Steels with ultrafine (α+θ) duplex structure, consisting of ferrite matrix (α) with average grainsize of about 1μm and dispersed cementite particles (θ), have been investigated widely in recent years formaking better the work-hardening capability of ultrafine-grained steels. In fact, the ratio of yield strength to tensilestrength for plain carbon steels with ultrafine (α+θ) duplex structure is commonly larger than 0.85. Forstructural material, the low ratio of yield strength to tensile strength is beneficial to absorb external energy anddelay theoccurrence of destruction. However, the ratio of yield strength to tensile strength is still relatively high for steelswith ultrafine (α+θ) duplex structure to act as the structural material. Namely, the work-hardening capability ofultrafine (α+θ) duplex steel needs further improving. It could be feasible for improving the work-hardeningcapability of ultrafine (α+θ) duplex steel to change the form, size and distribution of the cementite. Therefore, it isnecessary to investigate the work--hardening behavior of steel with different cementite states. In the present research, four different microstructures of eutectoid steel were obtained by different thermo-mechanicaltreatments, i.e., lamellar pearlite, spheroidized pearlite, ultrafine (α+θ) duplex structure and fine-grained (α+θ)duplex structure. The effect of different microstructures on the room--temperature work-hardening behavior of theeutectoid steel was analyzed using room temperature tensile tests, SEM and TEM. The results indicated that thework-hardening characters of lamellar pearlite,which initial work hardening rate is large but decreases quickly with strain, have direct relationship with its large tensile strength, small yield ratio and low uniform elongation.Although the initial work hardening rates of the three ferrite/cementite particles duplex structures are lower, theydecrease much slowly with strain comparing with that of lamellar pearlite. Therefore, three types offerrite/cementite particles duplex structures demonstrate good plastic deformation capability. In comparison withspheroidized pearlite, ultrafine (α+θ) duplex structure and fine-grained (α+θ) duplex structure demonstrate better balance between strength and plasticity due to microstructure refinement.

 Samples of ultrafine-grained 304L stainless steel with an average grain size of (130±30) nm wereprepared by using equal channel angular pressing (ECAP) technique after 6 passes with route Bc at 500 ℃.The electrochemical behaviors and the passive properties of the material in 0.05 mol/L H₂SO₄+0.25 mol/L Na₂SO₄solution were examined by using polarization curves and Mott-Schottky analysis. As compared with the coarse-grained counterpart, the ultrafine-grained 304L stainless steel sample exhibits a lower corrosion potential(-505 mV) and a higher corrosion current density (11.7μA/cm²), indicating the accelerated trend of the activedissolution, and has a broadening of passive region (-341-847 mV) with lower passive potential (-341 mV) andpassivation current density (4.5μA/cm²). Both of the passive films exhibit n-type semi-conductor behaviorsat potentials ranging from 0 to 0.7 V. The ultrafine grains of 304L stainless steel are helpful in forming more stablepassive films with the reduced donor density and the diffusion coefficient of the charge carrier.

The mechanical properties and microstructure of the TG (third generation) X90 pipeline steelswhich have been industrially trial-produced were investigated. The results showed that the microstructure of the18.4 mm thick X90 pipeline steels consist of quasi-polygonal ferrite, lath bainite and M/A (martenite/austenite) island. The yield strength was between 615 and 660 MPa, and the tensile strength was above 720 MPa, and theyield ratio was below 0.9, the impact absorbed energy at --30 ℃ was over 200 J, and the shearing area ofdrop-weight tear test (DWTT) at -15 ℃ was more than 80%. The ratio of the soft phase (quasi-polygonal ferrite)and the hard phase (lath bainite) was about 3∶2, and the dislocation density in the lath bainite was higher, whilethat in the quasi-polygonal ferrite was lower. The EBSD results showed that the lath of the hard phase met theshear transformation characteristics, indicating that the effect of microstructure control on hard and soft phase has been achieved.

The morphology evolution of the continuous precipitate Mg₁₇Al₁₂ in aging process was studiedusing KKS phase field model. In the model, the chemical free energy and chemical potential for the precipitatephase and matrix were obtained by using Thermo-Calc software and database. The effects of interfacial energyanisotropy were taken into account by introducing interface anisotropy function, and the effects of micro elasticstrain energy were introduced based on the theory of micro elastic strain energy formulated by Khachaturya. It isdemonstrated that the precipitate phase has a lath shape when only the interfacial energy anisotropy wasconsidered regardless of the micro elastic strain energy.The precipitate phase grows into a diamond shape whenonly the micro elastic strain energy was taken into account regardless of the interfacial energy anisotropy. Whenboth the effects of the interfacial energy anisotropy and the micro elastic strain energy were considered in thesimulation, the precipitate phase has a lath shape with lozenge ends,which is in agreement with experimental observations. The interfacial energy anisotropy affectsthe overall morphology of the precipitate, and combiningthe interfacial energy anisotropy and the micro elastic strain energy results in the lath shape with lozenge ends.

Aluminum alloy castings have been widely applied in automotive and aerospace fields. One of themost common defects of cast aluminum alloys is pore, which often results in poor mechanical properties suchas limited strength and ductility, short fatigue life and increasing variability in properties. In this investigation,micropores in practical ADC12 aluminum high pressure die castings were detected with 3D high resolution X-raycomputed tomography technology. The 3D morphologies, volumes, surface areas and sphericity coefficients ofthe pores were presented and analyzed. The pore's general characterization was summarized. Based on theirdifferent morphologies and characteristics, there were three types of pores, namely, gas, gas-shrinkage andshrinkage pores, detected in the die casting alloys. The volumes, sphericity coefficients and pressures of the threetypes of pores were compared and discussed. Then their formation mechanism was proposed. It was concludedthat the gas pore was with low volume, high pressure and near round shape. The appearance of shrinkage-gaspore was sphere with convexes or long tails. Compared with the gas pore, the shrinkage-gas pore volume washigher while its pressure was lower by one order of magnitude. For the shrinkage pore formed during the highsolid fraction period, it was characterized with low volume and tortuous and complex shape in space. The sphericity and pressure of the shrinkage pore were the lowest among the three types of pores and its pressure wasapproximate three orders lower than that of the gas pore. Furthermore, the volume distribution of pores wasinvestigated by statistical analysis, which showed that the three-parameter lognormal can fit the volume distribution better than lognormal.

Strengthening mechanism related to {1012} twinning plays an important role in improvingmechanical properties of Mg alloy. In this work, a hot-rolled AZ31 Mg alloy sheets were subjected to dynamicplastic deformation (DPD) along the rolling direction and tests were interrupted at strains of 1%, 3% and 5% withthe aim of introducing {1012} twins, and then were annealed at 200℃ for 3 h to eliminate the dislocations. The tensile deformation behavior and the microstructural evolution of these pre-deformedsamples containing {1012} twin lamellar structure with the aim of analyzing effects of untwinning and{1012} twin lamellar structure on the mechanical properties of materials were investigated. When subsequent tensile deformation is carried out along the DPD direction, untwinning causes a significant increase in the maximum flowstress. And the tensile yield stress and the maximum flow stress increases significantly with the volume fractionof twins. The texture hardening which is caused by the texture change attributable to untwinning plays an importantrole in improving mechanical properties of materials. When tension is carried out along the initial transversedirection, slip dominated plastic deformation and {1012} twinning activity is restrained. The tensile yield stress increases slightly with pre-strain, suggesting that the hardening contribution of initial grain refinement by {1012} twinlamellae is not very significant during deformation.

Cladding metals have been widely applied to many fields because they have many excellentphysical, chemical and mechanical properties that can not be obtained from the single metals. There are manyconventional processes for manufacturing cladding metals, for example roll bonding, diffusion bonding, explosivewelding, extrusion cladding and casting cladding. Among these processes, continuous casting is an ideal processto prepare cladding metals which has the advantages of high production efficiency, low production cost and goodinterface bonding. This process can make two metals contact directly by the ways of one liquid-one liquid, oneliquid-one solid or one liquid-one semi-solid and then the good metallurgical bonding can be obtained. So, thisprocess is extensively studied by researchers engaging in material processing. The process of the directwater-cooled continuous casting to fabricate cladding 3003/4004 aluminum alloy circular ingot is researched inthis paper. The 3003 aluminum alloy has excellent corrosion resistance, low strength and high melting point, whilethe 4004 aluminum alloy has poor corrosion resistance, high strength and low melting point. The cladding3003/4004 aluminum alloy material can combine the advantages of two metals and can be widely used in manyfields, especially in car engine and air conditioning heat sink. To obtain the good interface bonding, a special innermold with the single-side cooling capability was applied in this process. By the special inner mold, the two alloyscan make the contact of one liquid-one solid or one liquid-onesemi-solid on the interface. The solidification structure and elemental distribution near the interfaceof cladding ingot were systematically detected by OM, SEM and EPAM. Tensile test was carried out to evaluate theinterface bonding strength. The OM results indicated thatthe interface of cladding 3003/4004 aluminum alloy ingot was clear without gas holes and slag inclusion. Mostgrains were equiaxed in the cross-section of cladding 3003/4004 aluminum alloy circular ingot. The EPAM resultssuggested that the interdiffusion of alloy elements in 3003 and 4004 alloy occurred and there was an about 30μmwide diffusion layer near the interface. The entire tensile specimen fractured in the sides of 3003 alloy with theaverage ultimate tensile strength of 107.7 MPa, indicating that the interface bonding strength of cladding ingotwas higher than the ultimate tensile strength of 3003 alloy and the good metallurgical bonding near the interface wasobtained by this process.

With the rapid development of automotive industry, the requirements for engine performance hasbecome increasingly higher and higher, such as dynamic performance, environmental performance, fuel economyperformance and so on. Obviously, using lighter aluminum engine has become the future of the automobile industry.When start or stop the engine, the engine generates drastic temperature field that will make parts of the enginecomponents into the plastic deformation zone. If the engine components continue like this, extremely high thermalstrain will cause the thermal fatigue failure of aluminum alloy for automotive engine. Thermal fatigue is a more seriousform of material fatigue failure. The thermal fatigue is prevalent in many important engine castings, such as cylinderbody, cylinder head, piston and so on. Therefore, study on thermal fatigue properties of multivariate Al-Si-Cu castalloy has the most important significance. Based on this thermal fatigue behaviors of multivariate Al-7.5Si-4Cu alloys under different heat treatmentwere investigated in the temperature ranges of room temperature to 350℃. Thethermal fatigue cracks of the tested specimens were observed using OM and SEM. The results demonstrate that thermalfatigue properties of the alloy after T6 heat treatment is better than the alloy after cast-quenching+aging heattreatment and the cast alloy. Reasonable heat treatment process can improve the strength, the plasticity and thethermal fatigue property of multivariate Al-7.5Si-4Cu alloy, including reduce the rate of crack propagation.The main reason of crack initiation is oxidative microstructure induced by thermal stress. Fatigue crack expands alongthe grain boundary bypassivation-sharpen of crack tip in the early. In the late, fatigue crack expands with both intergranular way andtransgranular way by passivation-sharpen of crack tip and siamesed holes on front edge of crack tip. The Si phasescan affect fatigue crack propagation. When the angle between the fatigue crack propagation and the short axis of theSi phase is more than 60°, fatigue crack expandsas “bypass-wall expansion”. When the angle between the fatiguecrack propagation and the long axis of the Si phase is more than 60°, fatigue crack expands as “through-wallexpansion”. Oxidation behavior effects significantly at the crack initiation period. After T6 heat treatment, theantioxidant property of multivariate Al-7.5Si-4Cu alloy is the best. Oxidation kinetics curves of multivariateAl-7.5Si-4Cu alloy under the different heat treatment are all logarithmic relationship.

The Al－12%Ni (mass fraction) hypereutectic alloy from pure Ni and Al (99.9%) was induction melted and directionally solidified at constant growth rates ranging from 1 μm/s to 100 μm/s and abrupt change of growth rate were carried out in a Bridgman－type furnace. After solidification, the samples were quickly quenched into liquid Ga－In－Sn alloy to preserve the microstructure. The microstructures of the samples were observed using OM and SEM. It was indicated that at a growth rate of 1 μm/s, after experiencing a certain growth distance, the primary Al₃Ni phase disappeared and the coupled growth of eutectic could be obtained. The morphology of Al₃Ni phase was faceted when it was the leading phase at growth rates from 2 μm/s to 100 μm/s. The result of experiments with abrupt change of growth rate indicate that the initial microstructure before abrupt change of growth rate determine the microstructure after abrupt change of growth rate. Only if there existed no coarse primary Al₃Ni phase before abrupt change of growth rate could entirely coupled eutectic structure be obtained at relatively higher growth rates. After abrupt change of growth rate, the growth of primary Al₃Ni phase was suppressed and the coupled eutectic could grow continuously without any coarse primary phases. The strength and plasticity could be improved effectively through directional solidification.Besides, the elongation of Al－12%Ni alloy could be greatly improved by the abrupt change of growth rate during directional solidification.

Alloy films could form substitutionally supersaturated solute solution because of the non-equilibrium characteristics of physical vapor deposition (PVD) and gained grain refining and mechanical properties improving. In order to reveal the structure characteristics and strengthening effect of substitutionally and interstitially supersaturated solid solution films, a series of aluminum-based composite films with different Ti and C contents were synthesized by magnetron co-sputtering Al and TiC targets. EDS, XRD,TEM, STEM and nanoindenter were used to characterize the microstructure and mechanical properties of the composite films. The results showed that Ti and C dissolved in the grains with much higher solute contents than their solubility limits at thermodynamic equilibrium state and enriched at the boundary. The composite film formed “dual-supersaturated solid solution” exhibiting both substitutional and interstitial features. In lower solute contents, the grain size of the composite film decreased to less than 100 nm rapidly because of severe lattice distortion. The hardness of the film increased to 2.1 GPa from pure Al 1.3 GPa when containing 0.6%(Ti, C). With the increase of the solute contents, the film hardness increased gradually and achieved the highest value of 7.0 GPa when containing 6.4%(Ti, C). Then the composite film transformed into amorphous and its hardness also slightly reduced. The study showed significant grain refining and strengthening effects of dual-supersaturation of Ti and C in lower content on aluminum--based film and provided a way to improve the mechanical properties of metal films.

As a pollution_-free contact material, Ag/SnO₂ has shown a promising prospect to replace noxious Ag/CdO because of its remarkable resistance to arc_-erosion and welding. Especially La_-doped Ag/SnO₂ contact alloy shows excellent electrical properties during arcing test. In this paper, Ag/(Sn_0.8La_0.2)O₂ coating on the Cu substrate was prepared by atmospheric plasma spraying. The phase structure and microstructure of Ag/(Sn_0.8La_0.2)O₂ coating were characterized using XRD and SEM, respectively. The tests of tensile and arc erosion were employed to evaluate bonding strength and electrical properties of coating. The results show that the prepared coating has the uniform dense microstructure. The nanosized (Sn_0.8La_0.2)O₂ particles are uniformly dispersed within the Ag matrix of coating. The bonding strength is 18.8 MPa. The breakdown strength and arc erosion rate of coating are 3.76×10⁷ V/m and 37.7 μg/C, and the value are close to the bulk material. After arcing test, the surface of coating presents the dispersion of cathode spots and a little erosion, and the arc erosion mechanism of Ag/(Sn_0.8La_0.2)O₂ coating has a change tendency from liquid splash to evaporation erosion.

MCrAlY alloy has served as overlay coatings or bond coats in thermal barrier coating systems ingas turbine engines, and its phase constituents play a vital role in determining the performance of these coatingsystems. In order to further understand the influence of Re on the phase constituents of MCrAlY coatings, the phase evolution of a Ni-20Cr-10Al-20Co coating alloy with 0-9% (mass fraction) Re addition was predicted the Thermo-Calc thermodynamic software and TTNi7 Ni-based superalloy database. Based on the calculation results,the four NiCoCrAlY alloys with different levels of Re (0, 3%, 6% and 9%) were prepared, and their phase constituents were experimentally investigated in the temperature range of 800-1250℃. Both the calculation result and the experimental microstructural observation indicated that the addition of Re slightly increased the fraction of β-NiAl phase, but dramatically enhanced the amounts of σ and α-Cr phases. Based on the combined analysis of microstructural observation and EPMA composition identification, it was proposed that the increased amounts of σ and α-Cr phases should result from the changed partition of Cr in all phases caused by Re addition. The σ phase is a low temperature stable phase, and would change to α-Cr phaseas temperature rises. The transformation temperature of σ→α-Cr phase increases as the addition of Re increases.The experimental results also showed that the γ’-Ni₃Al→γ-Ni transformation temperature decreases with the addition of Re. In addition, the calculation results were compared with the experimental results and were found to be in reasonable agreement with the experimental observations from the aspect of the MCrAlY phase evolution. Some deviations of the calculation and experiment results about the precise phase-change temperatures, such as γ’-Ni₃Al→γ-Ni and σ→α-Cr phase transformation temperatures, were observed, and the reason were discussed in the light of both limited availability of thermodynamic database and experimental problems.

In recent years, the electronic packaging technologies have become more diversified and sophisticated, and among these technologies the interfacial metallurgical reaction between the solder and metallic substrate is significantly different from each other. However, in contrast to plenty of studies about the microstructure evolution in solder joints prepared by the conventional soldering processes, such as wave soldering and reflow soldering in surface mount technology (SMT), relatively little is known about the solder's melting feature, interfacial mass transition and metallurgical reaction as well as dissolution behavior of the under bump metallization (UBM) in the micro－scale solder joints manufactured by using the novel soldering technologies, for example, the local melting reflow process. In this study, the differential scanning calorimetry (DSC) experimental approach was employed to simulate the local melting reflow soldering process, and the melting behavior of ball grid array (BGA) structure Sn－3.0Ag－0.5Cu/Cu joints, the interfacial reaction and intermetallic compound (IMC) growth behavior of the joints during the liquid isothermal aging at the temperatures near or above the solder's melting point were studied systematically. Results showed that only a part of the solder matrix adjacent to the interface of the joints melted during the liquid isothermal aging at the solder's melting point of 217 ℃. Then, a slight increase of the aging temperature from 217 ℃ up to 218 ℃ has a significant influence on the melting status of the solder matrix and the Cu substrate consumption during isothermal aging process, that is, all the eutectic phases and partial β－Sn phase in the solder matrix melted, and the proportion of the Cu substrate dissolved into the Sn matrix in the total consumption of Cu substrate also increased greatly. However, the thickness of the interfacial Cu₆Sn₅ layer and total Cu－Sn IMC layer had a slight decrease due to the fact that Cu substrate rapidly dissolved into the solder matrix and resulted in less supply of Cu in the growth of interfacial IMC layers. When the aging temperature was increased to 230 ℃, the content of Cu in the solder matrix nearly reached saturation, and the thickness of interfacial Cu－Sn IMC layers also reached the highest value. The results of interfacial IMCs growth kinetics show that the growth of the interfacial Cu₆Sn₅ and Cu₃Sn is mainly controlled by grain boundary diffusion and bulk diffusion, respectively; and the grain boundary grooving and grain coarsening can also influence the growth kinetics of the interfacial Cu－Sn IMCs.

The tensile deformation behavior, surface morphology, fracture mode and fracture morphology of SRR99 superalloy bicrystals were systematically investigated at room temperature in air. It is shown that the bicrystal with misorientation of 4° has little influence on the tensile properties, however the effect of grain boundary (GB) becomes obvious when the misorientation is 10°, for the bicrystals with misorientations of 16° and 18° the effect of GBs becomes much more serious. It is experimently found that slip is the most important deformation mode in the SRR99 superalloy bicrystals at room temperature. For the bicrystal with misorientation of 4°, slip bands would transfer through GB easily, and finally the specimens fractured along slip bands with typical slip characteristic on their fractography. In the bicrystal with misorientation of 10°,cracks nucleated and propagated either along slip bands or along GBs due to the impingement of slip bands and the strain accumulations, and finally the fracture instability happened along slip bands. The fracture morphology showed not only slip characteristic, but also the features of interdendritic cracks. In the bicrystals with misorientations of 16° and 18°, cracks initiated after yielding, and then propagated along GBs and the fracture surfaces mainly consisted of interdendritic cracks.

The presence of grain boundary (GB) premelting can alter macroscopic properties of polycrystalline materials, which lead to catastrophic material failure as exemplified by hot cracking during high temperature processing of metallic alloys. Since the premelting is considered to occur in a very limited region, it is difficult to image and to measure thermodynamic properties of nanoscale width of liquid films. At the microscopic level capturing crystalline details, atomistic modeling techniques such as Monte－Carlo (MC) or molecular dynamics (MD) have been widely adopted, however, these methods can't be used to observe morphology of GB at atomic level. Phase field crystal (PFC) has the advantage of resolving the atomic scale density wave structure of a polycrystalline material, it naturally incorporates elastic and plastic deformations and multiple crystal orientations and can be used to study a host of important material processing phenomena. The GB premelting and melting were investigated by PFC method, microstructure evolution of premelting and melting under different misorientation angles were discussed and the film width of GB was quantitively compute in terms of the excess mass method. The results showed that the liquid film appears as the melting point is approached from below, and the morphologies of liquid film are related to misorientation angle. When misorientation angles are high－angle GB, the liquid film are homogeneous. For low－angle GB, the individual dislocations which are surrounded by liquids are uniform distributed in the GB, as the melting point is approached, structure transition occurs as follows: the dislocations form pairs and the original liquids are also combined a big “liquid pool”. This structure transition not only appears during the period of premelting, but also occurs in the overheated stage, which presents a jump in the film width picture. Critical wetting angle is 12° computed by PFC method, it is more closer to the actual results than the value obtained from the Read－Shockley theory.

The dendritic growth with the different solid/liquid (S/L) interface energy anisotropies in theunidirectional solidification has been investigated using the self-consistent front tracking model. It is found that,for a given solidification condition, there were two kind of interface shape solutions with the different spacing Peclect number ranges. The interface shape with the small spacing Peclect number range was similar with cellular tip, and that with the large spacing Peclect number range referred to dendritic tip.The higher S/L interface energy anisotropy was in favor of the widening of the dendritic growth solution range. There was a certain power exponential relationship between the dendritic tip marginal stability parameterσ^* and the S/L interface energy anisotropic parameter E₄. A modified Fisher dendritic tip solution, which considered the effect of S/L interface energy anisotropy, was obtained as follows: R_IMS=2.5646[гD_L/Vk₀△T₀]^0.5E₄^-0.1905,△T₀=mC₀(k₀-1)/k₀. The undercooling in front of the S/L interface decreased with increasing the anisotropic parameter. The primary dendritic spacing mainly depended on the interaction of solute diffusion field between the adjacent dendrite, and the S/L interface energy had little influence on the primary dendritic spacing due to its localized effect on the solute diffusion field near the dendritic tip.

It is an urgent thing how to control the quality of large size GH4169 ingots nowadays. The high Nb element content in this alloy can increase the tendency of freckle defect formation. Though almost all the investigators consider that the segregation of Nb-riched Laves phase is the key factor of the freckle defect, how to avoid this phenomenon is still a hard-to-solve problem in engineering practice. In this work, a new prototype system based on classical dynamical model related to basic metallurgy theory was established to simulate the dynamical dissolving process of precipites evolution in nickel base superalloys. In this prototype system, the parameters related to the thermodynamic equilibrium state can be got from thermodynamic software of Thermo-Calc, the solute element diffusion coefficient at any temperature and time iterative can be got from dynamic software of Dictra. By using this prototype system, the dissolution process of Laves phase during homogenization process with different initial particle sizes for GH4169 alloy was simulated, and then series remelting experiments with different cooling rates and different Laves phase distributions were carried out, and the calculated results were in good agreement with the experimental results. This newly developed prototype system may give great help to homogenization process design in engineering use.

A series of creep ageing tests under different temperatures and stresses have been designed to characterize the creep aging behavior of 2124 Al alloy, and the microstructure and properties of the samples in the interrupted tests have been observed. On the basis of the experimental results, combining the creep mechanism and age strengthening theory, a unified constitutive model has been developed for creep age forming of 2124 Al alloy. The constitutive relations, aging strengthening effect and the microstructure are closely correlated in the model, which could be suitable for different ageing temperatures and stress levels. Finally, the model hasbeen used to predict the creep strain curves of added creep aging tests with different temperature, the computed results tally well with the experimental data.