Dislocation Pile-ups Research Articles

Tailoring microstructure and twin-induced work hardening of a laser powder bed fusion manufactured Haynes 188 alloy

Co-based and Ni-based superalloys have been widely used in aerospace, with much attention given to additive manufactured Ni-based superalloys. However, the equally important Co-based superalloys have received less attention. In this work, a Co-based Haynes 188 superalloy is fabricated using Laser Powder Bed Fusion (LPBF). This research shows the impressive mechanical properties of Haynes 188 superalloy before and after heat treatment at both room and high temperatures for the first time. In the as-built specimen, arrayed stacking faults, Lomer–Cottrell locks, dislocations, and M6C/M23C6 eutectic carbides are found. The yield strength, ultimate tensile strength, and elongation of the as-built specimen are 642 MPa, 928 MPa, and 57.3 %, respectively. For the heat-treated specimen, the grain size increases, dislocation density reduces and the eutectic carbides dissolve into the matrix, which reduces the yield strength to 488 MPa. However, annealing twins are found in the heat-treated specimen, and the twin-twin interaction networks further enhance the pile-up of dislocations and work hardening ability during the deformation, which leads to an increase in the ultimate tensile strength and elongation to 1002 MPa and 68.1 %, respectively. This work provides an effective method to improve the mechanical properties of Co-based superalloys by additive manufacturing, which is essential for the development of high-performance superalloys and their manufacturing methods.

Stress distribution variations during nanoindentation failure of hard coatings on silicon substrates

Regarding quality inspection of technologically important nanocomposite hard coatings based on Ti, B, Si, C, and N and bioceramics such as hydroxyapatite that are used in small-scale high-precision devices and bio-implants, it is essential to study the failure mechanisms associated with nanoindentation, such as fracture, delamination, and chipping. The stress imposed by the indenter can affect the fracture morphology and the interfacial fracture energy, depending on indenter shape, substrate type, crystallographic properties, pre-existing flaws, internal micro-cracks, and pre-strain. Reported here are finite-element-based fracture studies that provide insights into the different cracking mechanisms related to the aforementioned failure process, showing that the fracture morphology is affected by the interaction of different cracking events. The interfacial fracture energy, toughness, and residual stress are calculated using existing models with minor adjustments, and it is found that increasing the indenter sharpness improves the shear stress distribution, making the coating more prone to separation. Depending on the prevailing type of stress, the stress distribution beneath the depression results in either crack formation or a dislocation pile-up leading to strain hardening. Different forms of resistances resulting from the indentation process are found to affect the tip–sample conduction, and because of its stronger induced plasticity than that of a Berkovich indenter tip, a sharper cube-corner tip produces more resistance.

Unambiguous coupling between acoustic and electromagnetic emissions in plastically deformed crystals

An evident coupling between acoustic (AE) and electromagnetic (EME) emissions has been proved experimentally during plastic deformation of LiF ionic monocrystals under uniaxial compression with simultaneous recording of both AE and EME. The strong correlation between AE and EME demonstrate clearly that the observed EME is caused by dynamical dislocations and charged vacancies in the ionic lattice during work hardening. The theoretical interpretation proposed to explain the observable EME is based on the well-known Stepanov effect that means sweeping-up the charged vacancies of a preferable sign by gliding edge dislocations and formation of charged Cottrell clouds. During work hardening dislocation pile-ups are formed, and a certain nonequilibrium charge density is accumulated at their heads, resulting to the dynamic electric polarization of the deformed crystal. As the external loading increases, a locked dislocation pile-up bursts through the stoppers and quickly loses its bound charge. The relaxation of this charge produces intrinsic polarization currents generating electric pulses strongly correlated with dynamic dislocation process during plastic deformation. To build the theoretical model, it is assumed that the relaxation current can be described as an athermic viscous motion of vacancies under the kinetic friction force ∼Bυ (B is the friction coefficient and υ is the vacancy velocity) in a self-consistent electric field determined by the distribution of the total charge density. The electrical signal generated by an acting slip system has been calculated. By comparing the calculated and experimentally measured electric signal patterns, the friction coefficient for the linear chain of vacancies (the analogue of an edge dislocation extra-plane) in LiF has been estimated to be B≃ 0.9⋅10–5 g cm–1⋅s–1. This value is in accordance with the corresponding coefficient for dislocations in ionic lattices.

Study on the mechanism of coarse grain formation of 6061 aluminium alloy forged wheels

The formation of coarse grains is easy to occur during the mass production of 6061 aluminium alloy forged wheels. The coarse grains and uneven grain structure will lead to the reduction and instability of the mechanical properties of the wheels, which will affect the fatigue strength, corrosion resistance and impact toughness of the wheels. In this paper, the effect of solution temperature and solution time on the coarse-grained formation of 6061 forged wheel was studied. The reasons and mechanism of coarse-grained formation were revealed by advanced materials characterization techniques such as scanning electron microscope, electron backscattering diffraction and transmission electron microscope. The results show that the deformation causes a large number of dislocation pile-up and entanglement in the grain, forming a large number of small angle sub-grains, and the heating process of sub-grains has more interfaces, which provides the driving energy for the abnormal grain growth.

Dynamic recrystallization of commercially pure titanium during cryogenic compression

In this study, a systematic investigation on the twin-induced dynamic recrystallization (DRX) of commercially pure titanium (CP-Ti) was carried out under uniaxial compression at room (RT) and cryogenic temperature (CT, −150 °C). The compression tests were intentionally interrupted at 0.02, 0.05 and 0.1 strain levels at both deformation temperatures and a detailed post-mortem analysis was performed via electron backscattered diffraction (EBSD) to examine the progressive evolution of microstructure. The results revealed that two major types of twins i.e. {10−12} extension twin (ETW) and {11−22} compression twin (CTW) were effectively activated at both deformation temperatures. During RT deformation, increased strain levels resulted in the higher evolution of ETWs and CTWs, where numerous twin lamellas, lateral twin thickening and twin-twin interactions (ETW-ETW, ETW-CTW) were observed. On the other hand, CT deformation led to significant grain refinement with the average grain size reduced by 94% (3.7 μm for CT-10 sample as compared to 63 μm of initial material). The recrystallization kinetics were understood with respect to three region of interests (ROIs). The DRX mechanism was identified as tDRX, where intersectional {10–12} ETWs and {11–22} CTWs network provided the preferential sites for the nucleation of DRXed grains due to high strain energy from the dislocation pile-ups at the TBs.

Rate controlling deformation mechanisms in SS316L stainless steel manufactured using laser powder bed fusion technique

We conducted uniaxial tensile strain rate jump tests to account for the strain rate controlling mechanisms in the additively manufactured SS316L stainless steel. Special emphasis is placed on the role of high dislocation density and chemical segregation on dictating the rate sensitive behavior. Lowest strain rate sensitivity is found, despite the highest dislocation density in the as build state, which signified the role of solute segregation at cell walls in increasing the activation volume and forms the strong basis for rejecting dislocation forest as the strain rate controlling obstacles. Consistent increase in the strain rate sensitivity with annealing is found consistent with the chemical homogenisation of the solutes causing smaller inter-obstacle spacing causing decreasing activation volume. Experimental findings are supported by CALPHAD simulations and mechanical threshold stress (MTS) modelling framework. Significant difference between the instantaneous and steady state rate sensitivity is understood using the dynamic pileup dislocation density readjustment mechanism causing considerable flow transient while adjusting the mobile dislocation density on strain rate change in the presence of solute decorated dislocation cells. Present investigation helps in gaining newer insights into the deformation mechanisms of the additively manufactured alloys featuring cellular microstructures.

Compressive creep behavior of selective electron beam melted high Nb containing TiAl alloy

In this study, the compressive creep behavior of Ti–45Al–8Nb alloy fabricated by selective electron beam melting technology at 800 °C and 850 °C was investigated. This alloy is mainly composed of equiaxed γ grains, and minor α2 and B2 grains. Creep results show that compressive creep behavior of this alloy is dependent on temperature, which is manifested in the significant increase of creep strain and creep rate with the increase of temperature. Through a multi-step creep test, the stress exponent n (2.65 at 800 °C and 3.65 at 850 °C) and activation volume V* (36.49 b3 at 800 °C and 53.51 b3 at 850 °C) were measured. Combined with kinetic analysis based on n and V* and TEM observation, compressive creep deformation of this alloy at 800 °C and 850 °C is dominated by dislocation motion, including dislocation slip and climb. Additionally, mechanical twinning also plays an important role in creep deformation. Driven by dislocation pile-ups or mechanical twins, discontinuous dynamic recrystallization (DDRX) occurs during creep. The degree of DDRX at 850 °C is greater. Sufficient DDRX greatly softens the alloy and accelerates creep rate, resulting in no steady-state creep stage at 850 °C.

Influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys

The excellent mechanical properties of Ni-based superalloys rely on their unique two-phase microstructure. Here we probe the influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys. The interfacial dislocation could generate a weak long-range interaction to the matrix dislocations at low stress, and react with the incoming dislocation to form junctions at high stress. The weak interaction results in the dislocation pile-up around the interphase boundary, while the strong dislocation reaction makes the gradual loss of interface coherency. With the activation energies, we predict the dependence of strain-rate sensitivity on temperature in a regime with two bounds correlated with these two kinds of interactions. The predicted values match well with that measured in experiments at elevated temperatures.

Void nucleation at dislocation boundaries aided by the synergy of multiple dislocation pile-ups

Void nucleation is of great significance in understanding ductile fracture. Recent experiments have shown that voids are nucleated via vacancy condensation and dislocation boundaries are the main nucleation sites. However, it is unclear what role is played exactly by dislocation boundaries in promoting void nucleation. Here we propose a new mechanism for dislocation boundary-induced void nucleation and develop a corresponding model based on the classical nucleation theory and vacancy diffusion theory. The model suggests that void nucleation is mainly influenced by hydrostatic stress, temperature, and relative vacancy concentration, whose contributions are systematically studied. It is also suggested that the vacancy formation energy and the interaction energy of hydrostatic stress and vacancy, which are absent in the previous models and introduced in ours, exhibit a clear tendency to lower the activation free energy barrier. Analysis of the nucleation kinetic suggests that the growth rate of void depends on the vacancy diffusion coefficient and vacancy concentration; the higher the values of these parameters, the faster the growth rate of the void. The kinetic feasibility of the newly proposed mechanism is examined using three-dimensional discrete dislocation dynamics simulations. The results predict that the size of incipient voids nucleated at the dislocation boundary is ∼35 nm, which is consistent with the experimental characterization value of ∼50 nm. Finally, when the relaxation of the dislocation boundary is considered, the synergistic effect is weakened.

Temperature variation and influence on local mechanical properties assessed by nanoindentation in AA6061-AA7075 dissimilar FSW

The present study aims to elucidate the temperature advancement along and transverse to the weld direction during dissimilar friction stir welding (FSW) between AA6061-T6 and AA7075-T651 with two different tools. Temperature measurements by thermocouples near surface and inside the plates exhibited faster rise in temperature for the threaded with intermittent flat faces pin. Experimental measurements and finite element simulation shows asymmetric temperature distribution in-and-near the stir zone (SZ) with high temperature towards retreating side (AA7075) whereas at distant, temperature is slightly higher on advancing side (AA6061) owing to its higher thermal diffusivity. Additionally, asymmetricity is also noted under shoulder influence region, as well along the weld line with higher temperature towards the trailing side. The electron back scatter diffraction analysis shows regions experienced high temperature yielded high angle grain boundaries. Higher modulus and hardness for AA6061 in the recrystallized region is noted from nanoindentation tests as grain refinement along with the intense strain and dislocation pileup at grain boundaries prevailed the other strengthening mechanism. Recrystallized AA7075 within SZ demonstrated reduction in hardness with respect to base metal due to the dissolution of strengthening precipitates.

Influence of microstructural refinement on cold deformation behavior of nanocrystalline Al–Zn–Mg alloy processed by solid-state mechanical milling and hot pressing

This research work was aimed at studying the workability behavior of Al–Zn–Mg alloys during a cold upsetting process. Nanocrystalline Al 7017 alloys were synthesized by mechanical milling (MM) at different milling times and consolidated by hot-pressing. The nanocrystalline Al 7017 alloy samples were incrementally deformed with a load step of 0.5 ton in a universal testing machine. Artificial image capturing and comparison techniques were introduced in this study and deformed images were captured using a high-resolution camera. For each incremental load (without removing the sample from the platens), different parameters such as the height of the deformed samples, and the corresponding contact diameters (top and bottom) were measured and analyzed using Matlab software (image processing). Cold upsetting was performed until the first crack formed. The influence of milling time on cold workability and strain hardening behavior was investigated under triaxial stress state conditions. The results revealed that the Al 7017-10 h nanocrystallite alloy exhibited improved deformation properties in terms of various tri-axial stress, strain hardening, formability stress index, and stress ratio parameters owing to the fine grain size, lower amount of dislocation pile-up, and fewer migrated atoms near the grain boundaries compared to other samples (after10 h). The microstructure and phase evolution were studied using an X-ray diffractometer and various electron microscopes. Several strengthening mechanisms are also discussed among which the dislocation strengthening mechanism plays a major role in the deformation behavior. Finite element analysis (FEA) was performed to examine the equivalent stress distribution during cold-upset forging.

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.

Impact of uneven distribution of grain characteristics on yield strength and martensitic transformation of as-hot-rolled medium-entropy alloys

As-hot-rolled medium-entropy alloys (MEAs) with unevenly distributed grain sizes of face-centered cubic grains exhibit better yield strength without uniform elongation loss compared to cold-rolled and annealed ones. Successive operation of dynamic recrystallization (DRX) during several hot rolling passes leads to a wide range of grain sizes from submicrons to tens of micrometers due to the grain growth after nucleation: early recrystallized grains are coarser than recently recrystallized ones. Not only the grain size but internal dislocation density of the recently recrystallized grain is low. During the tensile deformation of the hot-rolled MEAs at –196 °C, dislocation pile-ups in the relatively soft and fine DRX grains enhance yield stress and hetero-deformation-induced strain hardening. Thanks to the enhanced yield stress of the as-hot-rolled MEAs, stress-induced martensitic transformation easily occurs. Notably, partially DRXed MEAs hot-rolled at 800 °C have lower yield stress than fully DRXed ones, hot-rolled at 900 and 1000 °C. This is attributed to the softening effect of the stress-induced body-centered cubic martensitic transformation in unrecrystallized coarse grains prior to the yielding, which lowers the yield stress of the partially DRXed ones. After yielding, the martensitic transformation facilitates strain hardening and early necking is precluded. This study presents a fresh outlook on the uneven distribution of grain sizes by hot rolling beneficial to mechanical responses of uniform elongation of ∼45% despite the as-rolled states with an advantage of simplified thermo-mechanical processes.

Effect of a micro-scale dislocation pileup on the atomic-scale multi-variant phase transformation and twinning

In this paper, we perform concurrent atomistic–continuum (CAC) simulations to assess the contribution of the internal stress induced by the microscale dislocation pileup at an atomically structured interface to the atomic-scale phase transformations (PTs), reverse PTs, and twinning. The main novelty of this work is to unify the atomistic description of the interface and the coarse-grained (CG) description of the lagging dislocations away from the interface within one single framework. Our major findings are: (a) the interface dynamically responds to a pileup by forming steps/ledges, the height of which is proportional to the number of dislocations arriving at the interface; (b) the pileup-induced internal stress concentration profile follows neither the classical Eshelby model nor the super-dislocation model alone, but a combination of them; (c) when the pre-sheared sample is compressed, a direct square-to-hexagonal PT occurs ahead of the pileup tip and eventually grows into a wedge shape. The two variants of the hexagonal phases form a twin with respect to each other; (d) upon a further increase of the loading, part of the newly formed hexagonal phase transforms back to the square phase. The square product phase resulting from this reverse PT forms a twin with respect to the initial square phase. All phase boundaries (PBs) and twin boundaries (TBs) are stationary and correspond to zero thermodynamic Eshelby driving forces; and (e) the microscale dislocation pileup-induced internal shear stress and the structural change at the atomic-scale interface reduces the stress required for initiating a PT by a factor of 5.5, comparing with that in the sample containing no dislocations. This work is the first characterization of the behavior of PTs/twinning resulting from the reaction between a microscale dislocation slip and an atomically structured interface. The gained knowledge will advance our understanding of how the multi-phase material behaves in many complex physical processes, such as the synthesis of multi-phase high-entropy alloys or superhard ceramics under high-pressure torsion, deep mantle earthquakes in geophysics, and so on, which all involve dislocation slip, PTs, twinning, and their interactions across from the atomistic to the microscale and beyond.

High strain rate response and mechanical performance of tantalum carbide–hafnium carbide solid solution

Ultra-high temperature ceramics (UHTCs) based on the solid solution TaxHf1-xC have shown great promise as materials for extreme environments (>2500 °C) due to their improved thermo-mechanical properties. However, their dynamic response under high-impact loading remains unknown. In this study, we investigate the dynamic impact behavior and fracturing evolution of TaxHf1-xC samples (with x = 0, 0.2, 0.5, 0.8, and 1) at a strain rate > 103 s-1 using the Split Hopkinson Pressure Bar (SHPB) test. Among the different compositions, Ta0.5Hf0.5C exhibits the highest compressive strength of 1870 MPa, representing a 36% improvement over TaC and 54% over HfC. High frame rate videos and deformation analysis reveal that Ta0.5Hf0.5C, characterized by extensive Ta–Hf bonding, significantly reduces the crack propagation rate by approximately 1435% compared to TaC and 4050% compared to HfC. This effect is attributed to the dislocation pile-ups, nano-twin formation, inter grain twisting exhibited mostly along {111} plane as Hf substitutes Ta in TaxHf1-xC samples. Further, the surface energy is least for {111} plane eliciting the higher probability for bypassing crack propagation along this plane in TaxHf1-xC. The exceptional damage tolerance and improved mechanical properties of Ta0.5Hf0.5C, compared to other solid-solution TaxHf1-xC compositions, make it an extraordinary structural material for hypersonic applications. These findings provide valuable insights into the dynamic behavior of UHTCs and highlight the potential of Ta0.5Hf0.5C as a superior material for extreme environments requiring high-impact resistance.

Dislocations generated by pre-strain dominate the subsequent plastic deformation

Plasticity of sub-micron scale metals is well known to be controlled by dislocation source. Here, we report an in-situ study of how the dislocation structures generated by pre-strain in submicron-sized copper affect the subsequent plastic deformation by conducting nanocompression testing in a transmission electron microscope. The results show that short isolated dislocations first undergo directional intermittent jumping motion along specific glide directions under the applied force. And the jumping frequency and jumping distance of dislocation motion can be determined by the density and strength of obstacles distributed in the crystal. Then, the dislocation wall formed by dislocation pileup and entanglement during pre-strain can effectively pin the mobile dislocation in the early stages of compression, but at high stress levels it will perform overall motion in the form of dislocation avalanches and lead to the strain burst emissions of the material. The in-situ dynamic evolution of dislocation avalanche reveals the emitted dislocations and associated glide planes introduced by pre-strain can preferentially trigger dislocation avalanches of the dislocation wall.

A Hall-Petch-like relationship linking nanoscale heterogeneity to yield stress of heterogeneous metallic glasses

Metallic glasses (MGs) undergo plastic deformation through shear transformation zones (STZs) at small deformations and shear banding at large deformations. The STZ mechanism is linked to microstructural heterogeneities, including nano-heterogeneities associated with atom clustering and their spatial distribution. Shear banding occurs due to the coalescence of STZs. In this study, we investigate the yielding behavior of heterogeneous MGs by varying the spatial correlation and standard deviation of local shear moduli associated with clustering atoms on the nanoscale. Using a mesoscale shear transformation zone (STZ) dynamics model, we compute the yielding strength and observe deformation behaviors associated with the formation and propagation of STZs. We propose a Hall-Petch-like relationship where the yield stress of the MG scales inversely with the square root of the spatial correlation length. Our results show that the yielding of MGs corresponds to the percolation of STZs from soft to hard regions. Prior to macroscopic yielding, the activated STZs tend to accumulate inside soft regions, forming a Super-STZ array, which is similar to the formation of dislocation pile-up at grain boundaries. We derive the stress concentration in the front of the Super-STZ using the Eshelby inclusion model and formulate a Hall-Petch-like equation to quantify the yield of MGs with the spatial correlation length and standard deviation of the nanoscale heterogeneity. Our results provide insights into the structure-property relationship of MGs and have important implications for the design of nanoscale MGs with tunable properties.

Investigation of damage mechanisms related to microstructural features of ferrite-cementite steels via experiments and multiscale simulations

Spheroidized ferrite-cementite steel (SFC) is susceptible to micro-defects owing to the mismatched deformation between the ferrite and cementite during cold forming, which seriously deteriorates the fatigue life. However, the damage mechanism related to microstructural features in the SFC steel during cold deformation has not been fully investigated, hindering control of the microstructure and forming process. This study presents a method coupled with experiments and multiscale simulations to research the damage mechanisms and the dependence of microstructure features under uniaxial tension in SFC steel. In situ tensile test revealed three damage mechanisms in SFC steel under uniaxial tension: cementite cracking, ferrite/cementite interface debonding, and ferrite cracking. In particular, the final fracture was mainly dominated by the ferrite/cementite interface debonding. In multiscale simulations, the macroscale tensile and nanoindentation simulations were carried out to obtain the strain history and mechanical properties of the mesoscale simulations, respectively. The mesoscale IP-based RVE and CP-based unit cells simulations were performed to capture the driving forces from the three damage mechanisms and the dependence of microstructural features, respectively. The nanoscale molecular dynamics simulation was conducted to identify the model parameters of cohesive zone model (CZM) in the mesoscale simulations. The numerical results indicated that particle-cracking void occurred when the internal von Mises stress exceeded the ultimate strength of the cementite particles. Interface-debonding void nucleated when the load on the particle was insufficient to resist its strength. Matrix-cracking should be attributed to the stress concentration caused by the dislocation pile-up or the crossing of multiple slip bands. In addition, interfacial debonding were more likely to occur in the stable orientations and high-angle grain boundaries of ferrite matrix. Compared with a circular cementite particle, an elliptical particle was more prone to cause interfacial debonding. Also, the ferrite/ferrite grain boundary showed earlier interfacial debonding perpendicular to the loading direction than parallel to the loading direction.

The influence of L12 ordered precipitates on hydrogen embrittlement behavior in CoCrNi-based medium entropy alloys

The recently emerged multicomponent (or medium/high entropy) alloys have generated considerable excitement globally in the last 10 years because of their excellent mechanical and functional properties, particularly in terms of strength-ductility combinations that can surpass most other metallic materials. However, the achieved high strength level (above 1 GPa in many cases) fuels strong concerns about hydrogen embrittlement (HE). Detailed investigation in this field is still scarce, especially pertaining to the face-centered cubic medium entropy alloys (MEA) that are typically strengthened by ordered precipitates. Here, we unravel the effect of γ' (L12) ordered precipitates on H-induced damage behavior and the associated HE resistance in CoCrNi-based MEAs. Compared with the equi-molar CoCrNi MEA, the precipitation-hardened (CoCrNi)94Al3Ti3 MEA shows an enhanced HE resistance even at a higher strength level. Both alloys are fractured due to H-assisted intergranular cracking at the initial failure stage when loaded in the presence of H. The formation of intergranular cracks is primarily attributed to the H-induced decohesion at grain boundaries, where a high stress/strain concentration accompanied by a more intensive dislocation planar slip (or stacking fault formation) caused by H was observed. The presence of γ' precipitates serves to slow down the internal diffusion/migration of H due to the trapping effects. The precipitates with a relatively larger size (∼50 nm) also hinder dislocation planar slip thus decreasing the number of pile-up dislocations at grain boundaries. Both effects collectively reduce the tendency of H-induced intergranular cracking, leading to the improved HE resistance. The work reveals the positive role of ordered precipitates in H tolerance and thus provides some insights in further microstructure design of medium/high entropy alloys for applications in H-abundant environment.

Dynamic precipitation and dynamic recrystallization behaviors of Mg-Gd-Nd-Zr magnesium alloy during thermal compression deformation

In this paper, the dynamic precipitation and dynamic recrystallization (DRX) behaviors of Mg-7Gd-2Nd-0.5Zr (wt.%) alloy during thermal compression were investigated by optical microscopy (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The hot compression tests were performed at 350–500 °C with strain rates of 0.003–1 s−1 and a strain of 0.7. The results indicate that dynamic precipitation and DRX occur in the deformed region under appropriate conditions, and that dynamic precipitation precedes DRX. Elevated deformation temperature and strain rate facilitate DRX but weaken dynamic precipitation. The dynamic precipitates were β (Mg5Gd) phases and were mainly located at the grain boundaries in the deformed regions. DRX can be promoted when the size of the granular precipitates is larger than 200 nm. The second phase particles located at the grain boundaries of the recrystallized grains can pin the grain boundaries to refine the recrystallized grains. There are three mechanisms of recrystallization nucleation: grain boundary bowing nucleation, dislocation pile-up induced nucleation, and particle-stimulated nucleation (PSN). Moreover, a critical strain model and a kinetic model for the DRX of the alloy were developed.