Deformation Nanotwins Research Articles

Evolution of deformation structures in CrCoNiSi0.3 medium entropy alloy subjected to high-strain-rate and quasi-static compressive deformation

The objective of the present study is to fundamentally explore the evolution of deformed structures of CrCoNiSi0.3 medium entropy alloy subjected to high-speed compression deformation with true strains of 0.16, 0.33, and 0.46 (via split Hopkinson pressure bar system, strain rate of ∼6 × 103 s−1, denoted as HSR deformation), and quasi-static compression deformation with true strains of 0.20, 0.30 and 0.50 (via MTS compression system, strain rate of ∼10−3 s−1, denoted as LSR deformation). From the comparison between the results of HSR and LSR samples, it clearly indicates that the formation of deformation nanotwins is promoted under the HSR deformation and enhances the work hardening effect in the early stage of deformation; as the HSR deformation proceeds, the nanotwins effectively provide the dynamic Hall–Petch effect, leading to an increase in flow stress beyond the stress required for HCP phase transformation. The HCP phase also induces a stronger work hardening capacity at the late stage of deformation. Profuse deformation nanotwins and HCP phase promoted by HSR deformation result in a strong work hardening effect. Furthermore, the flow stress of the HSR deformed sample with a true strain of 0.46 is 1536 MPa, which is about 36% higher than that of the LSR deformed sample with the same true strain, 1102 MPa. It is clear that under the HSR deformation, the defect structures including deformation nanotwins and ultra-thin HCP platelets are much more favorably generated, greatly improving the working capacity.

Annealing-induced multi-heterogeneous microstructure strengthening in FeCoCrNiMo0.2 high-entropy alloys

In this study, by conducting cryogenic rolling (CR) followed by short-term annealing (CRA), we achieved the annealing-induced hardening in a single-phase FCC FeCoCrNiMo0.2 high-entropy alloy by introducing multiple heterostructures structures. Through this approach, the yield strength, ultimate tensile strength and tensile fracture elongation of CRA alloy were increased from 1116.8 MPa to 1286.1 MPa, 1363.2 MPa to 1576.3 MPa and 5.4–6.9 %, respectively. Furthermore, a broader steady state of strain softening behavior was found in the CRA alloy, which is attributed to the additional strengthening and hardening effects resulting from high density of deformation nanotwins and phase transformations. Additionally, the research results show that the ultra-high strength and sufficient ductility of CRA alloy are attributed to the synergistic effect of multiple mechanisms, such as strengthening induced by heterogeneous microstructure of σ precipitate/HCP phase/FCC matrix phase, and high work hardening ability caused by the heterogeneity of high density deformation nanotwins, geometrically necessary dislocations (GND), stacking faults (SFs) and Lomer-Cottrell locks (L-C locks) nano-scale defects. This work provides useful insights for alloys that involves multiple heterogeneous microstructures strengthening and provides a promising approach for the design and manufacture of high-strength structural materials.

High-temperature low-cycle fatigue and fatigue–creep behaviour of Inconel 718 superalloy: Damage and deformation mechanisms

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on Inconel 718 nickel-based superalloy at temperatures of 650 °C and 730 °C. LCF tests at elevated temperatures were performed with a mechanical strain rate of 1×10−3/s, while fatigue–creep tests involved either tensile or compressive strain dwell. Both the LCF and fatigue–creep tests revealed cyclic softening, with the mean stress evolving oppositely to the applied strain dwell in the fatigue–creep tests. Investigations into the damage mechanisms identified intergranular cracking as the predominant failure mode. Fatigue–creep loading with a compressive dwell resulted in multiple crack initiations from transgranular oxide intrusions, along with multiple creep cavities during loading at 730 °C. Deformation features such as persistent slip bands and deformation nanotwins were observed during cycling at 650 °C. In addition, fatigue–creep tests at 730 °C exhibited δ phase precipitation and a coarsening of strengthening precipitates, contributing to additional softening that increased over prolonged test durations. Finally, the observed lifetime during LCF tests decreased with increasing temperatures, and fatigue–creep loading was observed to be more damaging than LCF. On the other hand, fatigue–creep loading with a tensile strain dwell demonstrated a higher lifetime compared to LCF at 730 °C.

Achieving ultra-strong and ductile CoNi-based FCC multi-principal element alloys via alloying with refractory Mo and W

FCC single-phase multi-principal element alloys (MPEAs) often suffer from insufficient strength, which limits their wide applications. Here a CoNi-based FCC MPEA (Co44Ni44Mo9W3, at. %) was designed by alloying with refractory Mo and W. The Co44Ni44Mo9W3 alloy possesses an ultrahigh yield strength of ∼1050 MPa and good ductility of ∼30 % at ambient temperature. The excellent mechanical properties were correlated to its superior lattice friction stress and grain boundary strengthening compared to most reported FCC MPEAs and conventional alloys. By density-functional theory calculations, such extraordinary mechanical properties were further attributed to its high lattice distortion, low intrinsic stacking fault energy and high unstable stacking fault energy due to the addition of Mo and W. Transmission electron microscopy investigations revealed that the high strength-ductility combination arises from the synergistic effect of planar slip bands, stacking faults and deformation nano-twins.

Cryogenic strengthening of Fe27Co24Ni23Cr26 high-entropy alloys via hierarchical nanotwin-driven mechanism

FCC-based high entropy alloys (HEAs) with exceptional cryogenic mechanical properties have the potential for use in strategic applications. In this regard, we have studied via transmission electron microscopy (TEM) the microstructural evolution of hierarchical nanotwins in tensile-strained Fe27Co24Ni23Cr26 HEA alloys as a function of true strain at cryogenic temperature and compared the behavior at room temperature. At cryogenic temperature, the alloy had a high strength of ∼1211 MPa and large elongation of 87.2% compared to the room temperature tensile strength of 746 MPa and elongation of ∼61%. At 25 °C, the deformation initiated with the entanglement of wavy dislocations, which transformed into planar configuration and dislocation walls, and lastly to deformation nanotwins. In striking contrast, at −150 °C, numerous nanotwin bundles were formed through the coalescence of deformation nanotwins, which divided the matrix into microscale closed blocks. At higher strain, dense deformation nanotwins refined annealing twins into nanoscale closed blocks. Subsequently, in the interior of annealing twins, deformation nanotwin variants interpenetrated, producing serrated/curved interfacial structures. These interfaces are envisaged to promote the formation of ultra-fine deformation nanotwins and closed blocks that facilitate accommodation of a higher degree of deformation strain at cryogenic temperatures. The study provides new insights and guidelines in the futuristic design of FCC high entropy alloys for use at cryogenic temperatures by embracing the hierarchical nanotwin-driven mechanism.

Strength and ductility improvement of the axial gradient microstructure TC21 titanium alloys manufactured by electropulsing plus step-quenching treatment

In order to reduce the weak area caused by traditional connection method and improve the structural efficiency of aircraft, the design concept of an integral component with dual microstructure is proposed. The electropulsing treatment (EPT) was used to produce an axial gradient microstructure (AGM) in TC21 titanium alloy, followed by the step-quenching treatment (SQT) to tailor the precipitation behavior of the α phase. Results demonstrate that the AGM produced by the combination of EPT and SQT achieves a simultaneous enhancement in both strength and plasticity of the alloy. Compared to EPT alone, the SQT results in the dissolution of the bimodal microstructure region with small-sized α phase, and an increased density of secondary α phase precipitation in both the transition and lamellar microstructure regions. During room temperature tensile deformation, coordinated distribution of deformation occurs sequentially in various regions, the 101̅0112̅0type of prismatic slip and deformation nanotwins exist in the equiaxed and lamellar α phase, respectively. Moreover, the deformation gradually declines from the bimodal region to the lamellar region, leading to the fracture of AGM in the bimodal region and an overall augmentation of the strength and plasticity of TC21 alloy. Hence, the results of this paper can provide research basis for the strength and plasticity improvement and deformation damage mechanism of high strength and toughness titanium alloy.

Thermally driven evolution of deformation nanotwins into nanolaminated structures with low-angle boundaries in 316L stainless steel

Although twin boundary is a kind of low-energy boundary, the interactions between dislocations and twin boundaries promote the detwinning of twin layers. In this work, we investigated the thermally driven evolution of deformation nanotwins in 316L stainless steel. After high-temperature annealing, deformation nanotwins evolve into nanolaminated structures with low-angle boundaries, which are difficult to form in materials with low stacking fault energies. We explored the roles of dislocations and twin boundaries, as well as their interaction mechanism underlying microstructural evolution.

Evolution of microstructure and texture in Al-containing CoCrNi medium entropy alloys during severe warm-rolling

Al, as an alloying element, can influence the properties of the single-phase equiatomic CoCrNi MEA. In this study, different alloys of different Al concentrations added to CoCrNi MEA (AlxCoCrNi) were prepared by warm rolling to understand the effect of Al. The alloys were warm-rolled at 400 °C up to an 85% thickness reduction, followed by isochronal annealing at 700–1100 °C. The microstructure after deformation reveals the development of deformation heterogeneities such as shear bands and deformation nano-twins. The evolution of the weak brass component is seen after deformation. Different phases, such as BCC and sigma, evolve upon annealing treatment at different temperatures. The higher Al-content MEA (Al9.09) shows comparatively finer recrystallized microstructure irrespective of the annealing temperatures. Also, weak recrystallization texture develops after annealing, and the deformation texture components lying on α-fiber are preserved. This indicates the absence of preferential nucleation and growth during annealing. The hardness in the as-cast, deformed, and annealed conditions increases with increasing Al content, indicating the influence of Al in the strengthening of the AlxCoCrNi MEAs.

Effect of specimen size and crystallographic orientation on the nano/microscale mechanical properties and deformation behavior of CrCoNi medium-entropy alloy

CrCoNi medium-entropy alloy (MEA) shows an exceptional combination of macroscale mechanical properties, however, knowledge of its mechanical properties at nano/microscale is lacking. In this paper, nano/microscale single crystal MEA pillars with orientation of 〈116〉, 〈014〉, 〈102〉 were fabricated using focused ion beam milling, and the compressive strength was determined with micropillar compression testing using a flat-end tip in an indentation machine. Results showed that the yield and flow strength of these MEA pillars were orientation-dependent, i.e., the lower the Schmid factor, the higher the strength. At the same time the strength of the pillars was dependent on the pillar’s size, the strength increasing as the pillar diameter reduced. From the results of the micropillar compression testing, the bulk critical resolved shear stress (CRSS) was determined to be 92 MPa, which is the highest value of the group of CrCoNi-based medium and high-entropy alloys. Analysis of transmission electron microscope images and large-scale molecular dynamics simulations revealed a group of complimentary deformation mechanisms, including dislocation slides, dislocation annihilation, dislocation-slip band interaction, formation of nanoscale stacking-faulting networks, and deformation nanotwins.

New insight into the strengthening mechanism of AlCoCrFeNi2.1 eutectic high-entropy alloy with dual-phase nanolamellar structures achieved via laser powder bed fusion

Recently, the AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) prepared by laser powder bed fusion (LPBF) could reach a good balance of strength and ductility. The strengthening effect was only qualitatively clarified, however, quantitative analysis remained unclear. In this study, the quantitative analysis of strengthening mechanisms and the relationship between microstructural characteristics and tensile properties were revealed in depth. The AlCoCrFeNi2.1 EHEA with FCC and BCC dual-phase nanolamellar structures (interlamellar spacings of 200–300 nm) fabricated by LPBF achieved an excellent combination of high yield strength (1116 MPa) and fracture elongation (20.3 %). Compared with as-cast EHEA samples, the FCC/BCC nanolamellar interfaces in the LPBF-printed AlCoCrFeNi2.1 EHEA were still semi-coherent but with a higher degree of coherency, and the Al concentration in the FCC phase increased. Post-deformation analysis showed that there were high-density dislocation pile-up and slip, stacking faults (SFs), and Lomer-Cottrell (L-C) locks in the FCC phase, and there were deformation nano-twins and SFs in the BCC phase. The back stress generated in FCC/BCC phases during tensile deformation could maintain the plastic co-deformation of the two phases. The theoretical calculation revealed that the strengthening mechanisms mainly stemmed from the nanolamellar structures, dislocation strengthening, and the solution strengthening induced by the increased Al concentration in the FCC phase. The calculated result (1096.3 MPa) was consistent with the experimental value, and the nanolamellar structures strengthening (∼720.7 MPa) was the main contributor. This work provides a theoretical quantitative understanding of the strengthening mechanism of the AlCoCrFeNi2.1 EHEA fabricated by LPBF and references for the strengthening mechanism of additively manufactured EHEAs.

Enhanced fatigue resistance and fatigue-induced substructures in an additively manufactured CoCrNi medium-entropy alloy treated by ultrasonic surface rolling process

There is a significant need to elucidate the underlying mechanisms of cyclic plastic damage mechanism for additively manufactured materials and develop effective surface modification techniques to improve their fatigue life. This study investigates the efficacy of ultrasonic surface rolling process (USRP) technology in the creation of a ∼300 μm gradient nanotwinned structure on the surface of additively manufactured CoCrNi medium-entropy alloy (AM-MEA), which results in a beneficial result that yield strength and 107-cycle fatigue endurance limit are significantly improved, achieving the increment of 192.1 MPa and ∼130 MPa, respectively. The superior fatigue property is attributed to multiple factors that suppress crack initiation from sample surfaces jointly, including the presence of a gradient nanotwinned layer and the reduction in irregular defects located both on and beneath the surface. The cyclic plastic deformation behavior of AM-MEA samples with and without USRP under both high and low stress levels was studied in-depth through multiscale characterization techniques. When exposed to cyclic loading at a low stress level of 480 MPa, the fatigue damages of both samples were dominated by accumulation of statistical stored dislocations (SSDs) and persistent Lüders bands. There is no significant difference in the increase in dislocation density between both samples. However, under cyclic loading at a high stress level (660 MPa), the fatigue damage of the AM-MEA sample primarily originated from the accumulation of deformation nanotwins, stacking faults, geometrically necessary dislocations and SSDs. Conversely, the fatigue damage observed in the AM-MEA sample with USRP at the same stress level was dominant by an increase in stacking faults and SSDs. Notably, this increase in total dislocation density was visibly lower than that observed in the AM-MEA sample, which is ascribe to the stable gradient layer providing enhanced hetero-deformation induced stress for the core region in the AM-MEA sample with USRP at high stress level.

Introduction of nanotwins into nanoprecipitations strengthened CoCrNiMo0.2 alloy to achieve strength and ductility trade-off: A comparative research

Normal strengthening methods through precipitations and deformation obviously enhance the strength of metallic materials while resulting in the sacrifice of ductility, and synergistic improvement of strength and ductility is currently an urgent requirement. Herein we developed a cryogenic deformation combined with an annealing method to fabricate CoCrNiMo0.2 medium entropy alloy, which achieved an ultrahigh strength of 1.8 GPa with synergistic improvement in strength and ductility. Microstructure, mechanical performance, and strengthening mechanisms of the developed alloys were investigated compared with that prepared by the regular room temperature deformation method. It was found that high-density nanotwins were produced in CoCrNiMo0.2 MEA via cryogenic deformation. Fine grains, hard precipitations, and high volume fraction of nanotwins greatly strengthened the alloy, obtaining a yield and ultimate tensile strength of 1400 MPa and 1800 MPa. Ductility improvement of the developed alloy was mainly attributed to the production of deformation nanotwins due to the lower stacking fault energy, which greatly increases the dislocation storage ability, and thus, the ductility of the alloy was enhanced.

A novel wear-resistant Ni-based superalloy via high Cr-induced subsurface nanotwins and heterogeneous composite glaze layer at elevated temperatures

Tribo-induced structural and chemical modifications at elevated temperatures play paramount roles in the tribological performance of Ni-based superalloys. Here, we report on a novel face-centered-cubic Ni-27Cr superalloy with a low stacking fault energy exhibiting remarkable tribological properties at 25–800 ℃, especially the low wear rates at 400 ℃ (1.97 ×10−5 mm3/N·m) and 800 ℃ (3.1 ×10−6 mm3/N·m). First, at 25–600 ℃, tribo-induced deformation nanotwins show superior structural stability, contributing to the high wear resistance. Second, at 800 ℃, the tribo-induced nanotwins are not formed. Instead, the topmost glaze layer transits from homogeneous nanostructure to heterogeneous microstructure consisting of hard nanograined oxides and soft ultrafine grained matrix. The strong and ductile heterogeneous composite glaze layer accommodates large plastic deformation and reduces wear loss.

Achieving ultrahigh cryogenic yield strength and sufficient ductility in a medium-entropy alloy via bimodal grain design

This work proposes an efficient strengthening and toughening method. A CrCoNi medium-entropy alloy (MEA) with a bimodal grained structure can be designed by controlling intermediate-temperature-annealing after cold rolling. The bimodal grained MEA consists of ultrafine-grained domain and fine-grained domain (volume fractioñ37%), showing an ultrahigh yield strength (YS) of 1600 MPa and a remarkable uniform elongation of 19% at cryogenic temperature, whose cryogenic YS improves by ∼2.5 times compared to that of coarse-grained counterpart. The inhomogeneity of the bimodal grained structure can significantly improve the YS while maintaining stable strain hardening ability, and activate multiple deformation mechanisms including significant dislocation activities, massive stacking faults, deformation nanotwins, and Lomer-Cottrell locks. Our results demonstrate that tailoring bimodal grained structures can enrich the applications of MEAs in cryogenic engineering.

Twin and twin intersection phenomena in a creep deformed microalloyed directionally solidified high Nb containing TiAl alloy

• The combination of directional solidification and microalloying can significantly refine the columnar grains and homogenize the microstructure of TiAl alloy. • Compared with ternary TiAl alloy, the microalloying high-Nb TiAl alloy exhibits more superior creep property at 760 °C/275 MPa. • The decrease of stacking fault energy can promote the dislocation dissociation and the formation of deformation nanotwins. • Twin intersections will create more boundaries in the lamellar matrix, which will act as obstacles to dislocation movement. The creep behavior of a directionally solidified TiAl alloy with a high Nb content after microalloying by the addition of small amounts of W, Cr, and B elements was investigated by scanning electron microscopy and transmission electron microscopy. By means of directional solidification and microalloying, a TiAl alloy with a fine and uniform microstructure and continuous columnar crystals was obtained. High-density dislocations, deformation nanotwins, and twin intersections were observed in γ lamellae. The results show that, in comparison with the ternary TiAl alloy, the microalloyed high Nb containing TiAl alloy exhibited better creep properties at 760 °C and 275 MPa. The decrease in stacking fault energy can promote dislocation dissociation and the formation of deformation twins, and the twin intersections can hinder the movement of dislocation to enhance the creep performance of the TiAl alloy.

Hot corrosion behavior and mechanism of cryo-rolled MP159 superalloy with long rod-like γ′ phase

The hot corrosion behavior and corrosion mechanism of cryo-rolled MP159 superalloy in a salt mixture of 75 % Na2SO4 + 25 % NaCl (wt %) at 800 °C were investigated by using XRD, SEM, HRTEM and first-principles calculations. Many of long rod-like γ′ phases with stacking faults and deformation nano-twins can be generated during hot corrosion, which promotes the outward diffusion of Cr element. Moreover, a high content of Nb element in the long rod-like γ′ phase can increase the activity of Cr element, which is conducive to the formation of a denser Cr2O3 film by the selective oxidation of Cr element, and results in a better hot corrosion resistance.

Multiple strengthening mechanisms induced by nanotwins and stacking faults in CoNiCr-superalloy MP159

The cold-drawing deformation behavior of CoNiCr-superalloy MP159 with designed strains was investigated, and the substructural evolution was observed by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The unique network structure in the MP159 superalloy was found to consist of intersecting nanotwins and stacking faults, and the corresponding formation mechanism was revealed. Different fractions and thicknesses of deformation nanotwins were obtained at different cold-drawing-induced strains, and the effect of deformation nanotwins on the mechanical strength was further analyzed. Furthermore, the ultimate tensile strength at 48% drawing-induced strain increased by approximately 75% compared to that of the initial solution sample. The multiple deformations induced by Lomer–Cottrell locks, nanotwins, stacking faults (SFs) and their interactions contributed to the strength enhancement during cold-drawing deformation.

Microstructure evolution and strengthening mechanisms of MP159 superalloy during room temperature rolling and cryorolling

Microstructure evolution and strengthening mechanisms of MP159 superalloy during room temperature rolling (RTR) and cryorolling (CR) were studied by using the XRD, EBSD and HRTEM methods. The results suggest that the grains size is finer and the density of dislocations is higher in the CR specimens with the same rolling reduction compared with the RTR specimens. Moreover, the deformation nano-twins and L-C locks can form in the RTR and CR specimens. The strengthening mechanism of the MP159 superalloy during RTR and CR processing is mainly ascribed to the dislocation strengthening, grain refinement strengthening and deformation nano-twins strengthening. After rolling deformation, a higher yield strength (YS) and ultimate tensile strength (UTS) were obtained in the CR specimens. Especially, at the rolling reduction of 48%, the YS and UTS of the CR specimens are 342 MPa and 314 MPa higher than that of the RTR specimens, respectively.

High strength and ductility in a novel Ni-based superalloy with γ′ and nanotwins / stacking faults architectures

The variation of microstructure, which depends on the processing history, significantly affects the mechanical properties of alloys. Here, thermo-mechanical treatments were employed to control the microstructure and tune the corresponding mechanical properties of a Ni-based superalloy. The nanotwins can be successfully introduced into the superalloy after cold rolling reduction amount of 70%. Meanwhile, the nanotwins are thermally stable during subsequent annealing at 950 °C for 2 h and two-step aging processes (650 °C/24 h/AC and 760 °C/16 h/AC). The high yield strength of 1691 MPa has been obtained in the partially recrystallized structure. The high strength originates from the dislocations and deformation nanotwins induced by cold rolling, and γ′ particles. When compared to the initial condition, the yield strength and elongation of the fully recrystallized structure are simultaneously increased from 1043 MPa to 19.5% to 1386 MPa and 20.5%, respectively. The strength-ductility combination is derived from the fine-grains, annealing twins and γ′ particles coupled with a high density of intersected SFs with LC locks. In addition, the unique work hardening behavior is mainly related to the intersected SFs and LC locks configuration. The present study provides a strategy to obtain the desirable mechanical properties of Ni-based superalloy via proper thermo-mechanical treatments.

Deformation nanotwins in a single-crystal Ni-based superalloy at room temperature and low strain rate

The deformation microstructure of single crystal superalloys CMSX-4 was investigated during uniaxial tensile with a strain rate of 5 × 10 −3 s −1 at room temperature. The objection of the current study is to evaluate the deformation twinning and investigate the deformation mechanism. Experimental results followed by transmission electron analysis show that deformation nanotwins and elemental segregation in single-crystal Ni-based superalloys were observed after tensile deformation at room temperature with a strain rate of 5 × 10 −3 s −1 . Furthermore, the relationship among deformation twinning, elemental segregation, and high local stress due to plastic deformation with increasing plastic strain was estimated and discussed. These findings provide valuable insights into understanding twinning and elemental segregation in the plastic deformation of single-crystal Ni-based superalloys. • Deformation nanotwins in CMSX-4 are initiated under tensile tests with a strain rate of 5 × 10 −3 s −1 at room temperature. • Element segregation can be observed at the twin zone at room temperature. • Deformation nanotwins in CMSX-4 is not necessarily accompanied by an atomic reordering process at room temperature.