above about 0.6 TM the grain boundary region is thought to have a lower shear strength than the grains themselves, probably due to the looser atomic packing at grain boundaries Rounded and wedge shaped voids are seen mainly at the grain boundaries M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  6. The boundary sliding may account for 10% to 65% of the total creep strain Ball-Hutchinson model of GBS accommodated by dislocation movement: (Sub-grain formation) M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  7. Coble Nabarro-herring Diffusion Creep or Dislocation Creep Coble • Diffusion occurs through the grain boundaries • creep rates is reduced by increasing grain size, as structure with larger grains have fewer grain boundaries. Nabarro-herring • Diffusion occurs through the main body of the grains • Rate of the creep can be reduced by increasing the size of the grains M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  8. More than one creep mechanism will operate at the same time (In parallel), but they operate independentlyeach other. The fastest mechanism will control the creep behavior, the slowest mechanism will control the creep deformation M. E. Kassner, M. T. Perez-Prado, Fundamentals of Creep In Metals and Alloys, 2004

  9. A metal matrix composite (MMC) is composite material with at least two constituent, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite.

  10. where K and are the bulk and shear moduli, respectively, and f is the volume fraction of reinforcing particles. B. Budiansky, J. Mech. Phys. Solids, 1965, 13, 223. E. Kroner, Z. Phys., 1958, 151, 504.

  11. A model to predict the creep rate of a same sized aligned short fiber MMC

  12. Strengthening Mechanisms in Metal Matrix composites: Direct strengthening: Under an applied load, the load is transferred from the weaker matrix, across the matrix/reinforcement interface, to the typically higher stiffness reinforcement. In this manner, strengthening takes place by the reinforcement “carrying” much of the applied load. [1] Indirect strengthening: the thermal mismatch between the high expansion metallic matrix and the low expansion ceramic is typically quite high. Thus, upon cooling, dislocations form at the reinforcement/matrix interface due to the thermal mismatch. In this manner, thermally induced dislocation punching results in “indirect strengthening” of the matrix. [2] Interfacial sliding may be affected by internal thermal stresses arising at the interfaces on heating the specimen to the test temperature (the thermal expansion coefficients of the metallic matrix and the ceramic particles differ considerably). Since SiC particles are very rigid, interfacial sliding is not accommodated sufficiently and, hence, cavities form at interfaces.[3] [2]K. K. Chawla, M. Metzger, J. Mater. Sci., 1972, 7, 34. [1] V. C. Nardone, K. M. Prewo, Scripta Metall., 1989, 23, 291. [3] Florian Moll, Frantisek Chmelik, PavelLukac, Barry Leslie Mordike, Karl-Ulrich Kainer, Materials Science and Engineering A291 (2000) 246–249

  13. in the presence of thermal stresses a lower slope is observed at the early stage of deformation, due to the slightly lower apparent Young's modulus arising from prior plastic deformation. [1] [3] [2] [1] Y.-L. Shen, M. Finot, A. Needleman, S. Suresh, ActaMetall. Mater., 1995, 43, 1701 [2] LIU CHENG, ZHANG FAN, ZHANG GUODING, JOURNAL OF MATERIALS SCIENCE 39 (2004) 2923 – 2925 [3] V. SKLENICKA, M. PAHUTOVA , K. KUCHAROVA, M. SVOBODA, T.G. LANGDON, Metall. Mater. Trans. A 2002, Vol 33,13, pp 883-889

  14. Predicted tensile stress-strain curves for the Al/SiC/20p composite with the various reinforcement shapes.[1] unit-cylinder and double-Cone particles result in, respectively, the highest and lowest degrees of disturbance of the local plastic flow paths in the matrix. An enhanced creep resistance was also obtained with the higher aspect ratio whiskers than with particles, presumably due to more effective load transfer from the matrix to the high stiffness reinforcement [2] [1] Y.-L. Shen, M. Finot, A. Needleman, S. Suresh, Acta Metall. Mater., 1995, 43, 1701 [2] D. Webster, Metall. Mater. Trans., 1982, 13A, 1511±1519.

  15. With an increase in volume fraction, higher elastic modulus, macroscopic yield and tensile strengths were observed, coupled with lower ductility void nucleation composite, which may result from processing of composites with fairly coarse particulate reinforcement, do not contribute to load transfer or strengthening and would decrease strength. N. Chawla, C. Andres, J. W. Jones, J. E. Allison, Metall. Mater. Trans., 1998, 29A, 2843.

  16. Yield stress improvement of the composites could be predicted by the following equation: Assuming a well bonded particle where the strengthening coefficient ofˇ is 1.25, is the difference between CTE of matrix and reinforcement, and T is the difference between the processing and the test temperatures where Kis the Hall-Petch coefficient and is the matrix grain diameter. The parameter K was calculated from the slope of Hall-Petch− plot using different grain sizes of the rolled Mg–2Al2O3 material before and after annealing and their corresponding yield stress values where Gmis the shear modulus of matrix, b is the Burgers vector, is the inter-particle spacing, and dpis the average diameter of particles M. Habibnejad-Korayema, R. Mahmudi, W.J. Pooleb, Materials Science and Engineering A 519 (2009) 198–203.

  17. The addition of nano-particles resulted in significant improvements in both hardness and tensile properties. However, this caused a considerable reduction in the ductility of the composites relative to the monolithic materials CTE mismatch between the matrix and the particles was the most effective one M. Habibnejad-Korayema, R. Mahmudi,W.J. Pooleb, Materials Science and Engineering A 519 (2009) 198–203.

  18. Transition from quasi-cleavage fracture to amore advanced brittle state M. Habibnejad-Korayema, R. Mahmudi,W.J. Pooleb, Materials Science and Engineering A 519 (2009) 198–203.

  19. Creep experiments on pure Mg at temperatures up to ∼600 K have shown that, as in f.c.c. metals, the steady state creep rate is related to the applied stress through a power-law in which the stress exponent is ∼1 at very low stresses, ∼5 over a wide range of intermediate stresses and with a breakdown in the power-law behavior at very high stresses. [1] They have been interpreted in terms of transitions from diffusion creep at the lowest stresses, dislocation climb with basal slip at intermediate stresses and power-law breakdown at very high stresses, where the diffusion creep and dislocation climb regions have activation energies of ∼135 kJ mol−1 which is equal to the value for lattice self-diffusion in magnesium [2] pyramidal slip mode can offer five independent slip systems to enhance ductility. Therefore, deformation of Mg polycrystals is normally required prismatic and pyramidal slip system [1] S . S . VAGARALI and T. G. LANGDON, Acta Metall. 29(1981) 1969. [2] X. W. Wei, X. T. Zu, H. Fu and W. L. Zhou, Materials Science and Technology 2006 VOL 22 NO 8 907

  20. The Creep Curves in MMCs In many experiments on MMCs, the secondary stage is of a sufficiently short duration that it is best described as a minimum creep rate. • an abrupt and possibly poorly defined minimum creep rate is not an intrinsic property of all MMCs. K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1990, vol. 38, pp. 2149-59. K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1994, vol. 42, pp. 667-78.

  21. extent of the steady-state stage is also often of a very short duration in the creep of unreinforced powder metallurgy (PM) materials relatively short duration of the steady-state stage in the creep of MMCs is not necessarily related to the advent of damage mechanisms within the matrix such as the occurrence of debonding at the interfaces between the matrix and the reinforcement. K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1994, vol. 42, pp. 667-78.

  22. The Nature of the Threshold Stress in MMCs 1) the particulates used as the reinforcements, which may have diameters of up to 20 m, cannot represent the source of the threshold stresses measured experimentally Attention has, therefore, focused instead on the presence of fine oxide particles, with sizes in the nanometer range, which may exist in MMCs fabricated using standard PM techniques.[1, 2] 2) It is possible also that load transfer to the reinforcement may account for the origin of the threshold stress. based on creep experiments on Al-based composites, in which the values of the measured threshold stresses were dependent upon the precise volume fractions of the reinforcement. [3] 3) reinforcing particulates may crack as a result of the thermo-mechanicalprocessing, thereby giving fine SiCparticles with sizes of 100 nm. [4] [1] K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1990, vol. 38, pp. 2149-59. [2] F.A. Mohamed, K.-T. Park, and E.J. Lavernia: Mater. Sci. Eng., 1992, vol. A150, pp. 21-35. [3] A.B. Pandey, R.S. Mishra, and Y.R. Mahajan: Acta Metall. Mater., 1992, vol. 40, pp. 2045-52. [4] Y. Li and F.A. Mohamed: Acta Mater., 1997, vol. 45, pp. 4775-85.

  23. Threshold stresses in this composite are not associated primarily with a transfer of load from the metallic matrix to the ceramic reinforcement. Experimental It has been shown that attractive interactions between these particles and moving dislocations may account for the measured threshold stresses in the high-temperature creep of MMCs A higher volume of reinforcement may lead to the presence of a larger number of nanoparticles and higher threshold stress values. significant values for the threshold stresses were observed in an Al 6061-20 volpctAl2O3(p) and in an Al 7005-20 volpct Al2O3(p) composite, both were fabricated using an ingot metallurgy (IM) procedure. [3] [1] K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1990, vol. 38, pp. 2149-59. [2] K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1994, vol. 42, pp. 667-78. [3] Y. Li and T.G. Langdon: Acta Mater., 1998, vol. 46, pp. 1143-55.

  24. Calculating the threshold stress An extrapolation of against plot to a creep rate of s−1, which is in the range of the lowest measurable strain by a creep test machine, can then give the threshold stress for each temperature level. The true or effective stress exponents “n” in equation can then be obtained from the slope of against . [1] threshold stress for this composite is essentially independent of temperature for the condition where n =8. [2] [1] Y. Li, T.G. Langdon, Scr. Mater. 36 (12) (1997) 1457–1460. [2] A.B. Pandey, R.S. Mishra, and Y.R. Mahajan: Metall. Mater. Trans. A, 1996, vol. 27A, pp. 305-16.

  25. Q0 is an energy which has been tentatively associated with the binding energy between dislocations and obstacles in the glide plane. B.Q. Han, D.C. Dunand, Materials Science and Engineering A300 (2001) 235–244

  26. It is well documented that the values for the apparent stress exponent and the apparent activation energy are often exceptionally high and variable in creep experiments on MMCs. threshold stress is incorporated into the analysis these high and variable values for naand Qaare reduced to lower and constant values for simple unreinforced materials: n=3 representing viscous glide of dislocations [1] n=5representing climb of dislocations (lattice diffusion) [2] n=7 representing low temperature climb (core diffusion)[4] n=8representing (lattice diffusion) controlled by constant substructure model in which microstructure remain constant during creep [3] When n=3, the anticipated activation energy is close to the value for the inter-diffusion energy of solute atoms. When n=5, 8 the anticipated activation energy is close to the value for the self-diffusion in the crystalline lattice. [1] J. Weertman: J. Appl. Phys., 1957, vol. 28, pp. 1185-87. [2] J. Weertman: J. Appl. Phys., 1957, vol. 28, pp. 362-64. [3] O.D. Sherby, R.H. Klundt, and A.K. Miller: Metall. Trans. A, 1977, vol. 8A, pp. 843-50. [4] SherbyOD, Klundt RH, Miller AK. Flow stress, subgrain size, and subgrain stability at elevated temperature. Metall Trans A 1977;8(6):843–50.

  27. Metal-based composites reinforced with ceramic particulates may have a fine substructure which develops during processing and they anticipated this would be invariant with stress, thereby inferring that the creep data for MMCs may be successfully analyzed using the constant substructure model where n=8. [1] [2] Al-4 pct Mg MMC viscous glide Q = 125 kJ/mol Inter-diffusion of Mg atoms in an Al lattice (130.5 kJ/ mol) n=24 kJ/ mol n=8 n=8 [1] O.D. Sherby, R.H. Klundt, and A.K. Miller: Metall. Trans. A, 1977, vol. 8A, pp. 843-50.. [2] A.B. Pandey, R.S. Mishra, and Y.R. Mahajan: Metall. Mater. Trans. A, 1996, vol. 27A, pp. 305-16. [3] Y. Li and T.G. Langdon: Metall. Mater. Trans. A, 1997, vol. 28A, pp. 1271-73.

  28. [1] Al 6061 alloy MMC viscous glide Q = 150 kJ/mol Inter-diffusion of Mg atoms in an Al lattice (130.5 kJ/ mol) n=8 Inter-diffusion of Mg in an Al lattice [1] Y. Li and T.G. Langdon: Acta Mater., 1997, vol. 45, pp. 4797-4806. [1] O.D. Sherby, R.H. Klundt, and A.K. Miller: Metall. Trans. A, 1977, vol. 8A, pp. 843-50..

  29. creep properties of the MMCs are governed by creep in the matrix material. whereas values of n of 3 or 5 give threshold stresses which exhibit a consistent dependence on temperature, a value of n =8may lead either to negative threshold stressesor to threshold stresses showing no significant variation with temperature. [1] Y. Li, T. G. Langdon, Mater. Sci. Eng., 1998, A245, 1±9. [2] P. Yavari, F. A. Mohamed, T. G. Langdon, Acta Metall., 1981, 29, 1495. [3] Y. Li and T.G. Langdon: Acta Mater., 1998, vol. 46, pp. 1143-55.

  30. In class M (pure metal type) materials dislocation climb is the rate-controlling mechanism, with a stress exponent of around 5 and an activation energy similar to the value for self-diffusion in the matrix. In class A (alloy type) metals, viscous dislocation glide is the rate-controlling mechanism, with a stress exponent of around 3 and an activation energy associated with the viscous drag of the solute atmospheres. [1] It should be noted that in unreinforced solid solution alloys a transition exists between class M behavior at low stresses to class A behavior at higher stresses.[2] in dilute solid-solution alloys, representing either glide or climb as the rate-controlling process it is possible to modify the standard approach for solid solution alloys. in order to describe the two different classes of behavior in the MMCs [3] e is the solute-solvent size difference c is the concentration of the solvent, is the stacking fault energy of the matrix alloy Dg is the appropriate diffusion coefficient for glide [1] Y. Li, T. G. Langdon, Mater. Sci. Eng., 1998, A245, 1±9. [2] P. Yavari, F. A. Mohamed, T. G. Langdon, Acta Metall., 1981, 29, 1495. [3] Y. Li and T.G. Langdon: Acta Mater., 1998, vol. 46, pp. 1143-55.

  31. [2] [1] [1] Y. Li and T.G. Langdon: Acta Mater., 1997, vol. 45, pp. 4797-4806. [2] Y. Li and T.G. Langdon: Acta Mater., 1998, vol. 46, pp. 1143-55.

  32. Effect of Processing Procedure on the Creep Properties of MMCs It is apparent from Figure 16 that the PM composite is consistently stronger than the IM material under these testing conditions. the higher apparent strength recorded in Figure 16 for the PM composite is due only to the occurrence of a higher value for the threshold stress. It was shown earlier by calculation that the change in slope in Figure 17 is associated with the breakaway of dislocations from their solute atmospheres [1] K.-T. Park, E.J. Lavernia, and F.A. Mohamed: Acta Metall., 1990, vol. 38, pp. 2149-59. [2] Y. Li and T.G. Langdon: Acta Mater., 1997, vol. 45, pp. 4797-4806.

  33. – squeeze casting For the AZ 91 composite: 0.7 to 0.9 V. SKLENICKA, M. PAHUTOVA , K. KUCHAROVA, M. SVOBODA, T.G. LANGDON, Metall. Mater. Trans. A 2002, Vol 33,13, pp 883-889

  34. In the composite, the fibers act as nucleation centers in the precipitation process promoting precipitation of Al2Nd, Mg(Ag)12Nd, and Mg3Agphases 1) Massive precipitates along grain boundaries. 2) Continuous precipitates of bcc in grain interiors. (platelets, responsible for age hardening) 3) discontinuousprecipitates of bcc in grain interiors. (cellular, detrimental to the age-hardening) During the creep exposure, the Mg(Ag)12Nd particles at fibers grew slightly massive precipitates often form a continuous interconnectionamong the alumina short fibers At temperatures above 423 K, these particles are replaced by hexagonal phase or tetragonal Mg(Ag)12Nd precipitates, which nucleate often on dislocations, MgO particles in the vicinity of fibers The creep resistance of squeeze-cast AZ 91 and QE 22 magnesium alloys reinforced with 20 volpct Al2O3 short is shown to be significantly improved by comparison with the unreinforced alloy V. SKLENICKA, M. PAHUTOVA , K. KUCHAROVA, M. SVOBODA, T.G. LANGDON, Metall. Mater. Trans. A 2002, Vol 33,13, pp 883-889

  35. Higher stresses ~> the sudden weakening of composites~> dominance of the load transfer Creep resistance of AZ91 composite > Creep resistance of AZ91 alloy ~> good adhesion between carbon fiber surfaces and the matrix ~> load transfer Creep resistance of QE22 composite < Creep resistance of QE22 alloy ~> interfacial reaction and debonding A strong bonding between carbon fibresurfaces and MgO reaction zones and SiC/matrixinterfaces (Fig. 3a) implies very important role of the load transfer in creep strengthening of the AZ91 alloy M. Svoboda, M. Pahutova, K. Kucharova, V. Sklenicka, K.U. Kainer, Materials Letters 39 _1999. 179–183

  36. The creep resistance of the MMCs is poor by comparison to the unreinforced QE22 alloy. Interfacial sliding may be affected by internal thermal stresses arising at the interfaces on heating the specimen to the test temperature (the thermal expansion coefficients of the metallic matrix and the ceramic particles differ considerably). Since SiC particles are very rigid, interfacial sliding is not accommodated sufficiently and, hence, cavities form at interfaces. [1] [1] Florian Moll, Frantisek Chmelik, PavelLukac, Barry Leslie Mordike, Karl-Ulrich Kainer, Materials Science and Engineering A291 (2000) 246–249

  37. - squeeze casting- machining It is concluded that fibrecracking androrbreakage are not mechanisms controlling the damage process in the high temperature creep of magnesium-based metal matrix composites Inspection reveals a large number of Al2O3 short-fibre breaks and fibrefragmentation due to the high mechanical and thermal loads. Fibre damage was clearly evident in a subsurface zone to approximately 100 m below the specimen surface. Damage A ~> cracking Damage B ~> breakage M. Pahutovaa, J. Brezinaa, K. Kucharovaa, V. Sklenicka a, T.G. Langdon, Materials Letters 39 1999. 179–183

  38. 15%vol. Saffil short fibers and 5% vol. SiCparticulates 10%vol. Saffil short fibers and 10% vol. SiC particulates 10%vol. Saffil short fibers and 15% vol. SiC particulates (longitudinal direction) the plane containing random fiber orientation was parallel to the loading direction A.K. Mondal, S. Kumar, Composites Science and Technology 68 (2008) 3251–3258

  39. Initially the matrix deforms slowly with the generation and movement of dislocations. The movement of dislocations is hindered by the rigid network of the fibers and dislocation pile-ups are formed. As the formation of pile-ups is continued, the local stress around the fibers is increased accordingly. The increase in local stress concentration leads to the fiber fracture. This helps the dislocations start moving, inducing failure of further fibers, thereby acceleratingthe deformation. A.K. Mondal, S. Kumar, Composites Science and Technology 68 (2008) 3251–3258

  40. the broken fibers in the plastic deformation zone are clogged, i.e., all broken fibers are totally entrapped and segregated below and surrounding the indenter so that the further movement of the indenter is slowed down. The broken fibers also change their orientation and tend to align in the direction of flow within the bulk material A.K. Mondal, S. Kumar, Composites Science and Technology 68 (2008) 3251–3258

  41. Viscous glide as well as climb of dislocationis found to be the dominant creep mechanism in the stress and temperature range employed for all the composites. A.K. Mondal, S. Kumar, Composites Science and Technology 68 (2008) 3251–3258

  42. ) – squeeze casting . low-stress regime ~> (Qapp = 48 kJ/mol) = 1/2 energyof grain boundary diffusion in pure magnesium(Qgb = 92 kJ/mol) high-stress regime ~>(Qapp = 230 to 325kJ /mol ) >> activation energy of latticediffusion in magnesium (Q1=35 kJ/mol( The low-stress behavior can be described by an existing model of grain-boundary sliding inhibited by grain-boundaries dispersoids The high-stress behavior ~> threshold stress ~> dislocation climb B.Q. Han, D.C. Dunand, Materials Science and Engineering A300 (2001) 235–244

  43. AE42+alumina short fibers (Saffil)and SiC particles (SiCp)-squeeze casting-T4 The stress exponents calculated were very high, revealing threshold creep behavior as expected from these hybrid composites. The values of effective stress exponents were in general, between the range of 3–7 strongly indicating, that ‘viscous glide and high temperature climb of dislocations’ as the predominant creep mechanism involved. The T4heat-treatment could not provide any remarkable variation in the creep behavior of these composites, thus, it is suggested from the study that these composites be used in its as-cast state. A. Arunachaleswaran, I.M. Pereira, H. Dieringa, Y. Huang, N. Hort, B.K. Dhindaw,, K.U. Kainer, Materials Science and Engineering A 460–461 (2007) 268–276

  44. Mg–14Li–Al–(5-10-15)%vol. MgO/Mg2Si & Mg–14Li–Al - casting For Mg–14Li–Al-MgO/Mg2Si n= 7.38-9.42 Q= 109.9-135.2 kJ/mol For Mg–14Li–Al n= 4.95 Q= 76.67 kJ/mol The more content of MgO/Mg2Si particulates, the smaller interparticle spacing of the MgO/Mg2Si particulates. Constant structure implies that the barrier spacing to a dislocation motion (i.e. either subgrain size stabilisedby particles or the interparticle space) is independent of the applied stress. A stress exponent of >8 is reasonable for the Mg–14Li–Al–MgO/Mg2Si composites because a constant structure presents in the form of subgrainspinned by the MgO/Mg2Si particulates or by the presence of the particulates alone if they are closely spaced and are effective dislocation barriers. The compressive creep rate is controlled by dislocation climb and diffusion of Li in the testing materials. It is possible that the interparticlespacing of the MgO/Mg2Si particulates is smaller than the average subgrainsize, leading to a constant structure condition. X. W. Wei, X. T. Zu, H. Fu and W. L. Zhou, Materials Science and Technology 2006 VOL 22 NO 8 907

  45. Thank for your attention!

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