In order to comply with more stringent emissions and fuel economy regulations worldwide, the operation temperature of exhaust components for automotive gasoline engines is now reaching to as high as 1000 ℃, about 200 ℃ higher than the conventional standard. As a result, the incumbent materials for exhaust manifolds and turbine housings are being pushed beyond their high-temperature strength and oxidation limitations. Therefore, there is an urgent demand from automotive industries to develop novel and cost-effective alloys those durable against these increased temperatures. In this work, the effect of W additions on the creep behavior of a series of Nb-bearing austenitic heat-resistant cast steels is investigated at 1000 ℃ and 50 MPa. Microstructures before and after creep rupture tests are carefully characterized to investigate the microstructural evolution during creep deformation. The minimum creep rate of these alloys shows a trend from decline to rise as the W addition is increased. Microstructural analyses reveal that the W addition does not affect the formation of primary Nb(C, N), whereas significantly improves the precipitation of Cr-rich carbides, as well as accelerating the phase transformation from (Cr, Fe, W)7C3 to (Cr, Fe, W)23C6. Moreover, the excessive addition of W leads to the formation of the interme tallic χ-phase. During creep deformation, the secondary precipitation of nano-scale Nb(C, N) also aids in the strengthening of the creep resistance through pinning the dislocations. However, the cellular Cr-rich phase that contains χ-phase significantly accelerates the nucleation and propagation of creep cracks, thereby increasing the creep rate and decreasing the creep life.
TC4 titanium alloy is highly promising for aerospace and medical implant applications due to its low density, high strength, corrosion resistance and biocompatibility, and the ultra-fine grains of TC4 alloy by accumulative roll bonding (ARB) can efficiently improve the low temperature super-plasticity and biocompatibility for its widespread applications. However, the ARB process for TC4 alloy has been limited due to the high deformation resistance and low anti-oxidant ability. In this work, ARB was conducted for the ultra-fine grains of TC4 titanium alloy, and the effects of ARB temperatures and layer numbers on the bonding interface and microstructure were investigated as well as the deformation mechanism of the mixed α /β phase structure, and the influences of ARB processing on the mechanical properties were studied. The good interface bonding could be fabricated by the proper ARB temperature (near 720 ℃), the anti-oxidation treatment and the multilayer with the high deformation, which always takes on the hardened interface with the high oxidation contents, and the interface bonding strength increases with the increase of the ARB layers and temperature through the process of the diffusion and the necking fracture. The deformation process is composed by the cooperation deformation of α /β structure and the shear deformation during ARB processed TC4 alloy, during which the β phase at the grain boundary changes from the long strips to the short bands to deform with hcp α phase, while the shear bands with severe local-deformation is used to adapt the severe plastic deformation. The deformed microstructure is composed of the equiaxed structure (about 300 nm spacing) and the elongated deformation structure (about 400 nm spacing), in which the equiaxed structure comes from the function of the deformation temperature, local shear deformation and the local overheat. Additionally, the inhomogeneous microstructure and properties along the thickness direction can be observed, and the high hardness can be obtained at the bonding interface, which gradually distributes homogeneous with the increase of ARB layers. The strength of ARB processed TC4 sheets increases with the increase of ARB layers, which can get to 1325 MPa after 16 ARB layers, and simultaneously the plasticity decreases to 5.4%. The ductile fracture can be observed with the low ARB layers, while the mixed structure of the quasi-cleavage and ductile fracture is obtained with the increase of ARB layers.
Titanium alloys have been widely used in bearing force components in aeronautical structures, such as blades and beams to withstand the high frequency dynamic loads, which requires an outstanding fatigue resistance performance in very high cycle regime during their service life. In this work, very high cycle fatigue failure property of TC17 alloy used as aircraft engine blade material was studied by ultrasonic fatigue test and electromagnetic resonance fatigue test under 110 Hz and 20 kHz sinusoidal load, and crack initiation mechanism of different failure mode was analyzed. The results showed that, fatigue failure modes of TC17 alloy could be divided into surface induced failure and interior induced failure. Surface induced failure was caused by the machine defect and surface slide trace that triggered by the asymmetric loading. Interior induced failure was caused by slid fracture of primary α phase under asymmetric loading. Fatigue resistance of TC17 alloy was influenced by the fatigue crack initiation mechanism but concerned little about the loading frequency. The variation of the fatigue failure mechanism resulted in the S-N curves presenting bilinear. A fatigue strength predicted model is established based on the parameter of the weak crystal orientation area, which is in good agreement with the fatigue test result.
TiAl alloys are highly promising for high temperature structural application due to their excellent mechanical properties. However, the widespread applications of TiAl alloys have been limited for their low temperature brittleness and poor workability. The further thermo-mechanical treatments is applied for fine microstructures and improved ductility to promote the commercial applications, during which the investigations of hot deformation behavior and microstructural evolution are necessary for the improved microstructure and mechanical properties. The canned forging and subsequent heat treatments of Ti-43Al-4Nb-1.5Mo alloy have been conducted, during which the hot deformation behavior, flow softening mechanism, microstructure evolution and mechanical properties were investigated. The results show that the flow softening process of the canned forging TiAl alloy can be attributed to the soft β phase, α2/γ lamellae decomposition and the dynamic recrystallization induced by dislocation slipping and twinning in γ phase, and the final microstructure is composed of remnant α2/γ lamellae and equiaxed α2, γ and B2 phases. With the increasing heat treatment temperature, the microstructure changes from the multi-phase structure (remnant α2/γ lamellar, equiaxed α2, γ and B2 phases) at 1250 ℃ to the α2/γ lamellar and γ phase at 1285 ℃, and then the fully α2/γ lamellar structure at 1300 ℃, during which the B2 phase is gradually dissolved due to the solution diffusion, and the remnant α2/γ lamellae change to equiaxed α2/γ colonies according to the α2/γ→γ+α2+B2 transition, and the final fully α2/γ lamellar structure is promoted by γ→α transition at high temperature. Moreover, the tensile tests of the hot isostatic pressed (HIPed) samples, canned forged and heat treated samples at 800 ℃ are conducted, in which the fully lamellar structure shows the high properties with the ultimate strength of 663 MPa and the elongation of 26%. The deformation process of the fully α2/γ lamellar can be strengthened by the lamellae twisting, microvoid inhibition and wavy growth of the cracks, leading to the optimal high temperature performance. Moreover, the disordered bcc β phase can promote the deformation during the hot working process at the high temperature (≥1200 ℃), while the hard-brittle B2 phase severely deteriorates the service properties, which should be controlled accurately for the high mechanical properties during the thermo-mechanical processing.
The titanium alloy parts, which have been formed by traditional laser additive manufacturing (LAM) method, usually have obviously different microstructure from wrought microstructure of titanium alloy and show room temperature mechanical anisotropy. In order to make the LAMed titanium alloy parts get the same microstructure and mechanical properties as wrought titanium alloy, a new technology of LAM called consecutive point-mode forging and laser rapid forming (CPF-LRF) has been proposed. During CPF-LRF process, deposited TC11 titanium alloy by laser rapid forming (LRF) was deformed by consecutive point-mode forging (CPF), and then on the surface of the deformed TC11 titanium alloy, new LRF process started over again. Both LRF and CPF were performed alternatively throughout the process of the fabrication of a TC11 titanium alloy part. Microstructures and mechanical properties of the CPF-LRFed TC11 alloy sample have been investigated. The average grain size of equiaxed grains of the CPF-LRFed TC11 alloy sample is 48.7 μm. The equiaxed grains have continuous grain boundary α phase. The microstructure of the equiaxed grain is bimodal microstructure consisting of primary α phase lath and transformed β. During CPF-LRF process, being plastically deformed by CPF, the surface deformation zone of the thick-wall TC11 titanium alloy part is 1.5 mm depth and its deformation degree is 20%. During a new layer deposited on the surface of the CPF cold deformed TC11 titanium alloy part, when laser beam scans through, about 1 mm thick (four layers) cold deformed titanium alloy in the heat affected zone of laser melting pool is heated up above β-transus temperature of TC11 titanium alloy in which static recrystallization complete within time interval of 0.86 s. The mechanical properties indicate that compared with the tensile properties at room temperature of TC11 alloy forged piece, the CPF-LRFed TC11 alloy has higher strength and less ductility. Fracture analysis indicates that intergranular fracture is mainly responsible for the poor ductility of CPF-LRFed TC11 alloy.
The high strength or flow stress as well as low plastic deformability of 7000 series Al alloys makes it difficult to improve their microstructures and mechanical properties by cold processing, and many advanced alloying methods and processing technologies are continually developed for higher mechanical properties and acceptable elongation. In this work, the cryogenic deformation (rolling) was applied to process high-strength 7050 Al alloys, and its effects on the microstructures and mechanical properties were studied. The results showed that after the pre-cooling with liquid nitrogen, the quenched 7050 Al alloy can obtain much higher rolling reduction, similar to that under warm or hot rolling, and a great number of substructures and high-density dislocations were formed which greatly increased the strength. The higher cryogenic deformability would be mainly related with the higher work-hardening ability at low temperature, while the strength enhancement would be largely attributed to the solution strengthening and dislocation strengthening. The cryogenic deformation can obviously stimulate the ageing process of the quenched 7050 Al alloy, but the direct ageing of the cryogenic-rolling 7050 Al alloy can assure higher strength and acceptable elongation, which would be greatly attributed to the precipitation strengthening and dislocation strengthening, while the recovery and ageing-induced precipitates help improving the tensile elongation. During room-temperature rolling, the formation of GP zones and η′ phases caused by the heats transformed from the deformation as well as their interaction with dislocations leads to the appearance of amounts of shear bands (instability areas), which will easily cause the cracking or edge-cracking of the rolling sheets. However, the cryogenic rolling with distinctly impeding the solute diffusion can result in the suppression of precipitation of the strengthening phases so as to decrease the occurrence of the shear instability areas, and uniform and stable plastic deformation or good work-hardening as well as high-quality rolling sheets are obtained. The excellent plastic deformability of high-strength Al alloys at cryogenic temperatures could be suggested as an effective way to improve the processing of high-strength Al alloys.
Grain refinement may not only promote the formation of a fine quiaxed grain structure, which endows the Al alloy castings with good mechanical properties, but also cause a reduction in the casting defects, such as segregation and hot tearing, which has a dominating effect on the processability of Al alloys. It is, thus, essential for both the cast and wrought Al alloys. Although many techniques, e.g. mechanical vibration, electromagnetic stirring, ultrasound vibration, etc. may be used for the grain refinement nowadays, inoculation remains the most widely applied method in the industrial production due to its simplicity and high efficiency. For most Al alloys, Al-Ti-B master alloy is used as the grain refiner. Much work has been done to investigate the solidification behaviors of the Al alloys inoculated with Al-Ti-B master alloys since the 1970 s. Models were developed to describe the microstructure formation under the effect of inoculants. These researches clearly demonstrate that the grain refining efficiency or the heterogeneous nucleation rate is closely related to the concentration of solute Ti as well as the number density and size distribution of TiB2 particles in the melt. One shortcoming of the previous research work in this field is that the kinetic behaviors of TiB2 particles during the heating or cooling processes of the melt, i.e. dissolution/growth, coarsening and precipitation of TiB2 particles, are neglected. Generally the size distribution of TiB2 particles in the Al-Ti-B master alloy was used in the modeling and simulation of the solidification of Al alloys. In this work, solidification experiments were carried out to investigate the kinetic behaviors of TiB2 particles in the melt and the effect of solute Ti. A model was developed to describe the kinetic behaviors of TiB2 particles during the whole process from the beginning of the addition of TiB2 particles to the melt until the solidification of the melt. Calculations were carried out according to the experiments conditions. The results demonstrate that TiB2 particles may dissolve and coarsen during the holding temperature period, and grow during the cooling period of the melt. The kinetic behaviors of TiB2 particles have an obvious effect on the grain refining efficiency of the master alloys. The addition of solute Ti can significantly suppress the growth/dissolution, the Ostwald ripening of TiB2 particles and thus affects the grain refining efficiency of the master alloy.
Due to the increasing demands for lightweight parts in various fields, such as bicycle, automotive, aircraft and aerospace industries, hydroforming processes have become popular in recent years. Since tubular materials during tube hydroforming are under a bi-axial even tri-axial stress state, which is different from that in the tensile test, it is necessary to test the mechanical properties of the material under bi-axial stress state. Tube bulging test is an advanced method for characterizing the mechanical properties of tubular materials under bi-axial stress state. But there are excessive physical quantities in the theoretical model of tube bulging test for testing the mechanical properties of tubes under bi-axial stress state which are difficult to be obtained during the experiment. In order to solve the problems, a method for directly testing the mechanical properties of tubes under bi-axial stress state was proposed in this work, which will be referred to as "one point method". Because of circular model is characterized by a dominant function expression, theoretical models of both the pole axial curvature radius and the pole thickness during bulging test are derived under supposing the geometrical models for bulging zone as circular. Thus, the mechanical properties of tubes under bi-axial stress state can be obtained only through measuring the bulging height at the pole point during the bulging test, which laid the foundation for the establishment of a simple and reliable method for testing the mechanical properties of the tube online. Based on the above proposed method, the extruded aluminum alloy tubes AA6061 were tested. The results showed that both the pole axial curvature radius and the pole thickness during bulging test can be expressed as display functions pertaining to the bulging height at the pole point. For the theoretical model of the pole axial curvature radius, as the bulging rate increases, the prediction accuracy increases at beginning, and decreases at the end when using circular as the theoretical geometrical models for bulging zone. The prediction accuracy is the highest as the bulging rate is about 13%, the prediction accuracy decreases after the bulging rate is more than 20%. Fortunately, the overall prediction error is small. The maximum error does not exceed ±0.9%. The prediction accuracy of the pole thickness using the theoretical model is almost unaffected by the specimen geometry. When the ratios of length to diameter and diameter to thickness change, the difference is very small, the prediction error is not more than 0.8%. This is very helpful to ensure the accuracy of mechanical testing under bi-axial loading conditions. Using the "one point method", the stress and strain components along the circumferential and axial directions can be simultaneously measured, this laid the foundation for further analysis of the anisotropic property impacting on the flow and subsequent yield under complex stress state.
Al-7Si-Mg alloy castings have extensive applications in automotive industries, and the tensile properties of these alloys including yield strength, ultimate tensile strength and elongation are commonly used to judge their mechanical properties. In this work, the modified precipitation kinetics model, yield strength model and strain hardening model have been proposed to predict the tensile properties of Al-7Si-Mg alloys. The precipitation kinetics model can be used to predict the precipitate microstructure parameters including the precipitate density, size, size distribution, volume fraction, and composition and so on in these alloys, combining which with the strength model, their yield strengths can be obtained. The strain hardening model can be applied to simulate the stress-strain curves during tensile process, and the ultimate tensile strengths and elongations can be obtained by combining this model with the experimental data fitted with the expression (σUTS-σY)=mσY+n+f (Tss). First, the evolution of precipitate microstructure parameters and yield strengths as a function of ageing time were simulated, and then their comparisons with the experimental results were performed. The possible reasons resulting in the deviations between simulated and experimental yield strengths were analyzed. The stress-strain curves during tensile process of Al-7Si-0.36Mg alloy were simulated using strain hardening model, and the influences of ageing treatment and as-cast microstructure refining scale on the parameters of dislocation storage rate, dynamic recovery rate and the stress-strain curves were analyzed. Then, the ultimate tensile strengths and elongations of Al-7Si-0.4Mg alloy aged at different temperatures were predicted which are in better agreement with the experimental results, and the influence of secondary dendrite arm spacing on tensile properties was also analyzed. Finally, the limitation of present model and the factors influencing the prediction precision of tensile properties were outlined.
Effective grain size prediction for aluminum alloy die castings is of great significance to the rational formulation of die casting process parameters and to the improvement of casting mechanical properties. The traditional grain size prediction method cannot give consideration to both the efficiency and accuracy because of its inherent defects. To improve the efficiency and accuracy of predicting grain size for aluminum alloy die castings, this study proposes a prediction method that is based on the genetic algorithm-extreme learning machine (GA-ELM) model. ELM has the characteristics of few parameter settings, fast learning and good generalization performance, but the algorithm randomly generates the initial input layer weight matrix and the hidden layer threshold matrix, which greatly affects the prediction result. By exploiting GA's excellent global optimization ability, the optimal initial input layer weight matrix and the hidden layer threshold matrix for ELM can be found. The establishment of GA-ELM model can considerably improve the prediction accuracy of ELM model. This study uses grain size as the output parameters and relevant die casting process parameters as the input parameters. The castings produced under different die-casting process parameters are obtained experimentally, and the microstructures of specified sections of key casting positions are analyzed and measured to obtain the average grain size of the sec tions, i.e. the output parameters. The GA-ELM model is trained and tested using these data. To verify the superiority of the GA-ELM model in grain size prediction, this study compares the prediction results of GA-ELM model with the GA-BP neural network model and the original ELM model, and eventually verifies the reliability of GA-ELM model prediction results through metallographic structure measurement experiment. The results show that the GA-ELM model has higher prediction accuracy than the GA-BP neural network model and the original ELM model. Besides, its prediction efficiency is higher than the GA-BP model, while is lower than the original ELM model. With fairly high prediction accuracy and efficiency, the GA-ELM model can meet the actual engineering requirements. Furthermore, its prediction reliability and excellent prediction effect are verified by the results of metallographic structure measurement experiment.
As one of the lightest metal materials in current industrial applications, Mg alloys are being widely used in automotive, aircraft, aerospace and biomedical industries because of their super high strength-to-weight ratio and biodegradability. However, their limited ductility and workability at room temperature have become a bottleneck for many applications. Therefore, it has become critically important to obtain the Mg alloys with improved strength and ductility. On the other hand, Zn is a transition metal element, often applied to improve the mechanical properties. Also it has basic safety for biomedical applications. So the Mg-Zn alloys have attracted considerable attentions in recent years. Extensively investigated experiments indicated that the hardness of Mg-Zn alloys increases with increasing Zn content. However, there are only a few reported works about their mechanical properties and theoretically thermodynamic properties of Mg-Zn alloys. In this work, first-principles investigations have been performed on lattice parameters, elastic properties and thermodynamic properties of hcp Mg and eight kinds of Mg1-xZnx alloys with different contents of Zn less than 2% (atomic fraction), using the virtual crystal approximation in the frame of the density functional theory and the density functional perturbation theory. The elastic constants of Mg and Mg1-xZnx alloys with different Zn contents have been investigated by using optimized lattice, and their Young's moduli, Poisson ratios and elastic anisotropies have been analyzed in detail. Also, the thermodynamic properties, including Helmholtz free energies, internal energies, entropy and constant volume heat capacities of these alloys as a function of temperature were discussed. The results show that with increasing Zn content in Mg1-xZnx alloys, the lattice constants a and c, the entropy and constant volume heat capacity of Mg1-xZnx alloy decrease, while the elastic constants, Helmholtz free energy and internal energy of Mg1-xZnx alloy increase correspondingly. On the other hand, further discussions find that the effects of Zn content on free energy and entropy of Mg1-xZnx alloy are enhanced and the effect on heat capacity of each alloy at constant volume first increases, then decreases as the temperature rises. In summary, it can be given the conclusions that the high content of Zn in Mg1-xZnx alloy is beneficial to increasing the hardness and ductility of such Mg1-xZnx alloy, but decreasing its isotropy.
Investment casting is widely used in producting complex thin-wall titanium alloy components. In this process, the β→α phase transformation decides the final microstructures of these components. However most of present studies on phase transformation of titanium alloys focus on the microstructure evolution in heat treatment process or after deformation rather than in casting process now. It is a main reason only this work aims at the solid phase transformation of Ti-6Al-4V alloy in investment casting. In this work, the growth model of edge of α phase plates based on multi component Zener-Hiller model, and the growth model of broad face of α phase plates based on diffusion and conservation of multi components were established. The growth competition of different colonies, which consist of α phase plates in same orientation, was simulated and the microstructures and their evolution with temperature were obtained. The comparison between simulated microstructures and their evolution with temperature and experimental data indicated that the proportion of undercooling degree caused by impurities in the alloy is about 45% of the total undercooling degree in broad face of α phase plates and a much smaller portion in edge of α phase plates. The comparison also showed that the enthalpy change of solid phase transformation of titanium alloy is about 70 kJ/kg. The simulated and experimental morphologies look like similar and the simulated growth rate is also in good accordance with experiment inferred growth rate.