High Temperature Deformation Mechanisms Research Articles

Construction of processing maps combined with deformation mechanism maps using creep deformation equations

In this study, the construction of processing maps was attempted using the constitutive creep equations for various high-temperature deformation mechanisms to predict the effect of processing parameters (temperature and strain rate), the material constant and grain size on the power dissipation efficiency and the onset condition for flow instability. The developed processing maps were compared with the processing maps constructed using a conventional method, and the usefulness and limitations of the proposed method are discussed. The power dissipation map was constructed by considering the contributions of the creep mechanisms competing with one another. A flow instability map was constructed by observing that the onset of flow instability occurs near the transition from lattice diffusion controlled dislocation climb creep (or other creep deformation mechanisms) to pipe diffusion controlled dislocation climb creep. Following the proposed approach, processing maps and deformation mechanism maps could be incorporated into single maps in 2D or 3D.

Hot deformation behavior and processing maps of an equiatomic MoNbHfZrTi refractory high entropy alloy

The high-temperature deformation mechanisms and processing maps of an equiatomic body-centered-cubic(BCC) structured MoNbHfZrTi refractory high-entropy alloy (RHEA) were studied over the temperatures ranging from 1100 to 1250 °C and different strain rates varying from 10−3 to 0.5 s−1. Flow-stress analyses were performed, using an Arrhenius type relation. The activation energy (Q) for high-temperature deformation was calculated as 326.1 kJ/mol, and the related strain rate sensitivity (m), the power dissipation (η), and instability parameter (ξ) were also derived. The optimal processing area was at 1110–1170 °C and 10−3–10−2.5s−1, with only one unstable region. Detailed microstructural analyses including the surface cracks were carried out in both stable and unstable regions to confirm the derived. Dynamically recrystallized grains with necklace morphologies were observed along grain boundaries and shear bands, indicating that softening was caused by dynamic recrystallization (DRX). The size and proportion of the DRX grains increase with the strain rate decreasing and the temperature increasing.

Elevated temperature compressive deformation behaviors of γ-TiAl-based Ti–48Al–2Cr–2Nb alloy additively manufactured by electron beam melting

Ti–48Al–2Cr–2Nb (Ti4822) alloy was manufactured by the electron beam melting (EBM) process, and its microstructure and compressive deformation behavior at room and high temperatures (25, 600, 750, 900, and 1050 °C) were investigated. In addition, plasma-melted Ti4822 alloy was manufactured as a reference material to compare the microstructure and mechanical properties. EBM-built Ti4822 has a near-gamma structure composed of equiaxed γ phase (L10 structure) with α2 (D019 structure) phase at the interface of γ phase, whereas plasma-melted Ti4822 has a fully lamellar structure. Temperature-dependent compression tests identified that EBM-built Ti4822 has relatively low yield strength in all temperature ranges compared to plasma-melted reference material. However, in the case of ductility, EBM-built Ti4822 has higher fracture strain compared with plasma-melted Ti4822. The reason for this behavior is the microstructural differences found between EBM-built and plasma-melted Ti4822. In the high-temperature compressive results, yield stress anomaly (YSA) phenomena occurred in a certain temperature range with both alloys. The room temperature deformed microstructure shows that EBM-built Ti4822 accommodated deformation by dislocation glide and twinning, while plasma-melted Ti4822 could not fully accommodate the deformation. In addition, dynamic recrystallization (DRX) occurred at above 900 °C in the EBM-built Ti4822, and above 750 °C in the plasma-melted Ti4822, suggesting that different DRX behavior appeared in high temperature deformation. Based on the above findings, this study further analyzed the correlation between the microstructure and the room- and high-temperature deformation mechanism of EBM-built Ti–48Al–2Cr–2Nb.

Theoretical predication of dislocation-included high-temperature deformation mechanism maps for GH4742 alloy

ABSTRACT Based on high-temperature compression tests and focusing on the mechanical behaviour characteristics and microstructure evolution laws during high-temperature compression, laws relating to the strain-rate sensitivity index m and the activation energy of deformation Q-value at varying strain rates and deformation temperatures are clarified. Deformation mechanism maps involving dislocation quantity are plotted. Dislocation evolution laws and deformation mechanisms of the grain size with Burgers vector compensation, and the flow stress with modulus compensation during high-temperature deformation are predicted using the deformation mechanism maps.

A Comparison of Strengthening Mechanisms of Austenitic Fe-13Mn-1.3C Steel in Warm and Cold High-Pressure Torsion

A study on the role of deformation temperature on a twin-assisted refinement of austenitic structure and phase transformations in high-pressure torsion of high-Mn Hadfield steel single crystals (Fe-13Mn-1.3C, in mass. %) has been carried out. In high pressure-torsion, twinning has been experimentally confirmed as a high-temperature deformation mechanism and has been observed at the temperature 400 °C. An increase in deformation temperature of up to 400 °C decreases the activity of mechanical twinning but does not fully suppress it. A dense net of twin boundaries, which has been produced in cold deformation by high-pressure torsion at room temperature, possesses high thermal stability and stays untransformed after post-deformation annealing at a temperature of 400 °C. In high-pressure torsion at a temperature of 400 °C, the complex effect of high temperature and severe plastic deformation on the strengthening of high-carbon Fe-13Mn-1.3C steel has been observed. A synergetic effect of severe plastic deformation and elevated temperature stimulates a nucleation of nanoscale precipitates (carbides and ferrite) along with deformation-induced defects in austenitic structure. These fine precipitates are homogeneously distributed in the bulk of the material and assist high values of microhardness in high-pressure torsion-processed specimens, which is similar to twin-assisted microstructure.

High temperature deformation mechanism and microstructural evolution of relatively lightweight AlCoCrFeNi high entropy alloy

The high temperature compressive characteristics of relatively light weight AlCoCrFeNi HEA were investigated via compression tests in a wide deformation temperature (T) range of 1073–1373 K and strain rate (ε˙) range of 10−3–1 s−1. Hot deformation led to the phase constitution transformation from the disorder BCC (A2) + order BCC (B2) at the room temperature to FCC + A2+ B2 +σ at 1073 K and 1173 K, and then to FCC + A2+B2 phase at 1273 K and 1373 K. The kinetic analysis was performed based on the flow stress data by using the hyperbolic sine law constitutive models with a higher the correlation coefficient (R2). Using combination of kinetics analysis and microstructure evolution results, it can be concluded that there was a gradual transformation of the dominant deformation mechanism from the symbiosis effects of grain boundary sliding (GBS) in fine grains and local dislocation gliding in coarse grains at 1073 K to the dislocation climbing mechanisms at 1373 K. The flow stress decreased with the decreasing strain rate and/or increasing true strain, as well as the deformation temperature. At 1073 K and 1173 K, the flow softening was primarily associated with the GBS of ultrafine dynamic recrystallization (DRX) grains in the local phase transformation regions, as well as the dynamic precipitation of FCC phases. While, the softening effect was primarily controlled by the dynamic precipitation of FCC phases and dynamic recovery (DRV) at 1273 K. When the deformation temperature rose to 1373 K, the main softening mechanisms were the continuous DRX (CDRX) and DRV, as well as the coarsening and dissolution of A2 phases. Moreover, the clear serration behaviors at 1373 K/1s−1 and 1373 K/10−3s−1 were induced by the interaction effect between the dislocation and cluster.

New insights into high-temperature deformation and phase transformation mechanisms of lamellar structures in high Nb-containing TiAl alloys

The paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [Song, et al. Intermetallics 109 (2019) 91–96], in which a unique twin-like morphology in the α2 (Ti3Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α2 phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α2, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α2 phase is a diffusive-displacive process. Within the α2 phase, ωo is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.

Microstructure and mechanical properties of (B4C+Al2O3)/Al composites designed for neutron absorbing materials with both structural and functional usages

To meet the demand for the new generation of neutron absorber materials (NAMs) for the dry storage of the spent nuclear fuels, (B4C + Al2O3)/Al composites were fabricated by powder metallurgy technique using ultrafine Al powders. The composites designed with various fabricating parameters and fabricated at various sintering temperatures were characterized by electron microscopy and mechanically tested. The sample sintered at 450 °C shows the best strength-ductility balance at 350 °C (106.2 MPa in ultimate tensile strength and 9.6% in elongation). Addition of B4C particles and increase of the Al2O3 film thickness could enhance the strength of the composites at room temperature but showed no obvious effect on the strength at 350 °C. When sintering temperature of the composites increased from 450 °C to 550 °C, the transformation of amorphous Al2O3 lamellae to γ-Al2O3 particles led to deterioration of the strength of the composites. Based on the analyses of both high-temperature deformation mechanism and strengthening mechanism, it was considered that the amorphous Al2O3 could pin the grain boundaries and prevent them from gliding, which was the main factor to significantly increase the high-temperature strength. Based on the results, a strategy to design the aluminium matrix NAMs with excellent high-temperature strength was proposed.

Effects of Al on the mechanism of high temperature deformation of Mg-xAl alloys

High-temperature tests were conducted to investigate the tensile behaviors of Mg–xAl (x = 2, 3, 4, 6, 9 wt%) alloys. The mechanisms of deformation were investigated by analyzing the microstructures, stress exponents and deformation activation energy values. The results show that with the increase in the temperature and the decrease in the logarithmic strain rate, the mechanism of deformation of the Mg–xAl alloys changes from dislocation climb creep to solute drag creep, differing from the pure Mg that is consistently controlled by dislocation climb creep. Three regions are classified based on the stress exponents of the Mg–xAl alloys, and it is found that the stress exponent decreases with the increase in the Al content in each region. The Mg–xAl alloys exhibit enhanced tensile ductility of greater than 100% at 450 °C and 1 × 10−3 s−1, which is attributed to the solute-drag creep with a lower stress exponent.

The effect of Al to high-temperature deformation mechanisms and processing maps of Al0.5CoCrFeMnNi high entropy alloy

The high-temperature deformation mechanism and processing maps of cast Al0.5CoCrFeMnNi that comprises face centered cubic (FCC) and body centered cubic (BCC) phases (where BCC phase is a minor phase) were studied at temperatures in the range of 1023–1323 K and at strain rates in the range of 10−3 to 101 s−1. During hot compressive deformation, the hard BCC phase provided nucleation sites for dynamically recrystallized (DRXed) grains in the soft FCC matrix through particle stimulated nucleation (PSN). The PSN-induced DRX occurred by continuous dynamic recrystallization (CDRX) and CDRX accelerated as temperature increased. Due to the effect of PSN, the fraction of DRXed grains was notably higher and the size of the DRXed grain size was considerably smaller in Al0.5CoCrFeMnNi than those in the equiatomic CoCrFeMnNi with a single FCC phase. The effect of the BCC phase on the fraction of DRXed grains was pronounced even at the high strain rate of 10 s-1 in the temperature range between 1173 and 1323 K, which is advantageous for the practical use of cast Al0.5CoCrFeMnNi in the hot-working industry. Solute drag creep appeared at low strain rates and at high temperatures as the rate-controlling deformation mechanism due to the presence of Al solutes in the FCC matrix phase. The activation energy for plastic flow associated with solute drag creep was estimated to be 251 kJ/mol. This value most likely represents the activation energy for the diffusivity of aluminum solutes in Al0.5CoCrFeMnNi. The solute drag creep behavior could be predicted by the Weertman model, suggesting that Al0.5CoCrFeMnNi behaves similar to a typical class I type solid solution metal alloy. By adding aluminum to CoCrFeMnNi, the hot workability can be greatly improved according to the processing maps. The power dissipation efficiency notably increases, and the flow instability regime decreases in size.

Hot working process optimization of Fe–20Mn–19Cr–0.5C–0.6N steel by activation energy, power dissipation and microstructural evolution

Hot deformation behavior of an Fe–20Mn–19Cr–0.5C–0.6N high-nitrogen austenitic steel has been studied by isothermal compression tests in deformation temperature range of 800–1200 °C and strain rate range of 0.01–10 s−1. Results indicate that the Fe–20Mn–19Cr–0.5C–0.6N steel has high deformation resistance due to strong hindering effect on dislocation moving by nitrogen-induced lattice misfit. The twinning-induced plasticity effect is gradually suppressed with the increase in deformation temperature, and high-temperature plastic deformation mechanism by twinning is gradually replaced by dislocation planar slip. The deformation resistance is up to 343 MPa at deformation conditions of (1000 °C, 0.01 s−1), which is over 100 MPa higher than that in martensitic steel and 50 MPa higher than that in austenitic steel. Besides, value of deformation activation energy for the Fe–20Mn–19Cr–0.6N steel is up to 784 kJ mol−1. Power dissipation efficiency is lower than 0.13, while hot processing map exhibits a very wide range of working area. The optimum hot working process obtains at deformation temperature range of 950–1200 °C and strain rate range of 0.01–10 s−1, when deformation activation energy is less than 662.6 kJ mol−1, power dissipation efficiency exceeds 0.22, dynamic recrystallization fraction is over 46.1% and microstructures are without instable characteristics.

Effect of Annealing Temperature on Microstructure and Mechanical Properties of a Severe Cold-Rolled FeCoCrNiMn High-Entropy Alloy

An ultrafine-structured FeCoCrNiMn high-entropy alloy (HEA) was produced by severe cold rolling (SCR) with a 95 pct reduction, and the effect of annealing temperature over a wide range from 573 K to 1373 K (300 °C to 1100 °C) on the microstructure and mechanical properties of this SCRed HEA was systematically investigated. The results show that the microstructure of the single-phase FCC HEA after SCR is greatly refined and mainly comprises ultrafine crystalline, high-density dislocation cell and networks, which substantially improve the hardness and strength. The SCRed HEA starts to recrystallize at an annealing temperature of 773 K (500 °C) and is fully recrystallized at 973 K (700 °C) after 1 hours. The σ phases with a tetragonal structure form upon annealing at 773 K (500 °C), leading to a further enhancement in hardness and strength for the annealed HEA compared to the SCRed HEA. The average grain size remains below 500 nm when the SCRed HEA is annealed at 923 K (650 °C) for 1 hours, indicating that the ultrafine-structured HEA after SCR and subsequent annealing treatment possesses excellent microstructural thermal stability. The SCRed HEAs annealed at 923 K and 973 K (650 °C and 700 °C) exhibit excellent high strength–ductility combinations due to the formation of the fully recrystallized ultrafine-grained microstructure. The strain-rate sensitivity parameter (m value) with different grain sizes, from ultrafine to coarse, was evaluated by strain-rate jump tests. The m value of the HEA increases monotonically with increasing grain size and annealing temperature. Moreover, high-temperature tensile tests demonstrated that the annealed HEAs exhibit superplastic deformation behavior at 973 K and 1073 K (700 °C and 800 °C), and the formation of a Cr-rich phase is found during high-temperature deformation. It is believed that the plastic deformation facilitates the phase decomposition of fine-grained HEAs and grain boundary sliding is the key mechanism of high-temperature deformation.

High-temperature deformation mechanisms and processing maps of equiatomic CoCrFeMnNi high-entropy alloy

The hot compressive deformation mechanism and processing maps of the equiatomic FCC CoCrFeMnNi high-entropy alloy (HEA) were studied in the temperature range between 1023 and 1323 K and in the strain rate range between 10−3 and 10 s−1. At high strain rates above 1 s-1, strain hardening was dominant even at the very high temperature of 0.84Tm, which may be attributed to the sluggish diffusion coefficient and low stacking fault energy of the CoCrFeMnNi HEA, leading to suppression of dynamic recovery. According to the processing maps, the best condition for hot working was near 10−3 s−1 at 1323 K. Power-law breakdown and unstable flow occurred at low temperatures and high strain rates where the strain hardening was pronounced. The activation energy for plastic flow measured in the power-law creep regime when considering the dependence of elastic modulus on temperature was 312.2 kJ/mol; this value is close to the activation energy for the weighted diffusion coefficient calculated by weighting the contribution of each element in the CoCrFeMnNi HEA (284 kJ/mol). The size and fraction of the dynamically recrystallized grains increased as the strain rate decreased and the temperature increased, as in conventional metals. Both discontinuous dynamic recrystallization and continuous dynamic recrystallization (CDRX) occurred. CDRX became more distinct as the temperature increased. The deformation mechanism and behavior of the CoCrFeMnNi HEA were very similar to those of FCC pure metals in terms of the stress exponent and the effect of the stacking fault energy and diffusivity on the creep rates.

High-temperature deformation mechanisms and physical-based constitutive modeling of ultra-supercritical rotor steel

This study presents an investigation that characterizes the evolution of the dynamically recrystallized structure and flow behavior of FB2 rotor steel during hot deformation in the temperature range from 900 °C to 1200 °C at strain rates from 0.001s−1 to 0.1s−1 using Gleeble-3500 thermo-simulation machine. Dynamic recrystallization (DRX) plays a key role in softening mechanism for hot deformed FB2 rotor steel. The fraction of LABs greatly increases with the decrease of deformation temperature. Low angle boundaries (LABs) always have higher local misorientation angle, which means the lower deformation temperature is capable of providing not enough driven force for the migration of LABs and thereby decelerates the DRX process to consume the dislocations. Furthermore, the critical stress and critical strain for initiation of DRX are calculated by setting the second derivative of the third order polynomial. Based on the classical stress–dislocation relation and the kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of FB2 steel. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformations for the studied steel.

Effect of Strain Rate and Temperature on the Tensile Flow Behavior and Microstructure Evolution in Fe-0.3 Pct C-CrMoV Grade Steel

The effect of temperature and strain rate on the tensile flow behavior of Fe-0.3 pct C-CrMoV grade steel was studied over a wide range of strain rates (10−4 to 10−1 s−1) and temperatures (700 °C to 950 °C). The flow curves of the steel showed typical dynamic recovery (DRV)-type characteristics at low temperature, high strain rate, and dynamic recrystallization (DRX) type at high temperature > 775 °C. Stress regimes with stress exponent (n) of 3.6 to 5.5 for low–high stresses were observed. The ‘n’ values at temperatures of 850 °C and 900 °C were found to be > 4, which correspond to dislocation climb as the rate controlling mechanism. At 950 °C, ‘n’ value was found to be < 4, where viscous glide is the rate controlling mechanism. The apparent activation energy (Q) was found to be 320 ± 12 kJ mol−1. Hence, the dominant high-temperature deformation mechanism was identified as high-temperature climb of edge dislocations. The strain rate sensitivity index (m) of the steel was evaluated using jump strain rate tests and cyclic temperature and strain rate jump tests over temperatures of 700 °C to 950 °C and strain rates of 10−4 to 10−3 s−1 . Although, ‘m’ value as high as 0.5 was observed, cavitation resulted in premature failure during deformation resulting in low elongation. The volume fraction of cavities was inversely proportional to the strain rate at all temperatures. The fine-grained microstructure aids grain boundary sliding, and diffusion thereby favors the cavity growth at low strain rates. Microstructures evolved during the high-temperature tensile tests were analyzed and the optimum conditions for hot deformation i.e., hot rolling/hot forming schedules were determined as the temperature range of 850 °C to 950 °C and strain rate range of 10−3 to 10−4 s−1. The flow stress data for the steel were found to follow the universal Dorn sine hyperbolic equation.

High-Temperature Deformation Behavior of Ti-6Al-2Sn-4Zr-2Mo Alloy with Lamellar Microstructure Under Plane-Strain Compression

High-temperature plane-strain compression of a Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy with a lamellar structure was applied by a thermal simulation machine Gleeble 3800 at a temperature of 1223-1370 K and a strain rate of 0.01-10 s−1. Constitutive relations between the flow stress and strain at different temperatures and strain rates were constructed based on the Arrhenius equation. Processing maps based on the dynamic material model at strains of 0.5, 1.0, and 1.25 were also constructed to analyze the mechanism and instability of high-temperature deformation. Dynamic recrystallization was found to occur at 1223-1313 K and 0.01-0.1 s−1, with a peak efficiency of power dissipation of 70%. Observations of the microstructure demonstrated that α platelets and dynamic globularization were responsible for dynamic softening in the α + β phase field. Flow instability behaviors occurred at 1263-1343 K/1.78-10 s−1, 1233-1283 K/0.1-3.16 s−1, and 1353-1373 K/0.56-1.78 s−1, and adiabatic shear bands and microcracks were observed in these domains. Thus, for the lamellar Ti-6242 alloy, the recommended hot rolling parameters were 1243-1333 K and 0.01-0.1 s−1. Electron backscattered diffraction results revealed that the dynamic restoration mechanism in the β phase field was dynamic recovery when the strain rate was as high as 10 s−1.

Creep study on alumina and alumina/SWCNT nanocomposites

Alumina and alumina/SWCNT nanocomposites have been sintered by spark plasma sintering, obtaining relative densities higher than 99%. Microstuctural characterization revealed a grain microstructure in the submicron range, where alumina/SWCNT nanocomposites exhibited a good CNTs dispersion thought the ceramic matrix. Creep experiments performed in both materials showed a similar mechanical behavior, where the addition of CNTs seems to have a negligible effect on the strain rate, in contrast to results reported by other authors. Grain boundary sliding accommodated by lattice diffusion has been identified as the high temperature deformation mechanisms in both samples, alumina and alumina/SWCNT nanocomposites. We have discussed about the role of CNTs influence on the plasticity of these composites.

The temperature dependence of high-temperature strength and deformation mechanism in a single crystal CoNi-base superalloy

The thermal stability and high-temperature strength of single crystal CoNi-base superalloys, Co-30Ni-10Al-7W (at%) and Co-30Ni-10Al-7W-2Ta-3Ti (at%), containing γ′(L12) phase have been investigated. The investigated CoNi-base superalloy with alloying additions of Ti and Ta exhibits excellent γ′ solvus temperature, 1236 ℃, and high-temperature strength, 622 MPa at 900 ℃, compared to reported γ′-strengthening Co-base superalloys. Detailed analyses of the deformation mechanisms for the investigated alloys were carried out by using the transmission electron microscopy (TEM) characterization. Two different transitions of deformation mechanisms for the γ′ phase, from superpartial dislocation shearing to dislocation bypassing and to stacking fault shear, occurred in the investigated alloys as the deformation temperature increased above the peak temperatures. Meanwhile, the high-temperature strength of the investigated alloys largely decreased as the transitions of deformation mechanism took place. The influence of alloying effects on energies of faults and the relationship between high-temperature strength and deformation mechanisms were discussed.

High-temperature tensile properties and deformation mechanism of polycrystalline magnesium alloys with specifically oriented columnar grain structures

A Mg–4.78Zn–0.45Y–0.10Zr (wt%) alloy with specifically oriented columnar grain structures (crystal growth direction 〈101¯0〉) was prepared using directional solidification. The columnar grain structures possessed parallel-growing primary arms and straight longitudinal grain boundaries. The results from an electron backscatter diffraction analysis demonstrated that the angle between the c-axis and the growth direction was 74–87°. As the tensile tests were performed along the 〈011¯0〉 growth direction, the Schmid factors for the <a> cylindrical and <c+a> conical plane slips were as high as 0.45. When the tensile test temperature was 300 °C, dynamic recovery was observed and the <a> cylindrical, <a> conical, and <c+a> conical slip systems were all activated. This phenomenon, together with the high orientation consistency between the adjoining grains, resulted in an elongation increase up to 42%. Fractography of the Mg alloy revealed extended or parabola-shaped dimples with specifically oriented columnar grain structures, suggesting good ductility for this material.

Cooperative grain boundary sliding in creep deformation of FeCrAl-ODS steels at high temperature and low strain rate

The high-temperature deformation mechanism of the FeCrAl-ODS ferritic steel was investigated at 1000 °C for the creep loading perpendicular to the elongated and aligned grains. The strain rate was varied in the range from the order of 10−2 to 10−7s−1. With decreasing strain rate from 10−2 to 10−5s−1, creep mechanism shifts from conventional dislocation creep pinned by oxide particles to grain boundary sliding (GBS) assisted concomitantly by diffusional creep. With further decreasing strain rate to 10−7s−1, deformation mechanism is drastically changed; group of three grains can move cooperatively, and cooperative GBS (CGBS) was originally recognized. The threshold stress for onset of CGBS was designated as σthI(CGBS). Rate limiting process of CGBS is dominated by dislocation movement over the oxide particles so as to relieve the stress accumulation due to CGBS. The σthI(CGBS) for CGBS corresponds to one third of the conventional threshold stress for dislocation creep.