Investigation of time dependent deformation behaviors by creep test Chapter Investigation of time dependent deformation behaviors by creep test 6.1 Introduction More than one creep mechanism frequently operates at the same time and the main creep deformation mechanisms can be categorized as follows [1]: (a) Dislocation glide - involves dislocations moving along slip planes and overcoming barriers by thermal activation. This mechanism occurs at high stress, /G >10-2 where G is shear modulus. (b) Dislocation creep - involves the movement of dislocations which overcome barriers by thermally assisted mechanisms involving the diffusion of vacancies or interstitials. It occurs for 10-410-2. (c) Diffusion creep - involves the flow of vacancies and interstitials through a crystal under the influence of applied stress. It occurs for /G [...]... (1  2v  ) 16 Dl  GG  (1  v  ) 2 t (6. 14) where G* is the shear modulus (210 GPa for AlN) and * the Poisson’s ratio of the reinforcement (0.287 for AlN) On the other hand, the strain rate from GBS is given by [57]:   a 3dt (6. 15) where a is the unit of sliding (=b) and t the period of the time over which sliding occurs By combining equations (6. 14) and (6. 15), the critical strain rate for... strength and ductility [58,59] It can also result from stress-assisted grain growth developed in- situ during constant stress testing [60 ] Nc materials are composed of small grains that possess large driving force for grain growth, and the applied stress during plastic flow can also be very high From the practical point of view, particle (10-274 nm range in AlN particulates) and grain size (20-70 nm range in. .. (equation 6. 6) 1E-4 1E -6 (s-1) 1E-5 1E-7 1E-8 1E-9 1E-10 4 10   (M P a) 100 Figure 6. 4 Comparison of theoretical predictions and experimental results In diffusional creep, energy dissipation occurs in the following three irreversible processes [32]: (i) diffusional transport of atoms, (ii) grain-boundary sliding and (iii) interfacial reaction for the sink and the creation of vacancies in grain boundaries... piled up at the particles since the gliding dislocations cannot bypass the particles Besides the low volume fraction of second phase constituents, since the Mg grain size is in nanoscale and thus less likely for the reinforcement and in- situ formed particles to exist in the interior of the Mg grain, this effect should be negligible in the present material An appropriate grain size distribution could... particles residing at the grain boundaries Therefore, the stress concentration is suppressed for continuous sliding to take place, avoiding cavity formation as seen in Fig 6. 5 Instead of the particles at the grain boundaries impeding GBS leading to stress concentration, they facilitate to enhance sliding and hence show a much higher creep strain rate practically Grain 1 Grain 3 Grain 2 Figure 6. 5 Schematic... the origin of nano-voids The enhanced diffusivity and low activation energy for grain boundary diffusion may be understood in terms of free volume in the cores of the grain boundaries of nc materials [ 36- 38] An essential change in the grain boundary state in metals is caused by diffusion fluxes of impurity atoms from external sources It has been reported that the effect of diffusion induced loss of strength... of particles located at the grain boundaries Deformation in the aggregate of the grains cannot occur by diffusion alone Lifshitz [44] and Stevens [45] pointed out that GBS is necessary in creep deformation in order for the relative motion of individual grains to accommodate the macroscopic deformation There are two possible sliding modes during the creep deformation of MMC: (a) grain boundary sliding... dissipated in both the grain-boundary sliding and interfacial reaction is negligibly small for conventional grain sizes since grain boundaries will act as perfect 143 Investigation of time dependent deformation behaviors by creep test sinks and sources of vacancies [32] Therefore, the rate of total energy dissipated as heat can be equated to that of energy dissipated in the diffusional process In such... is the fraction of the total strain carried by GBS, d the grain size, dp the particle diameter and T the absolute temperature (Room temperature 298K) The critical strain rates are plotted as a function of effective applied stress at room temperature as illustrated in Fig 6. 6, where d, dp and c are taken to be 33 nm, 274 nm and 0 .6 [47] respectively As shown in Fig 6. 6, the critical strain rates at various... around the AlN particles when diffusional assisted flow operates as an accommodation process In such a case, superplastic-like large elongation is unlikely to be attained in the present Mg composite samples as verified by the highest attainable tensile elongation of only 351% Except for 1 wt% of AlN reinforcement, additional ~0.11% of second phase particles (MgAl2O4, MgO, etc.) of size between 4 and 10 . for grain boundary diffusion may be understood in terms of free volume in the cores of the grain boundaries of nc materials [ 36- 38]. An essential change in the grain boundary state in metals. the unit of sliding (=b) and t the period of the time over which sliding occurs. By combining equations (6. 14) and (6. 15), the critical strain rate for the relaxation at the intragranular particle, 3c   ,. likely for the reinforcement and in- situ formed particles to exist in the interior of the Mg grain, this effect should be negligible in the present material. An appropriate grain size distribution