In the production process of ultra-high strength steel, heating schedule of casting slab is one of primary controlling parameters in hot rolling techniques. Heating temperature and holding time affect the prior austenite grain size and the solution of microalloyed elements directly, which both affect austenite recrystallization, precipitation and mechanical properties. A plenty of researches have been made to get better understanding and controlling the austenite grain size or precipitation behavior during austenitizing process over the past half a century, but it lacks systematic researches. Hence it is very important to confirm a reasonable heating schedule. In this work, the austenite grain coarsening behavior and microalloying carbonitrides dissolving behavior in 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel during isothermal holding at different temperatures were studied by OM, TEM and EDS. The results showed that the precipitates in the slab can be obviously classified into three kinds by their size and shape. The average dimension of the bigger cubic precipitates is over 1 μm, and that of the smaller spherical, ellipsoid or cubic precipitates is below 500 nm or less. EDS results showed that the bigger cubic precipitates are TiN, and the smaller spherical, ellipsoid or cubic precipitates are mainly composed of Nb, Ti composite precipitates and a bit of TiS or Ti (C, S). With the increasing of holding temperature, increase of the original austenite grain size showed a monotonous increase trend, and the austenite grain grows rapidly when the heating temperature exceeds 1200 ℃; the amount of precipitates decreased and their size increased, the atomic ratio of Ti/Nb increased gradually, and EDS results showed all the precipitates contain Nb, Ti elements. With the increasing of holding time, the average austenite grain grew up in parabolic law, the amount of small sized spherical and ellipsoid precipitates dissolved gradually, and that of the large sized cubic precipitates increased gradually and their edges become blurred. The effects of heating temperature and holding time on austenite grain size and precipitation behavior were considered synthetically, the heating temperature at 1250 ℃ by holding 80 min will be more appropriate for the 1000 MPa grade Nb, Ti microalloyed ultra-high strength steel.
With industrial developments of aerospace vehicles, marine devices, biomedical and bones, pure Ti and its alloys have gained a great deal of attraction due to their superior properties. Despite having promising properties, limitations of lower hardness, inferior weldability, and poor brittle fracture have restricted their applications. So the objective of this work was to make surface electrodeposition of nanocrystalline Ni coatings on the surface of TA2 substrate using pulsed electrodeposition. Scratch tests was used to compare how effects of two typical HF and DMF activating solutions on modifying interfacial adhesion between TA2 substrate and surface coatings. In order to disclose crystal growth of Ni coating without and with CeO2 addition, a variety of characterizations such as FESEM, TEM and XRD were employed. A novel decussating-type microhardness with different loading forces attached with nanoindentation tests was conducted to make a comparative study of toughness and strengthening mechanisms between surface coatings and the un-coated TA2 substrate. Besides, wear behaviors of specimens was carried out using the ball-disc dry sliding tests. Results indicated that the addition of CeO2 nanoparticles into electroplating solution has effectively modified textural growth of Ni grains. This result was attributed to the presence of nano-sized CeO2 particles that adsorbed onto the preferred locations of crystal Ni growth, leading into an increasing catalytic site of nucleation to reduce Gibbs energy for grain refinement. According to observations of edges for hardness indentations, a smaller size with convergence feature for Ni-CeO2 coatings was indicative of effects of CeO2 nanoparticles or its precipiated Ce solute atoms on alloying-dispersion strengthening for completing defective grain boundaries. While for the case of a divergency state of indentations edges within obviously spalling cracks, it exhibited poor surface toughness for pure nickel. Furthermore, An analytic modeling validated here was based on the by-passing Orowan for dislocations pile-up mechanisms, in which this was contributed to the co-existence of Ce-rich worn products and NiO passive film to be expected as solid lubricants and make the self-lubricating effect, thereby improving wear resistance of Ti alloys where subjected to harsh conditions.
Microbiologically influenced corrosion (MIC) is a major corrosion related problem for steel pipelines. The great loss caused by microbiologically influenced corrosion (MIC) on buried pipelines has been paid considerable attention domestically and internationally. Various physical, chemical or biological strategies have been used for MIC control, including biocides, coatings, cathodic protection and biocompetitive exclusion. These strategies have limitations of being expensive, subject to environmental restrictions, and sometimes inefficient. There is an urgent need for oil industry to develop environmentally friendly strategies for microbial corrosion control. Cu could play many benefical effects in steels, such as exerting a vigorous effect on hardenability, enhancing strength via precipitation strengthening, improving fatigue resistance, reducing susceptibility of hydrogen embrittlement, promoting formation protective layer etc.. Cu is well known for its inherent antimicrobial properties and is the focus of interest for potential application as a component in antibacterial materials. The Cu-bearing antibacterial stainless steel, characterized by continuous release of Cu ions with antibacterial function, provides analogy to develop a new type of MIC resistance pipeline steel. In this work, three different Cu contents (1.06Cu,1.46Cu,2.00Cu, mass fraction, %) pipeline steels, named 1.0Cu, 1.5Cu and 2.0Cu, were fabricated by making proper Cu alloying designs for X80 steel that currently used in oil/gas industry. Study on antibacterial performance and MIC behavior of novel Cu-bearing pipeline steels against Escherichiacoli (E.coli), Staphylococcusaureus (S.aureus) and Sulphate reducing bacteria (SRB) was carried out by antibacterial tests, electrochemistrical monitor, corrosion product analyses and confocal laser scanning microscope (CLSM). The results showed that Cu-bearing pipeline steels had strong antibacterial performance against E.coli and S.aureus compared with X80 steel. 1.0Cu steel with the microstructure of polygonal ferrite showed excellent resistance to SRB with remarkable strength enhancement by nano-scale Cu-rich precipitates and good impact toughness compared with X80 steel. Cu-rich precipitates in Cu-bearing pipeline steels were found to be responsible for the antibacterial capability. The linear polarization resistances (RLPR) of both X80 and 1.0Cu steels in the soil-extract solution with SRB were dramatically decreased after 2 d, leading to the corrosion current density (icorr) value of X80 steel was much higher than that of 1.0Cu steel. The corrosion product analysis results showed that much biofilm produced by SRB was the reason that many pits and larger pit depth on X80 steel than that of 1.0Cu steel.
Oxide scale on hot rolled strip steel has been successfully applied to decrease the corrosion loss of the steel during its transport and storage. In recent years, some efforts have been made to improve atmospheric corrosion resistance of weathering steel by oxide scale on its surfaces. However, the structure and electrochemical properties of oxide scale and their evolution during atmospheric corrosion still need to be characterized. In this work, XRD, electrochemical test and scanning electron microscopy have been carried out to investigate structures of oxide scales on surfaces of weathering steel samples and their influence on subsequent atmospheric corrosion of the samples. To produce oxide scales, the samples had been held isothermally at 400~700 ℃ in open or close spaces for different times. It has been found that oxide scales consist of Fe3O4 and Fe2O3. The electrical resistance of oxide scales is far higher than that of oxide film on sample which has not been subjected to oxidation treatment. Meanwhile, oxidation results in obviously raised free corrosion potential. Oxide scales are composed of loose outer layers and compact inner layers from which protective action is derived. The relatively compact oxide scales form at 500~600 ℃ while prolonged holding time promotes oxide scales to become compact. The limited oxygen providing inhibits oxide scales to become compact. The compact oxide scales slow down atmospheric corrosion in initial stage while they accelerate atmospheric corrosion after long time. These results indicate that compact oxide scales are difficult to transform into corrosion products and remain as inclusions and defects in the rust layers which accelerate corrosion.
High Co-Ni secondary hardening steels have received great attention for a long time. In several decades, many previous studies were made to improve their strength, toughness, corrosion resistance ability and even stress corrosion sensibility. Although many new kinds of high Co-Ni secondary hardening steels were developed by researchers, the mechanisms of strength and toughness were still a problem, which would inhibit the further development of high Co-Ni secondary hardening steels. In this work, both microstructure and mechanical properties of high Co-Ni secondary hardening steels were analyzed by simulation and experiment. The strengthening and toughening mechanism was explained, which included the transformation induced plasticity (TRIP) effect of austenite layer in nanometer size and the precipitation of M2C in nanometer size. According to the previous studies about AerMet100 steel, the design standard for high Co-Ni secondary hardening steels was established, which included the mole volume change for austenite, the stability of austenite, the austenite thickness, the austenite equilibrium content, the M2C size, the M2C equilibrium content and cost control. Based on this design standard, the ageing process for a new high Co-Ni secondary hardening steel was analyzed. By controlling the ageing process as 515 ℃ for 10 h, based on the design standard, M2C with the size of 1~5 nm and austenite layers with the thickness of 10~20 nm was formed in new high Co-Ni secondary hardening steels. The equilibrium content of M2C and austenite layers were controlled as 19.5% and 3.8%, respectively. The simulation results were basically consistent with the microstructure observation results. The high Co-Ni secondary hardening steel treated by the designed heat treatment process has considerable strength (2021 MPa) and toughness (115 MPam1/2). Both simulation and experimental results showed that this design standard of ageing process for high Co-Ni secondary hardening steels can help to obtain steels with high strength and toughness.
Different from monolayers of same components, nanoscale multilayers have different mechanical properties owing to their relatively high interfacial density, such as extremely high yield strength, high ductility and outstanding wear resistance. Furthermore, their precise modulation period and unique interfacial structures contribute to investigate the plastic deformation mechanism of metal materials. As the plastic deformation behaviors of nanoscale multilayers were reflected in a thermal activation process, strain rate sensitivity index m can be used to characterize the tendency of material strengthening as the strain rate increases. To investigate the impacts of modulation period and interfacial structures upon strain rate sensitivity of nanoscale multilayers, Cu/Ni nanoscale multilayers with different periods (Λ=4 nm, 12 nm, 20 nm) were prepared on Si substrate with e-beam evaporation technologies, while Cu/Nb nanoscale multilayers with different periods (Λ=5 nm, 10 nm, 20 nm) were prepared on Si substrate with magnetron sputtering technologies. Under vacuum conditions, the Cu/Ni nanoscale multilayers of different periods were annealed at 200 and 400 ℃ for 4 h respectively, and the Cu/Nb nanoscale multilayers of different periods were annealed at 200, 400 ℃ and 600 ℃ for 4 h respectively. Microstructures of Cu/Ni and Cu/Nb nanoscale multilayers were characterized with XRD and TEM. Besides, the hardness of nanoscale multilayers was measured by nano-indentation techniques under different loading strain rates (including 0.005, 0.01, 0.05 and 0.2 s-1). The results suggested that strain rate sensitivity was impacted by interfacial structures and grain size. Both increased density of incoherent interfaces and grain size could result in weaker strain rate sensitivity. As the period increases, the density of incoherent interfaces and the grain size of Cu/Ni nanoscale multilayers increased, leading to a decline in the strain rate sensitivity. While for Cu/Nb nano scale multilayers, the density of incoherent interfaces decreased and their grain size was enlarged with longer period, the m value kept unchanged as a result. As the annealing temperature increasing, the strain rate sensitivity of Cu/Ni and Cu/Nb nanoscale multilayers generally tended to decline, which should be ascribed to increased density of incoherent interfaces and grain size in the course of annealing.
The grain refinement mechanism of hypoeutectic Al-7%Si alloy under low voltage alternating current pulse (LACP) has been investigated in this work. In which LACP generated by the homemade low voltage modulation pulse generator is imposed in different solidification stages of the alloy and the wire mesh tubes of different diameters which have the effect of limiting the melt convection is embedded in sand mould. The experimental results show that the grains of casting alloy are evenly refined under LACP. The grain refinement will not appear, when LACP is imposed in the stages which are the alloy melt temperature is above 620 ℃ and late stages of crystal growth of primary phase. The grain refinement mainly occurs in nucleation stage and early growth stage of primary phase. The grains of inside and outside of wire mesh tube are refined together under LACP. But the grains of outside of wire mesh tube are much finer. The solidification microstructure of outside of wire mesh tube changes from large dendritic crystal to rose-shape crystal, nevertheless, it is still large dendritic crystal inside of wire mesh tube. Inocu lation effect and Joule heat effect of LACP have little effect on grain refinement of Al-7%Si alloy. The main reasons of grain refinement are the embryos fell off from chilling walls under LACP and the nucleation kinetics of the alloy was changed by LACP, which cause the nucleation rate of alloy increased. In addition, the α-dendrites became fragmentation under the forced melt flow which was induced by electromagnetic force can also lead to the grain refinement during the early growth stage of primary phase.
Titanium alloys are widely used as structural material in aerospace, automobile, biomedical and other fields because of its low density, high specific strength, good corrosion resistance and good biocompatibility. But high coefficient of friction, poor wear resistance and high-temperature oxidation resistance are the main reasons for limiting the use of titanium alloy in complex working conditions. In order to improve the high-temperature oxidation resistance and optimize the microstructure of titanium alloy, high Nb content Ti-Al intermetallic composite coating was fabricated by laser in situ synthesis technique on BT3-1 titanium alloy surface. Phase structure of the composite coating was analyzed according to XRD spectra. Unit area oxidation weight gain of the titanium alloy substrate and coating before and after heat treatment were tested by GSL-1600X tube furnace under 950 ℃. The oxidation kinetics curves were drawn and the high temperature oxidation resistance was compared. The microstructures of coating before and after oxidation were observed by OM and SEM, and the high-temperature oxidation resistance mechanism was analyzed. The results show that the coating mainly consists of Nb, intermetallic γ-TiAl, α2-Ti3Al and Ti3Al2 phases before heat treatment. But after heat treatment, Nb is dissolved in γ-TiAl and α2-Ti3Al, and a new phase Ti3AlNb0.3 is generated in the coating. The coating is approximately γ-TiAl+α2-Ti3Al duplex structure. The oxidation kinetics curves of coating is between linear and parabolic rule before heat treatment, its high temperature oxidation resistance increased by 2 times of titanium alloy substrate. The oxidation kinetics curves of coating is approximately parabolic law after heat treatment, and the rate of oxidation is small, its high temperature oxidation resistance increased more than 20 times of titanium alloy substrate. Under 950 ℃ cyclic oxidation conditions, the oxide layer surface of coating forms a continuous dense capsule oxide, and oxide layer closely connect the unoxidized coating portion, the oxide layer plays a good protective role of the composite coating. But for titanium alloy substrate, the oxide layer surface is loose and porous oxide, the oxide layer is fractured and removed from the substrate surface. Nb alloying significantly improves the high temperature oxidation resistance of Ti-Al intermetallic coating.
Mg-Al-Ca base alloys have great potential for application because of its low cost and good high temperature creep properties, but its higher hot cracking susceptibility greatly limits the application of the alloy. The effect of Ca addition on the hot cracking susceptibility of Mg-5Al-xCa (x=0.5, 1.0, 2.0, 3.0, 4.0, 5.0, mass fraction, %) alloys at the pouring temperature 700 ℃ and mold temperature 200 ℃ was studied by using hot cracking curve test, solidification curve test, OM, XRD and SEM. The results showed that the hot cracking susceptibility of alloys decreased with increasing Ca addition until to 4.0%, and the Mg-5Al-4.0Ca alloy had minimal hot cracking susceptibility, which cracking susceptibility coefficient was only 0.824. But when Ca addition increased to 5.0%, the hot cracking susceptibility of the alloy increased, and the cracking susceptibility coefficient increased to 0.96. The appropriate Ca addition can reduce the precipitation temperature of α-Mg in Mg-5Al-xCa alloys, inhibit precipitation of Mg17Al12 phase, narrow solidification temperature range of the alloy and increase eutectic content, which are helpful for the alloy having a stronger ability of compensation at the solidification end to decrease hot cracking susceptibility. But excessive Ca addition will increase the numbers of brittle phases containing Ca and coarsen the microstructure, resulting in the increase of hot cracking susceptibility.
Mg and its alloy have large potential in weight reduction usages because of their low density. However, the relatively low strength and modulus hinder their widely applications. Accumulative roll bonding (ARB) is one kind of severe plastic deformation (SPD) process which can produce bulk ultra-fine-grained (UFG) metallic materials. In order to improve the strength, elastic modulus and corrosion resistance of Mg sheet, accumulative roll bonding was utilized to fabricate UFG Mg/Al laminated composites at ambient temperature in this work. Synchrotron radiation-based computer tomography, SEM and TEM were employed to characterize the global bonding condition and the interface structure of Mg/Al lam inated sheet ARBed after 3 cycles. No obvious cracks could be observed along the bonding interfaces during ARB, although small amount of tiny pores existed in some area. Mg17Al12 phase with thickness of 150 nm formed at Mg/Al interface after 3 cycles. There existed a definite orientation relationship between Mg17Al12 and Mg which is [11?1?]Mg17Al12//[12?10]Mg, (110)Mg17Al12//(1?011?)Mg. Nevertheless, the orientation relationship between Mg17Al12 and Al is not very obvious.
The energy density of chip is becoming increasingly higher with the power electronic devices developing toward miniaturization, high power and integration, which will lead a higher operating temperature. However, the traditional Sn-based soldering process fails to meet the elevated temperature. Transient liquid phase (TLP) soldering, which can form high-melting-point joints at relatively low temperatures, has been proven to be a promising bonding method for solving this technological challenge. Nevertheless, a common drawback for TLP soldering is that it will consume a very long time for the complete formation of intermetallic joints, up to tens of minutes, which will lead extra thermal stress and seriously negative effects on the reliability of packaging systems. Recently, this technological puzzle has been proven to be solved by a novel ultrasonic-assisted TLP soldering process, in which the ultrarapid formation of complete intermetallic joints was achieved due to the accelerated diffusion of Cu from the substrates into the molten Sn interlayer under the complex sonochemical effects of acoustic field on the interfacial reaction. In this study, the microstructure and mechanical properties of complete Cu-Sn intermetallic joints ultrarapidly formed by ultrasonic-assisted TLP soldering process were investigated. The sandwich Cu/Sn/Cu system was placed on the heating platform, and then the ultrasonic vibrations and the bonding force were applied on it. The horizontal ultrasonic frequency, pressure, power, bonding temperature and time were fixed as 20 kHz, 0.5 MPa, 300 W, 250 ℃ and 5 s. In summary, the complete intermetallic joints composed of Cu6Sn5 interlayer with a thickness about 15 μm and Cu3Sn boundary layers with a thickness about 1 μm were ultrarapidly formed by ultrasonic-assisted TLP soldering process. The formed Cu6Sn5 grains were remarkably refined to be with an average grain size less than 5 μm. Compared with the intermatllic joints formed by traditional TLP soldering process, the resulted intermetallic joints performed more uniform mechanical properties with elastic modulus and hardness of about 123 GPa and 6.0 GPa respectively, as well as a higher reliability with a shear strength of 60 MPa.
As for actinide metallic glasses, a minor branch of metallic glasses, almost all of them are binary alloys and their glass-forming rule has been insufficiently studied. Considering that binary alloy systems are the base of ternary or more component glass systems that possess better glass-forming ability, binary glass systems U-Fe, U-Co and U-Cr are chosen for study. After earlier investigation on the first two systems, the glass formation in U-Cr system is explored in this work. According to the eutectic criterion, a series of U-Cr alloys were designed at the Cr-side of the eutectic point U81Cr19. Under the preparation of melt-spinning, these alloys can be formed into a single amorphous phase with the capacity of crystallizing at about 700 K. The reduced crystallization temperature (Trx) of some U-Cr alloys exceeded 0.6, higher than those of U-Fe and U-Co metallic glasses, and comparable to those of ordinary bulk amorphous alloys. Being inconsistent with the prediction based on thermodynamics, kinetics and efficient structural packing, U-Cr alloy system shows anomalous strong glass-forming ability among reported actinide binary glasses. This abnormal behavior might be related to the existence of comparatively more mediate metastable phases in U-Cr system, which can be speculated from the multi-peak crystallization phenomenon. This system could be a potential system model for studying the glass formation of actinide amorphous alloys further.
To transform the columnar grain in the as-deposited GH4169 alloy to the exquiaxed grain and get better mechanical properties, a block sample of GH4169 alloy has been formed by using a technology of consecutive point-mode forging and laser rapid forming (CPF-LRF). During the process of CPF-LRF, GH4169 alloy was deposited by laser rapid forming firstly and then the deposited GH4169 alloy was deformed by consecutive point-mode forging. Both consecutive point-mode forging and laser rapid forming were alternately carried out until the completion of the forming of an objective part. The effects of heat treatment on the microstructures and mechanical properties of CPF-LRF GH4169 alloy have been investigated. The result shows that 980STA heat treatment fails to lead to recrystallization of CPF-LRF GH4169 alloy, and the tensile properties of 980STAed CPF-LRF GH4169 alloy can't meet the wrought standards. After the 1020STA heat treatment, the average recrystal grain size of GH4169 alloy is about 12.8 μm, and Laves phase can not be dissolved completely. The tensile properties of the 1020STAed CPF-LRF GH4169 alloy is superior to the wrought standards. Compared to the 1020STAed CPF-LRF GH4169 alloy, the tensile strength of 1050STAed CPF-LRF GH4169 alloy drops and its ductility increases due to complete dissolution of Laves phase and grain size increasing to 25.3 μm. The average grain size of the 1080STAed CPF-LRF GH4169 alloy is about 123.6 μm. Compared to 1020STAed and 1050STAed CPF-LRF GH4169, the tensile properties of 1080STAed CPF-LRF GH4169 has fallen substantially, which just satisfy the wrought standards.
Systematic study was conducted on the variation regularity of stress-strain curve, feature point stress, dissipated energy and equivalent damping ratio of shape memory alloy (SMA) wires changed with wire diameter, strain amplitude, loading rate and loading cyclic number. By nonlinearly modeling experimental results for SMA using the neural network intelligent algorithm (a neural network algorithm with back-propagation training) and optimizing the initial weight and threshold value of neurons using genetic algorithm, a new BP neural network constitutive model for SMA optimized with genetic algorithm is established. This model successfully overcomes the shortcomings of other mathematical models such as the phenomenological Brinson, by which the various influence factors to mechanical properties in an experiment for SMA are hardly simulated exactly. In fact, the average error between experimental and simulated results is only 1.13% by using this model, much better than conventional BP neural network models. The results show that the BP neural networks constitutive model optimized with genetic algorithm can not only predict accurately the superelastic performance of SMA under cyclic loading, but also avoid the no convergence problem caused by concussion of BP network due to the improper initial weight and threshold value set up. Furthermore, this model would be a better model than others because of fully considering the dynamic influence of loading/unloading rate on SMA experiments.