Ni-based speralloys have been widely used to make the blade parts of the advanced aeroengines for their high temperature tolerance and good mechanical property. During high temperature service, the materials endure the effects of temperature and alternating load, causing high-cycle fatigue deformation on the hot-end components. Meanwhile, the fatigue behaviors of the alloy are closely related to the deformation mechanisms and its microstructure characteristics, such as the size, distribution and morphology of γ' phase and carbides, and the fatigue fracture of the using materials possesses unpredictability. Therefore, investigating fatigue behaviors of the material is of significance in alloy design and life prediction. But the high-cycle fatigue behavior of K416B superalloy with high W content is still unclear up to now. For this reason, by means of high-cycle fatigue property measurement and microstructure observation, the high-cycle fatigue behavior of K416B Ni-based superalloy at 700 ℃ has been investigated. The results show that at 700 ℃ and stress ratio R=-1, the high-cycle fatigue life of K416B superalloy decreases with the stress increasing, and high-cycle fatigue strength of the alloy is 175 MPa. At the condition of low stress amplitude, the deformed dislocations may slip along different orientations in the matrix. With the stress amplitude increasing, the dislocations may shear into γ' phase and form the stacking fault. During tension and compression high-cycle fatigue, multiple slip systems are activated in the alloy, and the distortion occurs along various directions, resulting in stress concentration on the regions of γ +γ' eutectic and carbides. The crack sources may be initiated at the eutectic and blocky carbide near the surface of the alloy. As high-cycle fatigue goes on, the cracks propagate along the inter-dendrite in expansion region, and the typical cleavage fracture occurs in the final rupture region.

Most wrought magnesium alloys exhibit a significant tension-compression asymmetry in yield and work hardening behaviors. To some extent, the widespread implementation of wrought magnesium alloys is hindered due to this disadvantage in some special conditions. In this work, in order to quantitatively analyze the effects of the deformation mechanisms on the tension-compression asymmetry of wrought magnesium alloys, the plastic deformation behavior of the as-extruded ZK60 magnesium alloy under uniaxial tension and compression at room temperature is investigated by the crystal plasticity simulation and experimental methods. The crystal plasticity constitutive model including slip and twinning mechanism is established by modifying the viscoplastic self-consistent (VPSC) model. The activation and evolution of basal slip, prismatic slip, pyramidal slip, {1012}<1011> tensile twinning and {1011}<1012> compression twinning are quantitatively studied during the process of uniaxial tension and compression deformation. Tensile-compression asymmetry of the as-extruded ZK60 alloy with fiber texture is analyzed based on the microscopic plastic deformation mechanism. The results show that the tension and compression twinning in the axial tension-compression process are difficult to active, basal slip is the main deformation mode in the early stage of deformation, but the orientation factor of basal slip is low and has a hard orientation resulting in higher yield stress. With the rotation of grains, the critical shear stress of basal slip reduces, stress continues increasing and prismatic slip becomes the main deformation mechanism, moreover, pyramidal <c+a> slip also has a high activity. At this stage, the strain hardening rate is low and the stress-strain curve is smooth. In the early stage of compression, the tensile twinning has a high activity due to its low critical shear stress (CRSS), leading to a lower yield stress. The tensile twinning gradually saturated after the strain reaches 6.0%. And then, the relative activity decreases rapidly and the hardening rate increases at the same time. Since a large number of twin boundaries hindered the movement of dislocations, slip is no longer the major mechanism. In the later stage, the compression twinning startes activation and its relative activity rises rapidly, the accumulated stress during plastic deformation could be released and then the hardening rate decreases. It can be seen that, the variation in the relative activity of each deformation mode during compression deformation is much more complex than that during tension. The yield asymmetry and different work hardening behavior could be attributed to the combined effects of the strong fiber texture and the polar nature of twinning.

The influence of Fe-rich phase particle with different contents on the bendability of the Al-Mg-Si-Cu alloys was investigated by means of bending and tensile tests, OM, SEM and TEM characterization. The results reveal that, with the increase of Fe-rich phase particle content, the bendability of the alloy sheets in the longitudinal and transverse directions was quite different, and the outer surface of the alloy sheets after bending of 180° along the two directions became much rough, especially along the transverse direction. When the Fe-rich phase concentration increased to the medium level (0.2%Fe), the quality of outer surface after bending was very good. With further increasing Fe-rich phase to the high level (0.5%Fe), micro cracks were produced after bending along the transverse direction. Although increasing Fe-rich phase concentration did not give a great effect on the elongation of the alloys in the two directions, according to the tensile fracture and microstructure in the slid surface of the specimen after bending or tension test, the roughening of outer surface of the alloy sheet without Fe-rich phase was closely related with the formation of shear bands, while for the alloy sheet with high concentration of Fe-rich phases, the formation of micro cracks after bending was mainly related with the size, morphology and distribution of coarse Fe-rich phases. In addition, based on the quantitative relationship between Fe-rich phase concentration and bendability of the alloy sheets, the models of outer surface roughening and micro cracks forming during bendingare proposed.

Weld deformation of the thin-wall weldment used in fighter aircraft not only hinders its subsequent procedure of fabrication and assembling, but also reduces its fatigue strength. As a result, weld deformation shortens its service life essentially. Dustpan deformation is always produced in the thin-wall weldment after multiple-pass weld. In this work, combining with the experiment, the finite element method was adopted to analysis the deformation of the thin-wall weldment by multiple-pass weld and its shape correction by post weld heat treatment. For obtaining the fundamental properties such as thermal parameters and mechanical parameters of TA15 titanium alloy, a series of experiments were conducted at room temperature and high temperatures. Additionally, creep behaviors of TA15 titanium alloy were studied at the temperatures of 500, 550, 600, 650, 700 and 750 ℃, and the parameters of creep constitutive equations of the alloy were obtained with considering the analysis of post weld heat treatment. A thermal coupled temperature-displacement analysis for welding and post weld heat treatment was performed on a three dimensional shell model of protective grille. Experiments of multiple-pass weld and post weld heat treatment were used to testify the reliability of the finite element model of welding and post weld heat treatment. With using the reliable finite element model, the parameters of heat treatment were studied. The study indicates that, the fabrication on the crossing of structure section and fillet after fillet-wallboard weld leads the compression deformation release along the fillet, after that, the shrinkage distortion produced during spot welding of fillet-structural section mainly contributes the large dustpan deformation of the thin-wall weldment; increasing temperatures, enlarging loads and prolonging the hold time can improve the shape correction of the thin-wall weldment during post weld heat treatment, hence the guide maps of the post weld heat treatment for shape correction of the thin-wall weldment under 700 and 750 ℃ are worked out.

Empirical approaches to characterize the variability of high cycle fatigue have been widely used. However, little is understood about the intrinsic relationship of randomness of microstructure attributes on the overall variability in high cycle fatigue. The ability of quantifying the dispersivity of high cycle fatigue with physics based computational methods has great potential in design of minimum life and can aid in the improvement of fatigue resistance. To investigate the effects between microstructure attributes and high cycle fatigue dispersivity, the microstructure-sensitive extreme value probabilistic framework is introduced. First, the Voronoi algorithm is used to construct random polycrystalline microstructure representative volume elements. Different kinds of periodic boundary conditions are proposed to simulate the interior and surface constraints in polycrystalline microstructure representative volume elements. Then mechanical responses of both interior and surface microstructure representative volume elements under different strain amplitudes are simulated by internal state variable based crystal plasticity. The fatigue indicator parameter is introduced to characterize the driving force for fatigue crack formation dominated by maximum shear plastic strain amplitude. By computing a limited number of random polycrystalline microstructure representative volume elements, the distributions of fatigue indicator parameter under different strain amplitudes are obtained and analyzed with extreme value probability theory. The study with a kind of titanium alloy with material grade TC4 supports that the high cycle fatigue dispersivity increases with the decrease of the strain amplitude, especially under elastic limit. The extreme value of fatigue indicator parameter from random polycrystalline microstructure representative volume elements correlates well with the Gumbel extreme value distribution. Besides, the lower the average stress under different strain amplitudes, the fewer grains in polycrystalline microstructure representative volume element yield. Moreover, the grains on surface tend to have higher probability to initiate fatigue cracks and lower dispersivity in fatigue crack formation.

9Cr2WVTa steel is one kind of reduced activation ferritic/martensitic (RAFM) steels, which are considered as the candidate structural materials for the accelerator driven subcritical system (ADS). Effects of Al and Si on the microstructure, tensile properties, impact toughness and corrosion behavior in liquid lead-bismuth eutectic (LBE) of 9Cr2WVTa steels were investigated by SEM, TEM, EPMA and micro hardness tester. The results showed that the addition of Al and Si promoted the formation of δ-ferrite, and Al was a much stronger ferrite stabilizer than Si. The presence of δ-ferrite significantly degraded the impact toughness of 9Cr2WVTa steels. M₂₃C₆ carbides were observed to precipitate at the δ-ferrite grain boundaries, and stress concentrations were created at the carbide/matrix interface, resulting in the intergranular cracking after deformation. Static corrosion tests were conducted in oxygen-saturated LBE at 550 ℃ for 5000 h to study the effects of Al and Si on the corrosion behaviors in LBE. It is shown that the addition of Al and Si improved the corrosion resistance in LBE due to that appreciable enrichments of Al and Si in inner oxide layer increased the compactness of oxide layer and reduced the diffusion rates of alloy elements and oxygen atoms.

Heat stability of nanostructure can be related to alloy element, in order to investigate the effect of external element diffusion, asymmetrical rolling was adopted to roll 3% non-oriented silicon steel to realize the surface nanocrystallization, heat-treatment with different parameters was carried out for the rolled sheet in vacuum and Si+1% (mass fraction) halide powder respectively, and different techniques were used to examine the microstructural evolution, phase transformation and Si distribution along the depth. Experimental results show that nanocrystallines about 10~20 nm in size with random orientations form in the top-surface layer after the asymmetrical rolling with the mismatch speed ratio 1.31 and rolling passes 20 for 91% reduction. In the heating process in vacuum, the recrystallization temperature of the nanocrystallines in the top surface layer of the rolled sheet was found to increase obviously comparing with that obtained after keeping at this temperature for a long duration. In the heating process in Si+1% halide powder, a further enhancement of the recrystallization temperature was observed for the nanocrystallines in the top surface layer of the rolled sheet due to the fastly diffusion of Si atoms along the defaults, then the larger volume fraction of grain boundaries can act as fast diffusion channel at higher temperature (750 ℃), that can accelerate the diffusion of Si atoms, therefore dense compound layer can be obtained within shorter duration and with lower fraction of halide (acts as activator).

316 stainless steel (316SS) is widely used due to a combination of good mechanical properties and excellent corrosion resistance. However, the intergranular stress corrosion cracking (IGSCC) is a serious problem for 316SS exposed to aggressive environments, which could result in unexpected failures and lead to huge losses. The grain boundary structure and local stress applied on the grain boundary are proved to have significant influence on the initiation of the IGSCC. In this work, thermal-mechanical processing was applied to the 316SS to yield a large-grained sample. The sample plates with a single-grained thickness were subjected to three-points bending SCC tests in an acidified boiling 25%NaCl solution. The result shows that the random grain boundaries (GBs) have the highest propensity to IGSCC initiation, while the Σ3 GBs shows very low tendency to IGSCC initiation. The absolute values of Schmid factor mismatch (Δm) between the grains on both sides of the GBs were analyzed for a large number of GBs. The distribution of the Δm for the Σ3 GBs is obviously different from that of the random GBs. The Δm has significant influence on the IGSCC susceptibility in the range of 0<Δm<0.1. The larger value of the Δm, the higher propensity for the IGSCC initiation at the GBs, for both the random GBs and the Σ3 GBs.

Localized corrosion such as pitting and mesa attack caused by the presence of solid deposits on a metal surface is defined as under deposit corrosion (UDC). UDC is frequently observed in oil and gas transition pipelines where sand, debris biofilm and carbonate deposit are present. Studies have found that the introduction of oxygen would accelerate the galvanic corrosion behavior between the deposit covered area and the area without deposit. Some experiments have been carried out and demonstrated that high concentration inhibitor should be used for the migration of UDC. However, the inhibition effect of the organic phosphonic inhibitor for UDC is rare in the previous studies. In this work, the evaluation of UDC behavior of X65 pipeline steel and the performance of corrosion inhibitor Ethylene Diamine Tetra (Methylene Phosphonic Acid) Sodium (EDTMPS) in the oxygen contained solutions are studied by using polarization dynamic scan (PDS), electrochemical impedance spectra (EIS) and linear polarization resistance (LPR) methods. The galvanic effect caused by the deposit is studied by using wire beam electrode (WBE). The measurement results show that the corrosion rate of deposit-covered electrode is lower than that of bare electrode, but localized corrosion is observed on the deposit-covered steel surface. After 35 mg/L EDTMPS is introduced into the solution, the corrosion rate of the bare steel decreased from 0.17 mm/a to 0.082 mm/a and the corrosion rate of the deposit covered electrode decreased from 0.051 mm/a to 0.026 mm/a. Protective films are observed on both deposit covered steel surface and bare steel surface after EDTMPS added. In the galvanic corrosion monitoring experiment by using WBE, the under deposit area has a lower potential and performs as the anodic area with serious localized corrosion. After 35 mg/L EDTMPS is injected, the average potential begins to decrease. The maximum anodic current density and the total anodic current respectively decrease from 0.21 mA/cm² and 0.056 mA to 0.078 mA/cm² and 0.021 mA. The electrochemical measurement results reveal that EDTMPS has an excellent inhibition effect for the corrosion of both bare electrode and deposit covered electrode. The WBE test illustrates that EDTMPS also has an inhibition effect on the galvanic corrosion caused by the covering deposit. However, EDTMPS cannot completely prevent the localized corrosion behavior on WBE surface.

With the development of industry, the atmosphere in many cities along the coastal lines such as Qingdao in China has been polluted with SO₂, and has been changed to coastal-industrial atmosphere with the co-existence of SO₂ and Cl^-. The corrosion and stress corrosion cracking (SCC) behavior and mechanism of steel in this environment is different from that in the coastal atmosphere containing only Cl^- or the industrial atmosphere containing only SO₂. Previous study have indicated that SO₂ in the marine atmosphere can greatly promote the stress corrosion cracking of high-strength steel due to acidification of thin electrolyte layer and reproduction of H⁺ through FeSO₄. E690 steel, as a newly-developed high strength steel, is very promising to be widely used in offshore platform in the near future for its excellent performance. However, there is few research about its SCC behavior in marine atmosphere, especially in SO₂-polluted atmosphere. Therefore, it's of great importance to investigate the SCC behavior and mechanism of E690 steel in this environment. In this work, U-bend specimen corrosion test under dry/wet cyclic condition, electrochemical measurements, crack morphology observation and rust layer analysis, were conducted to investigate the effect of SO₂ on SCC behavior of E690 steel in simulated SO₂-polluted marine atmosphere. The results indicated that E690 steel has a high SCC susceptibility in SO₂-polluted marine atmosphere with a combined mechanism of anodic dissolution (AD) and hydrogen embrittlement (HE). SO₂ in the atmosphere can facilitate the densification of inner rust layer by promoting the formation of α-FeOOH and enrichment of Ni and Cr in the inner rust layer, leading to the concentration of Cl^- under the rust layer, which may result in the initiation and propagation of SCC cracks significantly and therefore enhance the SCC susceptibility.

Element interdiffusion will accelerate failure of surface coating systems after a long time service at high temperature. To extend service life of the coatings, developing a diffusion barrier between the coating and the substrate is considered as an efficient way. Many research results showed that a diffusion barrier with single function such as metallic or ceramic one can not meet requirements for strong barrier ability and strong interfacial strength of the coatings onto the substrate at the same time. Anodic aluminum oxide (AAO) film with porous surface structure, which has an effective role for element diffusion so as to strengthen the interfacial adhesion rapidly, and a dense Al₂O₃ sublayer to suppress the interdiffusion was effectively used as diffusion barrier in this work and interdiffusion barrier ability was investigated. The AAO film was obtained by anodizing Al film deposited on C103 niobium alloy by vacuum evaporation technology, and an electroplating Ni plating was prepared as an overlayer. Vacuum heat treatment was applied to promote element diffusion. The results indicated that substantial diffusion occurred in the Ni/C103 specimen without an interlayer and in the Ni/Al/C103 specimen with Al film as an interlayer. In the Ni/AAO/C103 specimen, hardly any interdiffusion was observed. After 4 h vacuum annealing at 900 ℃, NbNi₃ phase was detected on the Ni/C103 and Ni/Al/C103 specimens, which could not be found in the Ni/AAO/C103 specimen. Nb content in the Ni overlayer of Ni/C103, Ni/Al/C103 and Ni/AAO/C103 specimens was 7.05%, 5.08% and 3.55%, respectively. Ni content in the substrate of Ni/C103, Ni/Al/C103 and Ni/AAO/C103 specimens diffusing from the overlayer was 6.84%, 3.62% and 2.85%, respectively. Thus, AAO film exhibited strong barrier ability in suppressing element diffusion. From calculation of the Fick's law, it was found that diffusion coefficient of Ni and Nb in the AAO film at 900 ℃ was 3.28×10^-14 m²/s and 2.16×10^-14 m²/s, respectively, and it was raised to 1.03×10^-13 m²/s and 3.58×10^-14 m²/s at 1000 ℃, respectively.

Periodic-layered structure during solid state reactions is one of the most complicated and interesting structures in the solids, which consists of a periodic sequence of layers that grow perpendicularly to the expected macroscopic diffusion flow. Since the Zn/Fe₃Si system was first discovered, much research work has been done on the characterization of the microstructures, the understanding of the formation mechanism and discovery of new systems. However, the exact nature of this phenomenon still remains a controversial topic. In the spirit of thermodynamic instability mechanism, the periodic-layered structure consists of single phase α layer and single phase β layer arrange alternately, while in that of dynamic instability mechanism, which is based on a diffusion-induced stress model, the structure is considered to be composed of regular multilayers of single phase α and two-phase α+β. In the present work, the solid state reactions of various Zn/Cu_xTi_y diffusion systems annealed at 663 K for different times were investigated by using melting contact method, SEM and EDS. The results show that both the polished sections and the in situ fracture surfaces of periodic-layered structure, 5 new systems, i.e. Zn/Cu₉Ti, Zn/Cu₄Ti, Zn/Cu₇Ti₃, Zn/Cu₃Ti₂, Zn/Cu₄Ti₃ are found to form periodic-layered structure within the diffusion zones. The periodic-layered structure is composed of the CuZn₂ single phase and CuZn₂+TiZn₃ two-phase layers distributing alternately within the reaction area near the Cu_xTi_y side. Furthermore, the thickness of the periodic layers relates to the composition of Cu_xTi_y substrates: the higher the content of Cu atom in the Cu-Ti substrate, the thinner the layer will be. In addition, the adjacent two-phase layers show mated topography and the interface between the periodic layers illustrates typical tear characteristics in mechanics, which are in good accordance with the prediction of the diffusion-induced stresses model. Therefore, the present work provides new and convincing evidence for the dynamic instability mechanism in the interpretation of periodic-layered structures in solids.

MnCo₂O₄ spinel was coated on the surface of SUS 430 alloy by using the sol-gel method. The oxidation kinetics behavior and electrical property of coated SUS 430 alloy in solid oxide fuel cells (SOFCs) cathode atmosphere were investigated. XRD, EDS and SEM were used to characterize the phase structure, surface and cross-section morphology, and composition of the surface oxides; the area specific resistance (ASR) of the surface oxides was measured by using the four-probe direct current technique. The result shows that a 2 μm thick oxide scale, mainly consisting of an inner layer of Mn-Cr spinel and an outer layer of doped Mn-Co spinel, was formed during cyclic oxidation at 750 ℃ in air for 1000 h. The growth of Cr₂O₃ and Fe₂O₃ was depressed. The oxidation kinetics obeys the parabolic law with two rate constants 3.74×10^-15 g²/(cm⁴s)(0~200 h) and 7.06×10^-15 g²/(cm⁴s) (200~1000 h), respectively, which is 1 order of magnitude lower than that of the SUS 430 alloy without coating. The ASR is in the range of 5.21~22.65 mΩcm² at 600~800 ℃. MnCo₂O₄ coating was proved to be effective in enhancing the oxidation resistance and electrical property of SUS 430 alloy.

Intermetallic compounds have unique natures. Due to the natures of high temperature resistance, high strength and high hardness, intermetallic compounds always exist as strengthening phase in many alloys. The primary Al₂Cu phase in Al-Cu alloys is an intermetallic phase. The morphology, size and distribution of intermetallic compounds phases have largely effects on the mechanical properties of materials. Morphological evolution of intermetallic compounds is necessarily theoretical basis for controlling the size, morphology and improving the performance of intermetallic compound materials in the solidification process. At present, there are many reports on the research of Al-Cu alloys, the main research is focused on the eutectic point and 4.7%Cu (mass fraction) of Al-Cu alloys, but other composition alloys are less considered. The growth mechanism of Al₂Cu phase and the primary Al₂Cu phase structure of Al-Cu alloy are studied recently. However, the specific growth mechanism of Al₂Cu phase is currently not very clear. Alloy composition and cooling rate are often encountered in the ordinary metal melting and solidification. The change of solidification conditions will lead to the transformation of heat and solute in the melt, which will form different morphologies. In this work, the effect of alloy composition and cooling rate on the morphologies and growth behavior of Al₂Cu phase in Al-Cu alloys was studied. Through the microstructure observation of Al-xCu (x=30, 40, 45, 50, mass fraction) alloys, it was found that primary Al₂Cu phase morphologies transformed from dendritic shape to regular bulk with the Cu content increased from 30% (mass fraction) to 50% in the alloy, which indicated that Al₂Cu phase growth changed from non-faceted growth to faceted growth. Cooling rate also had a vital influence on primary Al₂Cu phase morphologies. Under low cooling rate, primary Al₂Cu phase morphologies were regular bulk. The morphologies of primaryAl₂Cu phases were transformed into dendritic shape with the increasing of cooling rate. The specific morphology transformation rule of primary Al₂Cu phases in Al-Cu alloys was also studied in the solidification process. It was found that when Cu content was 45%, the morphology transformation of primary Al₂Cu phases was from dendritic shape to square morphologies. While when Cu content was increased to 50%, the morphology transformation of primary Al₂Cu phases was from dendritic shape to reticular morphologies.

Cu-Zn alloys are one of the most commercially important metallic materials because of their excellent physical and mechanical properties, ease of fabrication and low cost. Ultrafine grained (UFG) metallic materials intrigue great interest due to their high strength, and most UFG materials are produced by severe plastic deformation (SPD). However, utilizing SPD to produce UFG materials needs large strain. Moreover, most UFG alloys produced by SPD have limited thermal stability and ductility which restrict the application in practical production. In this work, a UFG Cu-30Zn-0.15Fe alloy with good comprehensive properties and high thermal stability was prepared. Effect of minor Fe addition on the microstructure evolution of UFG Cu-Zn-Fe alloy subjected to cold rolling and subsequent isothermal annealing at 573 K was investigated through OM, TEM and SEM/EBSD observations. The results show that second phase particles are introduced into Cu-Zn-Fe alloy with trace P element by Fe addition. The second phase particles are identified as hcp structured Fe₂P phase with diameters ranging at 50~300 nm. The hardness-annealing time curves of Cu-30Zn and Cu-30Zn-0.15Fe alloys have three stages, corresponding respectively to recovery, recrystallization and recrystallized grains growth. It takes longer time for Cu-Zn-Fe alloy to get recrystallization started; after fully annealed, the hardness of Cu-Zn-Fe alloy is much higher, with 30 HV increment than that of Cu-Zn alloy. The UFG Cu-Zn-Fe alloy has highly stable average grain size of 1.3 μm during the process of annealing, which results from Fe₂P particles suppressing the growth of recrystallized grains. The Fe₂P particles retard grain boundary migration and dislocation movement, resulting in less mass fraction of Σ3 twin boundaries, lower increasing speed, higher dislocation density and local stored energy. The main strengthening mechanisms for present UFG Cu-Zn-Fe alloy are second phase strengthening, fine-grain strengthening and dislocation strengthening.

Superalloy In718 enjoys wide application in such crucial parts as turbine engine disks due to high strength, great toughness and corrosion resistance in different temperature environment. Since the mechanical properties of superalloy In718 are greatly influenced by the grain size, a nondestructive detection method is studied in order to determine the grain size quickly and effectively. In this work, superalloy In718 samples of different grain sizes were produced and the empirical mode decomposition (EMD) method was employed to find the characteristics of the time-frequency domain of the ultrasonic backscattering signals. Then the effects of the grain size over the intrinsic mode function (IMF) of different frequency bands were analyzed to seek the relations between the grain size and the power of the IMF signals. The original backscattering signals and IMF1 (the first IMF) signals barely respond to the change of the grain size because of their wide frequency bandwidths; the distribution of the frequency domain of the IMF2 signals is centralized and the amplitude of the peak frequency increases with the grain size, and the correlation coefficient between the power and the grain size is 0.995, much higher than that of other modes. This method eliminates the components irrelative to the grain size and takes the IMF2 components which fully reflect the intensity of the grain scattering as the characteristic signals of the grain size evaluation to build an ultrasonic backscattering EMD model evaluating the grain size of superalloy In718. The actual measurement results of the grain size show that the sensitivity of this method is 3.7 times over the traditional backscattering method; the evaluation errors over the two verification test samples are -3.72% and 2.87%, apparently more accurate than the ultrasonic velocity method; compared with the attenuation method, this method requires no information of the thickness so that the evaluation results are independent of the thickness measuring error; compared with the metallographic method, this method is more efficient and requires no damage on the components to be evaluated.