The NiAl-Cr(Mo) eutectic alloy has better fracture toughness and high temperature strength among NiAl alloys. Hf addition can raise the high temperature strength of NiAl-Cr(Mo) eutectic alloy further, but decreases its room temperature compressive properties obviously, which is caused by the microsegregation of Heusler phases. The NiAl-Cr(Mo)-0.5Hf alloy was rapidly solidified and then hot isostatic pressed (HIP) or high temperature treated (HTT) in order to improve the structure and property of this eutectic alloy. The results reveal that rapid solidification can well refine the microstructure of eutectic alloy and keep more amount of Hf solid solution phase in the alloy. Simultaneously, the shapes and distributions of the Heusler (Ni2AlHf) phases and Hf solid solution phases are well improved. After the HIP treatment, more Heusler phases are transformed into Hf solid solution phases, and the distributions of Heusler phases and Hf solid solution phases become homogeneous, and moreover the primary NiAl phases become coarsening obviously. After HTT, the amount of primary NiAl phases decreases a little, the Heusler phases and Hf solid solution phases become finer. The rapid solidification improves the room temperature compressive properties of the alloy significantly, the HIP and HTT improve its high temperature compressive properties further.
Nb--16Si refractory alloy was prepared by mechanical milling and hot--press sintering from high--purity Nb and Si powders. The milling process was carried out in a planetary ball mill for 24 h. The milled powders were consolidated by hot pressing in the argon atmosphere at 30 MPa and 1500 ℃ for 1 h. The powders ball--milled and material hot--pressed were characterized by XRD and SEM. The size of milled particles was refined and the Si atoms were dissolved into the Nb lattice to form interstitial solid solution. The results reveal that Nb--16Si refractory alloy consists of Nb solid solution (Nbss), Nb5Si3, Nb3Si and another Nb solid solution (NbssI) with high Si content. The average grain size is about 2 μm and the grains are nearly equiaxed. The predominant fracture mode is transgranular fracture with river patterns in Nbss and relatively flat cleavage planes in silicides. Nano-hardness values of Nb5Si3, Nb3Si and Nbss determined by nano--indentation are 13.9, 12.7 and 4 GPa, respectively. The fracture toughness of the alloy reaches 10.98 MPa?m1/2, indicating ductile phase toughening plays a positive role in improving the fracture toughness. A model of trust chamber was fabricated by sinter--forging and its microstructure is similar to the hot--pressed material.
Nb-silicide-based ultrahigh temperature alloys have attracted considerable attentions as potential high temperature structural materials because of their high melting point, suitable density, good elevated temperature creep strength and acceptable room temperature fracture toughness. However, the shortcoming in both high temperature strength and high temperature oxidation resistance retarded their practical applications. Directional solidification and alloying can be used in overcoming these deficiencies at certain degree. In this paper, the alloy with the composition of Nb-22Ti-16Si-6Cr-4Hf-3Al-3Mo-2B-0.06Y (atomic fraction, %) was designed and the master alloy ingot was prepared by firstly vacuum non-consumable arc melting and then vacuum consumable arc melting. The integrally directional solidification of this alloy was conducted with the use of special ceramic crucibles in a self-made resistance heating directional solidification furnace with ultrahigh temperatures and high thermal gradients. The microstructure and solid/liquid (S/L) interface morphology evolution of directionally solidified alloy were investigated under the condition of different melt superheat temperatures θs (1950, 2000, 2050, 2100 and 2150 ℃) but with a constant withdrawing rate of\linebreak 15 μm/s. The results revealed that when the melt superheat temperature θs=1950 ℃, the directionally solidified microstructure is composed of straight primary Nbss dendrites and couple grown lamellar (Nbss+γ-(Nb, X)5Si3) eutectic colonies (petal--like) along the longitudinal axes of the specimens. When θs=2000 and 2050 ℃ respectively, the directionally solidified microstructure is completely composed of straight petal-like eutectic colonies. As θs increased to 2100 and 2150 ℃ respectively, the directionally solidified microstructure evolves into straight coarse primary Nbss dendrites and fine lamellar eutectic colonies along the longitudinal axes of the specimens. The S/L interface morphology changes from coarse dendrite to cellular, then to coarse dendrite with the increase of melt superheat temperature.
Although Ti-V and Ti-Nb binary systems are subjected to many investigations, there remain some issues open for discussion, among which are the lattice parameter misfit and phase boundary between the non-equilibrium ω and β phases. On the other hand, the experimental elastic moduli of the non-equilibrium phases are rarely reported due to the difficulty of the measurement. In this paper, the lattice parameters, bulk moduli and phase stabilities of α(α'), ω, and β phases of binary Ti-V(Nb) alloys are investigated by the use of first-principles exact Muffin-Tin orbital method in combination with coherent potential approximation. It is shown that, with the increase in the V content, the lattice parameter aα of the α(α') phase decreases, whereas cα/aα slightly increases; aω and cω/aω of the ω phase and aβ of the $\beta$ phase decrease. For Ti-Nb alloy, with increasing Nb content, aα keeps almost unchanged whereas cα/aα increases; aω increases and cω/aω deceases; aβ does not change significantly. The lattice parameter misfit between the ω and β phases increases with increasing V or Nb content. Both V and Nb harden the bulk modulus of Ti and improve the phase stability of the β phase relative to the α(α') and ω phases. The theoretical predictions are compared in detail with the available experimental data.
Intermetallic alloys based on Ti3Al are potential high-temperature structural materials due to their low density, high specific strength, excellent creep behavior and good oxidation resistance, but their application has been hampered by the low room--temperature ductility and ambient brittleness. Numerous experiments have shown Nb is most effective additive to improve their ductility and toughness at low temperature, but the influence of Nb content on the mechanical properties of Ti3Al-based alloys has not been understood. In this work, using the first-principles pseudo--potential plane wave method, ultimate tensile strength σb of α2-Ti-25Al-xNb (x=0-12, atomic fraction, %) single crystal with D019 structure and bulk modulus B, Young's modulus E as well as shear modulus G of α2-Ti-25Al-xNb polycrystalline alloys have been calculated, and their ductile/brittle behavior is characterized and assessed by the Cauchy pressure (c12-c44) and the G/B ratio. The results reveal the ultimate tensile strength σb of α2-Ti-25Al-xNb crystals and the elastic moduli (B, E, G) of α2-Ti-25Al-xNb alloys monotonously increase with the addition of Nb in the whole range of x=0-12. Meanwhile a very sensitive ductile/brittle behavior of α2-Ti-25Al-xNb alloys to Nb content is also detected. The addition of Nb with low content is demonstrated to be profitable for weakening of the brittleness of α2-Ti3Al alloys, and the toughening tendency of α2-Ti-25Al-xNb alloys increases as increasing Nb addition in the range of =0-6. Whereas in the range of x=7-9, relative to α2-Ti3Al alloys no toughening effect can be seen as Ti in Ti3Al being partially substituted by Nb. As x≥10, the toughening effect of Nb addition is activated again, and an obvious improvement in the ductility and strength of α2-Ti-25Al-12Nb alloy is observed as comparing with α2-Ti-25Al-6Nb alloy. For this toughening and strengthening effect of Nb addition a reasonable explain was given by means of the analysis of the density of states (DOS) and the projective density of states (PDOS) of α2-Ti-25Al-xNb (x=0, 6, 7, 12) crystals.
In order to overcome some problems in variable polarity gas tungsten arc welding (VP-GTAW) employed for welding aluminum alloy components, poor strength and ductility of weld metal, solidification cracking and weld porosity, a novel ultrafast-convert high frequency pulsed current VP-GTAW technique is developed. The current converting speed in the novel technique is enhanced from less than 10 A/μs to more than 50-100 A/μs and high frequency pulsed current which has a frequency of more than 20 kHz is exactly integrated in the positive polarity current duration. It is expected that the novel pulsed VP-GTAW technique can improve the weld quality of aluminum alloy significantly. Thus, it is imperative to understand the effect of pulsed current parameters on the weld characteristics in the pulsed VP-GTAW process. The measured results of 2219-T87 high strength aluminum alloy weld joints show that the application of ultrasonic pulse current makes the coarse grains in weld zone change to the fine equiaxed grains, and a band-like zone with finer equiaxed non-dentrites appears. The width of weld fusion zone is obviously decreased and the mechanical properties of weld joints are predominantly improved. The improvements of the structure and weld properties can be significantly enhanced with the increases of the ultrasonic pulse current amplitude and pulse frequency, and decrease of the pulse duty cycle in certain ranges. Compared with the conventional VP-GTAW, tensile strength and elongation of weld joints are increased by about 22% and 111%, respectively, under the conditions of pulse current amplitude of 100 A, pulse frequency of 40 kHz and pulse duty cycle of 20%.
There is a strong demand within the steel industry to develop high strength microalloyed steels and matching welding materials for satisfying the ever increasing industrial needs. Nb microalloyed steel is one of the important structure materials. The weldability determines the industrial application prospect of Nb microalloyed steel. The majority of previous studies concerning Nb bearing steels have been focus on the transformation behavior of Nb bearing steels in thermomechanical process and the effect of Nb element on the process. However, the research on the matching welding materials for Nb bearing steels and the effects of heat treatment process on the microstructure and mechanical properties of Nb bearing weld metal were seldom reported. In this paper, Nb bearing S355J2G3 steel plates for high-speed train bogie were welded using welding wires with and without Nb addition. Differences of properties at the different regions in the as-welded joint, and the effects of the Nb element and the different post weld heat treatments on the microstructure and the mechanical property of the weld metal were analyzed systematically. Experimental results showed that the weld metal toughness is the weakest link of the welding joint properties. Nb addition can improve the strength of the weld metal, but has no obvious effects on the plasticity and impact toughness. After stress relief annealing, the strength of the Nb free weld metal decreased, while the elongation and impact toughness increased. However, for the Nb bearing weld metal, stress relief annealing can improve the strength of the weld metal significantly, but deteriorate the elongation and impact toughness. NbC particles were found in the as-annealing weld metal. With the increase of the normalizing temperature, the microstructure and mechanical property of the Nb free weld metal have no obvious change, while, for the Nb bearing weld metal, the strength increases obviously and the elongation and impact toughness decrease. Therefore, setting the normalizing temperature properly is the key to get higher toughness for the Nb bearing weld metal. It was simultaneously found that the content of widmanstatten ferrite in the Nb bearing weld metal increases obviously with the increase of the normalizing temperature. Furthermore, when the normalizing temperature was set at 920 ℃, the size of the NbC particles in weld metal is larger than that in the as-annealed weld metal. However, when the normalizing temperature was raised to 1200 ℃, the NbC particles will disappear because of its dissolution at\linebreak higher temperature.
High speed GMAW (gas metal arc welding) is an effective way to improve the welding productivity, however, its application is usually limited by the occurrence of several weld bead defects, such as humping bead. Based on the experimental results, a mathematical model is developed to analyze the forming mechanism of humping bead for high speed GMAW through considering both the momentum and heat content of the backward flowing molten jet in weld pools. One term related to the momentum of backward jet is added to the equation of weld pool surface deformation, and the heat content of overheated droplets is distributed within the layer covering the whole pool. The humping bead forming process and its dimension and 3D geometry are numerically simulated, and compared with the experimental measurement under some welding conditions. It is found that the model can describe and characterize the humping formation in high speed GMAW quantitatively.
The intergranular fracture characteristics in nanocrystalline and ultra--fine polycrystalline metallic materials present intensive size effect and microstructure geometry effect. The conventional elastic--plastic constitutive theory is unable to describe these effects because it doesn't contain any length parameters to characterize the scale changing. Regarding this, a micro--structured model was proposed for the study on intergranular fracture of nanocrystalline and microcrystalline metals (mainly for the fcc metals). The hardening and size effects of material plastic deformation are described by the computational model based on the conventional theory of mechanism--based strain gradient plasticity (CMSG). A cohesive interface model was used to simulate the processes of grain--boundary sliding and separation, the initiation and propagation of intergranular cracks until the material fracture. The tensile experiment and stress--strain curves of nanocrystalline Ni were simulated by using the present model. Then the relation between macroscopic mechanical behaviors and intergranular crack's initiation and propagation in nanocrystalline Ni was investigated. Through the simulation to the experimental result in literature, the validity of the proposed model calculated nanocrystalline and ultra--fine polycrystalline mechanical properties was confirmed. At the same time, the simulation results show that the high strain gradient effects and severely plastic hardening of grain are induced by inhomogeneous plastic deformation, and the grain boundary induced deformation has a significant influence on the overall mechanical properties of nanocrystalline metals.
TWIP (twinning induced plasticity) steels possess very high plasticity and high strength. It has been pointed out that deformation twinning plays an important role in controlling the deformation behavior, which divides grains into nano--scale layer--like structures to result in high strain-hardening rate or the so--called “TWIP” effect. The formation of deformation twins is affected by deformation temperature, strain rate, pr--deformation and grain size. The generation of deformation twins in austenitic steel with low stacking fault energy (SFE) is closely related to grain size. However, the relationship between strain hardening rate and grain size in TWIP steels has yet to be clarified, which is important for optimizing the parameters of solution treatments. In the present paper, the specimens of a typical TWIP steel with grain sizes of 7, 13, 30 and 63 μm were fabricated through solution treatments at different temperatures. Mechanical properties were measured by tensile tests, and microstructure evolution was observed by OM and TEM. The results show that the strain-hardening exponent rapidly increases with increasing true strain when it is less than 0.2, but levels off in the subsequent process of deformation. The relationship between strain hardening rate and true strain consists of two stages for the specimen with small grain size and three stages for the specimen with large grain size. Microstructure observation demonstrated that the number of deformation twins increases with the increase of grain size, induced to greater“TWIP”effect in the coarse-grained specimen than in the fine-grained specimen. This can be attributed to the dependence of the critical stress for formation of deformation twins on grain size of σT=σT0+KT}d-A.
Particle reinforced metal matrix composite (PR-MMC) has attracted extensive investigation in material science and engineering. Laser depositing of MMC coatings containing in situ carbide particles is a research focus in laser surface processing field. Recent literatures have indicated that the particle size and distribution play an important role on the wear resistance of laser clad coatings. It is necessary to analyze the morphologic characteristics of the particles. In this paper, particle reinforced Fe-based composite coatings were produced by laser cladding Fe-based alloy powders containing Zr+Ti, Ti+WC and Zr+Ti+WC, respectively, on the surface of a medium carbon steel. The carbide particles were analyzed by XRD, SEM and TEM. The results show that during the growth of composited carbide particles, a type of surrounding structure, the inside part of the particle is (Zr, Ti)C while the peripheral part is (Ti, Zr)C, forms when 12%Zr and 3%Ti (mass fraction) are added. When 1%-5% Ti and 10\%WC are added, the particles present petal-like shape with layered structure. When Zr, Ti and WC (7%-10% of Zr and Ti) are added, the particles present irregular polygon shape as low content of WC (5%) added, but become petal-like or even dendrite shape when the content of WC is high (10%-15%). Although the particles have high content of W (30%-50%), they still have TiC structure. The formation mechanism is discussed based on thermodynamic calculations. It is indicated that different strong carbide-forming elements (SCFE) play different roles in the nucleation and growth of particles. Zr and Ti are important elements for particle nucleation. The concentration of W into the particle has a great influence on the growth of the particles. Therefore, the addition of WC can enlarge the particle size and decrease the amount of particles meanwhile. The above results are helpful to select the suitable proportion of SCFEs on laser cladding PR-MMC coatings.
The influences of aging temperature and minor addition of Zr+Si on the damping property of Zn-22%Al alloy were investigated by dynamic mechanical analyzer (DMA) and microstructure observation. The results show that with the increase of aging temperature, the decomposition of super-saturated solid solution of Al (α' phase) is accelerated, resulting in increasing and coarsening of the lamellar structure, which will impair damping property. In addition, the damping property of Zn-22%Al alloy could be significantly improved by the minor addition of Zr+Si elements, which induced microstructure refinement and increase of interface. The dislocations induced by thermal mismatch between Si phase and matrix have little influence on the damping property of alloy, as the inter--diffusion of atoms makes dislocations decrease gradually.
There were some reports on superplasticity of ultra-high carbon steels in the last several decades, mainly referring to the superplasticity of fine-equiaxial double-phase microstructure (fine ferrite + cementite particles). However, in order to get the fine--equiaxial double--phase microstructure, very complicated pre-treatment was needed. An exploration to obtain superplastic microstructure through simple uniaxial compression at pearlite transformation incubation temperature was conducted using Gleeble 1500D in this paper. The microstructural evolution processes of the steel during deformation included (1) pearlite transformation, (2) cementite spheroidization and (3) ferrite recrystallization. The (2) and (3) processes start before the finish of pearlite transformation. Two micro-processes of cementite spheroidization were shown in the experiments. One is that the cementite lamellae were dissolved and broken. This process results in the formation of relatively coarse cementite particles (100-200 nm). Another is that finer cementite particles (10-30 nm) reprecipitated in the ferrite during ferrite recrystallization. Deformation during pearlite incubation period can accelerate pearlite transformation and cementite spheroidization. The above processes lead to form fine double-phase microstructure with sub--micrometer and nanometer cementite particles distributed uniformly in fine ferrite (about 1 μm). Samples with the fine double--phase microstructure show the m value of 0.40 in the strain rate range of 1×10-4-2×10-4 s-1 at 700 ℃. The flow stress under different strain rates reduces with the increase of the prestrain. For example, under the strain rate of 1×10-4 s-1 at 700 ℃, the flow stress of the samples with prestrain of 1.2 is only 70 MPa, much lower than 120 MPa of the samples without prestrain. The dispersed cementite particles can prevent ferrite grains from growing during deformation process at high temperature, as a result, the stability of the fine-equiaxial double-phase microstructure is ensured, which is the microstructural condition realizing superplasticity.
The influence of cooling process on microstructure of high-strain pipeline steel X80 with its low yield ratio has been examined by SEM and in situ TEM. The results illustrate that ferrite+bainite dual phase structure is obtained after proper relaxation and chilling down process, while the terminate temperature of relaxation is the decisive factor. When the stop temperature for relaxation ranges from 690 to 705 ℃, the combination of strength and ductility reaches the need for the use of X80 pipeline steel. The reduction of relaxation stop temperature results in increases of the volume of ferrite phase and grain size, which leads to lower yield ratio. Tensile test shows that the lower yield ratio mainly attributes to the cooperative deformation mechanism between soft ferrite and hard bainite.
Mg--1.5Mn--0.3Ce alloy was deformed by T-shape channel pressing (TCP) for four passes at 623 K, and the grain size is greatly refined from 35 μm to 2 $\mu$m, and a number of tiny Mg12Ce dispersively distributes in intragranular and intergranular regions. Superplastic deformation behavior of TCP deformed alloy was investigated at temperatures ranging from 573 K to 673 K and strain rates ranging from 1×10-1 s-1 to 4×10-4 s-1, and the microstructure evolution after tensile-to-failure was also analyzed. The experimental results indicated that the alloy deformed by TCP exhibits excellent superplasticity even in the condition of high strain rate at temperatures from 623 K to 673 K. The maximum elongation of 604 % is obtained at 673 K and a strain rate of 3×10-3 s-1, and its strain rate sensitivity m is 0.36. Grain boundary sliding is the primary mechanism of the superplastic deformation, and intragranular slip would become more obvious at lower strain rate and higher temperature, and plays an accommodated role in promoting grain boundary sliding during the deformation.
In the proton exchange membrane fuel cell (PEMFC) with widely applicated future, the bipolar plate plays an important role in supporting the cell stack, collecting current, separating the oxidants from fuels and channeling the oxidants and fuels. The ideal bipolar plate should be of good electric conductivity, high corrosion resistance, high mechanical strength, low gas permeability, low cost and easy processing. Although the stainless steels can be used as bipolar plate materials, their corrosion resistance in fuel cell environment is not satisfied, and the cations induced by metal corrosion would poison the proton exchange membrane. A series of Cr1-xNx (x=0.28-0.50) films were deposited on the surface of stainless steel by arc ion plating (AIP), the composition and phases of Cr1-xNx films and the electric conductivity and corrosion resistance of the modified bipolar plates were tested. The results show that as the value of x varying from 0.28 to 0.50, the phases in the films change from Cr+Cr2N to Cr2N, then to Cr2N+CrN and finally to CrN. The bipolar plates coated with the Cr1-xNx film with single phase structure show good electric conductivity and high corrosion resistance. Comparing with the original stainless steel, the electric conductivity and corrosion resistance of bipolar plates are enhanced by more two orders of magnitude and almost three orders of magnitude, respectively.
The influence of Cl- concentration in corrosion medium to crevice corrosion of X70 pipeline steel was investigated by wedge-type crevice model. The experimental results show that under natural corrosion condition, as the crevice opening thickness is 0.15 mm and the test periodic is 120 h,\linebreak with increase of the Cl- concentration (cmCl-) in corrosion medium outside the crevice the Cl- concentration (cmCl-) in the crevice solution from the opening to the bottom gradually increases, the pH value in the crevice solution and the electrode potential of X70 steel samples from the opening to the bottom in the crevice gradually decrease, which indicated that crevice corrosion tendency of X70 steel increases with increase of the Cl- concentration (cmCl-) in corrosion medium outside the crevice.
Sodium polystyrene sulfonate (PSS) doped polyaniline (PANI) nanoparticles were self-assembled on α-Fe2O3 nanoparticles, of which the surfaces have been modified by hexadecyl trimethyl ammonium bromide (CTAB), through controling the pH value. The effects of pH value, temperature and reaction time on the structure of composite particles were systematically studied, and the optimum conditions were determined. TEM, XRD, FTIR and cyclic voltammetry (CV) were used to characterize the composite particles. The analyses of CV curve and FTIR spectrum showed that compared with PANI nanoparticles, the Fe2O3-PANI composite particles present enhanced infrared absorption and well electrochemical property.
Ferromagnetic metallic flakes show high magnetic permeability in gigahertz frequency due to their large saturation magnetization and the effect of particles shape. However, their permittivity is too large to wave impedance matching and to application as microwave absorbent. In this paper, the surfaces of micrometer Fe flakes were modified by a thin layer of epoxy resin based on the reaction of hydroxyl groups anchored in the surface of Fe flakes with 3-aminopropyltriethoxy silane and diglycidyl ether of bisphenol A. The structure, morphology, surface state and microwave electromagnetic properties of the as-prepared products were characterized by Fourier transformed infrared spectra, scanning electron microscopy, atomic force microscopy, and network analyzer. The results show that compared with the pristine Fe flakes, the Fe flakes modified by a thin layer of epoxy resin exhibit a substantially decreased complex permittivity, particularly the imaginary part decreased by 30%-80%, but remain almost the same magnetic permeability, which is an absorber with excellent microwave absorbing properties. At the same time, the surface modification mechanism was prosposed. Compared with traditional core-shell modification, the method presented is effective for continuously tuning permittivity of electromagnetic wave absorbents.
In order to achieve the relationship between the melt thermal history and the solidification structure so as to explore new methods to effectively control the solidification process and the solidification structure of metal, the effect of the melt thermal history of Ga on the nucleation undercooling has been studied by using DSC, and some formulae among the atom cluster size in melt, the nucleation undercooling of melt, the melt temperature and the concerned physical and chemical parameters of metal have been proposed. The experimental results show that the nucleation undercooling increases with increasing the holding time at high temperature after a heating process and decreases with increasing the holding time after cooling to low temperature, but the change rates of the nucleation undercooling decrease with increasing the holding time. An equation between the atom number in the largest cluster in the melt and the melt temperature has been obtained by studying the effect of the liquid temperature on the cluster size thermodynamically and kinetically. Formulae between the homogenous nucleation undercooling, the heterogeneous nucleation undercooling and the temperature of liquid metal have been achieved. In terms of these formulae, the atom number in the largest cluster in the melt and the nucleation undercooling of the melt can be predicted if the temperature at which liquid metal is heated and hold is known. A method for predicting the hysteretic extent of nucleation temperature after changing the liquid temperature has been developed. The predicted results of the hysteretic extent of the nucleation temperature are in agreement with the experiential results. The predicted and experimental hysteretic extents of the nucleation temperature are -10.7 and -10.3 K for Ga heated from 303 K to 373 K, and 7.9 and 8.3 K for Ga cooled from 373 K to 313 K, respectively. The errors between the predicted hysteretic extent of the nucleation temperature and the experimental result are only 3.9\% for Ga heated from 303 K to 373 K and 4.8\% for Ga cooled from 373 K to 313 K, respectively.
