In this paper, research progresses on gradient nanostructured materials in recent years is briefly reviewed. It includes classification of gradient nanostructures, properties and processing techniques of the gradient nanostructured materials. Perspectives and challenges on scientific understanding and industrial applications of gradient nanostructured materials are addressed.
Carburization in Ni-Cr-Fe-based alloys is an important phenomenon, especially in ethylene cracking tubes which serve at high temperatures under highly carburizing environment. In this work, the Cr35Ni45Nb tube subjected to service condition for 6 a was carburized by low-pressure vacuum carburizing (LPVC) at 1080 ℃. The carburization behaviors and corresponding mechanisms of phase evolution in the inner wall were comprehensively analyzed through SEM, XRD and EPMA. The results showed that oxidation behaviors of the tube at high temperature were consisted of external oxidation of Cr and internal oxidation of Si, resulting in formation of composite oxide scales. Depletion of Cr in the subsurface caused by surface Cr2O3 leaded to carbide dissolution and formation of carbide free zone and carburized zone. The critical concentration of Cr for carbide dissolution is about 19.0% (mass fraction). By comparing carburization behaviors of specimens whose oxide scales were retained or removed, the carburization resistance of the composite oxide scales in carburizing environment was systematically investigated. The results showed that the composite oxide scales formed previously acted as an effective barrier to carbon infiltration. However, the outermost Cr2O3 scale tended to be carbonized to form carbide scale to spall from the surface in the strongly reducing environment with low oxygen partial pressure, while the SiO2 kept stable all along due to its excellent thermodynamic stability. However, a certain amount of carbon was still capable to penetrate the alloy interior through gaps of the SiO2 scale due to its discontinuity. Therefore, continuity, density and high-temperature stability of the oxide scales were crucial for the alloy to achieve excellent anti-carburizing performance. Once the oxide layers were removed or carbonized adequately, inconceivable internal carburization occured widely. Large amounts of secondary carbides precipitated again in the previous carbide free zone due to high carbon activity. Widespread precipitations of graphite called metal dusting in the range of about 0.5 mm in depth occurred after long exposure of specimens to the carburizing environment. The carbon activity gradually decreased with increasing distance from the surface. The primary carbides within the deeper carburized region were transformed from M23C6 to M7C3 in situ, which were accompanied by precipitation of vermicular g phase in the primary carbides, phase transition from h to NbC and decomposition of intragranular secondary carbides. Severe coalescing and coarsening of carbides and metal dusting caused the serious degradation of microstructure, formation of macro-cracks and final thinning of the Cr35Ni45Nb tube wall.
Recently, the steel plates used in the ship, pipeline and bridge generally required not only high strength but also excellent low temperature toughness. As a competitive candidate, the ultra-low carbon high strength low alloyed (HSLA) steel has been developed widely. The low temperature toughness depends on the microstructure of the steels. Therefore, the relationship of low temperature toughness and microstructure should be studied in detail. In the present work, the steel plates with 25 mm thickness after hot rolling were immediately water quenched to 550, 450 and 350 ℃(finish cooling temperature), respectively, and subsequently air cooled to room temperature. The effect of finish cooling temperature on the microstructure and low temperature toughness of Mn-series ultra-low carbon HSLA steel was investigated by SEM, TEM and crystallographic analysis. The results show that the granular bainite, lath bainite and martensite were obtained with finish cooling temperatures decreasing. There are three blocks with different orientations in a single packet for lath bainite microstructure in the sample with finish cooling temperature of 450 ℃, leading to the refinement of effective grain size and large amount of high-angle grain boundaries. Electron backscattered diffraction analyses of the cleavage crack path show that the bainite block boundaries can strongly hinder fracture propagation, and thus the refinement of bainite blocks can improve the low temperature toughness of Mn-series ultra-low carbon HSLA steel. Finally, the yield strength of 775 MPa and ductile-brittle transition temperature of -55 ℃can be achieved when the finish cooling temperature is 450 ℃.
High strength micro-alloyed steel has been widely used in automobile and machines because of the remarkable high strength and forming property which are attributed to nano-precipitates and refinement of the organization. Since the nano-precipitates are mostly nucleated between austenite/ferrite transition and ferrite can significantly advance strength, it is important to investigate precipitate behavior in coiling process. Nanoindentation technology provides a chance to study the special influence of nano-precipitates on the micro-properties of ferrite. The effect of cooling rate during continuous cooling process and coiling process on microstructural evolution and micro-hardness of Nb-Ti micro-alloyed steel were studied by using the thermal mechanical simulator, micro-hardness instrument, TEM and nanoindentation instrument. The precipitate behaviors of (Nb,Ti)C in coiling process and its effect on nano-hardnesss of ferrite were discussed. Experiments results indicated that the increase of cooling rate in continuous cooling process and coiling process could promote the microstructure transition from ferrite and pearlite to bainite. The micro-hardness of the tested steel increased with the increase of cooling rate in continuous cooling process, and decreased with the cooling rate in coiling process because of the large number of the dispersive nano-precipitate in ferrite which could improve the strength of matrix. The smaller cooling rate could promote volume fraction of (Nb, Ti)C particles in ferrite because there was enough time for the nucleation and growth of (Nb, Ti)C precipitates. When the cooling rate in coiling process was 0.1 ℃/s, precipitates were dispersive in ferrite matrix with a diameter of less than 10 nm. The nanohardness and Young's modulus of ferrite were 4.13 and 249.3 GPa for Nb-Ti micro-alloyed steel, 2.64 and 237.4 GPa for C-Si-Mn steel. The contribution of nano-precipitates to nano-hardness of ferrite reached 1.49 GPa.
In continuous unidirectional solidification process, an unidirectional heat transfer condition can be established to control grain growth direction along the solidification direction (SD). By this method, continuous columnar-grained (CCG) polycrystalline alloys without transverse grain boundary can be obtained, which possess high orientated texture and straight grain boundary morphology. High orientated texture can significantly improve the consistency among the grains, and the straight grain boundaries reduce the number of coordinated strain components, resulting in high plasticity and excellent extension behavior along the SD in the CCG alloys. For example, the CCG polycrystalline CuNi10Fe1Mn alloy has a high tensile elongation (>40%). However, as described above, the CCG polycrystalline alloy has an extremely anisotropic microstructure. In order to improve its performance, select the appropriate processing methods, and establish a reasonable process, its mechanical properties and deformation behavior were investigated with tensile direction along the SD or perpendicular to the solidification direction (PD) in this work. The electron back-scatter diffraction (EBSD) and digital image correlation (DIC) techniques were introduced to study the effects of microstructure anisotropy on the mechanical properties and deformation behavior. The results indicate that both SD and PD samples have [100] preferred orientation. All grains in SD samples (Taylor factor m=2.17) are nearby [100], while some grains in PD samples (Taylor factor m=2.93) scatter among [001]-[011]. Microstructure characteristics of low orientation dispersion and no horizontal grain boundary in SD samples contribute to the uniform stress distribution and consistent deformation behavior in each grain along the tensile direction. The yield strength, tensile strength and elongation are 85 MPa, 215 MPa and 42%, respectively. Compared to SD samples, PD samples appear to grain boundary stress concentration and zigzag surface morphologies due to the orientation dispersion and horizontal grain boundaries. As a result, the yield strength markedly increases to 115 MPa, and the elongation decreases to 36%. The SD and PD samples occur ductile and mixed fracture, respectively. The anisotropic deformation behavior of CCG polycrystalline CuNi10Fe1Mn alloy is attributed to the anisotropic grain orientation and the grain boundary distribution.
Amorphous alloy is a new type of material that exhibits exceptional properties or combinations of properties that are often not achievable in conventional crystalline materials. Fe-based amorphous alloys has attracted significant attention over the last few decades because of their low cost and enhanced mechanical performance. However, they are more suitable for the industrial application of coatings due to the fatal disadvantages of poor toughness. High velocity oxygen-fuel (HVOF) spraying is a good way to make amorphous alloy coatings (AMCs), for the individual droplets are cooled at a rate of around 107 K/s which is much higher than the critical cooling rate of the amorphous alloys during the thermal spraying. Fe-based AMCs obtained by using the HVOF spray method are important materials for industrial applications because of high glass-forming ability and exceptional performances, such as excellent corrosion resistance, high hardness, and superior wear resistance. In this work, FeCrMoMnWBCSi AMCs were prepared by HVOF thermal spray. The microstructure and amorphous characteristics of AMCs were characterized by SEM and XRD. Electrochemical corrosion behavior of AMCs was investigated in different concentration of NaCl and H2SO4 solutions compared with that of 304 stainless steel and ND steel. The surface film of materials after immersed in two solutions was analysed by XPS. The results indicated that HVOF thermal spraying Fe-based AMCs presented dense layered structure, high amorphous phase content and low porosity. The composite structure of AMCs was formed with some nanocrystallite phases embedded in the amorphous matrix. AMCs exhibited better resistance to pitting corrosion and relatively low uniform corrosion resistance due to the porosity, while the pitting potential of 304 stainless steel was sensitive to NaCl concentration. XPS results revealed that the presence of Cr, Mo and W oxides in the passive film of AMCs may result in the better corrosion resistance. The enrichment of Mo4+ oxides on the surface favored the formation of a more stable and protective layer which could be assumed to be responsible for the observed high stability of passive film. The diminishing or avording pores may be beneficial to further improve the pitting corrosion resistance of AMCs in NaCl solution. In all cases, AMCs showed better resistance to H2SO4 solutions corrosion due to the high stability of passive film. 304 stainless steel and ND steel presented stable passivation behavior only in high concentration of H2SO4 solution. In the lower concentration solution of H2SO4, the amorphous structure of the thinner coatings could facilitate the formation of thicker passivation film and lead to the higher corrosion resistance. The corrosion resistance of AMCs in H2SO4 solution could be enhanced significantly by formation of high amorphous phase.
7075 aluminum alloy is an ultra-high strength alloy containing Al, Zn, Mg, Cu and Cr elements, and is widely used in the aviation industry, but it has severe intergranular corrosion characteristics. The high-entropy alloys are composed of more than five major metallic elements and possess excellent corrosion resistance. When laser shock, featuring ultra high energy as well as the thermodynamic and kinetic loading characteristics far-from-equilibrium states, acts on the surface of alloys with multiple elements, high-entropy alloy surface layer with specific properties may be obtained. In this work, surface modification of 7075-T76 aluminum alloy by laser shock was investigated. The microstructure, formation cause of the amorphous/nano-crystalline composite high-entropy alloy surface layer obtained by laser shock, hardness and corrosion resistance of the laser were analyzed by means of SEM and TEM. The results show that the adiabatic shear thermal effect induced by super high energy, ultra-fast process of laser shock causes surface alloy system to occur entropy increase effect and partitioning. The high mixing entropy contributes to the randomization increase of the alloy system. Thus, the elements in the system spontaneously self-organize in accordance with the law of Boltzmann. The dynamical formation of the nano-crystalline grains coordinates the thermodynamic equilibrium during the process. The strain-hardened layer is composed of amorphous microstructure and nanocrystalline grains, and the total depth of it reaches up to about 100 μm. After 1 time laser shock,the depth of the surface high entropy layer is about 20 μm, of which the diameter of the nanocrystalline grains is 6~8 nm. After 3 times laser shock, the thickness of the layer can increase to more than 40 μm, and the diameter of the nanocrystalline grains is 2~3 nm. Meanwhile, the intense ultra high strain-rate induced by the laser shock makes precipitates deform, producing parallelly distribution of deformation twins in order to balance the laser energy. After repeated laser shocks, the hardness of the amorphous/nanocrystalline layer gradually closes to that of the matrix of the alloy because of the disappearing of the support of grain boundaries to the strength, the dislocation strengthening effect in nano-crystalline grains, and the coherent relationship between precipitates and matrix. Due to that the amorphous microstructure can prevent galvanic effect around precipitates, and nano-crystalline has good chemical stability, the nano-crystalline/amorphous composite high-entropy layer on surface of 7075-T76 aluminum alloy induced by laser shock can significantly improve the corrosion resistance, and effectively block the intergranular corrosion of the alloy.
The Ni-based superalloys are widely used as microstructural components of modern turbine engines due to its good high temperature strength, good fatigue and creep property and excellent hot-corrosion resistance. In order to increase their high temperature strength, more and more refractory elements, such as W and Mo, are added into these alloys while Cr content gradually decreases. During long-term aging, these alloys generally experience various microstructural changes, including coarsening of g' phase coarsening, formation of a continuous grain boundary (GB) carbide network, precipitation topologically close-packed (TCP) phase, and degeneration of MC carbide. However, there is limited available data about the effect of (W+Mo)/Cr ratios on the microstructural evolution of Ni-based superalloys. In this work, the cast Ni-based superalloys with different (W+Mo)/Cr ratios (mass ratios) are fabricated by vacuum induction furnace. After standard heat treated (1110 ℃, 4.5 h, air cooling+750 ℃, 10.5 h, air cooling), they are thermally exposed at 850 ℃ for different times. The stress-rupture tests are operated under the condition of 800 ℃, 294 MPa. Effects of (W+Mo)/Cr ratios on the microstructure evolutions and mechanical properties are investigated by the combination of OM, SEM, TEM and stress-rupture tests. The experiment results show that the (W+Mo)/Cr ratio has no obvious influence on the standard heat treated microstructure, which is mainly composed of g matrix, g' phase, MC carbide and secondary carbides distributing at grain boundaries. During long-term thermal exposure, the microstructure evolutions occur by g' phase coarsening, TCP phases formation, MC degeneration and grain boundary coarsening. The g' phase coarsening behavior is not affected obviously by the (W+Mo)/Cr ratio. However, the amount of TCP phases decreases significantly with decreasing of (W+Mo)/Cr ratio and the type of TCP phases transforms from m phase to coexist of m and s phases when (W+Mo)/Cr ratio decreases from 0.55 to 0.37. There are no TCP phases observed in the sample with (W+Mo)/Cr ratio of 0.22. The thermal stability of MC carbide is reduced obviously and the grain boundaries coarsen more severely by the decrease of (W+Mo)/Cr ratio. The degradation of stress-rupture property is attributed to the coarsening of g' phase and grain boundaries and the formation of TCP phases. Combined with the effect of (W+Mo)/Cr ratio on the solid solution strengthening, microstructure evolution and stress-rupture property, it can be concluded that the optimum stress-rupture property can be obtained when the (W+Mo)/Cr ratio is about 0.37.
The Ni-based single crystal superalloys are considered to be the major materials for advanced areo-engine blades. In order to improve the high temperature properties of Ni-based single crystal superalloys, many refractory elements are introduced into this kind of alloys. Recently Pt has been suggested to be the alloying elements of advanced Ni-based single crystal superalloys. However, there are no researches for the effects of Pt on creep rupture properties of advanced single crystal superalloys. In this work, the influence of Pt element on the creep rupture properties of a Re-containing single crystal superalloy was investigated. The high-temperature creep rupture properties of the Pt-containing Ni-based single crystal superalloy at 1100 ℃, 180 MPa and 1000 ℃, 310 MPa were investigated. The deformation microstructure and the morphology of dislocations were studied by SEM and TEM. The results show that the creep rupture life of Pt-containing superalloy decrease slightly at 1100 ℃, 180 MPa and decreased obviously at 1000 ℃, 310 MPa. The fracture models of different alloys are all ductile fracture, and many irregular microviods and microcracks can be observed in the fracture surfaces. After high temperature creep deformation, regular dislocation networks formed at the g/g' interfaces. The differences of creep rupture properties among those alloys are that Pt element may promote the formation of TCP phase, and the interface between the TCP phase and g matrix may be favorite sites of the initiation of microvoids and microcracks.
The electropolished (EP) alloy 690TT samples were first oxidized in the simulated B and Li containing primary water with 2.5 mg/L H2 at 325 ℃ and 15.6 MPa for 720 h, and then half of the samples were continuously immersed in this solution with 2.0 mg/L O2 for another 720 h. The microstructures and chemical composition of the oxide films formed under the above two conditions were analyzed. The results show that the dual layered oxide film formed under the single hydrogen water chemistry is mainly composed of spinel oxides. The outer layer is composed of big oxide particles rich in Ni and Fe and the underlying loose needle-like oxides rich in Ni. The inner layer is continuous Cr-rich oxides. The oxide film formed on EP alloy 690TT under the hydrogen/oxygen water chemistry also shows a dual layered structure. The surface morphology and chemical composition of the outer layer are similar to the oxide film formed under the hydrogen water chemistry. However, the inner layer is changed to the nano-sized NiO. The stable phase region in the potential-pH diagram for the Ni oxides is enlarged by the later dissolved oxygen. As a result, the oxygen promotes the fast growth of the outer needle-like oxides rich in Ni. Further, the oxygen promotes the dissolution of the inner Cr-rich oxides formed under the hydrogen water chemistry and increases the corrosion rate of the EP alloy 690TT. Electropolishing treatment can not reduce the corrosion rate of alloy 690TT in the simulated primary water with sequentially dissolved hydrogen and oxygen.
The effect of liquid-solid electromigration (EM) on the interfacial reaction in Ni/Sn-9Zn/Ni interconnects was investigated under a current density of 5×103 A/cm2 at 230 ℃. A reverse polarity effect was revealed, i.e., the interfacial intermetallic compounds (IMC) at the cathode grew continuously and was remarkably thicker than those at the anode. This results from the directional migration of Zn atoms from the anode toward the cathode, which is induced by the positive effective charge number (Z *) of Zn atoms but not the back-stress. A thin Ni5Zn21 layer formed at each interface after soldering. The initial Ni5Zn21 interfacial IMC gradually transformed into [Ni5Zn21+(Ni, Zn)3Sn4] after liquid-solid interfacial reaction for 8 h, due to the local equilibrium at the interface changed with decreasing of Zn atoms content. The interfacial IMCs at both anode and cathode were identified as Ni5Zn21, and no IMC transformation occurred undergoing liquid-solid EM, because the Zn atoms content at the cathode was enough under electron current stressing, and the diffusion of Zn atoms toward anode was inhibited. The reverse proving was proposed to explain the positive value Z * of Zn atoms. The abnormal directional migration of Zn atoms toward the cathode prevented the dissolution of cathode substrate, which is beneficial to improving the EM reliability of micro-bump solder interconnects.
In the past decades, the indentation test has been widely used to determine the mechanical properties of materials. For the micro- or nano- indentation, the indentation response is complex since only one or two grains can be indented by the indenter. In order to investigate the indentation behavior of the grain boundary, the crystal plasticity theory was implemented into finite element model to simulate the indentation behavior of single crystals and bicrystals. The stress distributions on the indented surface and grain boundary were obtained. The results showed that the crystallographic orientations of the neighboring grains had a remarkable influence on the depth-load response and the resolved shear stress distribution of the indented bicrystals. Under the indentation load, stress concentration occurred at the grain boundary, and the stress at the grain boundary increases with the increase of mis-orientation angle of the two neighboring grains.
The effect of defects in metal Ti such as vacancies, self-interstitial atoms and impurity He atoms on mechanical properties of metal Ti sample was studied using molecular dynamics simulation. First, the stress-strain curves of perfect Ti sample at different strain rates were calculated. The results show that the stretching process can roughly be divided into three stages, elastic deformation, plastic deformation and fracturing. For comparison the stress-strain curves of metal Ti samples with vacancies, self-interstitial atoms and impurity He atoms were researched, respectively, in which the strain rate was set as 2×109 s-1. Finally the corresponding Young's moduli were calculated. It is found that after carefully investigating that the mechanical properties of metal Ti are degraded by each of these effects in it and the degradation degree increases with increasing defect concentration. However, the stretching process of samples is not essentially affected by these effects (the stress-strain curves of Ti samples with defects have still 3 stages). In this process, self-interstitial atoms in samples always exist for they to be bonded by metal Ti atoms, but impurity He atoms in samples are released due to their extraordinarily low solution in metal Ti.
The height of floating zone and molten zone instability for five pure metals including Nb, W, Ta, Mo, and Ir with high melting points is investigated using electron beam floating zone method (EBFZM). The results show that the height level of floating zone for these five metals are in order with the sequence of Nb>Mo>W>Ta>Ir. The crystal growth angles for these metals are in the range of 8°~13° and the sample in large size can be developed by EBFZM as the growth angle is found not to be zero. Meanwhile, the actual growth angles are related with the interface growth mechanism. For continuous growth mechanism, the growth angles vary slightly with the solidification rate for rough interface, and for dislocation growth mechanism, the growth angles decrease with increasing the solidification rate. If faceting growth mechanism prevails, the growth angles drop remarkably at a low solidification rate and further increase with increasing the solidification rate. Additionally, by employing EBFZM growth of Ir and Mo pure metals, a solidification rate approaching 1 mm/min is available for controlling the growth angle and the height of floating zone. These calculations fit well with the experimental results of Mo single crystal prepared by EBFZM.
To solve such problems as sensitivity to noise and low accuracy of grain size evaluation using traditional ultrasonic time-domain attenuation method, an ultrasonic nondestructive evaluation model based on multi-scale attenuation coefficient was proposed. The distribution of time-scale of ultrasonic energy was obtained by means of wavelet transformation, then to calculate the distribution of attenuation coefficient with scale, and to make a comprehensive analysis of attenuation characteristics of various scales. After the weighted multi-scale ultrasonic attenuation coefficient was defined, a multi-scale ultrasonic attenuation evaluation model was established on the basis of combination of optimal dimension and normalized weight distribution strategy designed by particle swarm optimization. 304 stainless steel was used in the test. The distribution of attenuation coefficient with scale shows that ultrasonic wave of small scales attenuates fast, presenting the frequency characteristics of ultrasonic attenuation among high scattering materials. Following increase of the sample grain size, ultrasonic attenuation of all scales was intensified significantly. Test results show that the sound velocity method, the traditional evaluation method and the proposed method have maximum systematic errors of +12.57%, +5.85% and -1.33%, respectively. With these 3 methods, evaluation results of the sample with a mean grain size of 103.5 mm measured by metallographic method are (110.4±7.8), (98.2±6.6) and (101.7±3.9) mm, respectively, showing that the presented method can not only reduce the systemic error, but also can effectively control the random error by constant Q filtering properties of wavelet transformation. This model can be extended to grain size evaluation of other metals.
