Grain Boundary Sliding Research Articles

Creep and Superplasticity of Gadolinium‐Doped Ceria Ceramics under AC Electric Current

Shaping of dense ceramics is difficult due to their inherent brittleness. Nanograined ceramics like tetragonal zirconia (TZP) can be superplastically deformed and shaped at high temperatures owing to grain boundary sliding (GBS). Herein, the enhanced plasticity of gadolinium‐doped ceria (GDC) ceramics under mild and strong AC electric current in terms of steady state creep rate under both compressive and tensile loading is demonstrated. A current density of 25 and 200 mA mm−2 is used for the creep deformation. The creep rate increases by up to two orders of magnitude under electric current. The stress exponent remains unchanged for creep experiments at 1200 °C with and without electric current, suggesting a GBS mechanism of plastic deformation in both cases. The field‐enhanced creep rate is attributed to the interaction of space–charge layer and the electric field resulting in enhanced GBS. A higher current density results in enhanced ductility of GDC even when the Joule heating effect is compensated by reducing the furnace temperature.

Finite element simulations and experimental investigations on formability of magnesium alloys in superplastic forming

The paper presents the formability aspects of magnesium alloy (AZ31B) sheets into regular and intricate shapes during Super Plastic Forming (SPF). Finite Element (FE) simulations are performed at specific strain rates within the superplastic temperature range. The formability of 1 mm sheet into conical, cylindrical, box and groove shaped dies are simulated. The simulated Pressure-Time (P-T) curve is given as pressure control algorithm to form the shapes experimentally. Thickness strains of the experimentally formed components validated with simulation results are found to be in close agreement with each other. It is demonstrated by the analysis that regular shapes posses uniform thickness distribution throughout the profile. The components with complex intricate shapes recorded unevenness in thinning behavior. The unevenness in thinning is influenced by the Grain Boundary Sliding (GBS) and diffusion kinetics. GBS is well controlled by optimum P-T algorithm to produce sharp intricate shapes with uniform thickness.

Superplasticity of a hot rolled Mg–3Zn–0.5RE–0.5Zr (ZEK300) alloy

Superplasticity of a hot rolled fine-grained Mg–3Zn–0.5RE–0.5Zr (ZEK300) alloy was assessed via shear punch testing (SPT). The deformation behavior of the alloy having a grain size of 4.5 μm exhibited regions I, II and III, typical of superplastic materials. In region II, the strain rate sensitivity indices of ZEK300 alloy were obtained as 0.51, 0.48 and 0.41 at 350, 400 and 450 °C, respectively. The average activation energy of 87.6 kJ mol–1 implies that the principal mechanism of deformation is grain boundary sliding (GBS) facilitated by grain boundary diffusion.

Realizing impressive superplasticity in a low-alloyed Mg-Zn-Ca-Al-Mn alloy: The roles of grain boundary segregation and dense β-Mn particles

Achieving impressive superplasticity is an important strategy to manufacture Mg alloy products with complex shapes. In the present study, we report that an excellent superplastic deformation with elongation larger than 500% can be achieved at 623 K and 1.0 × 10−3s−1 in a Mg-1.51Zn-0.59Ca-0.59Al-0.70Mn (wt.%, ZXAM2111) alloy fabricated by equal-channel angular pressing. The superplastic deformation is mainly carried by grain boundary sliding (GBS), accompanied by a grain size growth from ∼3.0 µm to ∼6.0 µm after deformation. Before deformation, the ZXAM2111 alloy is mainly characterized by a strong co-segregation of Zn and Ca atoms at grain boundaries and uniformly distributed β-Mn particles. With deformation proceeding, the β-Mn particles further dynamically precipitate along grain boundaries that parallel the tensile axis, leading to improved resistance to grain coarsening. Although the enhanced stabilizing effects decrease the strain rate sensitivity value, the resulting impressive microstructure stability provides a cornerstone of the active operation of GBS, facilitating the achievement of superplastic deformation. The present work could provide insight into developing low-alloyed Mg alloys with high microstructure thermal stability to achieve superplasticity.

Dynamic recrystallization-dependent high-temperature tensile properties and deformation mechanisms in Al-Mg-Sc-Zr alloys

This research elucidates discontinuous dynamic recrystallization (DDRX)-dependent high-temperature tensile properties and deformation mechanisms in simply extruded Al-7Mg alloys tailored by Sc/Zr ratios. The bimodal-grained Al-7Mg-0.4Sc alloy presents a high elongation to failure of ∼540% at 400 °C and 5 × 10−4 s−1. However, such superplastic behavior has not been observed in both coarse-grained Al-7Mg-0.1Sc-0.3Zr and Al-7Mg-0.3Sc-0.1Zr alloys. Thermally stable dispersed nano-sized Al3(Sc,Zr) particles strongly retard DDRX, accompanied by the retention of heterogeneous grain structure in Al-7Mg-0.1Sc-0.3Zr till fracture. The predominance of dislocation slip is proved by analyzing the evolution of texture and dislocations. In contrast, the bimodal grain structure evolves into a homogeneous fine one in Al-7Mg-0.4Sc with tensile deformation proceeding since coarsening Al3Sc precipitates play a weak role in restricting DDRX. The superplasticity in Al-7Mg-0.4Sc is achieved by cooperated mechanisms, i.e., grain boundary sliding (GBS) and dislocation slip as dominant mechanisms, which is accommodated by DDRX in the initial tensile deformation stage, followed by dislocation slip-accommodated GBS in the late stage.

Effect of strain rate on the microstructure evolution and superplastic deformation behavior of cold-rolled medium Mn lightweight steel

This work investigates the high-temperature tensile behavior of a Medium Mn lightweight steel (MMLS) with a chemical composition of Fe-0.3C-7.0Mn-3.5Al. The MMLS was prepared by hot-rolling followed by cold-rolling to produce a complete martensite microstructure with a high density of dislocations. The tensile tests were conducted at 800 °C within the strain rate range of 5 × 10−4 s−1-2.5 × 10−2 s−1 to explore the effect of microstructural evolution and deformation mechanism by strain rates during the tensile deformation. The results showed that the deformation behavior and mechanism of the MMLS depend not only on the initial grain size and morphology but also on the microstructure evolution during deformation. The MMLS exhibits high strain rate superplasticity (HSRS) behavior, and the deformation mechanisms are dependent on the strain rate. At lower strain rate (5 × 10−4 s−1), dislocation slip/creep is more pronounced in the γ-phase, and grain boundary sliding (GBS) is inhibited during the early deformation period, contributing to the significant texture enhancement. At moderate rate (5 × 10−3 s−1), GBS dominates deformation, while at high strain rate (2.5 × 10−2 s−1), dislocation activity and dynamic recrystallization (DRX) play a significant role. The HSRS behavior of MMLS is related to the dynamic reverse phase transition from cold-rolled ferrite to austenitic/ferrite fine-grained dual-phase structure. This work provides a theoretical basis for the development of superplastic MMLS with improved HSRS behavior and controlled the microstructure/mechanical property of component parts.

The effect of Ti-Mo additions on microstructural evolution and superplastic deformation behavior of cold–rolled medium Mn steels

This study investigated the effect of Ti-Mo alloying on the microstructural evolution and superplastic deformation behavior of Fe–5.6Mn–1.1Al–0.2C steels. The steels were subjected to cold–rolling and intercritical deformation at a strain rate of 10−2 s−1 and temperatures between 655 and 775 °C. The results showed that Ti-Mo added steel exhibits a significant improvement in the strength due to the high density of nano–sized (Ti, Mo)C particles and optimized superplastic elongation of approximately 1000% at 745 °C. The superplastic flow behavior showed a strong temperature dependence under three typical temperature regions. This can be mainly attributed to the decreased interaction between precipitation and DRX with increasing deformation temperature. The grain boundary sliding (GBS) accommodation process varied with the deformation temperature due to the difference in precipitation nature of the (Ti, Mo)C particles.

Effects of strain rate and current density on plastic deformation of zirconia ceramics under a high electric field

The deformation behavior of ZrO2 ceramics were systematically investigated at different strain rates and current densities under a high electric field. We showed that the ceramic exhibited plastic deformation when the strain rate was lower than a certain value. Microstructure and hardness measurements revealed that rapid and stable deformation can be obtained at high current density and moderate strain rate. The strain rate of 7.2 × 10−3 s−1 can be achieved at the furnace temperature of 600 °C without decrease of mechanical properties. The deformation mechanisms were dislocation accommodated grain boundary sliding (GBS) with different control processes. The results showing here are very important to solve the critical problems of difficult to form ceramic materials.

Microstructural Changes Caused by the Creep Test in ZK60 Alloy Reinforced by SiCp at Intermediate Temperature after KOBO Extrusion and Aging.

In this study, we investigated the creep properties of ZK60 alloy and a ZK60/SiCp composite at 200 °C and 250 °C in the 10-80 MPa stress range after the KOBO extrusion and precipitation hardening process. The true stress exponent was obtained in the range of 1.6-2.3 for both the unreinforced alloy and the composite. The apparent activation energy of the unreinforced alloy was found to be in the range of 80.91-88.09 kJ/mol, and that of the composite was found to be in the range of 47.15-81.60 kJ/mol, and this indicated the grain boundary sliding (GBS) mechanism. An investigation of crept microstructures using an optical microscope and scanning electron microscope (SEM) showed that at 200 °C, the predominant strengthening mechanisms at low stresses were the formation of twin, double twin, and shear bands, and that by increasing the stress, kink bands were activated. At 250 °C, it was found that a slip band was created in the microstructure, and this effectively delayed GBS. The failure surfaces and adjacent regions were examined using SEM, and it was discovered that the primary cause of failure was cavity nucleation around precipitations and reinforcement particles.

Effect of dislocation-induced aging precipitate bands on creep resistance of Mg-Gd-Y-Zr-Ag alloy

This work investigates the effect of dislocation array induced β’ precipitation bands on the creep behavior in wrought Mg-8.9Gd-1.8Y-0.5Zr-0.2Ag (wt%) alloy. The alloy with β’ precipitate bands shows significantly enhanced creep resistance ranging from 448 to 548 K. Pipe diffusion-controlled grain boundary sliding (GBS) is suggested to be the main steady creep mechanism below 498 K. Cross-slip is suggested to be the main steady creep mechanism above 498 K. The characterization indicates that the precipitate bands separate the matrix into multiple units within the grains and restrict the long-distance slip of the dislocations. Bands with high thermal stability limit the diffusion of vacancies across the grain and prevent precipitation free zone (PFZ) broadening on the grain boundaries. For the alloy without precipitate bands, basal slip is initiated before the formation of β’, and it is easy to form broad PFZs on grain boundaries. Although cross-slip is still unavoidable at 523 K, these results provide guidance for improving the creep resistance of the deformed Mg-Gd series alloys.

Systematic analysis of distinct flow characteristics and underlying microstructural evolution mechanisms of a novel fine-grained P/M nickel-based superalloy during isothermal compression

Isothermal forging (IF) is an effective method for forming difficult-to-deform materials like P/M superalloys. Understanding the isothermal compression microstructural evolution mechanism of a novel P/M superalloy provides the basis for its optimized IF planning. In this study, the isothermal compression tests of a novel fine-grained P/M nickel-based superalloy were carried out at 1000–1150 °C with strain rates of 0.001–0.01 s−1. The results indicated that the alloy exhibits three distinct flow characteristics: continuous softening after reaching the peak stress, near-steady superplastic flow, and discontinuous hardening, corresponding to different strain rate sensitivity exponent (m) values. Varied microstructural evolution mechanisms, including grain boundary sliding (GBS), dynamic recrystallization (DRX), and grain growth, are dominated in different m-value domains. Meanwhile, different roles of primary γ' play in microstructural evolution were clarified. A moderate fraction of primary γ' with 8.5%–14.2% can well coordinate the GBS and hinder excessive grain growth at a high m value domain (m > 0.4). When 0.2 < m < 0.4, the role of the primary γ' is changed to promote dislocation accumulation, accelerating the nucleation of DRXed grains. As the primary γ' is dissolved at 1150 °C, obvious grain growth was observed after compression. Work hardening effect by overgrown grains competed with DRX softening results in the discontinuous rising stress.

Achieving superior superplasticity in CoCrFeNiCu high entropy alloy via friction stir processing with an improved convex tool

The investigation of the superplastic behavior of HEAs is significant since superplastic forming shows a great potential in fabricating complex components of HEAs. In this study, the superplastic behavior of the CoCrFeNiCu HEA fabricated by friction stir processing (FSP) was first time investigated at the temperature range of 900–950 °C and strain rate range of 3 × 10−4–3 × 10−2 s−1. The ultrafine grained structure with 320 nm in grain size and 91% of high angle grain boundaries (HAGBs) was achieved by FSP with the improvement of stirring tool based on the characteristics of HEAs, which exhibited a good thermal stability when annealing at 900 °C. The FSP CoCrFeNiCu HEA exhibited a largest elongation of 620% and a low flow stress of 5 MPa at 950 °C and 3 × 10−3 s−1. The flow stress in this study is the lowest in all the HEAs reported during superplastic deformation, which shows the great potential in the practical superplastic forming application. This is the first time to report the excellent superplasticity of dual-phase FCC HEAs. The superplastic deformation mechanism was mainly dominated by grain boundary sliding (GBS), accommodated by the diffusion of Cu-rich second phase. The failure mechanism was related to the aggregation of cavities. The excellent superplastic property was attributed to the stable equiaxed ultrafine microstructure, the high HAGB ratio, and soft Cu-riched second phase, which promoted the occurrence of GBS and its accommodation behavior. This study provides a way for fabricating superplastic HEAs and theoretical basis for superplastic forming of HEAs.

A high temperature manufacturability study of ultrafine grained Magnesium Rare-Earth alloy using processing map and constitutive analysis

The rare-earth (RE) contained ultrafine grained (UFG) Magnesium (Mg) alloys are excellent materials for aerospace and automobile industries due to their ultra-light weight, high strength, reduced tension to compression yield asymmetry, etc. Despite their excellent properties, the manufacturing potential of UFG Mg-RE alloys is yet to be realized. The present work is focused on engineering UFG microstructure in a Mg-Ag-Nd alloy (a Mg-RE alloy) and establishing the manufacturing potential of such engineered UFG Mg-RE alloy via processing map and constitutive analysis studies. The processing map is developed for a temperature range of 300 °C-450 °C and a strain rate range of 0.001 s−1-1 s−1, respectively. Three stability domains and one instability domain were identified. The governing mechanisms and scientific know-how of both types of domains are established via mechanical testing followed by detailed microstructural characterization, including Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM) analysis. A unique unexplored grain boundary sliding (GBS) mechanism-based stability domain is found as a dominant mode of deformation in one of the stability domains in the engineered UFG Mg-RE alloys.

Effect of Junction Disclinations on the Crack Initiation during Induced Grain-Boundary Sliding

Effect of Junction Disclinations on the Crack Initiation during Induced Grain-Boundary Sliding

Progression of creep deformation from grain boundaries to grain interior in Al-Cu-Mn-Zr alloys

Creep mechanisms are studied in θ′-Al2Cu-strengthened Al-Cu-Mn-Zr alloys at 300 and 350°C for (i) ACMZ, a base alloy without further alloying elements and (ii) RR350, a commercial alloy with additions of Ni and Co forming distinct grain-boundary precipitates. At high stresses, creep is dominated by dislocations bypassing θ′ precipitates within grains via the Orowan mechanism, as evidenced by (ⅰ) very high stress exponent (n∼20-25) and (ⅱ) α-Al and θ′ lattice strains (measured via in-situ neutron diffraction) evolving during creep in a manner consistent with load transfer from the plastically-deforming α-Al matrix to elastically-deforming θ′ precipitates. At intermediate stresses, both alloys exhibit a n∼3 regime, where α-Al and θ′ lattice strains scale near-linearly with applied stress while remaining largely unaffected by strain accumulation, indicating that Orowan looping or dislocation pile-up around θ′ is now inactive within the grains. Rather, dislocation motion occurs solely in θ′-precipitate-free zones (θ′-PFZ) where high dislocation densities are observed via TEM after creep deformation. Plastic flow at θ′-PFZ and/or localized pipe diffusion are expected to enable grain-boundary sliding (GBS), which is proposed as the rate-limiting mechanism in the n∼3 regime. Ni/Co-rich precipitates at RR350 grain-boundaries, with negligible θ′-PFZ around them, share load (as determined via neutron diffraction) with the α-Al matrix more effectively than θ-Al2Cu precipitates at ACMZ grain-boundaries, with wide surrounding θ′-PFZ. Thus, high creep resistance in the n∼3 GBS regime of RR350 is enabled by coarsening-resistant grain-boundary precipitates, forming without concomitant development of weak θ′-PFZ, which effectively share load with the grains.

Elucidating the influence of temperature and strain rate on superplasticity and microstructure evolution of near-α TNW700 titanium alloy

The superplastic deformation and microstructure evolution of a new near-α TNW700 titanium alloy with short-term service temperature up to 700 °C were systematically investigated in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1. The results illustrate that TNW700 alloy has excellent superplasticity with elongation above 300% and m value above 0.41 at 900–950 °C, and the maximum elongation of 523% is obtained at 950°C-0.001s−1 due to the appropriate grain size and phase ratio. In addition, TNW700 alloy exhibits high temperature softening and strain rate hardening behaviors. The increase of β phase volume fraction induced by dynamic phase transformation is a crucial cause for flow softening, and this phenomenon is accelerated with increasing temperature and decreasing strain rate. The refinement of primary α grains and the inconspicuous growth of β grains at lower temperatures are favorable for grain boundary sliding and superplasticity enhance, whereas the overgrowth of β grains at 975 °C results in superplasticity absence. The decrease of dislocation density in primary α grains caused by strain partitioning, dynamic recovery (DRV) or dynamic recrystallization (DRX) is another important factor for high temperature softening; the increase of dislocation density and velocity, and the decrease of annihilation time contribute to strain rate hardening. The appearance of random texture indicates the activation of grain boundary sliding (GBS) and grain rotation. GBS is the overarching mechanism of superplastic deformation of TNW700 alloy, accompanied by grain rotation, dislocation movement, dynamic phase transformation and grain refinement or coarsening.

Special deformation mechanisms dominated by grain boundary sliding of extrusion–refined Ti–22Al–25Nb alloy during compression in single BCC phase field

The deformation behavior of Ti–22Al–25Nb alloys with refined initial grain size was investigated under isothermal uniaxial compression in the BCC (B2) single-phase field, where a high-temperature extrusion process was used to produce the test materials with finer initial grains. As compressed at low to moderate strain rates, the present material exhibits significant continuous flow softening. To study the deformation and softening mechanisms during the compression, the samples' microstructures were examined using EBSD. The analysis shows that this alloy has a considerable grain boundary sliding (GBS) mechanism in addition to the conventional mechanisms of dynamic recovery (DRV), dynamic recrystallization (DRX), and texture softening. For the first time, this study offers direct evidence of the presence of GBS in current alloys. And the dominant role of GBS in high-temperature deformation and softening of current alloys was identified by comparing the impacts of various mechanisms on the dislocation density distribution. Besides, the impact of GBS on texture evolution was also discussed.

Superplasticity in a coarse-grained as-cast magnesium alloy

Superplasticity of a coarse-grained cast Mg–2.5Gd–0.5Zr alloy was investigated by assessing the strain rate sensitivity index (m-value) through the shear punch testing (SPT) method in the temperature range of 623–748 K and shear strain rate range of 2.8 × 10−3–2.2 × 10−2 s−1. The initial as-cast grain size of about 150 μm was reduced to about 3.5 μm at the end of the test. This severe grain refinement caused by dynamic recrystallization (DRX) facilitated superplasticity in the studied cast alloy. According to the respective m-values of 0.51 and 0.53 obtained at 698 and 723 K, and the activation energy of 80 kJ mol−1, the prevailing mechanism of deformation was suggested as grain boundary sliding (GBS) controlled by grain boundary diffusion.

Ductile Deformation of the Lithospheric Mantle

The strength of lithospheric plates is a central component of plate tectonics, governed by brittle processes in the shallow portion of the plate and ductile behavior in the deeper portion. We review experimental constraints on ductile deformation of olivine, the main mineral in the upper mantle and thus the lithosphere. Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding (GBS), and diffusion-accommodated grain-boundary sliding (diffusion creep). Deformation in most of the lithosphere is dominated by GBS, except in shear zones—in which diffusion creep dominates—and in the brittle-ductile transition—in which low-temperature plasticity may dominate. We find that observations from naturally deformed rocks are consistent with extrapolation of the experimentally constrained olivine flow laws to geological conditions but that geophysical observations predict a weaker lithosphere. The causes of this discrepancy are unresolved but likely reside in the uncertainty surrounding processes in the brittle-ductile transition, at which the lithosphere is strongest. ▪ Ductile deformation of the lithospheric mantle is constrained by experimental data for olivine. ▪ Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding, and diffusion creep. ▪ Observations of naturally deformed rocks are consistent with extrapolation of olivine flow laws from experimental conditions. ▪ Experiments predict stronger lithosphere than geophysical observations, likely due to gaps in constraints on deformation in the brittle-ductile transition.

Diffusion Creep Characteristics of Anorthite Revealed by Uniaxial and Pure Shear Deformation Experiments

AbstractPlagioclase, a major rock‐forming mineral in the Earth's crust, often shows microstructural evidence of ductile deformation in crustal rocks, suggesting that crustal flow is largely controlled by the high‐temperature deformation behavior of this mineral. Here, we present uniaxial and pure shear deformation experiments that reveal various diffusion creep characteristics of anorthite (CaAl2Si2O8), a calcium‐rich plagioclase endmember. We synthesized fine‐grained (∼1 μm) anorthite aggregates with different Al/Si ratios (Al/Si = 1 and 0.97) that were either undoped or doped with 1 wt% MgO. We determined that the synthesized samples deformed by interface (reaction)‐controlled and grain‐boundary diffusion creep mechanisms. At similar conditions of stress, temperature, and grain size, strain rates varied by ∼4 orders of magnitude among the samples. A reduction in Al/Si ratio weakens the aggregate by ∼2 orders of magnitude, while doping with MgO, which probably becomes concentrated at grain boundaries, further weakens samples with different Al/Si ratios to the same low strength level. The resulting low viscosity due to MgO is comparable to that of grain‐boundary diffusion creep in anorthite in previous studies. Grain boundary sliding (GBS)‐induced rigid‐body‐like grain rotation was identified by analysis of a marker‐etched sample surface after deformation. Crystallographic preferred orientation and shape preferred orientation developed after the samples were deformed to strains of ≥0.7, which is well explained by preferential GBS along grain boundaries parallel to (010) and also in the [100] direction within the grain‐boundary plane. These important, but previously unknown, characteristics of anorthite diffusion creep originate from the nature of the grain boundaries.