Rod-shaped Precipitates Research Articles

A coupled model for precipitation strengthening in Mg-Zn alloys

A recently developed precipitation model is coupled to an original mechanical model to predict the strength evolution in Mg-Zn alloys during aging. The proposed models consider the strengthening effects of rod and plate-shaped precipitates on the most important deformation mechanisms in Mg alloys: basal slip, prismatic slip, and twinning. It is found that shearing of rod precipitates dominates the strengthening on basal slip at early aging stages, while the bypass mechanism dominates medium and over-aging stages. On prismatic planes, the bypass of rod precipitates dominates across all aging stages. The influence of different precipitate arrangements and aspect ratios on modelling results is also discussed. The strengthening from plate-shaped precipitates is found to be very poor for the studied slip modes. While for twinning, the strength evolution is nicely represented by bypassing of rod-shaped precipitates. The evolutions of tensile and compressive yield strength during aging are well predicted by considering the hardening on prismatic and twinning planes, respectively.

Effect of lanthanum oxide on microstructure and mechanical properties of ZK60 magnesium alloy

Different amounts of La2O3 were added to ZK60 alloy to observe the changes in microstructure and mechanical properties of ZK60-XLa2O3 (X = 0, 0.2 wt%, 0.5 wt%) composites. Our findings reveal that within the as-cast ZK60 alloy, the Mg-Zn binary phase is manifested as rod-shaped precipitates, exhibiting a dispersed distribution predominantly along the grain boundary regions. With the increase of La2O3 addition, the rod-like Mg-Zn binary phases are gradually replaced by Mg-Zn-La ternary phases, and these Mg-Zn-La ternary phases are semi-continuously distributed around the grain boundaries. Both ZK602 and ZK605 alloys result in a corresponding improvement in ductility after extrusion. It is noteworthy that the ductility of the as-extruded ZK605 alloy is increased by 59.4% compared to ZK60 at the expense of a reduction of only 3.9% in yield strength. Taken together, the significant increase in ductility of ZK605 is mainly due to the weakening of the basal texture, which is weakened by the redox reaction of La2O3 with the α-Mg matrix, generating the La element. The decrease in yield strength of ZK605 in the extruded state is closely related to the observed grain coarsening phenomenon and the basal texture weakening. At the same time, the ultimate tensile strength of ZK605 shows a slight increase, which is mainly due to the second phase strengthening. The Mg-Zn-La ternary phase exhibits a notably hard character, and it is evident that the distribution of this phase becomes progressively denser as the La2O3 content increases.

Advancement of microstructural evolution and deformation mechanisms in AA6082 aluminum alloy under elevated-temperature tensile loading

This study explores the intricate interplay between strain-induced precipitation (SIP) and the underlying mechanisms that govern hot deformation in AA6082 aluminum alloy. Uniaxial tensile tests were conducted at elevated temperatures, ranging from 200 °C to 400 °C, and varied strain rates from 0.01 s−1 to 10 s−1. Employing advanced methodologies such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM), the research investigates the relationship between deformation, precipitation, and structural evolution. At lower strain rates, longer deformation times promote dynamic recrystallization, thereby promoting the growth of substructures with enhanced ductility. On the contrary, at higher strain rates (10 s−1), shorter deformation times inhibit such processes, resulting in a clear predominance of dislocation motion along specific slip systems and consequently influencing the mechanical response of the alloy. The results also reveal temperature-dependent deformation mechanisms, such as intragrain slip, continuous dynamic recrystallization (CDRX), and SIP phenomena, which influence the alloy's mechanical response. At 200 °C, prominent work hardening dominates, while 400 °C exhibits dynamic softening mechanisms. EBSD analysis elucidate temperature-specific deformation mechanisms, including intragrain slip at 200 °C and transitional phenomena at 300 °C. Microstructural analysis at 200 °C reveals the absence of needle- or rod-shaped precipitates. On the contrary, at 300 °C, the presence of β/β' precipitates significantly influences the nucleation of β'' precipitates at the grain boundaries. At 400 °C, substantial rod-shaped β' phase precipitates appear, demonstrating a complex interplay between diffusion mechanisms and strain-induced effects. In addition, the study also investigated the fracture behavior of aluminum alloy at different tensile temperatures, revealing the transition from transgranular fracture at 200 °C to intergranular fracture at 300 °C, as well as the shear effect of β/β' phase precipitates at this temperature. The findings provide a comprehensive understanding of the alloy behavior under diverse conditions, essential to optimize its processing parameters and mechanical properties.

Investigations on the diffusion bonding of titanium alloy and aluminum alloy using copper at low bonding pressure

This research investigates the diffusion bonding of hybrid aluminum (Al)–titanium (Ti) joints using a copper (Cu) interlayer in a vacuum environment. At 610 °C and 1 MPa pressure, partial melting of Al caused deformation and misalignment, while 520 °C showed no melting of Al–Cu at the interface. A rod-shaped precipitate with primarily Al was observed at 550 °C, and grain coarsening occurred at 580 °C at the interface, accompanied by increased Cu diffusion within the Al matrix for a bonding time of 45 min. A Ti–Al-based intermetallic layer formed on the Ti side, but voids and cracks were present in the Al–Cu diffusion region at 580 °C. Hardness variation resulted from microstructural changes due to bonding temperature, particularly near the bond line due to hard intermetallic compounds. Shear strength averaged 41 MPa at 550 °C and decreased to 33 MPa at 580 °C due to grain growth, voids, and crack propagation, causing brittle joint failure at the interface. X-ray diffraction on the fracture surfaces revealed that the failure of diffusion-bonded joints is attributed to fractures through the Ti–Al-based intermetallic compounds at the interface and unbonded region.

Effect of orientation relationship on the morphology and strengthening effect of chromium precipitation in Cu-0.67Cr-0.3Zr alloy

High strength and high conductivity Cu-0.67Cr-0.3Zr alloy was prepared by vacuum directional solidification equipment. The face centered cubic (FCC) and body centered cubic (BCC) Cr rich precipitations in Cu matrix were characterized by high resolution transmission electron microscopy. In the as cast and solution treated state, a large amount of coherent FCC Cr precipitates is uniformly distributed in the matrix. These FCC Cr precipitates are covered by close packed {111} crystal planes and presents regular morphology. Due to their small size and cube-cube orientation with the matrix, FCC Cr precipitates follows dislocation shearing mechanism, resulting in limited strengthening effect. After aging treatment, both incoherent ordered nearly spherical and incoherent rod-shaped BCC Cr precipitates were formed in the matrix. These incoherent precipitates always follow dislocation bypassing mechanism. Due to its small size and high-volume content, the spherical precipitation plays a major role in improving the strength of the alloy. On the contrary, the influence of rod-shaped precipitates on strength is minimal. According to the minimum free energy theory and local chemical potential equilibrium requirement, the precipitation with Kurdjumov-Sachs (K-S) orientation relationship tends to be rod-shaped extending along the direction of Cu [110] and Cr [111], while the precipitation with Nishiyama-Wassemann (N-W) orientation relationship tends to be nearly spherical.

Revisiting Precipitation kinetics in Mg-Zn alloy – a multi-characterization and modeling study

This work aims to provide new insights into understanding precipitation kinetics in Mg-Zn alloys. First, the evolution of Zn solute, precipitate size and volume fraction is characterized using a combination of thermoelectric power, X-ray diffraction, atom probe tomography and transmission-electron microscopy in an Mg-1.7at.%Zn alloy when aged at 150 °C and 200 °C. The experimental results are then supported by a precipitation model based on classical nucleation and growth theories for rod and plate-shaped precipitates. It is found that rod-shaped precipitates are prevalent in the early aging stage, while plate-shaped precipitates dominate the over-aging stage. The kinetics of both precipitates are depicted in detail to represent this competition. Finally, a time-temperature-transformation diagram for these two types of precipitates is proposed for this system.

Strengthening model development and effects of low diffusing solutes to coarsening resistance in aluminum alloys

A modified Orowan strengthening model is proposed to account for finite rod-shaped precipitates with hemispherical caps in aluminum alloy systems. A combined computational and experimental approach is used to study the influences of anisotropic Orowan looping and solute-dislocation interaction on temperature-dependent yield strength. The strengthening model is validated with a dataset containing 297 experimental precipitate geometries, chemistries, temperatures, and strength measurements, and achieves a strong predictive correlation of 0.8713 with experimentally measured yield strengths. Under conditions that are applicable to coarsening, constant particle volume fraction and aspect ratio, the model predicts that short rod precipitates provide far superior strengthening effects compared to plate precipitates. A cast Al-Si-Mg-Cu alloy with rod-shaped Q-phase (Al3Cu2Mg9Si7) precipitates was developed with a novel chemistry exploring the use of low-diffusivity elements (Mn, Ni, V, Zr) to limit precipitate coarsening. The thermodynamic behavior of Mn, Ni, V, and Zr across the Q-phase interface is examined using transmission electron microscopy (TEM), first-principles density-functional theory (DFT) calculations. and atom-probe tomography (APT). DFT calculations utilizing TEM identified Q-phase/Al-matrix interfaces show that Mn, Ni, V, and Zr preferentially segregate to the Q-phase precipitate boundaries which suggests inhibition of precipitate coarsening and higher strengths after temperature exposure. Atom-probe tomography confirms solute atom partitioning/segregation at the Q-phase/Al-matrix interface, found in the modified commercial AS7GU alloy (A356 +0.5%Cu), which supports these observations.

Effects of cooling rate and cryogenic temperature on the mechanical properties and deformation characteristics of an Al-Mg-Si-Fe-Cr alloy

The Al-Mg-Si-Fe-Cr alloy was subjected to water quenching (WQ) and air quenching (AQ) after solution treatment for 30 min at 843 K followed by a peak artificial aging treatment at 453 K for 8 h. Precipitates of different sizes and precipitate free zone (PFZ) of different widths were obtained. Uniaxial tensile tests were conducted to investigate the effect of the microstructure and deformation temperature on the mechanical properties and deformation behavior of the WQ and AQ samples at 298 K and 77 K. The results showed that the yield strength and work hardening rate of the AQ samples were lower than those of the WQ sample. In addition, the strength and ductility of the two samples are significantly improved at cryogenic temperatures. Furthermore, quasi-in situ electron back scatter diffraction (EBSD) was conducted to investigate the orientation evolution of grains with strains of 1%, 3%, 7% and 10% for the two samples at 298 K and 77 K. The different orientation evolution of the two samples was caused by the nonuniform deformation in the PFZ and grain interior. The dislocation characteristics and fracture morphology were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Because the rod-shaped precipitates hinder the dislocation movement, no obvious slip traces could be observed on the deformed surface of the AQ sample. In addition, two slip systems are activated for most grains of the WQ sample at 77 K, but only one slip system is activated at 298 K. Due to the weak grain boundaries deformed at 298 K, the stress applied to grains is consistent with the tensile direction. However, the grain boundaries are strengthened when tested at 77 K, and the stress field of the grain boundaries is more complex, resulting in the stress not only in the tensile direction. More slip systems are activated to coordinate the deformation, improving the ductility. TEM images reveal that the deformation of the PFZ is more severe for the AQ sample at 298 K than at 77 K. The deformation in the precipitate-free zone and within the grain was more homogeneous in the AQ sample at 77 K, which increased its ductility compared to the AQ sample at 298 K.

Microstructural and residual stress analysis of functionally graded friction surfaced Al-Cu-Mg-Ag alloy

This study analyzed the microstructure and residual stress of a functionally graded friction surfaced Al-Cu-Mg-Ag alloy. To prepare a multilayer material using the friction surfacing process, different layers of Al-Cu-Mg alloy containing 3, 7, and 12 wt% of silver powder were deposited on the Al-Cu-Mg alloy substrate. After homogenization and aging heat treatment, a functionally graded structure with different silver weight percentages in different layers was obtained. The results showed that silver powder reduces the dimensions of the clad layers compared to the sample without silver. After homogenization and artificial aging treatment, a bimodal microstructure formed in the silver-containing sample. It was determined that two types of spherical (S-Al 2 CuMg) and rod-shaped precipitates (Ω precipitates) are formed in the microstructure of different layers. By moving from the first layer to the third layer, the concentration of Ω precipitates increases as the weight percentage of silver added to the microstructure increases. After aging heat treatment, it is observed that the hardness changes are mild, and the average hardness increases from the substrate (134 HV0.1) to the third layer (165 HV0.1).

Remarkable cryogenic strengthening and toughening in nano-coherent CoCrFeNiTi0.2 high-entropy alloys via energetically-tuning polymorphous precipitates

In the present work, three kinds of precipitates with different morphologies, structures, sizes, and volume fractions were obtained via energetically-tuning the microstructures of the nano-precipitated CoCrFeNiTi0.2 high-entropy alloy (HEA). Subjected to the heavy cold rolling immediately after homogeneous precipitation, L12 structured spherical nano-particles with an average size of 16.5 nm rapidly grow into 200 nm-sized spherical ones due to Ostwald ripening. On the other hand, superfluous mechanical energy storage energetically facilitates the phase transformation from spherical L12 to rod-shaped D024 structures for initially formed nano-precipitates. Besides, some other newly formed nano-precipitates with an average size of 6.5 nm are available, originating from heavily plastically deformed-induced nucleated sites. Multi-scale precipitates interact with dislocations in different ways. The strengthening provided by dislocations cutting through smaller nano-particles and bypassing grown ones account for 57.7% and 42.3% of precipitation strengthening, respectively, while rod-shaped precipitates can act as equivalent interfaces to hinder dislocation movement. Their synergistic effect has achieved remarkable strengthening and toughening. Specially, dislocation slips dominate at 298 K, while stacking faults (SFs) assist plastic deformation at 77 K. Compared with 298 K, the yield strength (YS) and ultimate tensile strength (UTS) of the current HEAs at 77 K are increased by 38.9% and 38.2% to 1 GPa and 1.5 GPa, respectively, and the tensile strain is slightly increased to 35% instead of loss, realizing excellent strength and plasticity combination. Theoretically established strengthening models agree well room-temperature and cryogenic yield strengths experimentally. Moreover, the tensile elongation is effectively predicted by the Whitehouse-Clyne model. This strengthening strategy of energetically-tuning polymorphous precipitates provides the basic guidance to develop high-performance nano-precipitated alloys. The current strengthening and plasticity models can be employed to well predict the mechanical properties of such kinds of alloys at cryogenic temperatures.

Influence of hydrogen vacancy interactions on natural and artificial ageing of an AlMgSi alloy

The influence of hydrogen on the structural evolutions of an Al-Mg-Si alloy during natural and artificial ageing was investigated experimentally. The aim of the study was a better understanding of interactions between hydrogen and crystalline defects and especially vacancies. Experimental data demonstrate that during natural ageing in hydrogen environment, the hardening response is delayed. This is attributed to a slower recovery of excess vacancies linked to a lower mobility. To confirm and quantify the influence of hydrogen on the vacancy migration energy, artificial ageing was carried out in conditions where the vacancy concentration is constant. Hence, long-time annealing treatments were carried out to investigate the influence of hydrogen on the coarsening of rod-shaped precipitates. Using transmission electron microscopy and atom probe tomography, it was demonstrated that the precipitate volume fraction and composition are unchanged under H2 atmosphere but the coarsening kinetic is significantly reduced. This leads to a delayed softening, in good agreement with theoretical estimates. Thus, even a low concentration of hydrogen in solid solution significantly affects the mobility of alloying elements in the aluminium matrix. This is the result of hydrogen-vacancy interactions that lead to an increase of the vacancy migration energy. Based on classical coarsening theories, it was possible to demonstrate that this increase is of about 5% for a concentration of hydrogen close to the vacancy concentration.

Orientation Dependent Hardening by Rod-Shaped Misfitting Precipitates in Aluminium Alloys.

A coherent precipitate formed in a metallic alloy is of importance in its strengthening mechanism, owing to dislocation/precipitate interaction. Therefore, the present study investigated the effect of <001> rod-shaped precipitates on misfit hardening in aluminium alloys by means of parametric dislocation dynamics simulation based on Green’s function method. The simulation results revealed that the topological evolution of the dislocation microstructure is greatly influenced by local internal stress around the <001> rod precipitate. The strong orientation dependence of misfit hardening was observed for the gradients of the stress–strain curves and their maximum shear stresses, where the difference in the maximum stress values amounted to 30%. The strong and weak hardening behaviours associated with the internal stress of <001> rod precipitates were implemented in terms of the energy associated with the dislocation motion.

Coarsening kinetics of Cu particles in an Fe-1.5% Cu alloy

Abstract The Ostwald ripening of spherical and rod-shaped Cu precipitate particles in an Fe-1.5 mass% Cu alloy aged between 873 and 1023 K has been examined by measuring both the particle size by electron microscopy and the Cu concentration in the α-Fe matrix by electrical resistivity. The average radius of spherical and rod-shaped particles increases with ageing time t as t 1/3, in agreement with the growth by diffusion-controlled coarsening. The kinetics of the depletion of supersaturation with t obey the predicted t – 1/3 time law. The Cu/α-Fe interfacial energy and the diffusivity of Cu in α-Fe have been independently derived from the data on coarsening. The interfacial energies are measured as 0.57 and 1.1 Jm–2 for the spherical and rod-shaped particles respectively. The pre-exponential factor and the activation energy for bulk diffusion are found to be 4.54α10 –5 m2s– 1 and 241 kJmol – 1.

Effect of Annealing Time on Microstructure and Properties of P91 Heat Resistant Steel SMAW Joint

The microstructure and properties of shielded metal arc welding (SMAW) joint of P91 heat-resistant steel annealed at different holding time were observed and analyzed. The results show that the martensite and residual austenite structures in the weld zone and coarse grain heat affected zone (CGHAZ) were transformed with the extension of holding time. After 760°C+4h annealing, a large number of white rod-shaped precipitates appeared in the weld zone. The main components of precipitates were Fe, Cr, Mn, V, Si, and a small amount of oxides appeared. After 760°C+5h annealing treatment, relatively fine white granular precipitation appeared in the weld zone, and a large number of oxides and some precipitates have been dissolved. After 760°C+6h annealing treatment, the precipitated phases were composed of Fe-Cr, Fe3Si, Ni-Cr-Fe, Fe2MnAl. Compared with the annealing treatment for 5h, the microstructure of the precipitated phase is fine and uniform, dispersion distribution of second phase particles, and the material mechanics is relatively stable.

Microstructural and Micro-Chemical Evolutions in Irradiated UCO Fuel Kernels of AGR-1 and AGR-2 TRISO Fuel Particles

AGR-1 and AGR-2 tristructural-isotropic (TRISO) fuel particles were fabricated using slightly different fuel kernel chemical compositions, modified fabrication processes, different fuel kernel diameters, and changed 235U enrichments. Extensive microstructural and analytical characterizations were conducted to correlate those differences with the fuel kernels’ responses to neutron irradiations in terms of irradiated fuel microstructure, fission products’ chemical and physical states, and fission gas bubble evolutions. The studies used state-of-the-art transmission electron microscopy (TEM) equipped with energy-dispersive x-ray spectroscopy (EDS) via four silicon solid-state detectors with super sensitivity and rapid speed. The TEM specimens were prepared from selected AGR-1 and AGR-2 irradiated fuel kernels exposed to safety testing after irradiation. The particles were chosen in order to create representative irradiation conditions with fuel burnup in the range of 10.8 to 18.6% fissions per initial metal atom (FIMA) and time-average volume-average temperatures varying from 1070 to 1287°C. The 235U enrichment was 19.74 wt.% and 14.03 wt.% for the AGR-1 and AGR-2 fuel kernels, respectively. The TEM results showed significant microstructural reconstructions in the irradiated fuel kernels from both the AGR-1 and AGR-2 fuels. There are four major phases: fuel matrix of UO2 and UC, U2RuC2, and UMoC2—in the irradiated AGR-2 fuel kernel. Zr and Nd form a solid solution in the UC phase. The UMoC2 phase often features a detectable concentration of Tc. Pd was mainly found to be located in the buffer layer or associated with fission gas bubbles within the UMoC2 phase. EDS maps qualitatively show that rare-earth fission products (Nd et al.) preferentially reside in the UO2 phase. In contrast, in the irradiated AGR-1 fuel kernel, no U2RuC2 or UMoC2 precipitates were positively identified. Instead, there was a high number of rod-shaped precipitates enriched with Ru, Tc, Rh, and Pd observed in the fuel kernel center and edge zone. The differences in irradiated fuel kernel microstructural and micro-chemical evolution when comparing AGR-1 and AGR-2 TRISO fuel particles may result from a combination of irradiation temperature, fuel geometry, and chemical composition. However, irradiation temperature probably plays a more deterministic role. Limited electron energy-loss spectroscopy (EELS) characterizations of the AGR-2 fuel kernel show almost no carbon in the UO2 phase, but a small fraction of oxygen was detected in the UC/UMoC2 phase.

Aperiodic structures of rod-shaped precipitates in a Mg-Zn-Al alloy

Rod-shaped precipitates with their long axis parallel to [0001]α commonly form in Mg-Zn-Al alloys, providing strengthening effects. These precipitates are metastable and are generally accepted to be the MgZn2 and/or Mg4Zn7 phases. In this paper, the cross-sectional structure of precipitate rods in an aged ZA84 alloy is examined at the atomic scale using high-angle annular dark-field scanning transmission electron microscopy. It is found that these precipitates are neither pure MgZn2 nor Mg4Zn7. The structures of the rod-shaped precipitates do not have long-range translational symmetry nor long-range 5-fold rotational symmetry in the two-dimensional projection plane perpendicular to the rod long axis. However, these structures can be described by the tiling of different shapes, with four such shapes being most frequently observed. The structural details revealed in this work may help rationalize the many complex precipitate structures in Mg-Zn-Al and Mg-Zn alloys.

Probability-Dependent Precipitation Strengthening Effect of Anisotropic Precipitate in Al-Mg-Si Alloy Produced by T6 Heat Treatment

This study proposed a constitutive equation to predict the change in yield strength according to the behavior of β″ metastable precipitates, which have a profound effect on strength among materials precipitated during the T6 heat treatment of Al-Mg-Si alloy. The β″ precipitate is a metastable phase before it becomes a β (Mg2Si) precipitate, and is distributed in the form of nano-scale rods in the aluminum alloy matrix. Existing precipitation strengthening models assume the shape of the precipitate to be spherical, and in that case the equation that depends on the Orowan mechanism with the average precipitate size and distribution should dominate. However, precipitates are formed in various shapes and sizes by anisotropic growth. In particular, rod-shaped precipitates are not suitable for the existing precipitation strengthening model. In this study, an Al-Mg-Si alloy was fabricated by gravity casting followed by T6 heat treatment. The new precipitation strengthening effect equation proposes that the β″ precipitate affects yield strength during plastic deformation of the Al-Mg-Si alloy. The proposed precipitation strengthening effect equation probabilistically considers the Critical Resolved Shear Stress (CRSS), which varies depending on the angle between the dislocation and the precipitate, when the dislocation passes through a rod-shaped precipitate.

Ostwald ripening and stability of precipitates during two successive overaging in an Al-Mg-Li alloy

The precipitate coarsening and dissolution behaviors in an Al-Mg-Li alloy during overaging at 300 °C and then at 400 °C for various time were investigated. It is shown that the S1-phase tends to have rod-shaped morphology upon overaging at 300 °C. However, followed by overaging at 400 °C, which is the dissolution temperature for S1-phase, the rod-shaped S1-phase precipitates formed previously were not dissolved until their coarsening were finished. The ensuing reduction of the total precipitate–matrix interface energy due to the dissolution of small precipitates and spheroidization of the rod-shaped precipitates was suggested to provide the driving force for coarsening.

Remarkable Cryogenic Strengthening and Toughening in Nano-Coherent CoCrFeNiTi &lt;sub&gt;0.2&lt;/sub&gt; High-Entropy Alloys via Energetically-Tuning Polymorphous Precipitates

In the present work, three kinds of precipitates with different morphologies, structures, sizes, and volume fractions were obtained via energetically-tuning the microstructures of the nano-precipitated CoCrFeNiTi0.2 high-entropy alloy (HEA). Subjected to the heavy cold rolling immediately after homogeneous precipitation, L12 structured spherical nano-particles with an average size of 16.5 nm rapidly grow into 200 nm-sized spherical ones due to Ostwald ripening. On the other hand, superfluous mechanical energy storage energetically facilitates the phase transformation from spherical L12 to rod-shaped D024 structures for initially formed nano-precipitates. Besides, some other newly formed nano-precipitates with an average size of 6.5 nm are available, originating from heavily plastically deformed-induced nucleated sites. Multi-scale precipitates interact with dislocations in different ways. The strengthening provided by dislocations cutting through smaller nano-particles and bypassing grown ones account for 57.7% and 42.3% of precipitation strengthening, respectively, while rod-shaped precipitates can act as equivalent interfaces to hinder dislocation movement. Their synergistic effect has achieved remarkable strengthening and toughening. Specially, dislocation slips dominate at 298 K, while stacking faults (SFs) assist plastic deformation at 77 K. Compared with 298 K, the yield strength (YS) and ultimate tensile strength (UTS) of the current HEAs at 77 K are increased by 38.9% and 38.2% to 1 GPa and 1.5 GPa, respectively, and the tensile strain is slightly increased to 35% instead of loss, realizing excellent strength and plasticity combination. Theoretically established strengthening models agree well room-temperature and cryogenic yield strengths experimentally. Moreover, the tensile elongation is effectively predicted by the Whitehouse-Clyne model. This strengthening strategy of energetically-tuning polymorphous precipitates provides the basic guidance to develop high-performance nano-precipitated alloys. The current strengthening and plasticity models can be employed to well predict the mechanical properties of such kinds of alloys at cryogenic temperatures.

Quantitative analysis of {100}Al plate/lath- and Al rod-shaped precipitates in an aged Al-Cu-Mg-Si alloy using TEM

Precipitate parameters, such as length, diameter and thickness of rationally oriented {100}Al plate/lath-shaped and <100>Al rod-shaped precipitates in an age-hardenable Al-Cu-Mg-Si alloy have been examined quantitatively. An appropriate procedure of the plate- and rod-shaped precipitate separation was suggested to determine the precipitate parameters as well as their distribution histograms using bright-(BF) and dark-field (DF) transmission electron microscopy (TEM) images taken in the <100>Al and <110>Al zone axes (ZA).