High Stacking Fault Energy Research Articles

Effects of order and disorder on the defect evolution of NiFe binary alloys from atomistic simulations

The effects of the ordered and disordered arrangements of elements on radiation-induced defects production and evolution in NiFe alloys were investigated through atomistic simulations. Results present a sluggish evolution of the overall microstructure in ordered L10 NiFe. Although the disordered phase has fewer Frankel pair accumulation in cascade simulation attributed to the low thermal conductivity reduced by the intrinsic chemical disorder, the difference is negligible when PKA energy increases because of the direct formation of clusters. Interstitial diffusion is restricted in the ordered phase, where Ni and Fe layers are alternately arranged. This condition delays interstitials accumulation and leads to the formation of more Shockley partial loops rather than Frank loops which favor in the disordered phase. The higher stacking fault energy in the ordered phase renders it difficult to form stacking fault tetrahedra

Impact of short range ordering on the anomalous four-stage strain hardening behavior of low solid-solution hardening Ni–Cr alloys

The effect of short range ordering (SRO) on the strain hardening behavior of low solid-solution hardening Ni–Cr alloys with high stacking fault energies (SFEs) was systematically investigated under uniaxial compression to a high true strain ε t of 0.9 at room temperature. An unexpected four-stage (including Stages A, B, C and D) changing law in strain hardening rate was revealed, which is completely different from the case of conventional high-SFE alloys whose strain hardening rate just monotonously declines as the strain increases. Observations on the deformation microstructures strongly demonstrate that the recovery of strain hardening rate in Ni–Cr alloys (Cr ≥ 20 at.%) at Stages B (0.11 < ε t < 0.21) and D ( ε t ≥ 0.65) should be ascribed to the transformation in dislocation slip mode from wavy slip to planar slip, and the occurrence of the extensive deformation twins (DTs) together with the secondary DTs, respectively. Besides, in the later period of Stage C (0.21 < ε t < 0.65), microbands (MBs) and DTs have been locally formed in the Ni–Cr alloys with high SRO degree, which delays the decrease of strain hardening rate to a certain extent. Based on these experimental observations, the impacts of SRO on the formation of planar slip bands, MBs and DTs are analyzed, and the effect of SRO on the four-stage strain hardening behavior is also discussed systematically based on the different deformation mechanisms. In brief, an enhancement of SRO in low solid-solution hardening Ni–Cr alloys with high SFEs has been convincingly confirmed to be capable of improving the strain hardening capacity.

Numerical Study and Experimental Validation of Deformation of &lt;111&gt; FCC CuAl Single Crystal Obtained by Additive Manufacturing

The importance of taking into account directional solidification of grains formed during 3D printing is determined by a substantial influence of their crystallographic orientation on the mechanical properties of a loaded material. This issue is studied in the present study using molecular dynamics simulations. The compression of an FCC single crystal of aluminum bronze was performed along the &lt;111&gt; axis. A Ni single crystal, which is characterized by higher stacking fault energy (SFE) than aluminum bronze, was also considered. It was found that the first dislocations started to move earlier in the material with lower SFE, in which the slip of two Shockley partials was observed. In the case of the material with higher SFE, the slip of a full dislocation occurred via successive splitting of its segments into partial dislocations. Regardless of the SFE value, the deformation was primarily occurred by means of the formation of dislocation complexes involved stair-rod dislocations and partial dislocations on adjacent slip planes. Hardening and softening segments of the calculated stress–strain curve were shown to correspond to the periods of hindering of dislocations at dislocation pileups and dislocation movement between them. The simulation results well agree with the experimental findings.

Review on Dynamic Recrystallization of Martensitic Stainless Steels during Hot Deformation: Part I—Experimental Study

The evolution of the microstructure changes during hot deformation of high-chromium content of stainless steels (martensitic stainless steels) is reviewed. The microstructural changes taking place under high-temperature conditions and the associated mechanical behaviors are presented. During the continuous dynamic recrystallization (cDRX), the new grains nucleate and growth in materials with high stacking fault energies (SFE). On the other hand, new ultrafine grains could be produced in stainless steel material irrespective of the SFE employing high deformation and temperatures. The gradual transformation results from the dislocation of sub-boundaries created at low strains into ultrafine grains with high angle boundaries at large strains. There is limited information about flow stress and monitoring microstructure changes during the hot forming of martensitic stainless steels. For this reason, continuous dynamic recrystallization (cDRX) is still not entirely understood for these types of metals. Recent studies of the deformation behavior of martensitic stainless steels under thermomechanical conditions investigated the relationship between the microstructural changes and mechanical properties. In this review, grain formation under thermomechanical conditions and dynamic recrystallization behavior of this type of steel during the deformation phase is discussed.

Zr addition-dependent twin morphology evolution and strengthening response in nanostructured Al thin films

Manipulating the twin morphology to achieve the reinforcement of strength is a great challenge in Al with high stacking fault energy. In this work, the influence of Zr addition on the twin morphology and strengthening response of nanostructured Al films was symmetrically studied. The results showed that, for low Zr addition (≤ 4.0 at.%), the Zr atoms were homogeneously distributed within the matrix, while Zr segregation at grain boundaries was evident at higher Zr addition (> 4.0 at.%). Twins were substantially observed in all the films, and the twin morphology was highly dependent on the Zr addition. In the pure Al film, only twins with a single coherent twin boundary were detected. In comparison, nanotwins with coplanar twin boundaries (C-nanotwins) and 9R phase were predominant in the Al-Zr films with Zr addition ≤ 4.0 at.%. Further raising the Zr content, multiple nanotwins (M-nanotwins) coexisted with the C-nanotwins and 9R phase. In particular, a zero-strain twinning mechanism was applied to account for the C-nanotwins and 9R phase formation, and a zig-zag grain boundary feature induced by Zr segregation was responsible for the M-nanotwin formation. The hardness also exhibited a strong Zr dependence that increased monotonically with the Zr addition. The Al-13.4 at.% Zr film displayed a hardness of ~4.3 GPa, about 11 times greater than the pure Al film. Strengthening mechanisms were quantitatively evaluated, and the highly-promoted hardness was mainly ascribed to the 9R phase and solid solution strengthening.

Fabrication of FeCoCrNiMnAl0.5-FeCoCrNiMnAl gradient HEA coating by laser cladding technique

In this study, an FeCoCrNiMnAl 0.5 -FeCoCrNiMnAl gradient high-entropy alloy (HEA) coating was fabricated by laser cladding. The gradient HEA coating was subjected to compression at 600 °C, and the phase structure, grain orientation, recrystallization state, grain boundary distribution, dislocation content and plastic deformation resistance of the coating were systematically studied by EBSD technology. The gradient HEA coating had a compression strength of 504 MPa with an elongation of 82% at 600 °C, and the surface of the FeCoCrNiMnAl cladding layer exhibited no obvious cracks. The gradient HEA coating exhibited an excellent balance between strength and toughness. The FeCoCrNiMnAl cladding layer formed a fiber texture parallel to the compression direction, and its body-centered cubic (BCC) structure had a high stacking fault energy and more continuous dynamic recrystallization. The FeCoCrNiMnAl 0.5 cladding layer formed a slab texture perpendicular to the compression direction, and this cladding layer with a face-centered cubic (FCC) structure had low stacking fault energy, which formed deformed grains that stored many dislocations. The FeCoCrNiMnAl 0.5 cladding layer reduced the energy of the FeCoCrNiMnAl cladding layer by storing dislocations, and its cracking tendency during high temperature compression was decreased, thereby extending the service life of the coating surface. • The FeCoCrNiMnAl 0.5 -FeCoCrNiMnAl gradient HEA coating was prepared by laser cladding. • FCC phase in the FeCoCrNiMnAl 0.5 cladding layer had low stacking-fault energy and high dislocation storage capacity. • Cracking tendency of the FeCoCrNiMnAl cladding layer was effectively reduced. • The gradient HEA coating achieved an effective strength-toughness balance.

Influence of strain rate and crystallographic orientation on dynamic recrystallization of pure Zn during room-temperature compression

This work investigates the strain rate dependence of dynamic recrystallization behaviour of high-purity zinc in room temperature compression under strain rates of 10−4 s-1, 10-2 s-1 and 0.5 s-1. Results from electron backscatter diffraction provide insight into the deformation and dynamic recrystallization mechanisms operative. Continuous dynamic recrystallization, twin-induced dynamic recrystallization, and discontinuous dynamic recrystallization are all active under compressive deformation at room temperature. Due to the high stacking fault energy of Zn, continuous dynamic recrystallization is the dominant mechanism while discontinuous dynamic recrystallization only operates in the early stages of compression at 10−4 s-1. Dynamic recrystallization kinetics are enhanced at higher strain rates (10-2 s-1 and 0.5 s-1) due to an increased contribution from twin-induced dynamic recrystallization. The present study reveals that the controlling mechanisms for continuous dynamic recrystallization are basal < a> slip and 2nd order pyramidal < c + a> slip activity. Because the activation of slip systems is mainly determined by crystallographic orientation, continuous dynamic recrystallization behaviour varies with grain orientation according to their propensity for basal and 2nd order pyramidal slip.

The impact of the stacking fault energy of nanostructured metals on phenomena during annealing at the high hydrostatic pressure

Abstract The present study investigates the impact of stacking fault energy on the microstructure evolution and mechanical properties of nanostructured metals that differ in stacking fault energy, annealed under high hydrostatic and atmospheric pressure. Ag and Ni were selected as materials of low and high stacking fault energy, respectively. To this end, nanostructured metals were obtained by high pressure torsion and subsequently annealed by high hydrostatic pressure annealing, performed under 2 GPa at 0.4 homologous temperature for 1h. For comparison, similar experiments at the same temperature and time were performed under atmospheric pressure. After deformation and annealing, the microstructures were examined using transmission and scanning electron microscopy, and further analysed in terms of grain size, coefficient of grain size variation, and twinning frequency. The stored energy and peak temperatures were measured by differential scanning calorimetry. The mechanical properties were evaluated from microhardness measurements and tensile tests. It is demonstrated that the pressure applied during annealing leads to a more profound retardation of microstructure evolution in the low stacking fault energy material, mainly due to a higher deformation nanotwin density. The twinning deformation mechanism generates a higher dislocation density and a lower grain size than those achieved by dislocation slip.

The effect of austenite grain size on deformation mechanism of Fe–17Mn steel

Abstract The transition of deformation mechanism from transformation-induced plasticity (TRIP) to twinning-induced plasticity (TWIP) was investigated in a viewpoint of γ-austenite grain size (AGS) using Fe–17Mn steel. When the AGS decreased from 51.72 μm to 0.75 μm, the apparent stacking fault energy (SFE) value increased from 10.8 mJ/m2 to 23.4 mJ/m2. The transition of deformation mechanism from TRIP to TWIP occurred at the late stage of tensile strain in ultrafine-grained specimens with high apparent SFE values (>23.1 mJ/m2). This result matches well with the fact that a critical AGS for the transition of deformation mechanism is ~0.52 μm, which was calculated from the relationships between AGS and critical resolved shear stresses for mechanical twinning (τtwin) and e-martensitic transformation (τe-mart). UFG specimens with the AGSs below 1.12 μm exhibited a two-step rise at the stage II of strain hardening rate curves due to active mechanical twinning and possessed high tensile strength without a great loss of elongation.

Evolution of microstructure, texture and mechanical properties of Fe–30Mn–11Al–1.2C low-density steel during cold rolling

In the past decades, the microstructure and properties of tensile deformed high Mn–Al–C lightweight steels were extensively studied due to their excellent combination of strength, ductility and low-density. However, the effects of cold rolling were barely reported. This study systematically investigated the evolution of microstructure and texture as well as mechanical properties during cold rolling of Fe–30Mn–11Al–1.2C (wt%) steel. The stacking fault energy (SFE) of the steel was estimated as ~108 mJ/m2. With cold rolling, the yield strength of the steel was significantly enhanced and exceeding 2.0 GPa after 90% rolling reduction, which is attributed to the gradual refinement of the microstructure. Different deformation substructures were formed approximately in the sequence of Taylor lattice, microbands, nano-twins, shear bands and dislocation cells. This means the dislocation glide mode changes from planar to wavy glide with increasing rolling reduction. Among these structures, twining is widely believed to be impossible by tensile deformation due to the high SFE. But it happened during cold rolling with relatively low resolved shear stress, which may be attributed to the occurrence of short range order (SRO) in austenitic matrix and local stress concentration. The rolling texture of the steel was weak and characterized by the transition from “copper-type” to “brass-type” after 50% rolling reduction. The dislocation planar glide and deformation twining play an important role in texture evolution.

Microband-Induced Plasticity in a Nb Content Fe–28Mn–10Al–C Low Density Steel

Novel Fe–28Mn–10Al–C–0.5Nb low-density steel was developed and the room temperature tensile behavior in the solid solution state and the microstructure evolution process during plastic deformation were studied, aiming to clarify the dominant deformation mechanisms. The results show that the developed steel was fully austenitic with a low density of 6.63 g/cm3 and fairly high stacking fault energy of 84 MJ/m2. The present fully austenitic Fe–28Mn–10Al–C–0.5Nb low-density steel exhibited an excellent ultimate tensile strength of 1084 MPa and elongation of 37.5%; in addition, the steel exhibited an excellent combination of strength and ductility (i.e., the product of strength and ductility (PSE) could reach as high as 40.65 GPa%). In spite of the high stacking fault energy, deformed microstructures exhibited planar glide characteristics, seemingly due to the glide plane softening effect. The excellent combination of strength and ductility is attributed to plasticity induced by microbands and leads to the continuous strain hardening during deformation at room temperature. Moreover, the addition of Nb does not change the deformation mechanism and strengthening mechanism of Fe–Mn–Al–C low-density steel, and can optimize the mechanical properties of the steel.

Extra-Hardening of SPD-Processed Al-Mg Alloy with Minimum Grain Sizes

Severe plastic deformation (SPD) processing of Al alloys could obtain high strength by grain refinement mechanism. The minimum grain size of Al alloy, obtained at higher strain rate at low temperature, is determined the stacking fault energy of the alloy. SPD-processed pure Al metal, has high stacking fault energy, has relatively large grain size. During SPD processing, large strain is introduced, and the dislocation is rearranged in the specimen. The re-arrangement of dislocation in SPD-processed Al alloy with intermediate stacking fault energy significantly delayed, thus the strain remains in the grain interior. The extra-hardening, a kind of strain hardening, results from an incomplete of dynamic recrystallization during SPD processing. Al-Mg solid solution alloy has intermediate stacking fault energy and the minimum grain size of this alloy approaches about 200 nm after SPD. The mechanical property of this alloy is remarkably higher than the predictable strength by Hall-Petch relationship due to the extra-hardening. In addition, the increase in strength by the extra-hardening varies with the Mg content of Al-Mg alloy. In this study, the effect of Mg content, i.e. the stacking fault energy of the alloy, on the degree of the extra-hardening of SPD-processed Al-Mg alloy was investigated in terms of the dislocation density and low-angle grain boundary of the alloy.

Effect of Stacking Fault Energy and Solute Atoms on Microstructural Evolutions of FCC Metals Processed by Severe Plastic Deformation

Pure copper, pure silver, Cu-6.8at%Al, Cu-1at%Mn and Cu-5at%Ni (stacking fault energies (SFEs) are about 41, 22, 23, 43 and 105mJ/m2, respectively) were processed by equal channel angular pressing (ECAP) at the same homologous temperatures to investigate the effect of SFE and solute atoms on microstructural evolutions. The final grain size of Cu-6.8at%Al after eight passes of ECAP was the smallest followed by that of Cu-1at%Mn with little difference. The former alloy reaches saturation for grain size and grain boundary misorientation in early passes of ECAP while the latter continues decreasing even after eight passes. The role of shear bands and deformation twins is predominant for grain fragmentation in the early stage for Cu-6.8at%Al and Ag with low SFE while evolution from cell walls to grain boundaries is main mechanism for Cu, Cu-1at%Mn and Cu-5at%Ni with medium or high SFE. Solute Mn atom of Cu-Mn with high atomic size misfit and may suppress the dynamic recovery which transforms cell walls to grain boundaries, and allow accumulation of higher dislocations and reduction of cell size to smaller scales.

Co-free CuFeMnNi high-entropy alloy with tunable tensile properties by thermomechanical processing

An equiatomic CuFeMnNi high-entropy alloy (HEA) was synthesized and homogenized at 1000 °C to achieve a microstructure of single face centered cubic phase based on CALPHAD (CALculation of PHAse Diagrams) prediction. The strength and ductility of this Co-free (low cost) HEA can be tuned through thermomechanical processing (cold rolling and annealing at temperatures ranging from 650 °C to 950 °C). Electron backscattered diffraction, transmission electron microscopy and scanning transmission electron microscopy techniques were used to characterize the deformation microstructure of this new HEA. The results show that plastic deformation was achieved through the slip of full dislocations with Burgers vector of a/2 . No dissociated dislocations and deformation twins were observed, indicating that this alloy poses high stacking-fault energy, which is supported by first principle calculations. The research suggests that further improvement on the mechanical properties of this alloy can be achieved by reducing stacking-fault energy via alloying, providing a promising low-cost alternative to CoCrFeMnNi HEAs for engineering applications.

Friction surfacing of AISI 904L super austenitic stainless steel coatings: Microstructure and properties

Microstructure, texture, pitting corrosion resistance and hardness of surfaces and cross-sections of the friction-surfaced (FSed) super austenitic stainless steel 904L coatings fabricated at various spindle speeds were investigated. The microstructure of the coatings was governed by both dynamic recovery (DRV) and continuous dynamic recrystallization (cDRX) owing to the high stacking fault energy (SFE) of 904L. The grain orientations in DRV were in A 2 ⁎ shear texture component, while the rotated cube textures were related to the cDRXed grains. Immersion tests of the FSed coatings in 3.07 mol/L Cl − solution showed that the corrosion pits were slightly preferred to generate at the {111} DRVed grains on the coating surfaces, since these grains were likely to have a higher elastic strain during FS. Compared with AISI 304, the pitting potential of the FSed coatings in 3.5 wt% NaCl solution was significantly shifted to nobler direction and also insensitive to spindle speed owing to the high SFE of 904L. However, the passive films of the FSed coatings were thinner and more difficult to reform, as evidenced by slightly larger average pit diameters and lower protection potentials owing to the high residual stress caused by FS. • Microstructures of FSed 904L coatings were governed by DRV in A 2 ⁎ shear component and cDRX in rotated cube textures. • Increment of spindle speed tended to enhance cDRX but suppress DRV at the same time. • The FSed coatings could maintain excellent pitting corrosion resistance of as-received 904L. • Due to high SFE of 904L, elastic strain in {111} grains was reduced. • Crystallographic effect on pitting cororsion resistance of the coatings was weakened. • Increase in misorientation density induced by cDRX led to hardness improvement and ductility reduction for the coatings

Effect of grain refinement on sensitization of high manganese austenitic stainless steel

Effect of grain size on degree of sensitization (DOS) was been evaluated in Nickel free steel. Manganese and nitrogen contained alloy is a Ni free austenitic stainless steel (ASS) having type 202 grade. In the present investigation, the deformation of 202 grade analyzed using XRD and microstructural testing. Optical Microstructure of Ni free austenitic stainless steel (ASS) has been done for cold worked samples with thermally aged at 900 °C_6 Hrs. Ni free austenitic stainless steel (ASS) appears to be deformed more rapidly due to its higher stacking fault energy which gave results in rapid transformation from strain induced martensite to austenite in form of recrystallized grains, i.e., it concluded that as cold work percentage increases more rapidly recrystallization occurs. XRD results also indicate that more fraction of martensite formed as percentage of CW increases but as thermal ageing reverted those all martensite to austenite. Double Loop Electro-potentio Reactivation Method (DLEPR) test used for findings of degree of sensitization. So investigation gives the conclusion which suggest that with high deformation at higher temperature and duration gives very less degree of sensitization (DOS).

Nucleation of Twinning Dislocation Loop in Pt: A Computational Approach

Twinning plays a critical part in the plastic deformation of the materials and the strengthening mechanisms and is hence considered as one of the most prevalent deformation mechanisms in metals. Because to the high stacking fault energy of the fcc metals like Al, Pd, and Pt, the extended dislocations are believed to be energetically favored over isolated partials, thereby rendering deformation twinning unfeasible. Nevertheless, some recent experimental researches have confirmed a potential deformation twinning pathway in nanocrystalline platinum. This alternate-shear mechanism has a much lower energy barrier than the usual layer-by-layer twinning. We utilize computations involving atomistic calculations and continuum modeling in this study to examine the genesis of deformation twins in Pt. Atomistic simulations provides the generalized planer fault energy using an EAM (embedded-atom-model) potential. Moreover, a potential energy-based method, namely; a nudged-elastic band (chain of states) has been used to compute the activation energy barrier for the nucleation of the twinning dislocation loop in the alternate-shear model. The critical stress needed for the nucleation of the twinning dislocation loop in platinum is estimated using some of the parameters acquired from atomistic calculations and using them as fitting parameters in the continuum model. The minimum-energy path between the two end states can be identified using this methodology. Through the unusual alternate-shear approach, the results provide a rudimentary but essential dislocationbased perspective of the occurrence of deformation twins in fcc metals.

A crucial impact of short-range ordering on the cyclic deformation and damage behavior of face-centered cubic alloys: A case study on Cu-Mn alloys

Cyclic deformation behavior and damage mechanisms of Cu-Mn alloys with high stacking fault energies (SFE) were systemically investigated at constant total strain amplitude in the range of 0.2% - 1.0% at room temperature in air. The results demonstrate that the cyclic hardening capacity and fatigue life of the alloy are notably improved with the increase of Mn content. Increasing Mn content significantly affects the damage behavior of the alloy, including a transition of cracking mode from concentrated intergranular cracking to slip band (SB)/intergranular mixed cracking and an enhanced resistance to crack propagation. All these features above are attributed to the transformation of cyclic deformation micromechanisms, i.e., the changes of slip mode from wavy- to planar-slip with increasing Mn content. The existence of short-range ordering (SRO) in Cu-Mn alloys is the essential reason for promoting slip planarity. Based on microstructural characterizations and analyses, the intrinsic connection between the SRO and the cyclic deformation and damage behavior was clarified. In brief, the present study opens another promising window to improve fatigue performance in face-centered cubic alloys especially with high SFE and SRO degree.

Deformation twinning in pure aluminum under ultrahigh strain rate: Dependence of strain rate and crystallographic orientation

Deformation twinning rarely occurs in single or polycrystalline aluminum due to its high stacking fault energy (SFE), even when deformed at low temperatures and/or high strain rate. Here we report electron back-scattered diffraction observations that provide evidence of deformation twinning in single crystal pure aluminum under shock loading. In addition, deformation twinning prefers to occur as twinning plane roughly paralleled to shear direction. Under shock loading, dislocation motion becomes jagged or even stopped, resulting in self-pinning. Then deformation twinning takes over as the dominant deformation mode. Our results provide a comprehensive understanding of the dynamic transition from dislocation slip to deformation twinning in pure aluminum under shock loading.

Effect of thermal ageing and deformation on microstructural evolution of 304 and 202 grade steel

AISI 304 ASS was compared with AISI 202 SS in metallurgical aspects. Manganese and nitrogen contained alloy is a Ni free ASS having type 202 grade. In the present investigation, the deformation of two grade analyzed using XRD and microstructural testing. Optical Microstructure of AISI 304 and AISI 202 has been done for various thermal ageing temperature and duration at 15% deformation and without deformation. AISI 304 appears to be deformed more rapidly than AISI 202 due to its higher stacking fault energy which gave results in rapid transformation from strain induced martensite to austenite in 304, i.e., it concluded that 202 is more stable at low percentage of deformation as compare to 304 stainless steel. XRD results also indicate that more fraction of martensite formed in 304 stainless steel. So investigation gives the conclusion which suggest that AISI 304 is more suitable then AISI 202 at low strain rate or deformation.