Network Of Stacking Faults Research Articles

An excellent combination of strength and ductility via hierarchical precipitation structures in Co-free medium-entropy alloys

A Co-free non-equiatomic Ni2·5CrFeAl0·25Ti0.25 medium-entropy alloy (MEA) with an excellent strength-ductility synergy was fabricated, which shows a multiphase structure composed of face-centered cubic (FCC), L12 (ordered FCC), and Cr-rich body-centered cubic (BCC) phase by thermomechanical processing. Specifically, the aged sample displays the outstanding yield tensile strength (YTS, ∼1188 MPa), ultimate tensile strength (UTS, ∼1560 MPa) and work-hardening rate (WHR, ∼4.5 GPa) values as well as an acceptable plasticity of ∼16.6%. Theoretical calculations suggest that precipitation strengthening significantly contributes to achieving the fascinating tensile strength among various strengthening contributors. Further analyses reveal that multiple nanoscale stacking-fault (SF) networks are activated during plastic deformation in the aged alloy. Accordingly, the dual effects consisting of the hierarchical precipitation structure and SF networks lead to the combination of excellent tensile strength and strain-hardening capacity.

Achieving exceptional strength-ductility combination in a novel L12 nanoparticles-strengthened high-entropy alloy via conventional casting

A novel L12 precipitation-strengthened Co51.3Ni19.5Cr19Al6.4Ti3.4Nb0.4 medium-entropy alloy (MEA) with excellent combination of large ductility and high strength was prepared via direct casting. The coherent ultrafine L12 precipitates particles (∼30 nm) with a volume fraction of 65% formed uniformly in the FCC matrix after heat-treatment, which endows this newly-designed MEA with superior tensile properties, i.e., a yield strength of ∼710 MPa, an ultimate tensile strength of ∼1045 MPa and a uniform elongation of ∼31.3%, respectively. The blocking effect of ultra-fine L12 precipitates significantly contributes to the high strength of the Co51.3Ni19.5Cr19Al6.4Ti3.4Nb0.4 MEA. In addition, deformation-induced nano-spaced stacking-fault networks, nano-twins and high-density Lomer-Cottrell locks facilitate the high work-hardening capacity, leading to excellent tensile ductility. This alloy design strategy can potentially be applied to many other alloys and open new avenues for designing strong yet ductile materials for engineering applications.

Wear-resistant CoCrNi multi-principal element alloy at cryogenic temperature

Traditional high strength engineering alloys suffer from serious surface brittleness and inferior wear performance when servicing under sliding contact at cryogenic temperature. Here, we report that the recently emerging CoCrNi multi-principal element alloy defies this trend and presents dramatically enhanced wear resistance when temperature decreases from 273 to 153 K, surpassing those of cryogenic austenitic steels. The temperature-dependent structure characteristics and deformation mechanisms influencing the cryogenic wear resistance of CoCrNi are clarified through microscopic observation and atomistic simulation. It is found that sliding-induced subsurface structures show distinct scenarios at different deformation temperatures. At cryogenic condition, significant grain refinement and a deep plastic zone give rise to an extended microstructural gradient below the surface, which can accommodate massive sliding deformation, in direct contrast to the strain localization and delamination at 273 K. Meanwhile, the temperature-dependent cryogenic deformation mechanisms (stacking fault networks and phase transformation) also provide additional strengthening and toughening of the subsurface material. These features make the CoCrNi alloy particularly wear resistant at cryogenic conditions and an excellent candidate for safety–critical applications.

Investigation on solidification structure evolution of deeply undercooled single-phase Ni-Cu-Co alloys

In this work, the main focus was on exploring the effect of increasing the gradient content of Co element on the solidification process of Ni-Cu-Co alloy under deep undercooling conditions. The effects on its solidification rate, recalescence effect, and critical undercooling range were also explored. In the experiment, a combination of molten glass purification and cyclic superheating technology was mainly used to obtain gradient undercooling of Ni88Cu10Co2, Ni88Cu8Co4, and Ni88Cu6Co6 alloy samples at intervals of 10 K. By analyzing the microstructure evolution under different undercooling degrees through metallographic diagrams, and combining it with high-speed cameras to record the phenomenon of recalescence during the solidification process, the influence of the addition of Co element on the solidification rate, recalescence effect, and the critical undercooling range appearing in the microstructure during the overall solidification process was explored. Conduct EBSD analysis on the alloy samples under the maximum undercooling of Ni88CuCo6, and analyze the representative significance behind their characterization data. At the same time, TEM transmission analysis was conducted on the Ni88Cu6Co6 alloy sample with the highest Co content, and it was found that there are high-density dislocation and stacking fault networks as well as obvious twinning structures in the substructure area. The standard FCC diffraction pattern also represents that it is still a single-phase structure. Based on the above metallographic diagram, EBSD, and TEM data analysis, it is proven that the occurrence of grain refinement under high undercooling is mainly due to stress induced recrystallization.

High density of stacking faults strengthened TaN/TiN multilayer

Multilayer coatings with individual layer thickness on the nanoscale exhibit superior mechanical properties because of their high interface density. However, compared to artificial phase interfaces, planar defects like twin and stacking faults (SFs) are hardly investigated in ceramic multilayer systems. We choose to combine the group VB transition metal nitride (TMN) TaN with its extremely low stacking faults energy (SFE) and TiN to form a superlattice (SL). TiN/TaN multilayers with a bilayer period (Λ) from 8 nm to 100 nm were prepared by dc magnetron sputtering. These as-deposited films show a peak hardness 36 ± 2.4 GPa at Λ=20 nm. The extensive high-resolution transmission electron microscopy (HRTEM) observations reveal that the dissociation of full dislocations results in the network of SFs and the formation of Lomer-Cottrell lock arrays inside the TaN layer. Meanwhile, further dislocation analysis indicated the Shockley partials cross slip at the interface. These findings provide us with a new perspective for designing TMN multilayers with planar defects.

Hierarchical precipitates facilitate the excellent strength-ductility synergy in a CoCrNi-based medium-entropy alloy

An interstitial carbon-doped CoCrNiSi0.3 medium-entropy alloy (MEA) has been developed, which exhibits excellent strength-ductility synergy. This has been achieved through the implementation of a three-level hierarchical-precipitate-microstructure design. The CoCrNiSi0.3C0.048 MEA displays an ultra-high yield strength of 1.34 GPa, accompanied by a uniform tensile elongation of 8.62%, surpassing the performance of most carbon/silicon-doped MEAs. The three-level hierarchical precipitate structure comprises the primary Cr23C6 carbides (2–10 μm), the secondary SiC precipitates (200–500 nm) surrounding the grain boundaries, and the tertiary SiC precipitates (∼50 nm) within the grains. The strengthening and toughening behavior of the current MEAs are attributed to multiple mechanisms, including crack branching and blunting in Cr23C6 carbides, dislocation-bypass mechanism around the non-shearable SiC precipitates, hetero-deformation-induced strengthening caused by the interface between matrix/three-level hierarchical precipitates, as well as stacking-fault networks and dense nanotwins activated by precipitates in the matrix. These findings provide valuable insights into the development of high-performance alloys for engineering applications in the future.

C-modified stacking-fault networks inducing the excellent strength- plasticity combinations of medium manganese steel by simple two-stage warm rolling without annealing

The medium manganese steel with ultrahigh yield strength of 1421 MPa, a tensile strength of 1575 MPa, and an excellent total elongation of 28.4% was designed by simple two-stage warm rolling without annealing. The improvement of yield strength is mainly attributed to the extensive strengthening defects in the lamellar dual-phase microstructures, which including the high-density dislocations in ferrites and high-density dislocations, nanoscale VC precipitates, stacking faults and Lomer-Cottrell (L-C) locks in austenite (22 Vol.%). In addition, the abnormally enhanced strain-hardening ability benefits from the C-modified stacking-fault networks, whose stability and C-segregated characteristics are precisely clarified by the advanced in-situ heating HRTEM and APT analysis. This special strengthening and plasticizing effects of stacking faults induced by simple two-stage warm rolling can effectively break through the trade-off between strength and plasticity of ultra-strong medium manganese steel.

Enhancement of Strength and Plasticity by Nanoprecipitation Strengthening and Stacking Fault Deformation in a High Entropy Alloy

Precipitation strengthening can effectively improve the strength of high entropy alloys (HEAs), but usually severely reduces the ductility. In this study, a new interstitial carbon-doped HEA with a nominal composition of Ni33.5(CoFe)55V5Nb5C1.5 at.% was developed. The HEA exhibits a high yield strength of 1244 MPa, ultimate tensile strength of 1553 MPa and excellent elongation of ~ 30%. The stacking fault energy of the matrix is affected by the precipitation of L12-Ni3Nb, which enhances the stacking fault (SF)-induced deformation. Since the Taylor lattice domains are subdivided by the SF networks, the mean free path of dislocation is reduced. The enhanced strain-hardening ability leads to a good combination of strength and ductility.

Small-scale mechanical properties and deformation mechanisms of a laser welded CrCoNi medium-entropy alloy joint

Attributed to deformation twins, laser welded CrCoNi medium-entropy alloy joints exhibit best combination of strength and ductility among the joints of Cantor alloy and its derivatives. However, the nano/microscale mechanical properties and associated deformation mechanisms of the welded CrCoNi alloy joint remain mysterious. Here, we fabricated cylindric pillars with diameter from 500 nm to 2000 nm in a single crystal of the fusion zone, and then compressed these pillars in a nano-indenter. Results showed that the yield stress and flow stress of these pillars were size-dependent, with a “smaller is stronger” trend. Via examination and analysis of the deformed microstructure using electron backscattered diffraction, transmission electron microscope, and large-scale atomistic simulations, we found that deformation mechanism in the single crystal at nano/microscales was the formation of nanoscale stacking-fault networks. Large-scale atomistic simulations further revealed that dislocation-based mechanisms, such as entanglements, and annihilation from the free surface of the pillar, dominated the compressive deformation. Finally, a dislocation-based theoretical model was successfully utilized to predict the critical resolved shear stress and the size effect, which matched well with our experimental results.

High strength-ductility synergy in a laser welded dissimilar joint of CrCoNi medium-entropy alloy and stainless steel

CrCoNi medium-entropy alloy (MEA) and its welded joints has great potential applications in engineering structures since they present superior ultimate tensile strength (UTS) and ductility. However, the mechanical performance of the welded joints of CrCoNi and traditional metal alloys remains mysterious. Here, we applied laser welding to join CrCoNi and stainless steel (SS) 301L to achieve a dissimilar MEA-SS301L joint and investigated its microstructure evolution and mechanical properties. The fusion zone becomes a face-centered cubic single-phase non-equiatomic FeCrCoNi high-entropy alloy (HEA). Its UTS and ductility is 482 MPa and 46% at 300K, respectively; and is respectively enhanced to 791 MPa and 59% at 77K. Our dissimilar joint displays a strength-ductility synergy that exceeds that of all other dissimilar HEA joints and most similar HEA joints. Electron backscattered diffraction, transmission electron microscopy and molecular dynamics simulation uncover that extensive dislocation activities, stacking fault networks, deformation twins, and interactivity between dislocations and twin boundaries are the origin of this high strength-ductility synergy in our joint.

Mechanical properties and deformation mechanisms in CoCrFeMnNi high entropy alloys: A molecular dynamics study

We used molecular dynamics simulations to investigate the relationship between mechanical properties and deformation mechanisms in CoCrFeMnNi alloys having different compositions. According to deformation twinning activity, the deformation mechanisms in the different CoCrFeMnNi compositions under tensile loading can be classified into three major categories: easy-shear (ES), inter-locks (IL) and bulk hexagonal closed packed (hcp) transformation (BH). We found that the compositions with ES were more likely to rupture early because of the short and fragmented twins. The compositions following the IL pattern promoted the movement of interlocking stacking faults in the twin boundaries that could prolong tensile deformation. Moreover, BH could further increase ductility because the hcp transformation could absorb and dissipate the stacking faults. Experimental observations of immobile stacking-fault networks that impede dislocation motion and further provide preferred sites for the formation of the hcp phase were commonly found in the ES, IL and BH patterns. The calculated intrinsic stacking fault energies of CoCrFeMnNi and CoCrFeNi were consistent with the experimental measurements. The combination of high unstable stacking fault energy with the formation of the IL pattern results in high strength and high ductility in the CoCrFeMnNi HEA system.

Ultrahigh strength and ductility in newly developed materials with coherent nanolamellar architectures

Nano-lamellar materials with ultrahigh strengths and unusual physical properties are of technological importance for structural applications. However, these materials generally suffer from low tensile ductility, which severely limits their practical utility. Here we show that markedly enhanced tensile ductility can be achieved in coherent nano-lamellar alloys, which exhibit an unprecedented combination of over 2 GPa yield strength and 16% uniform tensile ductility. The ultrahigh strength originates mainly from the lamellar boundary strengthening, whereas the large ductility correlates to a progressive work-hardening mechanism regulated by the unique nano-lamellar architecture. The coherent lamellar boundaries facilitate the dislocation transmission, which eliminates the stress concentrations at the boundaries. Meanwhile, deformation-induced hierarchical stacking-fault networks and associated high-density Lomer-Cottrell locks enhance the work hardening response, leading to unusually large tensile ductilities. The coherent nano-lamellar strategy can potentially be applied to many other alloys and open new avenues for designing ultrastrong yet ductile materials for technological applications.

Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy

Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation.

Mechanisms of damage formation in Eu-implanted AlN

X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the evolution of damage during implantation of 300 keV Eu ions at room temperature in AlN. At low fluence, a strain increase is observed in a buried layer where clusters of point defects and stacking faults (SFs) coexist. At higher fluence, a saturation of the strain is observed in this layer, and the XRD curves exhibit characteristic features which coupled with TEM results enable the identification of additional, spatially separated, dilated and contracted regions. From these observations, the following damage mechanisms are proposed. As the SFs grow by trapping point defects, a dense network of basal and prismatic SFs forms, which leads to the ejection of point defects from the buried damaged layer and consequently to the saturation of the strain. In this process, interstitials in excess migrate towards the undamaged bulk where they form clusters inducing large strain values. In contrast, defects ejected towards the surface either remain isolated or form isolated dislocation loops and SFs depending on their nature, i.e., interstitial or vacancy. This is probably the main difference with GaN where the defects ejected from the buried damaged layer contribute to the fast propagation of the dense SFs network towards the surface due to their relatively low formation energies. As a consequence, whilst nanocrystallization occurs at the surface of GaN, the relative confinement of defects and implanted atoms in the buried layer of AlN results in its amorphization, although at extremely high fluences (∼1017 Eu/cm2).

Molecular dynamics study on the plastic behavior of monocrystalline copper under shock loading and unloading

Molecular dynamics simulations were used to study the plastic behavior of copper monocrystal under shock loading and unloading. We obtained the Hugoniot relation for both ［001］ and ［111］ orientations and the melting pressure, which turns out to be in agreement with experiment. The results showed that higher initial temperature increases the possibility for the dislocation loop generate and expansion. The release behavior after the shock wave reaching the back of the sample was also analyzed. We found that the release behavior was quasi-elastic and most of the stacking-fault networks disappear behind the release wave along ［001］ orientation while few stacking faults disappear along ［111］ orientation.