To reduce the weight of car body, Al-Mg-Si-Cu alloys are becoming increasingly attractive as a candidate for material substitution used to produce the outer body panels of automobiles because of their favorable bake-hardening response. However, the formability still needs to be further improved compared to steels. In this work, the effect of the thermomechanical processing on the mechanical properties and microstructure of Al-Mg-Si-Cu alloy is studied through tensile test, OM, SEM and TEM observation, as well as EBSD characterization. The results reveal that there is almost no change in both strengths and strain-hardening exponent n of the sheets in T4P condition after different thermomechanical processing, but the average plasticity strain ratio r-, planar anisotropy ∆r and elongations in the three directions show obvious differences. The sheet undergone hot rolling, cold rolling, intermediate annealing, cold rolling and solution (processing Ⅱ) has a better formability (r-= 0.6187) and a weaker planar anisotropy than that subjected to hot rolling, intermediate annealing and then cold rolling before solution treatment (processing I). Although the particle stimulated nucleation (PSN) effect of processing I is remarkable during solution treatment, due to the appropriate controlling cold deformation and distribution of second-phase particles with different sizes in processing Ⅱ, most of the recrystallization grains are equiaxial and the recrystallization texture is only consisted of CubeND, Cube and H with a low intensity. At last, according to the relationship between the microstructure and the thermomechanical processing, the microstructure evolution model during different thermomechanical processes is established.
Amorphous alloy composite is designed to prevent rapid propagation of shear bands in amorphous phase by introducing the second crystalline phase, which can improve the plasticity of alloy. In situ formed amorphous alloy composites have attracted much interest due to excellent properties and extensive application prospect, especially the dendrite reinforced amorphous alloy composite with excellent tensile plasticity. Recent studies show that the plastic deformation behavior of amorphous alloy composite is not only related to the mechanical properties of the crystalline phase, such as elastic modulus, but also with the size, volume fraction and morphology of the crystalline phase. In addition, the mechanical properties, especially the plastic deformation ability, of amorphous alloys are closely related to topological morphology of the samples, such as aspect ratio. For the amorphous alloy composite, the relationship between mechanical properties and topological morphology of the samples are of interest. In this work, by adjusting preparation process and size of the samples, the effect of cooling rates and aspect ratios on the mechanical properties of Ti45.7Zr33Ni2.9Cu5.9Be12.5 amorphous alloy composites were systematically studied. As decreasing the cooling rate during the preparation process, the sizes of dendrites in the amorphous alloy composites increases. And the crystalline phase presents evolution from branchlets to coarse dendrite. As the cooling rate decreases, strength of the composite decreases while plasticity increases. Moreover, different from the previous reports, the mechanical properties of amorphous alloy composite are not sensitive to the aspect ratio. It is attributed to the existing of the dendrites phase and deformation-induced phase transformation in the dendrites, which may adjust stress distribution of the amorphous alloy composites during deformation process.
Mg alloy has hexagonal structure and exhibits poor workability at room temperature, which is attributed to the difficulty in activating a sufficient number of independent slips to accommodate the deformation. Twinning plays an important role in plastic deformation of Mg alloys during low and medium temperature to accommodate the imposed strain, especially the strain along the c-axis. Therefore, the twinning behavior of AZ31 Mg alloy during plane strain compression at room temperature was investigated with EBSD in this work. Rectangular specimens with a dimension of 10 mm in length, 9 mm in width and 7 mm in thickness were cut from a hot rolled plate. The results show that {101-2} twinning is dominant when the compression and constraint direction are parallel to transverse direction (TD) and rolling direction (RD) of the plate, respectively. The twinning variant selection mechanism is dominated by the Schmid factor (m) along compression direction, and also related to the constraint direction. The differences of twinning behavior can be interpreted by the twinning strain tensor. For the case when single twinning variant occurs within a grain, the average twinning strain tensor of twinning variant in constraint direction will result in spreading; while for the case there are two or more twinning variants taking place within a grain, the average twinning strain tensor of the variant with higher m will induce spreading in the constraint direction, and that with lower m results in size reducing in the constraint direction. During plane strain compression, different twinning variants coordinate with each other, twinning won't be suppressed until the micro-strain in the constraint direction reaches 0.
Annealed 7B04 Al sheets in thickness of 2 mm were subjected to friction stir welding (FSW) under three rotation rate and welding speed parameters of 1600 r/min, 200 mm/min; 800 r/min, 200 mm/min and 400 r/min, 400 mm/min, respectively. The effect of welding parameters on the tensile property and microstructure of the FSW joints were investigated, with more efforts focusing on the low-temperature superplasticity of the nugget zones (NZs). The results showed that FSW joints with high quality could be produced by controlling welding parameters, with a joint strength coefficient of 100% being obtained. Dynamic recrystallization took place in the NZs with fine and equiaxed grains generated. The grain size of the base material was about 300 μm, while it was significantly decreased in the NZs with decreasing the rotation rate: about 2, 1 and 0.6 μm for the above three samples, respectively. The fine grain structure of the NZs could facilitate their superplastic deformation. The NZs exhibited superplastic elongations ranged from 160% to 590% at 300 ℃ at strain rates of 1×10-3 and 3×10-4 s-1. The maximum superplasticity of 790% was obtained at 350 ℃ at the strain rate of 1×10-3 s-1. The ability to superplastic deformation disappeared in the NZs at 400 ℃.
Metals exhibit inhomogeneous deformation features under plastic strain, leading to the appearance and evolution of the deformation bands. The quantitative characterization of this effect is significant for an in-depth understanding of the plastic deformation and strengthening mechanisms of metals. In this work, the full-field strain information are obtained using digital image correlation method, and the work hardening behaviour in Ni single crystals is investigated from the angle of strain heterogeneity. First, a digital image correlation method, adapted for characterizing single crystal deformation, is proposed for the precise evaluation of strain field. The tensile test results show that the plastic strain in Ni single crystal manifests a distinct localization characteristic, which is closely linked to the slip band formation and development process. Based on the characteristics of the strain field evolution, 3 deformation regimes can be determined, which demonstrate a one-to-one correspondence to the 3 work hardening stages of the material. Some reasonable interpretations of their correlation are proposed within the framework of dislocation theories, which are verified through the experimental observations on the microstructure evolution of the material.
Alloy 825 is widely used for chemical and petrochemical applications due to its good combination of mechanical properties and corrosion resistance. However, intergranular corrosion (IGC) is one of the serious problems for alloy 825 exposed to aggressive environments, which could result in unexpected failures and lead to huge losses. The grain boundary structure, which can partly be described by coincidence site lattice (CSL) model, can influence the grain boundary chemistry and the susceptibility to intergranular corrosion. The field of grain boundary engineering (GBE) has developed a lot over the last two decades since the concept of grain boundary design was proposed. The aim of GBE is to enhance the grain-boundary-related properties of materials by increasing the frequency of low ΣCSL (Σ≤29) grain boundaries (GBs) and tailoring the grain boundary network. It was reported that in some fcc materials with low stacking fault energy, such as Ni-based alloys, lead alloys, austenitic stainless steels and copper alloys, the frequency of low ΣCSL GBs can be greatly increased by using proper thermomechanical processing (TMP), and as a result the grain boundary related properties were greatly enhanced. In this work, GBE is applied to the manufacture of Ni-based alloy 825 tubes by cold drawing using a draw-bench on a factory production line and the subsequent annealing. The effect of thermomechanical processing on the grain boundary character distribution (GBCD) of alloy 825 was studied by means of the EBSD technique and orientation image microcopy (OIM). The results show that the proportion of low ΣCSL grain boundaries increase to more than 75% by the TMP after 5% cold drawing and subsequent annealing at 1050 ℃ for 10 min, and simultaneously the large-size highly-twinned grain-cluster microstructure is formed. The size of the grain-cluster and proportion of low ΣCSL grain boundaries decrease with the increase of pre-strain. The proportion of low ΣCSL grain boundaries decreases with the increase of the mean grain size. The annealing temperatures in the range of 1050~1125 ℃ have no obvious effect on the GBCD of the specimen with 5% cold drawing deformation; while the proportions of low ΣCSL GBs of the sample with 3%, 7% and 10% cold drawing deformation decrease with the increase of annealing temperature.
Ni-based single crystal (SC) superalloys have been widely used to produce turbine blades of aeroengines, but under the action of centrifugal force, creep damage is still the main failure mode. In service, the blades experience multiple cycles of various conditions of high temperatures, low stresses and intermediate temperatures, high stresses, and due to effective and efficient means of cooling and insulating the blades during operation, the actual temperature the blades bear can be smaller than the working temperature at the hot ends of aeroengines, so the systematical study on the creep behavior of SC superalloys at intermediate temperatures, high stresses is significant. It is generally considered that dislocations cutting γ′ phase is the main deformation mechanism of SC alloys at intermediate temperatures, high stresses, and dislocations cutting into γ′ phase can be decomposed into different configurations for different alloy systems, even under similar conditions. Moreover, large amount of dislocations cutting into γ′ phase means the degradation of creep performance of the alloys, so it is significant to study the cutting modes of dislocations. In this work, by means of creep tests, TEM observations and diffraction contrast analysis of dislocations, the deformation mechanisms of a Ni-based SC superalloy during steady-state creep at intermediate temperatures, high stresses are studied. Results show that, under the conditions of 760 ℃, 760 MPa and 800 ℃, 650 MPa, dislocations cutting into γ′ phase are decomposed to form partial dislocations plus superlattice intrinsic stacking faults (SISF). Thereinto, the leading α/3<112> super Shockley partial dislocations cut into γ′ precipitates, while the dragging α/6<112> Shockley partial dislocations remain at γ′/γ interfaces, and between them there exists SISF. Additionally, super dislocations shearing into γ′ phase can cross slip from {111} to {100} crystal planes to form Kear-Wilsdorf (K-W) locks with non-plane dislocation core structure, which can inhibit the slip and cross slip of dislocations to enhance the creep strength of the alloy. At 850 ℃, 500 MPa, stacking faults disappear in the alloy, and some a<110> super dislocations cutting into γ′ rafts can be decomposed to form the configuration of two partial dislocations with Burgers vector of α/2<110> plus antiphase boundary (APB), and K-W locks are released for high-temperature thermal activation results in the cross slip of dislocations from cubic slip systems to octahedral ones.
Automobile beam steel with high strength is the development trend of the automotive industry. With the development of heavy-duty vehicles, low cost automobile beam steel both with high strength and high toughness need to be developed. V-N microalloying method combined with thermal mechanical controlled process has a significant grain refinement function. Therefore, the relationship of different N content and technology should be studied in detail. In this work, the microstructure and precipitates of V and V-N microalloyed steel were investigated by using OM, SEM and TEM. And their strengthening mechanism was studied. The results show that both V microalloyed steel and V-N microalloyed steel mainly consist of ferrite and little pearlite. With the increasing of coiling temperature, the strength increased first and then decreased. The optimum mechanical properties were obtained when coiling at 600 ℃, the yield strength, tensile strength and elongation reached 605 MPa, 687 MPa and 24.5%, respectively. Compared with V microalloyed steel, the ferrite in V-N microalloyed steel present finer grain size which can be refined to about 4.5 mm. The precipitates were finer and more dispersed which distribute mainly between 3~50 nm and have the average size of 8.0 nm. And the dislocation density in V-N microalloyed steel is higher. Ferrite grain refinement strengthening, precipitation strengthening and dislocation strengthening make V-N microalloyed steel possess higher yield strength. Ferrite grain refinement strengthening is the predominant mechanism and contributes 43.05% to the yield strength. And the contribution of precipitation strengthening and dislocation strengthening to the yield strength is up to 34.44%.
The use of high strength low alloy steels provides several potential advantages including lower weight, lower manufacturing costs, and ease of handling and transport. The progress in steel manufacturing technology has continually called for new developments in welding processes and consumables to produce weld metal deposits with mechanical properties essentially equivalent to the base metal. Controlling the weld metal microstructures as well as raising the welding productivity is critical factor for the development of weld metal of high strength steel to secure satisfactory mechanical properties and to reduce production costs. In order to meet the demand to apply 1000 MPa class steel to the fabrication of large scale steel structures, a weld wire for the 1000 MPa class steel has been under development to obtain the required strength and toughness, which depend primarily on the microstructure. In this work, the effects of shielding gas composition on the microstructure and properties of 1000 MPa grade deposited metals produced by metal active gas (MAG) welding have been investigated. The shielding gas employed was a mixture of argon (Ar) and carbon dioxide (CO2) (5%~30%), and the weld heat input was 13 kJ/cm. The properties of deposited metal with shielding gas of 80%Ar+20%CO2 is the best, the yield strength is 980 MPa, meanwhile, its Charpy absorbed energy at room temperature and -40 ℃ are 72.6 and 52 J, respectively. The results show that the microstructure of the deposited metal, consisting primary of low carbon martensite and a few parallel bainite plate, became more interweaved bainitic packets as the CO2 content of the shielding gas was increased. The initial bainite nucleated at austenite grain boundaries and subsequent bainite plates can form at the oxide inclusions of intragranular, which presented an intersected configuration and the microstructure was refined. The content of bainite palte and distribution morphology of martensite/bainite is the intrinsic reason attributed to mechanical properties of deposited metals. The content of bainite for deposited metal has an optimal proportion and more isn't necessarily better. It was also found that the area fraction, the size and the compositions of oxide inclusions in deposited metals were changed with increasing CO2 content. The deposited metal as using 70%Ar+30%CO2 has the minimum toughness because more large size oxide inclusions formed which are known to be harmful to the toughness.
Low-temperature surface carburization has proven to be one of the most effective techniques for improving the mechanical properties of 316-type austenitic stainless steel (Fe-Cr-Ni alloy), including surface hardness, fatigue resistance and wear resistance. It is well known that carbon diffusion in austenitic stainless steel is a very complicated process and still not fully understood. So it is of great importance to figure out the carbon diffusion mechanism in steel and establish a model that can predict the carbon concentration along the depth direction in any given carburization conditions. Studies in recent years reveal that trapping effect should be considered in carbon diffusion in austenitic steels at low temperature. In this work, low-temperature surface carburization treatment was carried out with 316L austenitic stainless steel, and the carbon concentration along the depth direction was measured. A kinetic model based on the "trapping-detrapping" mass transport mechanism for simulating the carbon fraction-depth profile was developed. This model considered that the diffusion of carbon under the influence of trap sites formed by local chromium atoms. Then the calculated carbon concentration was compared to the experimental results in order to check the validity of the model. The results show as follow: (1) in low-temperature-carburized 316L austenitic stainless steel, the carbon fraction-depth profile exhibits plateau-type shape which is not consistent with the standard analytic solution of the diffusion equation (Fick's law of diffusion); (2) carbon fraction-depth profile based on "trapping-detrapping" model is in good agreement with experimental carbon fraction-depth profile, which indicates the trapping effect plays an important role in carbon diffusion; (3) carbon diffusivity decreases by the trapping effect of Cr atoms, and the detrapping energy of carbon deduced from fitting experimental data is 165 kJ/mol; (4) the proposed model can only be used to describe the carbon diffusion in austenitic stainless steel during low-temperature surface carburization without chromium carbide precipitation. In addition, the influence of stresses induced by incorporating the carbon into austenite lattice on the carbon transport mechanism is not included in the trapping-detrapping model.
Twinning induced plasticity (TWIP) steel exhibits high strength and exceptional plasticity due to the formation of extensive twin under mechanical load and its ultimate tensile strength and elongation to failure ductility-value can be as high as 5×104 MPa%, which provide a new choice for automobile in developing the lightweight and improving safety. Generally, due to the texture was formed during process of plastic deformation, metal material appear anisotropic behavior. The deformation mechanisms, responsible for this high strain hardening, are related to the strain-induced microstructural changes, which was dominated by slip and twinning. Different deformation mechanisms, which can be activated at different stages of deformation, will strongly influence the stress strain response and the evolution of the microstructure. In this work, to predict the texture evolution under different loading conditions and understand these two deformation mechanisms of plastic deformation process, a polycrystal plasticity constitutive model of TWIP steel coupling slip and twinning was developed based on the crystal plasticity theory and single crystal plasticity constitutive model. A polycrystal homogenization method to keep geometry coordination and stress balance adjacent grains was used, which connected the state variables of single crystal and polycrystal. And then the model was implemented and programed based on the ABAQUS/UMAT platform. The texture evolution was obtained by EBSD at strain 0.27 and 0.60, respectively. The finite element models of tensile, compression and torsion processes were built by using the constitutive model. The mechanical response and texture evolution during plastic deformation process of TWIP steel were analyzed. The results show that with the increasing of the strain, the strain hardening phenomenon and texture density enhanced during the tensile process. Although texture types changed, texture density unchanged during the compression process. Owing to deformation increasing along the diameter direction, there is no obvious texture inside the cylinder when torsion deformation is small, texture emerged and enhanced gradually with the increasing of strain.
Because of excellent corrosion resistance, good toughness and machinability, austenitic stainless steels are widely used in many industries. In order to improve the corrosion resistance, the carbon content of austenitic stainless steel is ultra-low, resulting in low surface hardness, poor wear and fatigue resistance properties which limit its application. Low temperature colossal carburization (LTCC) is a kind of novel surface strengthening technology for significantly increasing the surface hardness of austenitic stainless steels, while keeping their original excellent corrosion resistance because of no formation of carbides. The wear, fatigue and corrosion resistance of austenitic stainless steel of low temperature carburized layer have been investigated in recent years. However, the researches on key technical parameters, especially the carburizing atmosphere and the alloying element, have been rarely reported due to intellectual property protection limits. In this work, OM, EPMA, XRD and IXRD are used to investigate the effects of CO concentration on the microstructure, carbon concentration distribution, phase constitution and residual stress of the carburized layer on 316L austenitic stainless steel surface. Based on thermodynamic theory, the model of carbon transfer and diffusion was also built by software DICTRA to calculate the distribution of carbon concentration and activity of low temperature carburized layer. The results reveal that S phase is detected on 316L austenitic stainless steel surface treated by LTCC, and the compressive residual stress is formed at the same time. The increment of CO concentration can significantly increase the carbon concentration of carburized layer, which improve the hardness and compressive residual stress. The simulated carbon concentration and activity distributions are in accordance with the experimental results when the carbon concentration is lower, but when the carbon concentration is higher, the simulated carbon concentration is lower than experimental results due to the decrease of trapping sites and high compressive residual stress.
Magnetron sputtering ion plating (MAIP) is limited by the low density and low ionization of target atoms, which results in that the films deposited by MAIP have poor compactness, low adhesion and the quick decreasing in thickness along the target-to-substrate distance, so this disadvantages of the film quality and property can not satisfy the harsh need of modern society. Based on the physical gas discharging plasma theory, the gas discharge could be introduced into the glow-arc discharge section between the glow discharge and the arc discharge by increasing the target current density. By means of the collision kinetic energy of Ar+ and the Joule heating effect of electrons, the electrons and atoms could be spontaneously induced to emit by overcoming the surface work function. Thus the deposited particles with a high density, a high energy and a high ionization can be obtained. Two groups of the Ti films were deposited in glow discharge and glow-arc discharge sections respectively. The film thickness at different target-to-substrate distances was measured by the CLSM. The microstructure of films was characterized by XRD, SEM, AFM and TEM. The adhesion between the film and substrate was determined by the microscratch tester. The results showed that the Ti film deposited in the glow-arc section of gas discharge had nanocrystal size, dense structure, uniform thickness, high deposition rate and excellent adhesion.
Ti and Ti alloys with low elastic modulus, good mechanical properties and biocompatibility have been widely used for dental implant, arthroplasty and internal fixation material in spinal fusion. But the poor wear resistance of Ti and Ti alloys generally results in the aseptic loosening of the implants. TiN coating has good chemical stability and biocompatibility in physiological environment and plays an important role in improving the corrosion wear performance of Ti and Ti alloys. However, the adhesion strength of TiN film prepared by traditional technologies does not meet the requirement of long service life of the implants. In this work, the alternating Ti/TiN multilayer films were prepared by magnetron sputtering technology with constant Ti layer thickness and varying TiN layer thickness. The cycling periods were designed to be 1, 3, 6, 9, and 12. The total depositing time was 185 min. The main aims of this investigation were to clarify the effects of the cycling periods on the surface morphologies, hardness, bonding strength, friction and abrasion behavior in simulated body fluid of Ti/TiN multilayer films. The results show that the total thickness of Ti/TiN multilayer film is in the range of 5.5~6.0 mm. (111)TiN preferred orientation is found in TiN monolayer film, and (002)TiN preferred orientation is found in Ti/TiN multilayer films. In comparison with TiN monolayer film, Ti/TiN multilayer films exhibit lower surface roughness, higher hardness, bonding strength and wear resistance. The strengthening and toughening of Ti/TiN multilayer films result from the refinement of columnar crystals and interface coherent effect between Ti and TiN layer. The increase of cycling period decreases the hardness of Ti/TiN multilayer film, but is beneficial to enhancing the bonding strength to the substrate. The rupture and exfoliation of thin TiN layer at outer surface promote the abrasive wear and oxidation wear. At the condition of layer thickness ratio 30 for TiN and Ti and 3 cyc, the Ti/TiN multilayer film has good combined mechanical properties. Hardness is 15.8 GPa, adhesion strength is 50 N, coefficient of friction is 0.35, and volume wear rate in Hank's solution is less than 4.0×10-6 mm3/ (Nm).
With the development of hydrogen economy, the demand of structural materials with high strength suitable for service in hydrogen or hydrogen-bearing environments such as storage of hydrogen gas was incremental. An optional structural materials is J75 alloy, which is mainly strengthened by an ordered fcc γ' phase, Ni3(Al, Ti), coherent with the austenite matrix. Investigation on J75 alloy indicated that the commercial alloy free of B would lose about half its ductility when charged with hydrogen, accompanied by a change of fracture mode from ductile rupture to brittle-appearing intergranular fracture. Otherwise, an improvement in ductility and hydrogen resistant performance was observed in the J75 alloy with trace B, however, its role in the alloy is unclear. So, in present work, mechanism of B in the J75 hydrogen-resistant alloy was investigated by means of OM, SEM, TEM, EPMA, 3DAP, SIMS, hydrogen penetration, thermal hydrogen charging experiments and tensile tests. It was found that a lot of Ti segregated at grain boundaries (GBs) in the alloy free of B, resulted in abundant precipitation of cellular η phases. However, the cellular η phase was not observed in the alloy with B, and it could be attributed to the segregation of B atoms at GBs and inhibited the segregation of Ti. A lower hydrogen diffusion coefficient was observed in the alloy with B than that in the alloy free of B by hydrogen permeation, indicating that diffusion velocity of H atoms in the alloy had been decreased by the addition of B. Moreover, segregation of B at GBs could not only inhibit the precipitation of η phases but also decrease the number of H atoms there, which would improve the hydrogen-resistant performance of the alloy.
Zirconium alloys with low alloying content are mainly used in the nuclear industry as structural materials because of their superior properties in terms of thermal neutron transparency, mechanical strength and corrosion resistance. They are used for fuel cladding tubes and channels. The reaction between zirconium and water at high temperature forms oxide film on the surfaces. In order to further improve the corrosion resistance of Zr-based cladding tubes, research has continued on developing new zirconium alloys. The corrosion resistance of Zr-0.72Sn-0.32Fe-0.14Cr-xNb alloys (x=0, 0.12, 0.28, 0.48, 0.97, mass fraction, %) was investigated in a superheated steam at 500 ℃ and 10.3 MPa by autoclave tests. All the plate specimens of zirconium alloys with thickness of 2.8 mm have a similar texture. The microstructure of alloys and oxide films on the corroded specimens were observed by TEM and SEM. The results showed that no nodular corrosion appeared on these alloys for 500 h exposure. The thickness of oxide layers developed on the rolling surface (SN), the surface perpendicular to the rolling direction (SR) and the surface perpendicular to the transversal direction (ST) after 500 h exposure was close to each other. There was no anisotropic corrosion resistance for these alloys. The corrosion rate of the alloys increased with the increase of Nb content after 250 h exposure when the Nb content exceeded 0.28%. In the alloy with low Nb content, the fcc-Zr(Fe, Cr)2 or fcc-Zr(Fe, Cr, Nb)2 precipitate was mainly formed, while the hcp-Zr(Fe, Cr, Nb)2 precipitate was frequently observed in the alloy with high Nb content. The corrosion resistance of Zr-0.72Sn-0.32Fe-0.14Cr-xNb alloys was improved by decreasing the Nb/Fe ratio. From a point of view for the improving corrosion resistance, the addition of Nb no more than 0.3% is recommended.
