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In this work, the effect of microstructure on hot deformation behavior of ATI 718Plus (hereinafter refers to 718Plus) alloy was studied by isothermal compression test. The results showed that when the strain rate was 0.01–0.1 s−1 with deformation temperature of 980 and 1030 °C, hot deformation behavior was mainly affected by dislocation density. Dislocation density of the air-cooling alloy was larger than that of the furnace-cooling alloy, which makes its critical strain smaller and peak stress higher than that of the furnace-cooling alloy. When the strain rate was 1 s−1, hot deformation behavior of the alloy was mainly affected by twins and γ′ phase, and high-density deformation twins in air-cooling alloys resulted in higher critical strain. γ′ phase exists in furnace-cooling alloy, which makes its peak stress higher than that of air-cooling alloy. 718Plus alloy is sensitive to cooling rate; dislocation and γ′ phase have obvious effects on its hot deformation behavior.

Modification mechanism and uniaxial fatigue properties of A356.2 alloy treated by Al-6Sr-7La and traditional Al-5Ti-1B/Al-10Sr (hereinafter refers to traditional treated alloy) were investigated by constant stress amplitude method. Microstructure, dislocation and Si twinning of the alloys were studied by thermal analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that Al-6Sr-7La possesses better refining and modification effect than Al-5Ti-1B/Al-10Sr. Meanwhile, fatigue properties of the alloy treated by Al-6Sr-7La are higher than traditional treated alloy, and this is mainly owing to that Al-6Sr-7La treated alloy has more twins in eutectic Si and lower twin spacing. In addition, higher density of nanophases formed on twin faces and La-rich clusters appear at multiple twin intersections. Stacking faults and entrapped nanophases appeared on growing Si twin faces. Impurity induced twinning (IIT) mechanism and twin plane re-entrant edge (TPRE) mechanism are valid for eutectic Si which are important for mechanical optimization of A356.2 alloy.

Ni3Al-based alloys have drawn much attention as candidates for high-temperature structural materials due to their excellent comprehensive properties. The microstructure and corresponding mechanical properties of Ni3Al-based alloys are known to be susceptible to heat treatment. Thus, a significant step is to employ various heat treatments to derive the desirable mechanical properties of the alloys. This paper briefly summarizes the recent advances in the microstructure evolution that occurs during the heat treatment of Ni3Al-based alloys. Aside from γ′ phase and γ phase, the precipitations of β phase, α-Cr precipitates, and carbides are also found in Ni3Al-based alloys with the addition of various alloying elements. The evolution in morphology, size, and volume fraction of various types of secondary phases during heat treatment are reviewed, involving γ′ phase, β phase, α-Cr precipitate, and carbides. The kinetics of the growth of precipitates are also analyzed. Furthermore, the influences of heat treatment on the mechanical properties of Ni3Al-based alloys are discussed.

Recently, the γ′-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from the γ′ strengthening phase. For these γ′-strengthened superalloys, the changes in morphology, size and distribution of γ′ precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys. This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developed γ′-strengthened superalloys and its effects on the hot deformation behavior of superalloys, drawing specific examples on Allvac® 718PlusTM and Ni3Al-based intermetallic superalloys. It is found that the particle size plays an important role in morphological evolution of γ′ precipitates. For instance, the morphology of γ' precipitates evolves from cuboidal to strip-like or other complex structures in Ni3Al-based intermetallic alloys, while the γ' precipitates in Allvac® 718Plus alloy always present near-spherical morphology due to the relatively small initial particle size. The Lifshitz-Slyozof-Wagner (LSW) theory and its modifications, as well as Trans-Interface Diffusion Controlled (TIDC) theory have been applied to describing the coarsening kinetics of γ' precipitates. Additionally, the hot deformation behavior of γ′-strengthened superalloy is found to be greatly influenced by the coarsening of γ′ precipitates.

In this work, water cooling, air cooling (AC) and furnace cooling (FC) were applied to investigate the effect of cooling rate on microstructure evolution of primary γ′ in a newly designed Ni3Al-based alloy. The results showed that nucleation rate of primary γ′ increased with increasing cooling rate. In addition, higher cooling rate shortened growth period of primary γ′, which made its morphology close to the initial precipitated γ′. For AC and FC specimens, due to the lower cooling rate, primary γ′ possessed longer growth period and its morphology was mainly due to the evolution of lattice misfit between γ and primary γ′. Meanwhile, growth of primary γ′ depended on lattice misfit distribution between its corner and edge area. Moreover, primary γ′ morphologies of sphere, cube and concave cube with tip corners were illustrated by considering interaction between elemental diffusion and elastic strain energy.

Precipitation and growth behavior of γ′ (Ni3Al) phase are crucial to evaluate the service stability of superalloys. In this work, precipitation and growth mechanisms of γ′ phase in different areas of a newly designed Ni3Al-based superalloy were investigated under short-term thermal exposure conditions. Results showed that twinning γ′ phase and foliated γ′ phase precipitated in eutectic area and interface area, respectively. γ′ phase went through from sphere to precursor plate and plate twinning eventually in eutectic area, which was related to preferred orientation of γ′ phase and local segregation of Al element. Dislocation and sidestep interface between matrix and the precipitation were the main reasons for the formation of twinning γ′ phase. Meanwhile, the morphology of γ′ phases in interface area went through a process from sphere to rod-like, and foliated γ′ phases formed eventually which grew toward eutectic areas with thermal exposure time increasing. Vacancies, distortions caused by different thermal expansion coefficients, lattice mismatch between dual-phase area and eutectic area contributed to the formation and growth of spherical phase. Moreover, reduction in interface energy promoted the transformation of spherical phase to rod-like and foliated γ′ phases. In addition, concentration gradient of Al element and adequate holding time were responsible for the directional growth of foliated γ′ phase.

In present work, a multiphase Ni3Al-based intermetallic alloy with complex components of γ'+γ dendrite comprising extremely high volume fraction (81 vol %) of γ′ phase and interdendritic β phase is investigated, the microstructure evolution and phase transformation behavior tailored by different solution cooling rates are studied by applying different cooling methods of water cooling (WC, 138 °C/s), air cooling (AC, 72 °C/s), and furnace cooling (FC, 0.05 °C/s) after solution treatment at 1200 °C for 10 h. Results show that the cubic degree, size and volume fraction of cuboidal primary γ′ as well as size of spherical secondary γ′ particles in the γ'+γ dendrite are improved by decreasing cooling rates, and the average sizes of the primary and secondary γ′ particles are found to meet logarithmic relationships with cooling rates. There are three kinds of precipitates in the interdendritic β phase resulted from different cooling methods, i.e., the rod-like Cr3C2 carbides, the semi-spherical α-Cr particles, and the long acicular γ′ precipitates. The lower cooling rate decreases the number of rod-like Cr3C2 carbides, increases the number and size of the semi-spherical α-Cr particles, and promotes the formation of long acicular γ′ precipitates in the interdendritic β-matrix. The semi-spherical α-Cr particles play the role of pinning dislocations in the β-matrix and have the orientation relationship of [‾110]α-Cr//[‾110]β-matrix. In addition, the precipitation of semi-spherical α-Cr particles in β-matrix is observed to accomplish in two sequential stages and grow in an onion-like ordered way. Stacking faults existed in partial of α-Cr particles due to the nanotwins in β-matrix. Microtwins are observed in the interdendritic β-matrix after water cooling, which have a bimodal configuration of nanotwins at the microtwinning boundaries and interior stacking faults. Besides, γ′-envelope formed between the γ'+γ dendrite and interdendritic β phase dependent on the solution cooling processes.

The morphological evolution and coarsening kinetics of γ′ precipitates in the γ'+γ area of a Ni3Al-based alloy during isothermal aging at temperature range of 800 °C to 1000 °C have been investigated. For the aging temperature above 900 °C, the average size of γ′ precipitates increases and the morphology of γ′ precipitates gradually changes from cuboidal to strip-like and L-shaped with increasing aging time up to 50 h. The temporal evolution of the average size of γ′ precipitates is evaluated by the experimentally measured data and it suggests that the cube rate law fits better for experimental data at the early stage coarsening, while the square rate law fits better for the later stage coarsening. Moreover, using these determined values of coarsening rate constants which determined by using a coarsening rate equation based on the LSEM model, the activation energy of the early stage and later stage coarsening have been calculated as 174.2 kJ/mol and 186.3 kJ/mol, respectively.

Corrosion behaviors of Al-15%Mg2Si-1%Ni alloy in 3.5% NaCl solution were studied by characterizing the corrosion morphologies of Al3Ni, Mg2Si, and Al phases. The results show that there are two galvanic coupling systems (Al/Al3Ni and Al/Mg2Si) in Al-15%Mg2Si-1%Ni alloy. In the Al/Al3Ni coupling system, the intermetallic Al3Ni phase (nobler than the Al matrix) acts as the cathode throughout the corrosion process. For the Al/Mg2Si coupling system, electrochemical polarity conversion occurs between the Al matrix and the Mg2Si phase. The corrosion pits around the Al3Ni and Mg2Si are connected to form long corrosion channels, which accelerate corrosion of the Al-Mg2Si-Ni alloys.

Due to its excellent comprehensive performances, Al-Si-Mg alloy i widely used in automotive, transportation and other fields. In this work, tensile performances and fracture behavior of Al-Si-Mg alloy modified by dilute Sc and Sr elements (Al-7.12Si-0.36Mg-0.2Sc-0.005Sr) were investigated at the temperature of &minus, 60&ndash, 200 &deg, C for the first time, aiming to obtain a satisfactory thermal stability within a certain temperature range. The results showed that the new designed Al-Si-Mg alloy possessed a completely stable yield strength and a higher-level elongation under the present conditions. Fracture morphology analysis, fracture profile observation and strengthening mechanism analysis were applied to elucidate the evolution mechanisms of yield strength and elongation of the alloy. The fracture modes were significantly distinct in different temperature sections, and the reasons were discussed. In addition, the interaction among the nano precipitate phase particles, the deformation substructure and the dislocations were responsible for the thermal stability of the alloy within a certain temperature range.

To prolong the service life of heavy-duty vehicle engine parts subject to wind and sand exposure, a FeCrMoWBRe amorphous-nanocrystalline composite coating was prepared via high-velocity arc spraying for its higher wear resistance than the traditional one. The adhesive strength of the coating is approximately 60.5 MPa, the hardness is approximately 13.3 GPa, and H3/Er2 is 0.07 GPa, indicating that the coating has excellent wear resistance. Under the condition of oil lubrication, the wear rates of the coating under different loads (50–150 N) are in a range of (0.993–250) × 10−7 mm3·N−1·m−1; the coating performance is most stable under low and medium loads (50–100 N). It is proved that, abrasive and fatigue wear are the predominant wear mechanisms under low and medium loads (50–100 N), while abrasive, fatigue, and delamination friction as well as wear mechanisms are the dominant under higher loads (e.g., 125 and 150 N) for the coating. After adding abrasive particles, the wear rates of the coating under different loads are in the range of (3.6–259) × 10−7 mm3·N−1·m−1; the wear amount is not significantly increased under higher loads (125 and 150 N). The wear mechanism is almost the same under this condition. These results indicate that the coating performs stably under low and medium loads and shows resistance to abrasive particles, especially under higher loads.

Precipitation and growth behavior of mushroom-like Ni 3 Al phase along the dual phase (γ + γ′) and eutectic γ − γ′ area interfaces of as-cast JG4246A Ni 3 Al-based alloy were investigated by using short time cyclic heat treatment. Results showed that vacancies, small angle grain boundaries and dislocations formed along the dual phase (γ + γ′) and eutectic γ − γ′ area interfaces during the cyclic heat treatment, which promoted the precipitation of Ni 3 Al phase along this location. In addition, concentration gradient of Al elements between the eutectic γ − γ′ area and dual phase (γ + γ′) areas and the more rapid pipe diffusion provided by dislocations contributed to growth of directional mushroom-like Ni 3 Al phase.

The expansion test of twinning induced plastic (TWIP) steel tube was carried out at room temperature. Cracks appeared inside the tube edge when TWIP steel tube expands by 15.6%. The microstructure evolution, deformation mechanism and crack generation of TWIP steel after expansion were investigated by XDR OM TEM and EBSD. The results show that TWIP steel tube still austenite phase after expansion at room temperature; a great deal of dislocations are gathered around the grain boundaries and twin boundaries, the integral number of twins is high, and the mechanism of expansion deformation is the joint action of dislocations slip and deformation twins; Twins produce a large number of finer secondary or even multiple twins, intersecting each other; After expansion, the silk texture with direction of 〈111〉∥X0 dominated by rotating brass texture {110} 〈111〉 is gradually produced through grain deformation and rotation; The proportion of small angle grain boundaries increased greatly; It is deduced that the criterion of crack nucleation is based on the difference between the dislocation pile-up energy (DPE) and the crack nucleation energy (CNE), and the expansion deformation process of TWIP steel satisfies the condition of crack nucleation.

Polycrystalline Ni3Al-based superalloy with up to 70% γ ′ phase owns excellent strength at around 1100 °C. It has been successfully used as a substitute for turbine blade crown, which is one of the aircraft components working in the severest environment (i.e., sometimes nearly overheating). However, few studies concentrate on the coarsening resistance and phase thermal stability in this newly designed polycrystalline superalloy at the working temperature, hence, directional coarsening behavior of primary γ ′ ( γ I ′ ) has remained restricted. This paper investigated directional coarsening behavior of γ I ′ during aging treatment (1100 °C) after overheating at 1270 °C with furnace cooling (SFA), air cooling (SAA) and water cooling (SWA) respectively. It clearly showed that γ I ′ grew into lamellar and gradually became irregular during the directional coarsening process. Moreover, its morphological combination mode was illustrated. Since parallel short lamellar γ I ′ connected each other, long lamellar formed. When perpendicular long lamellar γ I ′ connected each other, it became L -type or T-type, i.e., precursor of irregular γ I ′ . The kinetics of directional coarsening regime of γ I ′ was in line with the matrix-diffusion-controlled (MDC) theory according to the evaluation of γ I ′ size. This was caused by higher migration of γ I ′ interfaces than the diffusion of solute elements. It was also confirmed that driving force of directional coarsening mainly depended on the competition of elemental diffusion, lattice misfit distribution and γ channel distance.

As structural materials, closed-cell aluminum foams possess obvious advantages in product dimension, strength and process economics compared with open cell aluminum foams. However, as a kind of structure-function integration materials, the application of closed-cell aluminum foams has been restricted greatly in acoustic fields due to the difficulty of sound wave penetration. It was reported that closed-cell foams with macrostructures have important effect on the propagation of sound waves. To date, the relationship between macrostructures and acoustic properties of commercially pure closed-cell aluminum foams is ambiguous. In this work, different perforation and air gap types were designed for changing the macrostructures of the foam. Meanwhile, the effect of macrostructures on the sound absorption coefficient and sound reduction index were investigated. The results showed that the foams with half-hole exhibited excellent sound absorption and sound insulation behaviors in high frequency range (>2500 Hz). In addition, specimens with air gaps showed good sound absorption properties in low frequency compared with the foams without air gaps. Based on the experiment results, propagation structural models of sound waves in commercially pure closed-cell aluminum foams with different macrostructures were built and the influence of macrostructures on acoustic properties was discussed.

As-cast microstructure of a designed polycrystalline Ni3Al-based superalloy is characterized using optical microscope, scanning electron microscope, transmission electron microscope equipped with selected area diffraction system, and the intermetallic phase transformations involved during solidification process are determined based on thermal analysis measurements. The as-cast microstructure is mainly composed of 80.63 vol% dendritic and 19.37 vol% interdendritic phases, and the dendrite is identified as quasi-cuboidal $$\gamma_{\text{I}}^{\prime }$$ phase connected by γ-channels where ultrafine $$\gamma_{\text{II}}^{\prime }$$ particles are distributed, and the interdendritic phases are determined as γ′–γ eutectic structure consisting of γ E phase with dotted quasi-spherical $$\gamma_{\text{E}}^{\prime }$$ particles. During solidification, the dendrite firstly nucleates from liquid melt near 1348 °C; subsequently, the residual liquidoid is transformed into interdendritic phases around 1326 °C. Afterward, γ′ phase will precipitate from dendritic γ-matrix with two-stage characteristics, resulting in the distinct precipitation of $$\gamma_{\text{I}}^{\prime }$$ and $$\gamma_{\text{II}}^{\prime }$$ phases when approaching to 1190 and 1043 °C, respectively. The corresponding transformations involved during the solidification process can be translated as

To study the hot deformation behavior of Ni3Al-based alloy, hot compression tests were conducted in the temperature range of 1050–1250 °C with the strain rates from 0.01 to 10 s−1. With the increase in deformation temperature and the decrease in strain rate, flow stress of the Ni3Al-based alloy would be decreased. Based on the obtained constitutive equation, the calculated values of peak flow stress are in good agreement with the experimental ones, and the activation deformation energy is determined as 802.71 kJ/mol. Moreover, by dynamic material model (DMM), processing maps of the hot-deformed Ni3Al-based alloys are established. It is indicated that the optimum processing parameters for the studied alloy correspond to deformation temperature of 1250 °C and strain rates from 0.01 to 0.1 s−1. Specimens deformed under the optimum processing conditions exhibit fine and uniform grains, which is a typical dynamic recrystallization (DRX) microstructure. The DRX degree could be effectively enhanced with the increase of deformation temperature and the decrease of strain rate.

A new thickening agent (hollow ceramic microsphere, CM) together with a novel bubble nucleation particle (Mg2Si) for magnesium composite foam (MCF) was obtained. Initial morphology and evolution process of Mg2Si during preparation process were described. Meanwhile, initial morphology of Mg2Si which is favorable for absorption of initial tiny bubbles was confirmed. Transition process of Mg2Si phase and the coalescence behavior of bubbles along with the foaming process were analyzed based on the experimental results. According to the results obtained from this work, MCF (using CM as thickening agent) with homogeneous pore structures, superior mechanical properties and excellent corrosion resistance performance could be foreseeable.

In order to investigate the synergistic effect of Sr and La on the microstructure and mechanical properties of A356.2 alloy, Al-6Sr-7La master alloy was fabricated and its microstructure and phase compositions were investigated. The optimal percentage of Al-6Sr-7La in A356.2 alloy was obtained according to the evolution of microstructure and mechanical properties. What's more, to clearly access the refinement and modification performance of Al-6Sr-7La master alloy, untreated and traditionally treated A356.2 alloys (using 0.2 wt% Al-5Ti-1B as refiner and 0.2 wt% Al-10Sr as modifier, respectively) were used for comparison. The results showed that α-Al, Al4Sr and Al4La were the main phases in Al-6Sr-7La. In addition, the aluminum alloy with 0.5 wt% Al-6Sr-7La exhibited optimal microstructure and mechanical properties, with the secondary dendrite arm spacing (SDAS) decreasing to 17.9 μm and acicular-like eutectic silicon transforming to fibrous. Due to the improved microstructure, the ultimate tensile strength, yield strength and elongation of the alloy (with 0.5 wt% Al-6Sr-7La) increased to 228.15 MPa, 108.13 MPa and 11.92%, respectively, which are much better than that of traditionally treated A356.2 aluminum alloy. Finally, the grain refinement and modification mechanisms were discussed. Keywords: Al-6Sr-7La master alloy, Modification and grain refinement, Synergistic effect, Mechanical properties

In the present study, closed-cell aluminum foams with different percentages of erbium (Er) element were successfully prepared. The distribution and existence form of erbium (Er) element and its effect on the compressive properties of the foams were investigated. Results show that Er uniformly distributes in the cell walls in the forms of Al3Er intermetallic compound and Al-Er solid solutions. Compared with commercially pure aluminum foam, Er-containing foams possess higher micro-hardness, compressive strength and energy absorption capacity due to solid solution strengthening and second phase strengthening effects. Additionally, the amount of Er element should be controlled in the range of 0.10wt.%-0.50wt.% in order to obtain a good combination of compressive strength and energy absorption properties.

Closed-cell aluminum foams with different contents of multi-walled carbon nanotubes (MWCNTs) were fabricated by using modified melt foaming method. In order to effectively disperse MWCNTs, orthogonal tests were utilized to determine the optimal ball-milling parameters. The existence forms of MWCNTs in aluminum foams and the compressive properties of the foams were investigated. Considered from the dispersion degree and structural changes of MWCNTs, the optimal parameters were obtained, the parameters mainly referred to weight ratio of MWCNTs to aluminum powder, weight ratio of ball to powder, milling rate and milling time, respectively. The results showed that MWCNTs mainly existed in three forms: totally embedded in cell wall, partly embedded in cell wall and totally exposed on cell wall surface, respectively. The reasons were mainly due to the existence of defects and amorphous carbon on the surface of MWCNTs, which promoted the wettability between the aluminum matrix and MWCNTs. In addition, with the MWCNT content increasing, the yield strength, structural stiffness and energy absorption capacity of the foams increased first and then decreased. Meanwhile, under the present conditions the foams with MWCNT content of 0.5% possessed the optimal comprehensive mechanical properties and the reasons were discussed. Keywords: Aluminum foam, Multi-walled carbon nanotubes, Orthogonal test, Compressive property

In this work, hot compression deformation behavior and processing maps of ATI 718Plus superalloy were studied by isothermal compression test on Gleeble-3800 thermal simulation testing machine. The results showed that under the present conditions (with deformation temperature range of 1030–1180 °C, strain rate of 0.01–10 s−1 and deformation amount of 50%), flow stress decreased with the increasing of deformation temperature and decreasing of strain rates. Meanwhile, simplified and strain-compensated classical Arrhenius constitutive equation models were established respectively, and strain-compensated constitutive equation improves the accuracy of flow stress prediction. In addition, processing maps showed that the optimal hot processing parameter ranges were 1140–1180 °C with strain rate of 0.01–0.1 s−1 respectively. The effects of strain rate and deformation temperature on microstructure evolution was investigated and low strain rate and high deformation temperature are conductive to DRX.

The microstructure evolutions during 1000 °C-1000 h long-term aging and relevant 800 ~ 840 °C/220 ~ 300 MPa creep behaviors of a novelly designed multiphase Ni3Al-based intermetallic alloy were investigated. Results showed this alloy presented excellent intermediate temperature creep properties even after 1000 °C-1000 h long-term aging. A kind of bimodal R-type γ′ rafting structures were observed in the 800 °C/220 MPa creep process of the long-term aged alloy, and the creep resistance levels of three kinds of γ′ rafting structures were evaluated to be in the order of bimodal R-type γ′ rafting structures > R-type γ′ rafting structure > N-type γ′ rafting structure. The 1000 °C-1000 h long-term aging can promote the inhomogeneous precipitation of Cr4.6MoNi2.1 phase at the edge γ′+γ dendrite area, which can play the role of pinning dislocations and inhibiting the further rafting and coarsening of the R-type γ′ rafts during creep. Differed from Ni-based alloys and other Ni3Al-based alloys, this long-term aged alloy exhibited abnormal four-stage creep characteristics, which consisted of atypical short steady-state creep stage with step fluctuant feature and atypical long accelerated creep stage with creep rate increasing at a constant rate.