High Stacking Fault Energy Research Articles

Experimental analyses and numerical modeling of the microstructure evolution of aluminum alloy using an internal state variable plasticity-based approach coupled with the effects of second phase

Continuous dynamic recrystallization (cDRX) is an effective way to refine the microstructures of materials with high stacking fault energies during their hot working. However, investigation of the influence of grain boundaries and second phases on the microstructure evolutions of these materials using conventional cDRX models is challenging. Accordingly, herein a cDRX model characterized with micron-scale second phases distributed along the grain boundaries was developed by an internal state variable plasticity-based approach. Geometrically necessary dislocation (GND) densities generated near the second phase and grain boundary were calculated by considering strain concentration in the grain, which affected the evolution of dislocation density at the subgrain boundary. The cDRX model considers an original grain as a collection of subgrains and represents the subgrains as a compound structure consisting of subgrain boundaries with relatively high dislocation densities and a subgrain interior with significantly low dislocation density. Changes in the misorientations of subgrains can be accurately determined by tracking and analyzing the evolutions of the dislocation densities of subgrain boundaries. Accuracy of the developed cDRX model was verified by comparing the hot compression data of 5052 and 2A14 Al alloys acquired using this model with the corresponding experimental results. The predicted values of variables, including flow stress, subgrain size, and the fraction of low-angle grain boundary, were in appropriate agreement with the experimental results. Effects of temperature and strain rate on the microstructure evolution of the 2A14 Al alloy were investigated. Based on the influences of strain concentration near the grain boundary and second phase on the dislocation density of the 2A14 Al alloy, the reasons for the enhancement of the occurrence of cDRX and improvement of the mechanical properties of the 2A14 Al alloy by the second phase and grain boundary are explained. The results of this study provide a direction for the subsequent improvement of the proposed cDRX model with second phases inside the grain.

Study on Mechanical Properties and Deformation Mechanism of TWIP Stainless Steel

In this study, based on the sensitivity of the chemical composition fluctuation to the thermodynamic parameter, which controls the level of the stacking fault energy (SFE), a series of high Cr–Mn–N twinning-induced plasticity (TWIP) stainless steels are designed by using a sublattice model, and their mechanical properties and micro deformation mechanism are analyzed The formation of mechanical twins (Mts) during the deformation makes the test steel show a perfect combination of strength and ductility after different solution treatments. Among them, after a solution treatment at 950 °C and 1050 °C, the 19Cr–0.7N and 19CrSi–0.7N samples have the maximum value with the product of the strength and plasticity reaching 60.7% and 64.6%, and 12Cr–CN has the maximum value after the solution treatment at 1200 °C, reaching 81.3%. The SFE values of the 19Cr–0.7N and 19CrSi–0.7N samples were relatively high, 48 mJ·m−2 and 45 mJ·m−2, respectively. The SFE of 12Cr–CN is 37 mJ·m−2, and the Mts grow rapidly during the deformation and maintain the highest twinning density under the same strain conditions. The characterization of the tensile samples occurs under different deformations by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The results of the EBSD local misorientation difference angle analysis showed the Silicon element addition with a good Mts saturation rate. It is observed from the TEM that the nucleation process of the Mts with a high SFE is difficult, and the Mts emit and grow inward along the grain boundary during the tensile process and present a cross shape with the increase in strain. The contribution of the grain boundary strengthening (σ0), dislocation strengthening (σf), and twinning strengthening effect (σt) under dynamic micro-refinement to stress were calculated. It is known that under a certain amount of strain, the ratio of σt and σf changes with increasing, and when the contribution of the twinning deformation to the stress exceeds about 25%, the reinforcement of the plastic deformation is dominated by the plane of σf.

Stacking faults in a mechanically strong Al(Mg)–Al3Mg2 composite

Stacking faults (SFs), similar to other planar defects such as grain boundaries, twin boundaries and heterogeneous interfaces, can be tailored to simultaneously increase strength and ductility of metal matrix composites. However, SFs are rarely observed in Al alloy matrix composites, resulting from the high stacking fault energy (SFE) (166 mJ/m2) of pure Al and its weak dependence on most alloying elements. Therefore, strengthening Al alloy matrix composites with SFs has been a longstanding challenge. In the current work, SFs with a density of 2.8 × 1015 m−2 are introduced into an ultrafine-grained Al(Mg)–Al3Mg2 composite fabricated by extruding compacted mechanically alloyed and nanostructured Al-11 at.% Mg alloy powder at 703 K. These SFs, together with grain boundaries and solute Mg, increase the yield strength of the as-extruded composite to 700 MPa at a tensile elongation of ∼4%. We show that the Al(Mg) alloy matrix-Al3Mg2 interfaces enable the formation of the densely arranged SFs. This finding offers a pathway to tailoring high strength Al alloy matrix composites through manipulating SFs by interface engineering.

Exploring the deformation behavior of nanotwinned Al–Zr alloy via in situ compression

Nanotwinned metals have demonstrated the capacity for concomitant high strength and ductility. However, metals with high stacking fault energies, such as aluminum (Al), have a low propensity for twin formation. Here, we show the fabrication of supersaturated solid-solution Al–Zr alloys with a high density of growth twins. Incoherent twin boundaries (ITBs) are strong barriers to dislocation motion, while mobile partial dislocations promote plasticity. These deformable nanotwinned Al–Zr alloys reach a flow stress of ∼1 GPa, as demonstrated using in situ micropillar compression tests. Density functional theory calculations uncover the role Zr solute plays in the formation and deformation of the nanotwinned microstructure. This study features a strategy for incorporating ITBs and 9R phase into Al alloys for simultaneous benefits to strength and deformability.

Effects of stacking fault energy on deformation induced grain boundary relaxation in nanograined Cu alloys

Deformation induced grain boundary (GB) relaxation with resultant enhancements in thermal stability was observed in various metals with grain sizes below a critical size. In this study, effects of stacking fault energy (SFE) on the GB relaxation and thermal stability are investigated in several Cu-Ni and Cu-Al alloys with gradient nanograined samples prepared using the surface mechanical grinding treatment. For each alloy, thermal stability drops with a decreasing grain size from submicrometers to about 70 nm. However, two distinct grain size dependences of thermal stability were observed below 70 nm. For Cu-10Ni and Cu-5Ni alloys with higher SFEs than Cu, thermal stability increases for smaller grains, similar to that in pure Cu. For Cu-10Al and Cu-5Al with lower SFEs than Cu, as grain sizes decrease the thermal stability elevates firstly and then drops, exhibiting a stability peak at a certain size. The observed thermal stability in the Cu alloys below 70 nm can be attributed to the GB relaxation induced by plastic deformation dominated by partial dislocation activities. The different behaviors of GB relaxation in these Cu alloys demonstrated its obvious dependence on SFE, which determined the governing deformation mechanisms and hence the degree of relaxation of GBs.

Effect of Warm Deformation on Mechanical Properties and Deformation Mechanism of Nano/Ultrafine‐Grained 304 Stainless Steel

To prevent the performance of 304 stainless steel with nano/ultrafine grain from being damaged during cold forming, the deformation temperature is increased to above the Md point, and its mechanical properties and deformation mechanism during warm forming are studied. The significant effects of rolling temperature on the microstructural evolution of nano/ultrafine‐grained 304 stainless steel deformation are characterized by X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy, and the relationships between temperature, stacking fault energy (SFE), and deformation mechanisms are discussed. The results show that the tensile strength of 304 austenitic stainless steel decreases with an increase in the tensile temperature from room temperature to 900 °C, and the total elongation decreases first with increasing temperature. The elongation at 400 °C is the worst, only 32%. The dislocation slip becomes the only deformation mechanism due to the high SFE. Then, the total elongation begins to increase, and the highest elongation is 288% at 800 °C, showing superplastic behavior and the deformation mechanism is dynamic recrystallization, and grain boundary sliding.

Coupled quantitative modeling of microstructural evolution and plastic flow during continuous dynamic recrystallization

Continuous dynamic recrystallization (cDRX) dominates microstructural evolution during the hot working of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. However, in reality, a lack of quantitative and visual modeling of the process hinders its widespread application in the hot working process. In this study, using a recently developed multilevel cellular automaton (MCA) that integrates the newly established cell switching rules and topology deformation technique, a novel mesoscale MCA-cDRX model was constructed to investigate the evolution of both microstructures and macroscopic mechanical response in the hot working of AA7075 aluminum alloy. By considering the evolution of dislocation density and the orientation angle of the local cells as the primary clues, the plastic flow, recrystallization kinetics, features of subgrain size and high-angle grain boundaries, and influence of initial matrix characteristics on the cDRX mechanism were analyzed. The model predictions are consistent with the experimental data. Quantitative analysis confirms that the incubation time for the initiation of subgrain formation is significantly short. The fine-grain matrix and high initial volume fraction of low-angle grain boundaries can significantly accelerate the progress of cDRX owing to a stronger accumulation of dislocations in the dislocation cell walls through the climb and cross-slip mechanisms in the deformed aluminum alloy. The subgrain size is dependent on the Zener-Hollomon parameter. The developed simulation framework offers an effective means to allow the visualization of the cDRX.

In situ neutron diffraction unravels deformation mechanisms of a strong and ductile FeCrNi medium entropy alloy

• An equiatomic medium entropy FeCrNi alloy with excellent strength-ductility balance at 293 and 15 K was presented. • The single-crystal elastic constants and bulk moduli were calculated by neutron diffraction and density functional theory simulation. • The role of severe lattice distortion and dislocations in strengthening the medium entropy alloy was explored and quantified in 293 and 15 K. • Severe lattice distortion made the greatest contribution to the yield strength of the alloy. We investigated the mechanical and microstructural responses of a high-strength equal-molar medium entropy FeCrNi alloy at 293 and 15 K by in situ neutron diffraction testing. At 293 K, the alloy had a very high yield strength of 651 ± 12 MPa, with a total elongation of 48% ± 5%. At 15 K, the yield strength increased to 1092 ± 22 MPa, but the total elongation dropped to 18% ± 1%. Via analyzing the neutron diffraction data, we determined the lattice strain evolution, single-crystal elastic constants, stacking fault probability, and estimated stacking fault energy of the alloy at both temperatures, which are the critical parameters to feed into and compare against our first-principles calculations and dislocation-based slip system modeling. The density functional theory calculations show that the alloy tends to form short-range order at room temperatures. However, atom probe tomography and atomic-resolution transmission electron microscopy did not clearly identify the short-range order. Additionally, at 293 K, experimental measured single-crystal elastic constants did not agree with those determined by first-principles calculations with short-range order but agreed well with the values from the calculation with the disordered configuration at 2000 K. This suggests that the alloy is at a metastable state resulted from the fabrication methods. In view of the high yield strength of the alloy, we calculated the strengthening contribution to the yield strength from grain boundaries, dislocations, and lattice distortion. The lattice distortion contribution was based on the Varenne-Luque-Curtine strengthening theory for multi-component alloys, which was found to be 316 MPa at 293 K and increased to 629 MPa at 15 K, making a significant contribution to the high yield strength. Regarding plastic deformation, dislocation movement and multiplication were found to be the dominant hardening mechanism at both temperatures, whereas twinning and phase transformation were not prevalent. This is mainly due to the high stacking fault energy of the alloy as estimated to be 63 mJ m −2 at 293 K and 47 mJ m −2 at 15 K. This work highlights the significance of lattice distortion and dislocations played in this alloy, providing insights into the design of new multi-component alloys with superb mechanical performance for cryogenic applications.

High stress twinning in a compositionally complex steel of very high stacking fault energy

Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m2, far above the SFE regime for twinning (<~50 mJ/m2) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.

Simultaneous Twinning and Microband-Induced Plasticity of a Compositionally Complex Alloy with Interstitial Carbon at Cryogenic Temperatures

Intermediate to low stacking fault energy (SFE) high entropy alloys (HEA) have shown an excellent combination of strength and ductility as a result of deformation twinning and martensite transformation. However, even in the absence of these mechanisms HEA can show a good strength-ductility combination, as is the case with non-equiatomic (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1. The room temperature mechanical behavior of this alloy has been associated with Taylor lattice and microband formation. The current research focuses on tensile cryogenic deformation of this alloy and investigates if these features and/or alternate mechanisms like deformation twinning are obtained. Surprisingly, it is not one or the other but both deformation twinning and microband formation that are observed during cryogenic deformation. The activation of both deformation mechanisms is a combination that is not often reported as the former is generally associated with intermediate to low SFE alloys and the latter with intermediate to high SFE alloys. The activation of twinning in (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 is attributed to the high yield stress-temperature variation, as a result of solid solution strengthening being far greater than in other commonly researched compositionally complex alloys. A ductility retention down to 4 K was observed, while simultaneously showing a significant increase in flow stress. Despite the intermediate to high SFE deformation behavior, (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 exhibits excellent cryogenic strength-ductility combination.

Achieving High Strength-plasticity of Nanoscale Lamellar Grain Extracted from Gradient Lamellar Nickel

Traditional metallic materials usually face a dilemma between high strength and poor strain hardening capacity. However, heterogeneous structured metallic materials have been found to obviously overcome the trade-off. Herein, gradient lamellar structure was fabricated through ultrasound-aided deep rolling technique in pure Ni with high stacking fault energy after heat treatment. The gradient lamellar Ni was successively divided into the four regions. In-situ micropillar compression tests were conducted in different regions to reveal the corresponding microscopic mechanical properties. Microscopic characterization techniques were performed to explore underlying deformation mechanisms and the effects of microstructural parameters on deformation behaviors. This work demonstrates that the micropillar with near nanoscale lamellar thickness possesses excellent strength and plasticity. On one hand, the reason for high strength of near nanoscale micropillar is that the strength of micropillar increases with the decrease of lamellar thickness according to the Hall-Petch effect. On the other hand, numerous lamellar grain boundaries perpendicular to the loading direction is found to hinder the motion of slip bands, resulting in great strain hardening capacity in the near nanoscale lamellar micropillar.

Extraordinary Strain Hardening from Dislocation Loops in Defect-Free Al Nanocubes.

The complex interaction of crystalline defects leads to strain hardening in bulk metals. Metals with high stacking fault energy (SFE), such as aluminum, tend to have low strain hardening rates due to an inability to form stacking faults and deformation twins. Here, we use in situ scanning electron microscopy (SEM) mechanical compressions to find that colloidally synthesized defect-free 114 nm Al nanocubes combine a high linear strain hardening rate of 4.1 GPa with a high strength of 1.1 GPa. These nanocubes have a 3 nm self-passivating oxide layer that has a large influence on mechanical behavior and the accumulation of dislocation structures. Postcompression transmission electron microcopy (TEM) imaging reveals stable prismatic dislocation loops and the absence of stacking faults. MD simulations relate the formation of dislocation loops and strain hardening to the surface oxide. These results indicate that slight modifications to surface and interfacial properties can induce enormous changes to mechanical properties in high SFE metals.

Effect of Annealing Process on the Microstructure and Texture of Cold-Rolled High-Purity Al-0.5%Cu Plates.

As a kind of typical high stacking fault energy materials, recrystallization behavior of high purity Al-0.5%Cu alloy is significantly influenced by the annealing process. In this study, different heating rate, target temperature, and holding time were discovered to have profound impact on the microstructures and textures of Al-0.5%Cu plates. Electron backscatter diffraction (EBSD), scanning electron microscope (SEM), and X-ray diffraction (XRD) were utilized for analyzing the evolution of the microstructure and texture in the subsequent microstructural characterization. Vickers hardness tests were employed for measuring hardness of specimens. The results showed that no obvious recrystallization was observed at lower temperature and the composition of texture influenced by rising temperature, heating rate affected initial recrystallization temperature, grain size, and strength of textures. After recrystallizing completely, the size of microstructures and the distribution of textures had little change with the extension of holding time.

In-situ TEM observation of the ITB evolution in aluminum deformation twinning

Deformation twinning is another important phenomenon besides dislocation slip during the plastic deformation of metals and alloys. In this work, the plastic deformation behavior of coarse-grained aluminum with high stacking fault energy was studied by in-situ tensile TEM. The results show that deformation twins appear at the crack tip under continuous applied stress, and the thickness of deformation twin increases with the appearance of incoherent twin boundaries (ITBs). Further analysis shows that Frank partial dislocation in deformation twins is decomposed into perfect dislocation and Shockley partial dislocation under the action of applied stress. These perfect dislocation and Shockley partial dislocation can slip along the crystal planes and react to form the dislocation combinations that constitute the ITB. The results of this work provide a new way for researchers to understand the role of ITBs on deformation twins and the formation of dislocation combinations in ITBs.

A study on formation mechanism of friction-surfaced stainless steel coating via a stop-action method

The formation mechanism of a friction-surfaced (FSed) coating of super austenitic stainless steel AISI 904 L with high stacking fault energy was investigated with a stop-action method. The advancing side, center and retreating side of the surfaces and cross-sections of the coating and the bump were studied in detail using EBSD. The microstructure of the bump formed at the end was found to be different from that of the FSed coating. A systematic analysis of the microstructure revealed the formation mechanism of the coatings. A two-step formation mechanism of the FSed coating was proposed.

Room temperature compressive superplasticity of low density steel

The present work deals with the room temperature compressive deformation behavior of a low density steel holding an apparent density of 6.8 gr/cm3. Surprisingly, it was found that the experimented materials tolerated a high amount of compressive true strain (∼2.5) without the occurrence of any micro-cracking or macro-localized shearing. Step by step study of the interrupted microstructures indicated the continuous and progressive development of the substructure in the course of room temperature compressive deformation. The extended dynamic recovery was justified considering the high stacking fault energy of the material and was followed by the occurrence of continuous dynamic recrystallization where the final microstructure was composed of ultrafine and equiaxed grains. Interestingly, the corresponding flow behavior was greatly affected by such specified microstructure evolution and an appreciable strain softening was obtained over a wide range of strain.

Hierarchy modification induced exceptional cryogenic strength, ductility and toughness combinations in an asymmetrical-rolled heterogeneous-grained high manganese steel

The strategy of heterogeneous-structured (HGS) high manganese steels was designed by asymmetrical-rolling (AR) with annealing to obtain superior mechanical performance for cryogenic applications. The grain size hierarchy can be modified by adjusting the annealing temperature based on the growth rate of recrystallized grains. Compared with the AR-740 sample, which is dominated by a large number fraction (∼72.51%) of fine-grained (FG) zone, the AR-630 specimen consisted of larger fractions of ultrafine-grains (UFG) and sub-fine-grains (SFG) not only shows significant yield strength (YS) and ultimate tensile strength (UTS) increments to ∼983 MPa and ∼1330 MPa, respectively, but also improved tensile ductility and impact toughness value to ∼64.9% and ∼183 J‧cm−2 at liquid nitrogen temperature (LNT), respectively. The difference in YS mainly originates from the different grain boundary strengthening degrees. Moreover, the ductility enhancement can be attributed to the different evolutions of deformed substructures and strain partitioning between heterogeneous domains. On one hand, the localized stress partitioned in the UFG and SFG zones of the AR-630 sample leads to the generation of deformation twins and geometrical necessary dislocations (GNDs) in all scales of grain, thus contributing to sustainable high back stress over the whole deformation stage. In contrast, the strain hardening of the AR-740 sample is dominated by the dislocation and twin hardening of FG zones with a homogeneous strain state. More importantly, the exceptional cryogenic toughness can be attributed to the following aspects: i) ductile fracture mode induced by twinning-induced plasticity (TWIP) effect generated by high stacking fault energy (SFE); ii) a more heterogeneous strain partitioning, which can alleviate the strain concentration in the weak FG zones, and also induce more nano-twins in the UFGs and SFGs to impede the crack propagation. The hierarchy modification is of great significance for the practical application of HGS to improve the cryogenic-temperature mechanical performance.

Deformation-activated recrystallization twin: New twinning path in pure aluminum enabled by cryogenic and rapid compression

SummaryBulk aluminum rarely forms deformation or annealing twins owing to its high stacking fault energy. We report a novel twinning mechanism mediated by dynamic recrystallization in 6N pure aluminum under high strain rate (∼1.3 × 104 s−1) impact at a cryogenic temperature (77 K). Discontinuous dynamic recrystallization occurs during rapid severe plastic deformation and generates inhomogeneous microstructures exhibiting low-angle and high-angle grain boundaries. Unexpectedly, Σ3 twin boundaries were able to develop during dynamic recrystallization. Although these recrystallization twins have similar morphology as that of annealing twins, their formation relies on deformation activation instead of thermal activation, which was suppressed by the cryogenic experiment. Besides, strong orientation dependence was observed for formation of these novel twins. Beyond annealing and deformation twin, deformation-activated recrystallization twin is a new path for pure aluminum twinning.

Effect of Rolling Temperature on Microstructural Characteristics and Deformation Mechanisms of a Metastable Austenitic Stainless Steel

Herein, the significant effects of rolling temperature and equivalent strain on the deformation microstructural evolution of a 304 stainless steel are characterized by X‐Ray diffraction, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy, and the relationships between temperature, stacking fault energy (SFE), and deformation mechanisms are discussed. The SFE of experimental steel gradually increases from ≈19 to ≈48 mJ m−2 with increasing temperature from room temperature (20 °C) to 600 °C. A transition in deformation mechanisms occurs from martensitic transformation to deformation twinning and finally dislocation glide‐only, which is attributed to the increasing SFE caused by higher deformation temperature. Dislocation glide and deformation‐induced martensitic transformation dominate the plastic deformation during cold‐rolling at room temperature. Deformation twinning is observed in the higher temperature range where the SFE is between 28 and 38 mJ m−2, acting as a complementary deformation mechanism to dislocation glide. When the rolling temperature is increased to 600 °C, deformation twinning is inhibited. Dislocation slip becomes the sole deformation mechanism due to the high SFE. Meanwhile, the dislocation cells and highly dense dislocation walls are formed due to the high SFE.

Outstanding improvement in the CSL distribution in interstitial free (IF) steel via strain annealing route

In the last two decades, grain boundary engineering (GBE) study has neglected the high stacking fault energy BCC materials. Researchers have tried many single or multi-step strain recrystallization routes for improving the grain boundary character distribution (GBCD) in BCC materials, but the results have not been satisfactory. So, in this study, a single-step low deformation (5%, 10%) strain annealing route is employed for obtaining optimized GBCD and triple junction distribution (TJD) in interstitial free steel. This low deformation (5%) and low temperature (923 K) short annealing (10 min) treatment led to the evolution of 48.2% of Σ3, 54.3% of low Σ CSL boundaries, and 31.1% of special triple junctions (J2 and J3 type) in the microstructure. To the best of the authors' knowledge, these are the highest values reported for BCC material in literature. A good correlation is found between {111}〈110〉, {111}〈112〉 gamma fiber texture components and low deviation Σ3 boundaries. Moreover, a thorough analysis of average grain size, residual strain, GBCD, TJD, and grain orientation spread indicates that the regeneration mechanism (due to strain-induced boundary migration (SIBM)) is responsible for establishing a GBE microstructure in the thermomechanically processed sample (5%, 923 K, 10 min). The present work also explains the role of abnormal grain growth (AGG) on the deterioration of GBCD and TJD in the highly deformed (10%) samples.