Precipitation via thermal treatments is among the most effective approaches to strengthening and is widely applied in the Al industry. Thermal treatments combined with deformation are capable of finely regulating the process of precipitation and distribution of precipitates. Deformation-induced defects exert significant impacts on the precipitation and already present precipitates, which however is often overlooked. In this study, the interactions between deformation and precipitation/precipitates, and their impacts on mechanical properties were systematically investigated in the solution-treated (ST) Al-0.61Mg-1.17Si-0.5Cu (wt.%), processed by multi-pass equal channel angular pressing (ECAP) and thermal treatments. Novel deformation-mediated cyclic evolution of precipitates is discovered: ST→ (1,2 passes: deformation induced precipitation) Guinier Preston (GP) zones→ (An250/30) Q’ and L phases→ (3-pass: deformation induced fragmentation/resolution) spherical precipitates→ (4-pass: deformation induced further fragmentation/resolution) GP zones. On this basis, we extend the quasi-binary phase diagram of Al-Mg2Si along deformation as the third dimension and construct an innovative defect phase diagram for the Al-Mg-Si-based system. To testify to the effect of deformation-mediated cyclic evolution of precipitation/precipitates on the optimum mechanical properties, peak-aging treatments were performed in samples of ST and 3-pass states. Based on the microscopic characterizations, a distinctive mechanism of peak-aging strengthening is proposed. Notably in the 3-pass ECAPed and peak-aged sample the dominant strengthening phases become the L precipitates that thrived from the segmented and spherical L phases, rather than β’’ precipitates in the solely peak-aged ST sample. Our work provides a feasible example for exploring the combined processing technique of multi-step deformation and thermal treatments, to optimize the mechanical properties.

Adding carbon nanotubes (CNTs) to metal composites changes their corrosion resistance, which is significantly affected by the distribution of CNTs. In this study, the effect of the content and distribution of CNTs on the corrosion resistance of composites was investigated by changing the electrodeposition process. The results indicated that could inhibit grain growth and act as an elemental channel for passivation film formation, which positively enhanced the corrosion resistance of the material. However, the annealing used to improve the bonding strength of CNTs to the matrix increased the grain size of the material, which had a weakening effect on the corrosion resistance. Using ultrasonic in electrodeposition had an obvious promoting effect on the uniform distribution of CNTs. The composites with 0.1 g/l CNT showed the best corrosion resistance after annealing for 30 min at 600 °C.

Ti-xNb (x < 24) shape memory alloys (SMAs) typically exhibit triangular self-accommodation morphology of α” martensite after thermal β→α” transformation, whereas α” variants contain no internal twins. In present work, we found an interesting transition from the self-accommodated triangular into nano-twinned martensite morphology during tensile deformation in solution treated Ti-20Nb-(3, 6)Zr SMAs. Such transition was not observed in binary Ti-20Nb. Moreover, the superelasticity in the deformed Ti-20Nb-(3, 6)Zr SMAs is greatly improved by several times as compared to that in Ti-20Nb. The underlying mechanisms of superelasticity and its improvement are briefly discussed based on the phenomenological models concerning the self-accommodation morphology transition.

Properties of metastable beta-Ti alloys can be optimized by tuning the rich phase transformations. Oxygen (O) introduction, inevitable during melting and processing, affects greatly on phase transformations and the associated properties in beta-Ti. We investigated the effects of oxidation on phase transformations in Ti-Nb alloys. Based on microscopic characterizations, the as-quenched microstructure of oxidized Ti-20 Nb is revealed to contain 3 distinct layers with respect to the phase constitutions and subsequent transformations, i.e. oxide, oxygen influenced (OI), and center (C) layers. On subsequent heating, beta -> wiso occurs in OI layer, and in C layer undergo successive two step alpha"->beta and beta -> wisotransformations. In C layer of low O-content, both the starting temperatures of alpha"->beta (ACs ) and beta -> wiso (wCs ) transitions increase slightly with O-content. In OI layer of high O-content, since the beta ->alpha" transition is suppressed upon quenched to 273 K, the retained beta starts to precipitate wisofar below wCs upon subsequent heating. In summary, with the increase of O-content, the As and ws slightly increase at low O-content and then undergo a drastic decrease. The underlying mechanisms are further discussed. Our work may shed lights on the high temperature thermomechanical processing and designing high performance metastable beta-Ti alloys.

Mechanical alloying through the process of severe plastic deformation can significantly enhance the mechanical properties of dissimilar metals. However, uneven mixing of these materials can adversely affect their mechanical performance and may even lead to material failure. To quantitatively assess the degree of mixing between dissimilar materials, a mechanical mixing efficiency index (η) has been introduced. This index is calculated accurately by combining an improved Non-dominated Sorting Genetic Algorithm II (NSGA-II) with computer image processing techniques. The study focuses on the Al-Zn mechanical mixture, establishing a parametric correlation between the mechanical mixing efficiency index and the microstructural evolution of mechanical mixing. The findings indicate that when the η value falls below 0.27865, the material transitions from the mechanical mixing stage to the chemical fusion stage. This research facilitates the controllable preparation of mechanical alloying, providing theoretical support and practical evidence for the process.

To unravel the determinants contributing to the remarkable strength observed in nanocrystalline binary alloys, we employed a high-pressure torsion method to fabricate Al-Mg alloys with varying Mg concentrations. Mechanical property analysis indicates that, with the increasing Mg content, both the hardness and yield strength exhibit an ascending trend. Notably, the nanocrystalline Al-8Mg alloy boasted an impressive hardness and yield strength of 295 HV and 889 MPa, respectively. Comprehensive characterizations of solute distribution and microstructural evolution were conducted using X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy, and atom probe tomography. We clarified the distribution mechanism of Mg atoms at grain boundaries, developed a method for the precise assessment of inhomogeneous solute distribution, and proposed a multi-mechanism strengthening model, incorporating the inhomogeneous solute distribution strengthening mechanism. The calculated results aligned well with the experimental data.

Martensitic transformation temperatures are the most important indicators for designing shape memory alloy (SMA) components. Subjected to alternating temperatures in service, they undergo functional fatigue, which affects little the thermal hysteresis. We investigate the thermal cycling induced functional fatigue in Ti44Ni47Nb9 wide hysteresis SMA, decorated with multi-scale Nb-rich particles. We identify an accelerated functional fatigue behavior leading to the hysteresis widening from 58 to 71 K after 20 thermal cycles, in contrast to which hysteresis maintains nearly constant in Ti-Ni binary SMAs. Compared to Ti49Ni51 with similar transformation temperatures and latent heats, the hysteresis widening in Ti44Ni47Nb9 is attributed to accelerated decrease in starting temperature of martensite transformation (Ms).Microscopic characterizations reveal an inhomogeneous microstructure, consisting of (Ti,Nb)Ni B2 matrix and multi-scale Nb-rich particles and (Ti,Nb)2Ni particles. The presence of unique multi-scale Nb-rich particles has been demonstrated at the core of the underlying mechanisms of accelerated functional fatigue. Densely distributed Nb-rich nanoparticles narrow the mean matrix distance of transformation to 156.3 nm, which restricts the size and cross-over of the transformation induced dislocation leaves. Our work provides a new mechanism to widen the thermal hysteresis of SMAs, which is probably applicable to the other SMAs systems with dense nanoparticles.

The deposition mechanism of Ni-Co alloys with different cobalt compositions plays an important role in the preparation and application of Ni-Co alloys by electrodeposition. Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy were used to study the effect of the Co content on the electrodeposition mechanism of the plating. The surface morphology and crystal structure of the plated layers were studied by scanning electron microscopy and X-ray diffraction. The corrosion performance of nickel-cobalt alloys with different Co contents was analyzed by anodic polarization curves and electrochemical impedance spectroscopy. The results showed that during the early stage of electrocrystallization, increasing the CoSO4 content in the plating solution decreased the charge transfer resistance during nucleation and decreased the activation energy of nucleation/growth. Nucleation occurred via a three-dimensional transient nucleation mechanism, which was controlled by both diffusion and electrochemical factors. Compared with pure nickel, the Ni-Co alloy had a more pronounced (1 1 1) orientation and superior corrosion resistance. Plating with 10 wt% Co provided the best corrosion resistance.

Surface mechanical attrition treatment (SMAT) was used on the surface refinement of pure copper mainly to study the electrochemical corrosion behavior of pure copper in a 3.5 wt% NaCl solution, under grain refinement and short-time annealing conditions. Fine crystalline copper showed higher corrosion resistance than coarse crystalline copper, and the samples with grain refinement exhibited increased corrosion resistance under annealing conditions. This improvement may be attributable to the enhanced passivation of the surface passivation film after grain refinement. In addition, the residual stress after annealing reduces the activation energy of atoms leaving the metal lattice and entering the solution, reducing the dissolution rate of the anode. Consequently, the corrosion resistance of fine crystalline copper is improved. Grain refinement and short-time annealing help design corrosion-resistant pure copper.

The type and content of particles in the plating solution greatly affect the electrodeposition process, making it important to study the effect of carbon nanotubes (CNTs) on the electrodeposition of nickel during the prepa-ration of CNTs/Ni composites. In this paper, cyclic voltammetry (CV), linear sweep voltammetry (LSV), chro-noamperometric analysis (CA), and electrochemical impedance spectroscopy (EIS) were adopted to study the nucleation mechanism in the early stage of electrocrystallization. Field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) were used to reveal the electrocrystallization properties. The results showed that Ni was electrodeposited by 3D nucleation/growth and was controlled by diffusion and electro-chemical kinetics. Increasing the CNTs concentration shortened the time required for 3D growth. Additionally, the polarizability of the electrodeposited nickel increased upon increasing the CNTs content, which helped in-crease the electro-crystallization probability of Ni. As the concentration of CNTs increased, the charge transfer resistance decreased, the Ni nucleation activation energy decreased, and the Ni nucleation became easier. The introduction of CNTs changed the preferred growth orientation of pure nickel, and the surface growth morphology was mainly a spiral in a stepwise pyramid.

In this paper, the effects of as-quenched, single aging, and double aging on the mechanical properties, microstructure, corrosion resistance, and electrochemical behavior of an Al-Cu-Mg-Li alloy were investigated. Most changes only improved the mechanical properties or corrosion resistance of aluminum alloy alone, and very few of them improved the mechanical properties or corrosion resistance at the same time. The results showed that double aging treatment improved the strength, hardness, and corrosion resistance of the alloy but decreased its elongation. During the aging treatment, the corrosion morphology of the alloy gradually transformed from pitting corrosion (as-quenched) to a combination of pitting corrosion and corrosion cracks (single aging) and finally to pitting corrosion (double aging). The fracture morphology gradually changed from an uniform distribution of a mixture of large and small dimples (as-quenched) to an intergranular fracture (double aging) as aging progressed. Transmission electron microscopy results showed that the T1 phase within grains and at grain boundaries (GBs) was different between the as-quenched and double aged states. In addition, a precipitate-free zone (PFZ) formed in the double aged state. The optimal process of Al-Li alloy was determined to be pre-aging at 100 degrees C for 4 h and final aging at 160 degrees C for 24 h.

Exploring the redistribution mechanism of Mg from the perspective of experimental observation has been a long-term yet challenging attempt in Al-Mg alloys. Here we demonstrate a simple but effective approach to obtain non-uniform Mg solute distribution (i.e., Mg-enriched/depletion zones) around typical grain boundaries (GBs) in a nanocrystalline Al-8 Mg alloy. This abnormal segregation was detected by both high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. The results show that local strain and GB migration during deformation leads to spatially inhomogeneous Mg solute distribution around the GBs. Both the non-uniform distribution and the broadened GBs can hinder GB migration and dislocation motion, thus enhance the strength. An inhomogeneous solute distribution mechanism of Mg atoms is proposed based on the extensive investigations. This study may help with developing new strengthening mechanisms of nanocrystalline materials.

Two half-disk samples of pure Al and pure Zn were mechanically alloyed via high-pressure torsion (HPT) processing, followed by post-deformation annealing (PDA). The microstructure evolution of the Al–Zn alloy was studied by scanning electron microscopy, transmission electron microscopy and molecular dynamics (MD) simulations. The results indicated that the HPT-processed Al/Zn assembly was presented as a mixture of nanocrystalline and amorphous phases. The deformation-induced special orientation of (0001)Al//(111)Zn facilitated the interatomic diffusion, and the dislocation density reached 2.17×1017 m−2 under the pinning effect of high solid solubility. Nanocrystalline, high diffusion degree, and high local dislocation density may primarily accounted for the crystalline-to-amorphous transformation in Al–Zn alloy. Moreover, the results indicated a bimodal grain size distribution of 150–250 nm and 500–900 nm, and Zn atoms were enriched at the grain boundaries, upon subsequent PDA. Under the effect of this special heterogeneous microstructure, the prepared alloy exhibited an excellent plasticity with 160% of tensile elongation.

In this work, mechanical alloying of the alternating stacked pure Al and Zn thin foils was accomplished via high-pressure torsion (HPT). In the alloyed Al-Zn system, an exotic phase transformation from hexagonal close-packed (HCP) to face-centered cubic (FCC) was identified. The atomic-scale evolution process and underlying mechanism of phase transformation down to atomic scale are provided by molecular dynamics simulation and high-resolution transmission electron microscopy. The HCP → FCC phase transformation was attributed to the sliding of Shockley partial dislocations generated at the Al-Zn grain boundaries, which resulted in an [2110]/[011] and (0001)/(111) orientation relationship between the two phases. This work provides a new approach for the in-depth study of the solid phase transformation of Al-Zn alloys and also shed lights on understanding the mechanical properties of the HPT processed Al-Zn alloys.

Al-Mg-Si alloys are widely used in automotive body panels and parts of the engine owing to their low density, medium strength, high specific strength, good corrosion resistance and other characteristics. Currently, there are many studies on the precipitation behavior of undeformed Al-Mg-Si aluminum alloy, but there is a lack of research on the precipitation evolution and precipitation strengthening mechanism of ultra-fine grained 6061 aluminum alloy at different post-aging temperatures. In this study, the microstructure and mechanical properties of an ultrafine grain 6061 aluminum alloy produced by combining the equal channel angular pressing (ECAP) and post aging methods was comparatively evaluated via TEM, XRD, microhardness tests, and tensile tests. The results indicated that the average grain size of the alloy after two ECAP passes was refined to 210 nm. The average grain size of the alloy after the ECAP pass at 80 degrees C and 20 min post aging was 278 nm; moreover, the fine needle beta '', L phase, and Q' phase precipitates at nanoscale were dispersed in the matrix. Furthermore, the tensile and yield strengths were 514 and 483 MPa, respectively, while maintaining a remarkably uniform elongation of 15.1%. These results indicate that numerous dislocations introduced by ECAP in the matrix provide a location for the nucleation of the precipitate, which accelerates the precipitation kinetics during the post aging process. The high strength and toughness of the ECAP alloy after low temperature post aging can be attributed to the grain refinement strengthening, dislocation strengthening, and nanoprecipitation strengthening. Thus, the evolution of the aging precipitates during the ECAP and post aging alloy was analyzed.