Search

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.

Al-0.5Mg-0.4Si-0.1Cu alloy possessing weather resistance and oxidation resistance can obtain good surface quality (metallic luster) without chrome plating. Therefore, it is an important material to replace polluting chrome-plated aluminum profiles for automotive decorative parts. At present, studies about the extrusion process of Al-0.5Mg-0.4Si-0.1Cu alloy are very few, which affects its further application. In this work, the effect of extrusion ratios on microstructure and mechanical properties of Al-0.5Mg-0.4Si-0.1Cu alloy is investigated by optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests. The results showed that the dynamic recrystallization degree of the alloy gradually increased with the extrusion ratio increasing, which is attributed to the driving force provided by the large extrusion ratio. Meanwhile, due to the occurrence of dynamic recrystallization, the texture changed from to orientation. In addition, grains were obviously refined and uniform with the extrusion ratio increasing. Due to the fine grain strengthening mechanism, the tensile strength and elongation of the alloy with an extrusion ratio of 30 reached 152 MPa and 32.4%.

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.

In order to understand the effect of high temperature pre-precipitation (HTPP) temperature on the precipitation behavior and tensile properties of A356.2 alloy treated by Al-6Sr-7La master alloy, SEM, TEM and tensile tests were applied to investigate the evolution of fracture morphology and precipitates of the alloys under different HTPP temperatures. The results showed that ultimate tensile strength (UTS) and yield strength (YS) of the alloys decrease and elongation (El) increases with HTPP temperature decreasing. When HTPP temperature decreased from 510 °C to 470 °C coarsen coherent β″ phase appear in α-Al matrix, continue to decrease HTPP temperature to 450 °C the main precipitate transformed into semi-coherent β′ phase, leading to the change in mechanical properties. In addition, coarsening and transformation of the precipitate were attributed to the reduction of Si concentration which decreases with HTPP temperature decreasing. Moreover, Si nanoparticles precipitated in α-Al matrix, leading to the decrease of UTS and YS to certain extent due to reducing Si concentration during aging process.

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 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.

In this work, precipitation behavior and mechanical performances of A356.2 alloy (treated by Al–6Sr–7La composite refinement-modification agent) were studied for the first time. Artificial aging treatment (T6) and long-time natural aging treatment (>10,000 h, T4) were applied to clarify the precipitation and growth behavior of Mg5Si6 phase. The results show that vacancies and dislocations formed during solution treatment promote the nucleation and growth of Mg5Si6 phase in the eutectic area during the subsequent aging process. During the T6 treatment, the growth of Mg5Si6 phase is promoted due to the interaction of atomic diffusion channel caused by higher Si concentration gradients and dislocations, leading to a large number of uniform distributed spherical Mg5Si6 phase in eutectic area. While, for the T4 treated alloy, atomic clusters with short rod-like Mg5Si6 phase exists inside the alloy. In addition, T6 treatment resulted in significant improvement of tensile strength (UTS), yield strength (YS) and vickers hardness (HV) compared with the T4 treated alloy and the reasons were discussed.

Closed-cell AZ31 magnesium alloy foams with different percentages of hollow ceramic microspheres (CMs) are synthesized using modified melt foaming method. The distribution of CMs is investigated and also the effect of CMs on the foaming behaviors (specifically for porosity and pore size) and quasi-static compressive behaviors of Mg-based composite foams are characterized. The results show that CMs distribute in cell walls homogeneously and most of them are penetrated by magnesium alloy melt. In addition, the mean pore size declines with the increase of CMs percentage. Moreover, the overall porosity of the foams increases first and then decreases with the increase of CMs content, and the variation tendency is more obvious when the foaming temperature is lower (namely 680 °C). Besides, proper percentage of CMs changes the compression fracture mode of the foams from brittleness to ductility. OM/SEM/EDS/XRD detections and finite element analysis are applied to explain the reasons.

In this paper, closed-cell aluminum foams with different kinds and contents of ceramic microspheres are obtained using melt-foaming method. The distribution and the effects of the ceramic microspheres on the mechanical properties of aluminum foams are investigated. The results show that both kinds of ceramic microspheres distribute in the foams uniformly with part in the cell wall matrix, some in adhere to the cell wall surface and part embed in the cell wall with portion surface exposed to the pores. Ceramic microspheres have an important effect on the yield strength, mean plateau stress, densification strain and energy absorption capacities of aluminum foams. Meanwhile, the content of ceramic microsphere in aluminum foams should be controlled in order to obtain good combination of compressive strength and energy absorption capacity. The reasons are discussed.

Corrosion behavior of closed-cell AZ31 foam in different concentrations of NaCl solutions is investigated, using commercial AZ31 magnesium alloy as comparison. Mathematical models and expressions are established to calculate surface areas of the foams. Microstructure observation, hydrogen evolution and potentiodynamic polarization are applied to characterize the corrosion behaviors. Results show that foams possess higher corrosion resistance than commercial AZ31 magnesium alloy. In addition, the mathematical models and expressions are confirmed to be simple and effective in calculating surface areas (expose to corrosive medium) of closed-cell metal foams.

The precipitation and evolution of intersected plate-like γ′ (γ′P) phase within interdendritic β during long-term aging and its relevant influence on the creep behavior of a multiphase Ni3Al-based intermetallic alloy are investigated. Results show that the precipitation and growth of the γ′P precipitates during 800 °C aging of original as-cast alloy are significantly rapid, which can be fully precipitated in 60 min to form the intersected morphology and continue to coarsen and grow during the extended aging and subsequent 800 °C creep processes. The γ′P precipitates have the Bain orientation relationship (OR) of (100)γ′P//(100)β and [0-11]γ′P//[001]β with the interdendritic β-matrix. The precipitation of intersected γ′P phase is sensitive to the elemental fluctuations in β-matrix, and the preceding high-temperature annealing treatment can significantly reduce the elemental inhomogenization and inhibit its generation within the interdendritic β. Precipitation of adequate intersected γ′P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic β during creep, resulted in the improved creep properties. Slightly rafted γ′ after long-term aging at 800 °C transitioned into R-type γ′-raft structure during creep at 800 °C/220 MPa, and can induce larger creep resistance than N-type γ′-raft structure.

Cyclic oxidation behavior of a polycrystalline Ni3Al-based superalloy under as-cast and solution treated conditions was investigated in detail. SEM equipped with energy dispersive X-ray spectrometer and high-resolution transmission electron microscopy was used for microstructure observation and mechanism analysis. Results showed that oxidation process occurred along eutectic area (γ-γ′) and dual phase area (γ+γ′)interfaces firstly for as-cast alloy and oxide scales along (γ-γ′)/(γ+γ′) interfaces spalled gradually with oxidation process proceeding. As for the solution treated alloy, though oxidation process also occurred along (γ-γ′)/(γ+γ′) interfaces, no obvious spalling of oxide scales was observed under the present conditions. Due to the precipitation of Hf-containing carbides which can increase the adhesion of scales and its segregation to Al2O3 grain boundaries during cyclic oxidation to hinder outward migration of Al, leading to slower growth rate of scales. During the long-time (8–200 h) cyclic oxidation process of the solution treated alloy, O atoms continuously diffused towards the matrix along the pores in scales, resulting in thickening of Al2O3 layer. Moreover, thickening of Al2O3 layer also hindered the outward diffusion of Ni and a small amount of NiO was observed on surface of specimen after long-term cyclic oxidation, meaning the improvement of oxidation resistance.

To investigate the effect of initial microstructure on the hot deformation behavior of a Ni3Al-based alloy, the isothermal compression tests were conducted on the studied alloy with two different microstructures at 1100 °C and 1200 °C with strain rate range of 0.01–1 s−1. The results show that the hot deformation behavior of the Ni3Al-based alloy is sensitive to the initial microstructure, particularly under low deformation temperature and high strain rate. The flow curves signified that the peak stress and the magnitude of the stress drop for the alloy with coarse γ′ precipitates are higher than those for the alloy with fine γ′ precipitates at a given temperature and strain rate. Besides, it is found that the critical conditions are affected by deformation temperature, strain rate and initial microstructure. Both the critical stress and strain of dynamic recrystallization significantly decreases with the increase of strain rate and decrease of deformation temperature. For different initial microstructures, the critical stress and strain for the alloy with coarse γ′ precipitates are higher than that for the alloy with fine γ′ precipitates. The critical ratios of the studied alloy were also determined. The dissolution of coarse γ′ precipitates easily occurs and the dissolution rate can be accelerated at the sufficiently high temperature, which leads to that the effect of initial microstructure on hot deformation behavior of the Ni3Al-based alloy is weakened.

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni3Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni3Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni3Al phase with ordered face-centered L12 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101]γ'-envelope//[01‾1]dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L12 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni3Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr23C6 carbides surrounded by the widened γ′-envelope.

Though metal foams possess excellent overall performances, its performances behave quite different due to its apparent aperture difference of pores diameter, pores roundness and significant macrostructure flaws, such as sharp edged pores and incomplete cell walls. It is meaningful to reduce macrostructure defects or improve pores structure uniformity of metal foams from the view point of practical applications. In this work, different thickening agents were added into AZ31B magnesium composite foams to balance foaming process and improve the homogeneity of pore diameter, pore roundness and cell wall integrity. The results showed that pore diameter, roundness of the pores and integrity of cell walls were obviously influenced by thickening agents. Based on a large number of experimental results, calcium granules and magnesium cerium intermediate alloy were used to control the number and morphology of solid particles in magnesium melt, and a kind of magnesium composite foams with fine roundness pores and complete cell wall structure were obtained.

In our current work, AZ31 magnesium alloy foams with closed-cell were successfully fabricated by melt foaming method using Ca and CaCO 3 as thickening and blowing agent, respectively. The influences of porosity and pore size on the quasi-static compressive properties of the foams were systematically investigated. The results showed that the yield strength, energy absorption capacity and ideality energy absorption efficiency were decreased with the increase in porosity. However, specimens with porosities of 60%, 65% and 70% possessed similar total energy absorption capacity and ideality energy absorption efficiency. Meanwhile, experimental results showed that mean plateau strength of the foams was increased first and then decreased with increase in mean pore size. In addition, energy absorption capacities were almost the same in the initial stage, while the differences were obvious in the middle stage. From the engineering point of view, the specimens with mean pore size of 1.5 mm possess good combination of mean plateau strength and energy absorption characteristics under the present conditions.

Closed-cell aluminum foams with different manganese contents were successfully prepared by melt-foaming method. The distribution of Mn elements in the foams and the effects of Mn elements on the compressive properties of aluminum foams were investigated. The results showed that Mn elements were uniformly distributed in the forms of MnO 2 , Al 6 Mn intermetallics, Al–Mn solid solution and un-dissolved Mn particles in the cell wall matrix. And Mn elements had an important effect on the quasi-static compressive properties of aluminum foams. The Mn-containing foams possessed much higher micro hardness, compressive strength and energy absorption capacities than commercially pure aluminum foams. In addition, Mn content in the aluminum foams should be controlled in order to obtain good combination of compressive strength and ductile deformation behavior. And the reasons were discussed.

Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. Homogenizing heat treatment was applied on the foams and the effects of heat treatment on compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that homogenizing heat treatment enhanced the compressive properties in terms of yield strength, mean plateau strength, available energy absorption capacity and ideality energy absorption efficiency of the foams. In addition, homogenizing heat treatment greatly reduced the stress drop rates of the foams. Specimens homogenized at the temperature of 753 K for 24 h possessed good combination of yield strength, compressive stability, available energy absorption capacity and ideality energy absorption efficiency under the present experiment conditions. And the reasons were discussed.

Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. The effects of specimen aspect ratio (the thickness/width ratio, AR) on the compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that the length of stress–strain plateau stage extended and ideality energy absorption efficiency improved with the specimen AR increasing and the yield strength decreased. Specimens with the AR = 1.00 possess good combination of yield strength, plateau stage length and compressive stability when compressed under the experiment conditions.

Magnesium composite foams with 10 vol. % of hollow ceramic microspheres (CMs) were prepared by modified melt foaming method. Specimens with homogeneous pore structures were subjected to various heating temperature (150, 250, 320, 400, and 500 &deg, C, respectively) and enduring times (1, 2, 4, 6, and 24 h, respectively). Evolution of microstructure and mechanical properties of the samples, before and after the heating processes were examined by applying X-ray diffraction technique (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and quasi-static uniaxial compression test. The results showed that as heating temperature and enduring time increasing, &beta, Mg17Al12 phases gradually dissolved, resulting in a solid-solution strengthening effect. Meanwhile, internal stress relaxation in the matrix leads to the decrease of yield strength and micro hardness of the specimens. When compared with the unheated foams, the treated specimens possessed lower micro-hardness, yield strength, and energy absorption capacity due to the dissolution of &beta, Mg17Al12 phases and the release of internal stress. However, higher strain hardening exponents for almost all of the treated composite foams were observed and the reasons were discussed. It is proposed that more factors should be taken into account when using heated composite foams in practical applications.

Macrostructure has an important effect on the compressive properties of closed-cell aluminum foams. Meanwhile, the decomposition behavior of a foaming agent has a significant influence on the macrostructure of closed-cell aluminum foams. In order to get optimal compressive properties on aluminum foams, it is important to obtain the optimal decomposition behavior of a foaming agent. In this paper, different heat treatment temperatures and fixed heat treatment were employed to investigate the decomposition behavior of titanium hydride. For a more intuitive understanding of their decomposition characteristics of the pretreated titanium hydrides, closed-cell commercially pure Al foams were prepared by melt foaming method using different types of pretreated titanium hydrides as foaming agent. In addition, the macrostructures and quasi-static compressive properties were used to evaluate the pretreatment effect. The results showed that pretreatments have a significant influence on the macrostructure and compressive properties of aluminum foams. The decomposition characteristics of titanium hydride pretreated at 753 K for 30 min are most suitable for the preparation of closed-cell aluminum foams under present conditions, as the foams possess good combination of pore size distribution, yield strength and energy absorption capacity. DOI: http://dx.doi.org/10.5755/j01.ms.20.4.6082