Semi-solid metal processing is a metal-forming technology that combines the advantages of casting and forging, realizing near-net forming high-performance parts with complex structures. Research on semi-solid processing of AlSi7Mg alloys mainly focuses on rheology, and the preparation of high solid fraction AlSi7Mg semi-solid billets by the solid phase method has been largely neglected. In fact, semi-solid technology is more significant than casting at higher solid fractions. The present study investigates semi-solid billets of AlSi7Mg aluminum alloy with a high solid fraction, prepared by the recrystallization and partial re-melting (RAP) method. The effects of upsetting temperature, compression ratio, semi-solid isothermal treatment temperature, and holding time on the billet microstructure were investigated by DSC test, upsetting experiment, semi-solid isothermal treatment experiment, OM observations, and Image Pro Plus image processing software. The microstructure of the semi-solid billet during isothermal compression was slightly affected by temperature but was beneficially refined by increasing the compression ratio. The optimal hot upsetting parameters were 240^oC and 40% deformation. During the semi-solid isothermal treatment, increasing the holding temperature gradually increased the size of the solid phase grains in the microstructure. As the holding time increased, the solid phase particles in the semi-solid structure initially grew slowly, and thereafter rapidly grew to a stable size. The changes in roundness of the solid particles were more complicated. The average grain size of the billet prepared by the RAP method was 64~117 μm, and the shape factor was 0.76~0.89. The linear relationship between cubic coarsening of the average semi-solid grain size and isothermal time was nonobvious at isothermal temperatures below 599^oC but was evident at temperatures of 599^oC. Below 599^oC, the grain coarsening is affected by Ostwald ripening, coalescence, recrystallization, and melting; while at 599^oC, the grain coarsening was dominated by Ostwald ripening.

To improve the properties of Ti-Ni shape memory alloys (SMAs), a third element can be added to them and assisted by a heat treatment process. After the Ni-rich Ti-Ni SMAs are doped with a small amount of Zr, the parent phase of the alloy is observed to exhibit enhanced stability; further, the alloys exhibit improved yield strength, elongation, and memory performance. The effects of the composition and annealing processes on the phase transformation behaviors and mechanical properties of the Ti-Ni-Zr SMAs have been studied; however, the microstructure, tensile properties, and shape memory behaviors of the aged Ti-Ni-Zr SMAs remain to be investigated. In this work, a Ni-rich Ti-50.8Ni-0.1Zr alloy could be obtained by doping the Ti-Ni alloys with 0.1%Zr (atomic fraction). The effects of the ageing processes on the microstructure, tensile properties, and shape memory behaviors of the alloy were investigated through TEM and tensile tests. The Ti₃Ni₄ precipitates in the Ti-50.8Ni-0.1Zr alloy samples aged at 300, 400, and 500^oC exhibit morphologies of fine particles, lenticular particles, and long strips, respectively. The effect of ageing temperature on the morphology, size, and dispersion of precipitates is greater than that of the ageing time. The alloy exhibited enhanced strength but reduced ductility after the ageing treatment. With the increasing ageing time (t_ag), the tensile strength (R_m) increased and the percentage elongation (A) decreased when considering the alloy sample aged at 300^oC. In case of the alloy sample aged at 400^oC, R_m initially increased and subsequently decreased, whereas A initially decreased and subsequently increased. The alloy sample aged at 500^oC exhibited a reduced R_m but an enhanced A. The alloy samples aged at 300^oC for 1-50 h or at 400^oC for 1 h exhibited superelasticity, whereas those aged at 400^oC for 5-50 h or 500^oC for 1-50 h exhibited the shape memory effect. In the alloy samples aged at 300^oC, higher t_ag values resulted in enhanced energy dissipation and lower critical stress values for stress-induced martensite transformation. Alloy ageing at 400^oC or 500^oC resulted in lower critical stress values for martensite reorientation and lower energy dissipation.

Duplex stainless steel with its exceptional corrosion resistance, mechanical properties, and proficient weldability has been widely used in ships and bridges, as well as petrochemical and seawater desalination industries. Friction stir processing (FSP) does not only induce dynamic recrystallization of the material but also achieves the purpose of repairing the crack automatically, which markedly improves the mechanical properties of duplex stainless steel. Thus, FSP is particularly useful for crack repair of duplex stainless steel structures. In the present study, microstructure, mechanical property, and corrosion property of FSP 2507 duplex stainless steel were investigated. FSP was performed at a constant welding speed of 100 mm/min and tool rotation speeds of 200, 300, 400, 500, and 600 r/min using a tungsten-rhenium-based tool. Due to the thermal and mechanical effects in the processing, the section of the processing zone can be divided into the thermo-mechanically affected zone (TMAZ) and the stir zone (SZ). Only under the sufficient parameters of thermoplastic flow, the internal faultless processing zone was obtained. In accordance with the increased tool rotating speed, the grain size of the SZ initially decreased and then increased. Processing heat cycle and stress deformation had an insignificant influence on the proportion of ferrite and austenite phases in the processing zone, and the ferrite content still remained between 40% and 60% in the standard specification. The σ phase was determined at the bottom of the processing zone, namely at the tool rotation speed of 200 r/min due to the low heat input. Microhardness distribution of the processing zone demonstrated a basin-like morphology, and the largest hardness value appeared at the bottom of the advanced side of the SZ, corresponding to the smallest grain size of the SZ. As the tool rotating speed increased, the longitudinal tensile strength of the SZ increased initially and then decreased, contrary to the elongation. According to the results of potentiometric polarization and electrochemical impedance spectroscopy, the refinement of grain enhanced the stability, compactness, and repassivation performance of surface passivation film. The corrosion resistance of the upper surface in the SZ exceeded that of the base material, rendering it more useful. When the tool rotation speed was 400 r/min, the SZ had the optimal corrosion properties.

Owing to its high creep rupture strength, good weldability, and low costs, T23 steel is an ideal material for manufacturing the heating components of water walls, superheaters and reheaters in ultra-super critical plants. However, its coarse grain heat affected zone (CGHAZ) is prone to stress-relief cracking (SRC) during post-weld heat treatment or high-temperature service. The mechanism of SRC is controversial and an effective method for forecasting and preventing SRC in T23 components is currently lacking. Clarifying the mechanism of SRC in the CGHAZ of T23 steel, and developing a practical engineering technique for predicting and preventing SRC generation, are therefore essential. In this work, CGHAZ specimens of T23 steel were simulated in a thermo-mechanical simulator, and aged at 650^oC for 0-48 h. After simulating the microstructure evolution of the as-welded CGHAZ during service, the SRC susceptibility of the CGHAZ was evaluated. The microstructural changes and carbide precipitation were observed by OM, SEM, TEM, and EDS. The as-welded CGHAZ of T23 steel was composed of mixed martensite and bainite with high hardness. After ageing at 650^oC, the structure recovered and recrystallized with a lower dislocation density and larger sub-grains than the as-welded CGHAZ. Carbides such as M₂₃C₆, M₇C₃, and MX gradually precipitated inside the grains and grain boundaries, decreasing the hardness. The SRC susceptibility was high in the as-welded CGHAZ, but decreased with increasing ageing time. When the ageing time exceeded 24 h, the sample was SRC-resistant. The main cause of SRC in the CGHAZ was precipitation and growth of M₂₃C₆ on the grain boundaries, which induced the formation of softened zones in the matrix near the grain boundary, and promoted the formation of micro-voids. During ageing, the unstable microstructure in the as-welded CGHAZ transformed as carbides precipitated and the matrix recrystallized, thereby reducing the intragranular strength. Meanwhile, the depletion of alloy elements near the grain boundary was eliminated. The microstructural evolution decreased the difference between the intragranular and intergranular strengths in the CGHAZ. Finally, the CGHAZ showed significantly improved ductility and low SRC susceptibility. The hardness of the aged CGHAZ was positively related to the SRC susceptibility. At hardnesses above 250 HB, the CGHAZ was SRC-susceptible, but at hardnesses below 250 HB, the CGHAZ was SRC-resistant.

Ultra-high-strength bainitic steels with excellent combinations of strength and ductility may be the new generation of metallurgical interest. However, there still exist some production problems, such as long transformation times due to low-temperature processing and difficulty in tailoring the elongation. In this work, both ausforming and austempering were used to investigate the effects of deformation on the transformation and microstructure in a medium-carbon bainitic steel. The Gleeble 3500 simulator, SEM, TEM, XRD, and tensile tests were used to analyze the effects of ausforming on retained austenite, the strength and plasticity of bainitic steel. The results show that ausforming at 300^oC with a strain of 0.2 not only accelerates the kinetics of isothermal transformation, but also refines the bainitic microstructure and optimizes the retained austenite and its stability. The stability of the retained austenite is affected by the carbon content and dislocation density, and the carbon content can be increased by prolonging the duration of the isothermal stage. The volume fraction of retained austenite is increased by ausforming because of the enhanced dislocation density, which leads to ultra-high-strength bainitic steel with excellent properties of a tensile strength of 1733 MPa and ductility of 15.7%.

Commercially pure titanium (CP-Ti) is a human-implant metal material commonly used for cardiovascular scaffolds and dental implants in the medical field. This is because CP-Ti has better biocompatibility and corrosion resistance compared to other alloys such as titanium-aluminum-vanadium alloy (Ti-6Al-4V). However, the low strength properties of CP-Ti have limited its wider application (e.g., load-bearing components). On the contrary, Novel β titanium alloys possess higher strength and lower elastic modulus, which has led to the consideration of Ti-Nb based alloys for biomedical applications, while also taking into consideration their biocompatibility and other mechanical properties. Recently, laminated metal composites (LMCs) have attracted a lot of attention due to the excellent properties of the constituent alloys. Direct laser deposition (DLD) is an additive manufacturing technology that can be potentially used to manufacture LMCs. In this work, the DLD process was used to manufacture Ti/TNTZO LMC, and CP-Ti and TNTZO alloy powders were the raw materials. Subsequently, the microstructure, phase composition, mechanical properties, and in vitro bioactivity of the Ti/TNTZO LMCs were analyzed. The results demonstrated that high-density, crack-free Ti/TNTZO can be fabricated using the DLD process. Ti/TNTZO is mainly composed of α/α' and β phases. Transmitted Kikuchi diffraction maps showed the presence of α" martensite, but due to its low content, there were no relevant peaks in the X-ray powder diffraction spectra. The hardness of the Ti region in the Ti/TNTZO increased due to the diffusion of alloy elements and refinement of the structure formed as a result of a faster cooling rate. However, for the TNTZO region, the hardness also increased due to the martensite transformation caused by the dilution of β-stabilizing elements compared with the TNTZO manufactured using the DLD process. In comparison with the CP-Ti and TNTZO made using the DLD process, the microstructure of the Ti/TNTZO multilayered materials was significantly different. The microstructure of Ti layers had coarse columnar grains and fine α/α' plates, and there was acicular martensite at the subgrain boundary of the TNTZO layers. As a result of the alloy elements diffusion, transition layer with a size of approximately 50 μm was found between the Ti layer and TNTZO layer. The tensile test results also showed that the multilayered materials have high yield strength and ultimate tensile strength. However, the presence of acicular martensite at the interface reduces the plasticity of the materials. Additionally, the Ti/TNTZO multilayered materials showed good ability to induce apatite formation after soaking in simulated body fluid for 14 d. Therefore, the results of this study showed that the Ti/TNTZO multilayered composites fabricated using the DLD process have potential application in the biomedical field.

Owing to their low density, high specific strength, biocompatibility, and good corrosion resistance, titanium and its alloys have been widely used in the aerospace, biomedical, and marine engineering fields. As engineering applications of titanium alloys continue to develop, especially in special engineering projects, the service safety and stability of titanium alloys must be satisfied under extremely complex conditions. Unfortunately, traditional titanium alloys usually exhibit low plastic-deformation ability and no significant work hardening behavior, which limits their applicability. Improving both the strength and ductility of these materials is expected to broaden their engineering applications. In recent years, β-type (body-centered cubic) titanium alloys have shown good formability and impact toughness, by virtue of their diverse plastic deformation modes and excellent ageing strengthening. Therefore, they are promising candidates for titanium alloys with a good strength-ductility combination. In this work, a multilayered Ti-10Mo-1Fe/3Fe alloy was manufactured by a multi-pass hot rolling and heat treatment, and the coupling effects of pre-strain and isothermal ageing on the mechanical properties of the alloy were studied by various techniques: laser scanning confocal microscopy, XRD, SEM, SEM-EDS, EBSD, a Vickers hardness tester, and a tensile testing machine. After pre-strain and isothermal ageing, the alloy exhibited {332}<113> twins and slip bands alternately multilayered deformation microstructures. The alloy demonstrated a relatively high yield strength and large uniform elongation. The high yield strength resulted from the initial plastic deformation, which was dominated by dislocation slips due to isothermal ω-phase precipitation. The early onset of plastic instability after yielding was hindered by the pre-strain induced twins, and the uniform elongation was enhanced not only by the dynamic Hall-Petch effect caused by further twinning activation, but also by interactions between the twin and layer-interface. As demonstrated on this multilayered alloy with twinning and dislocation-slip coupled deformation, the strength-ductility combination in β-type titanium alloys can be controlled through the coupling effect of pre-strain induced {332}<113> twins and the subsequently precipitated ω phase.

Pt-Al coating has been widely used in engine rotor blades because of its ability to improve the oxidation and hot corrosion resistance of Ni-based superalloys. However, the effect exerted by Pt on S and other refractory elements, as well as the rupture mechanisms, is under debate. To investigate the influence of Pt-Al coating on the corrosion resistance of single-crystal superalloy at high temperature, the hot corrosion test utilized Na₂SO₄ salt coated on the surface of the Pt-Al coating samples and the uncoated ones were carried out at 900^oC, respectively. Using several techniques, such as XRD, SEM, EDS, and EPMA, the influence of Pt-Al coating on the hot corrosion behaviors of a Ni-based single-crystal superalloy was analyzed. Moreover, the hot corrosion kinetics, hot corrosion products, and microstructure evolution during the process were analyzed. The results reveal that the hot corrosion resistance of the substrate alloy was enhanced by Pt-Al coating. The hot corrosion rate of the Pt-Al coating sample was lower than that of the uncoated one. Thus, it can be inferred that Pt-Al coating exhibited better hot corrosion resistance. Pt prevented the diffusion of S into the β-(Ni, Pt)Al phase. The S atom was present at the oxide-metal interface, which reduced the hot corrosion rate of the substrate alloy. The presence of Pt in the β-(Ni, Pt)Al obstructed the great mass of Ta in the inter diffusion zone, which led to the diffusion of only a small quantity of Ta atoms into the oxide, and reduced the formation of Ta₂O₅. Finally, Pt-Al coating was also found to restrain to some extent the void formation at the oxide-metal interface.

Owing to their low density and good mechanical properties at high temperatures, C/SiC composites are increasingly used in the aerospace industry. They are also being proposed as thermal-structural materials in the hypersonic field; however, C/SiC composites are easily oxidized in high-temperature air environments. In this study, a C/SiC composite was coated with a SiC-ZrC oxidation-resistant layer by a two-step sintering method using Si-25%Zr (mass fraction) alloy, and the phase evolution of the coating was studied during the sintering. The oxidation resistance of the material was then tested at 1400^oC in an air environment. The microstructural changes of the coating before and after oxidation and the effect of oxidation on the bending properties of C/SiC were analyzed. After the reaction with carbon, Si and ZrSi₂ disappear in the coating, leaving only pure ZrC and SiC. The ZrC phase refined the structure of the reactive SiC layer. The grain size of the sintered SiC was 2 μm, versus 5-20 μm for SiC sintered from pure Si. The refined grains created a dense and continuous SiO₂ film during the oxidation process. As the oxidation time was increased at 1400^oC, the C/SiC composite with the SiC-ZrC coating began losing weight at 200 s, but began gaining weight at 500 s as a dense SiO₂ film was formed. After 1000 s of oxidation, the flexural strength of the C/SiC composites was 335 MPa, only 5% lower than that of the initial C/SiC composite. According to this result, the sintered SiC-ZrC oxidation-resistant film effectively protected the mechanical properties of the C/SiC composite during the oxidation process.

Within the China standard (6061 GB/T 3190-2008) of the aluminum alloy 6061, there are a wide range of alloy compositions having multiple trace elements. From the viewpoint of scientific research and quality control in industries, it is important to understand the relationship between the different potential compositions and corresponding mechanical properties of the aluminum alloy 6061. In this work, high-throughput experiments on materials synthesis and an active-learning framework based on the Bayesian optimization sampling process were combined to develop effective machine learning (ML) models to describe the relationship between the composition and hardness of aluminum 6061 alloys. In this work, > 100 alloys with ML designed compositions were synthesized and their hardness data were obtained through high-throughput experiments. The composite ML features were introduced by combining elementary material properties and chemical compositions of alloys and were selected subsequently according to their importance and correlation among features. The efficiencies of two sampling strategies were compared in guiding the iterative experiments: manual sampling based on empirical experience and Bayesian optimization sampling trained within the active-learning framework using the efficient global optimization and knowledge gradient algorithms. These ML models were updated iteratively until the prediction accuracy approached the experimental error. Specifically, the error in the hardness values predicted by the Bayesian model using 64 aluminum alloy samples after three rounds of iterations was 4.49 HV (7.23%), which is much lower than the error predicted by the empirical sampling method (9.73 HV; 15.68%). The results show that Bayesian optimization sampling accelerates the optimization of alloys property more efficiently than manual empirical sampling. Finally, the machine learning models using Bayesian sampling were interpreted using the Shapley additive explanations method and analysis of the partial dependence plot discuss the effects of various trace alloying elements and composite ML features on the hardness of the aluminum alloys. It was found that the hardness value of the aluminum alloys became large when the ratio between Mg and Si (Mg/Si) was between 1.37 and 1.72. In addition, the machine learning models suggested that the lattice distortion, cohesive energy, configurational entropy, and shear modulus were positively proportional to the hardness of the alloy. This work demonstrated that active-learning-guided high-throughput experiments on composition refinement can not only improve the performance and quality control of aluminum 6061 alloys within its standard nominal composition range as used in industry but also provide a feasible approach for the design and property optimization of other multialloy materials.

Steel structures exposed to offshore atmospheric environment for a long time inevitably suffer from corrosion damage. Safety assessment of corroded steel structures largely depends on the quantification of corroded surface features as the irregular corrosion characteristics are the main factors causing decline in steel mechanical properties. To investigate the structural steel corrosion characteristics in offshore atmospheric environment, accelerated corrosion tests were conducted on 16 pieces of Q235B steel plates by periodic spraying to simulate the offshore atmospheric environment. Moreover, the surface morphologies and characteristic parameters were measured and analyzed using a ST400 3D Noncontact Profilometer and a self-written algorithm. The distribution characteristics such as corrosion depth, pit depth, and aspect ratio were elucidated, and the changing laws of statistical parameters such as mean value, standard deviation, and pitting shapes were revealed. The results indicated that in the simulated offshore atmospheric environment, the structural steel corrosion process generally goes through three stages: scab, swell, and spall. The scab and swell stages are dominated by pitting corrosion, whereas, the spall stage shows the general corrosion characteristics. Moreover, the corrosion depth of structural steel in the simulated offshore atmospheric environment conforms to the normal distribution, whereas, the pit depth and aspect ratio conform to the log-normal distribution. As the degree of corrosion increases, the mean value and standard deviation of the corrosion depth, peak value of the power spectral density of the corrosion depth, and logarithmic mean value of the pit depth also gradually increase, whereas, the logarithmic mean value of the pit aspect ratio decreases. Meanwhile, at different ages, the cone pits have the highest proportion, and the pit shape gradually changes from a cylinder or a hemisphere to a cone. Finally, based on the results of the statistical analysis of the corrosion depth and pit parameters, the stochastic field model of corrosion depth and random distribution model of corrosion pits were constructed, which achieved the accurate characterization and reproduction of the surface morphology of the corroded steel in a simulated offshore atmospheric environment. The research results would lay the foundation for the establishment of an accurate stochastic model and structural reliability analysis in the natural offshore atmospheric environment.

Cluster dynamics is a mesoscopic modeling technique describing the various kinetic stages of homogeneous precipitation by the same set of rate equations. However, when the simulated cluster size continuously increases, it easily causes an enormous computational workload, and the use of a particle-size-grouping method is often necessary to solve this problem. In this study, an ungrouped cluster dynamics model and certain existing grouping methods are reviewed. Next, a new grouping method with an assumed logarithmically-linear distribution of cluster number densities inside each group size is proposed. Comparing the results of all grouped models with the exact solution of the ungrouped model for simulating aluminum-scandium (Al₃Sc) precipitation in the Al-0.18%Sc (atomic fraction) alloy at 300^oC, the new grouping method was able to reduce computational costs considerably keeping enough total and local accuracies. Moreover, the reasonable agreements of the mean radii and size distributions as functions of time between experiments and simulations were obtained, demonstrating the ability of the new grouping method in modeling large-scale precipitation kinetics.