Hydrogen-induced cracking (HIC) resistance of pipeline steels is one of the most important properties for sour gas service pipelines. For the conventional pipeline steels, the strength level mainly depends on the Mn content. However, as the Mn content increases, the unfavorable microstructures such as large size martensite/austenite (M/A) islands, bainite or martensite will be generated, which will deteriorate the HIC resistance of the steels. Therefore, it is hard to simultaneously improve strength level and HIC resistance for pipeline steel. The nature of HIC in pipeline steel is hydrogen embrittlement, which is essentially the redistribution of H atoms into the matrix of steel. So, how to make the distribution of H atoms in the steel as evenly as possible without causing local enrichment is a key factor to improve the HIC of pipeline steels. In this work, the susceptibilities of HIC of traditional X80 and novel Cu-bearing pipeline steels were studied with comparison. The results showed that the X80 pipeline steel behaved bad HIC resistance. The hydrogen-induced cracks mainly expanded along the interface between M/A islands and the matrix. However, the novel Cu-bearing pipeline steels with different Cu contents exhibited excellent HIC resistance, showing no cracks were produced after HIC test. It was analyzed that nano-sized Cu-rich precipitates in the Cu-bearing pipeline steels are speculated to act as the beneficial hydrogen traps, and these uniformly dispersed fine Cu-rich phases in matrix provide many sites for the distribution of H atoms, which helps to avoid the localized high concentration H atoms enrichment leading to hydrogen embrittlement. Taking nano-sized Cu-rich phases as a type of beneficial hydrogen traps provides a new way for the development of new pipeline steels with high strength and excellent HIC resistance.

Spring steel is an important steel widely used in the national economic construction. Its application environment is quite harsh, so it strongly demands for a high quality of the surface. However, the decarburization behavior on the surface seriously affects the surface quality and reduces the fatigue performance of materials. Decarburization is caused by chemical reaction between oxidizing atmosphere and steel surface. Hydrosphere, as a component of the oxidizing atmosphere, could lead to a significant influence on decarburization process. Our experimental materials, taken from continuous casting billet, were polished carefully by abrasive paper in order to remove stains, original scale and decarburization layer. Components of furnace gas were controlled by mixing units designed independently. Spring steel samples were heated by a tube vacuum furnace in 600~950 ℃. The mixed atmosphere contains (15%~20%)CO₂, (2%~4%)O₂, hydrosphere of different contents and N₂ in balance. With the condition of the mixed atmosphere, influence of hydrosphere on surface decarburization of spring steel 55SiCr was investigated by 3D measuring laser microscope. The results show that, with the influence of the mixed atmosphere, decarburization is able to happen during low temperature interval, 650 ℃~A_c1 (A_c1: starting temperature of austenitization during slow heating). The thickness of total decarburized layer increases correspondingly with the temperature. In the mixed atmosphere, slight decarburization occurs at 650 ℃ and obvious ferrite decarburization layer can be detected within a temperature range of 700~950 ℃, which is more serious than that in the air. The peak temperature of ferrite decarburization is 850 ℃ under both mixed atmosphere and air. The surface decarburization in low temperature region is related to the dissolution of cementite in pearlite lamellae driven by carbon concentration gradient. When the decarburization degree deepens as time goes, partial pearlite colony begins to shrink, much line form cementite dissolves gradually, and the line cementite becomes short and punctate. The grain morphology of ferrite decarburization layer is different from that who generates within α+γ phase region, which is small in size and without a strong orientation. Hydrosphere in mixed atmosphere could increase porosity of oxidation layer and destroy the important protection mechanism of preventing decarburization by compact scale. It is deduced that existing of hydrosphere offers a chance for low temperature decarburization process occurring and hydrosphere plays an important role in deepening the decarburization degree of samples.

Since structural components in the nuclear power plant are unable to be disassembled during their in service process, it is an urgent and key problem how to quickly and non-destructively evaluate fatigue reliability of these key structural components by using miniature specimens. Fatigue properties of miniature specimens of CA6NM martensite stainless steel for impellers in the nuclear pump were obtained by using symmetrically bending fatigue loading and uniaxial tension-tension fatigue loading, respectively. A comparison of fatigue properties between the miniature specimens and bulk specimens was conducted to examine feasibility for the evaluation of fatigue reliability of the CA6NM steel using miniature specimens. The results show that tensile strength of the 40 μm-thick CA6NM specimens is slightly higher than that of the bulk specimens, but elongation of the 40 μm-thick specimens is lower than that of the bulk counterparts. In low cycle fatigue regime, fatigue strength of the 40 μm-thick specimens subjected to uniaxial tension-tension fatigue loading is lower than that of the standard bulk counterparts. With decreasing the applied stress amplitude, the difference in fatigue properties gradually decreases, and the fatigue limit of the miniature specimen is close to that of the bulk counterparts. Fatigue strength of the 40 μm-thick specimens subjected to bending fatigue loading is much higher than that subjected to uniaxial tension-tension fatigue loading, and also higher than that of the bulk counterparts. Fatigue strength of the miniature specimens is related to the loading mode. The difference in the fatigue mechanism between the miniature specimens and the bulk counterparts is discussed, and the feasibility to evaluate fatigue reliability of the steel using miniature specimens is addressed.

The low carbon bainite steel with high strength, excellent toughness and plasticity was widely used for pipeline, engineering machinery, ocean station vessel and other fields. The light weight of structure of construction machines puts forward higher requirements for performance of steel, which promotes the development and application of low carbon microalloyed steel. A low carbon bainite steel combined with V-N microalloyed was designed for engineering machinery, to upgrade performance by microstructure control and the refinement and dispersion control of precipitates. This steel was tempered on-line with rapid heating rate after controlled rolling and accelerated cooling process. Effects of different holding time under rapid induction tempering on precipitation strengthening mechanism and mechanical property of V-N microalloyed steel were investigated by using three dimensional atom probe (3DAP), SEM and TEM. The results showed that the main microstructures of tested steel are granular bainite before tempering, and granular bainite and ferrite appears after tempering at 600 ℃. The hardness and yield strength values reached its peak at 600 ℃ tempered for 300 s, which were 330.45 HV and 815 MPa, respectively. Compared with untempered sample, the measured strengthening increment in yield strength was 173 MPa which was due to the V-rich or VN-rich clusters with 20~100 atoms distributing similar to monoatomic layer and resembled the GP zones. These small nanoclusters have strong interaction with dislocation, and compared with V(C, N) particles, V or VN clusters have better strengthening effect.

Grain boundaries are sources of failure and weakness due to their relatively excess free volume compared to the lattice of polycrystalline materials exposed to aggressive environment. The control of grain boundary degradation has become one of the key issues of materials science and engineering. It has been found that the coincidence site lattice (CSL) boundaries, especially Σ3 (the twin boundaries), have stronger resistance to intergranular degradation than random boundaries. Materials with a high proportion of CSL boundaries that could disrupt the connectivity of random boundaries have better performance to resist intergranular failures. However, the distribution characteristics of twin boundaries in grain boundary network are still unclear. In this work, three-dimensional electron backscatter diffraction (3D-EBSD) was used to map the 3D grain boundary network of a 316L stainless steel. The topological characteristics of triple junction and quadruple junction in the presence of twin boundaries were investigated. The distribution of twin boundaries around grains and grain boundaries was analyzed. The results show that the twin boundary number fraction in the 3D grain boundary network is lower than the measured twin boundary area fraction, indicating that the average area per twin boundary is larger than random boundary. Most of triple junctions in the 316L stainless steel have one twin boundary. The proportion of triple junctions with two twin boundaries is about 9.4%. A quadruple junction has three twin boundaries at most. Most of quadruple junctions have one or two twin boundaries. About 7.9% of quadruple junctions have three twin boundaries. The 3D-EBSD data of 316L includes 1840 grains, 7353 random boundaries and 1824 twin boundaries. On average, a 3D grain in the 3D microstructure has 11 faces (39.85 neighboring faces that includes all boundaries of the grain and all boundaries that connected with the grain by lines or points), in which the number of twin boundaries is 2.03 (8.02) on average. A 3D grain boundary has 9.35 neighboring boundaries, in which the number of twin boundaries is 1.99 on average.

Lamianate structured metals have recently attracted extensive interests for their outstanding mechanical properties which are produced by synergetic strengthening of different heterogeneous layers. For Mn-rich maraging steels, austenite can precipitate in the martensite matrix due to Mn segregation during heat treatment, and then the austenite and martensite/ferrite duplex steel is produced. Besides, high Mn TRIP steel is regarded as a promising material for next generation automobile steel because of its high strength and good ductility. In this work, the microstructures and properties of Mn12Ni2MoTi(Al) steels produced by thermo-mechanical process were investigated using XRD, SEM, TEM, EBSD, EPMA, hardness tests and tensile tests. The results showed that laminate structures with austenite/ferrite band alternate arranging along the normal direction were formed in Mn12Ni2MoTi(Al) steels, which were processed by 65% cold-rolling and subsequent annealing at 745 ℃. Both of austenite bands and ferrite bands consist of ultrafine equiaxed grains. Moreover, a small amount of ferrite grains and austenite grains were found inside the austenite bands and ferrite bands, respectively. In the austenite/ferrite laminate structures, the austenite bands show the Brass texture of {110}<112> and Goss- type texture of {110}<001>. The ferrite bands show the rotate Cube texture of {001} <110> and Cube texture of {001}<100>. With the increase of annealing time, the laminate structures first become dominant and then disappear gradually, accompanying with the orientation transition of austenite from the Brass texture into the Goss one. The samples with laminate structures have high yield strength and good ductility. Otherwise, when the laminate structures disappear, the yield strength and ductility of the samples will decrease, and the yield strength deceases more.

Stress corrosion cracking (SCC) is considered as the main risk of tubing steels during the exploitation of oil and gas fields, which could result in sudden and catastrophic failures of downhole tubing. Especially in annular downhole environment, P110 tubing steel is prone to sulfide stress corrosion cracking and hydrogen embrittlement (HE) where S^2- could be originated from bio-reduction of SO₄^2- inspired by sulfate-reducing bacteria (SRB). Currently, extensive work have been performed to investigate the influence factors on SCC and mechanism of tubing steels, but limited researches have been conducted on the SCC of P110 tubing steel in annular downhole environment, particularly, on the early detection of SCC. In this work, the SCC behavior of P110 low alloy steel in simulating sulphur-containing annular fluid (SAF) and the effect of S^2- concentration on the initiation and propagation of crack were investigated by slow stress rate test (SSRT), non-destructive electrochemical noise (ECN), SEM and EIS techniques. The results showed that, during the elastic stress stage, the addition of S^2- accelerated the breakdown of passivation film on the surface of P110 steel tensile specimen. There are many short duration current transients caused by metastable pits on ECN curves. The transformation time of metastable to stable pits is shortened significantly by the addition of S^2-, which not only promotes the growth of pits but the initiation of cracks from the stable pits under the action of tensile stress. Compared with the ECN spikes from metastable pits, the spikes associated to the advance of cracks are featured by longer average duration (about 400 s), stronger amplitude (40 μA), and higher charge (about 4000 μC). As a result, the susceptibility of P110 steel to SCC increases with S^2- concentration, and the propagation of SCC is dominated by anodic dissolution characteristic of discontinuous advance.

Microbiologically induced corrosion (MIC) is known as one of the most damaging failures for pipeline steels. Especially, sulfate-reducing bacteria (SRB) is the most widespread strains in soil and seawater environments and is the typical bacteria associated with MIC. SRB may cause severe localized attack, leading to pipeline failures in forms of pitting, crevice corrosion, dealloying and cracking. In this work, SEM, Raman spectroscopy, XPS, scanning vibrating electrode (SVET) technique, EIS and other electrochemical techniques were used to study the formation of SRB biofilm, its electrochemical interaction with X80 pipeline steel and corrosion behavior of the steel in a simulated seawater. The results showed that barrier effect of the extracellular polymer substances (EPS) inhibits corrosion process of X80 steel in the initial formation of EPS and SRB micro-colony. After the formation of SRB biofilm, open circuit potential (E_OCP) of the steel decreases 20 mV, and SRB significantly promotes the corrosion process of the pipeline steel. In the later stage, due to SRB and its biofilm, the corrosion rate of X80 steel exposed in SRB inoculated environment is almost one order of magnitude higher than that in the sterile environment. The biofilm have complexation effect and chelation effect with corrosion products (Fe²⁺/Fe³⁺). SRB cells, metabolites and biofilms have direct and indirect electron interactions with the steel substrate. These various coupling effects promote occurrence and development of local corrosion on the surface of the steel beneath biofilm.

6061 Al alloy is widely used for structural components in the structural, building, aerospace and automobile industry because of their good extrude-ability, high strength and low density. 6061 Al alloy is easily corroded during the corrosive environments containing Cl^-, which lead to the decreasing of the alloy service life. Therefore, the Al alloy need to adopt surface treatment to improve the corrosion resistance. The chromate passivation was the most effective surface treatment technology of Al alloys in the past. However, Cr(VI) could pollute the environment and harmful to human body. Thus, it is necessary to develop chromium-free films to protect Al alloy. Layered double hydroxides (LDHs) are an environment-friendly and smart material, which could be used for anticorrosion film. The inhibitor can be inserted into the film layer due to its unique anion exchanging capacity. Therefore, the LDHs is acquired self-healing performance and applied to the anticorrosion field of Al alloy. Meanwhile, vanadate is a good inhibitor and it has many forms under different pH conditions. Moreover, different forms of vanadate have different influence on LDHs film. The corrosion resistance of LDHs and its modified form films on Al alloys need deep study. In this work, ZnAl-LDHs films with NO₃^- were prepared on the suface of 6061 Al alloy via a facile in-situ growth method, and then ZnAl-LDHs-NO₃ films were intercalated with the corrosion inhibitor VO₄^3- and VO₃^- to obtain the ZnAl-LDHs-VO₄ and ZnAl-LDHs-VO₃ films, respectively. The structure, morphology and composition of as-prepared ZnAl-LDHs films were investigated by XRD, fourier infrared spectrometer (FT-IR) and SEM; the corrosion behavior of ZnAl-LDHs films in the 3.5%NaCl (mass fraction) solution were studied by the electrochemical workstation and the 3D microscope. The results show that the plate-like ZnAl-LDHs microcrystals are perpendicular to the substrate and cover almost the entire Al alloy substrate surface. Compared with the 6061 Al substrate, ZnAl-LDHs films can not only decrease the corrosion current (I_corr), but also increase the corrosion potential (E_corr) and charge transfer resistance (R_ct) of the 6061 Al alloy. It is suggested that ZnAl-LDHs films could significantly enhance the corrosion resistance of 6061 Al substrate, indicating an effective protection for 6061 Al alloy by the ZnAl-LDHs film. The ZnAl-LDH-VO₃ film is of the highest corrosion resistance in the studied ZnAl-LDHs films.

With the increase of the alloying degree and structural complexity as well as larger size in Ni-based superalloy blades, it is essentially important to suppress the solidification defects. When the electromagnetic field is introduced into solidification process, the solidification properties of alloy can be modified without changing the alloy composition, which can well eliminate the casting defects, such as the composition segregation, and optimize the solidification microstructure and improve properties. The effect of induction coil magnetic field on solidification structure of DD90 single crystal superalloy is studied by changing the thickness of graphite sleeve. The distribution of magnetic field and flow field in alloy melt are analyzed by Ansys finite element analysis (FEM). The results show that when the thickness of the graphite sleeve is 10~30 mm, the monocrystalline remains intact and the primary dendrite arm spacing increases with increasing the thickness, while the second dendrites are the opposite rule. Moreover, the as-cast microstructures of γ′ phase size, eutectic structure and content increase significantly, and the element segregation increases simultaneously with increasing the graphite sleeve thickness. The Ansys FEM shows that the magnetic field and flow velocity in the melt decrease with the increase of the thickness of graphite sleeve. Based on the thermoelectric magnetic convection induced by the magnetic field during solidification and the effect of the convection on the microstructure, the above phenomenon is analyzed and discussed.

The preparation of single crystal involves the grain orientation competitive growth. For high melting point metals, Mo is a typical bcc crystal structure and its preferred growth orientation of single crystal was revealed to be the [110] direction, different from the known preferred growth orientation [100] for bcc metals during solidification. This disagreement remains unclear. For the high melting point metal Ir that is a fcc crystal structure, its preferred growth orientation of single crystal remains unknown. Based on electron beam floating zone melting (EBFZM), the orientation competitive growth of these two metals Mo and Ir with three different directions [100], [110] and [111] has been analyzed using solidification theory. It shows that the preferred growth orientation of Mo is the [110] direction at a planar front interface. When introducing the interface energy anisotropy, the preferred growth orientation of Mo could be the [100] or the [110] direction, depending on the magnitude of interface energy anisotropy parameters a₁ and a₂. This result matches well with the experimental results of Mo single crystal prepared by EBFZM. For the fcc structure Ir, its preferred growth orientation always keeps the [100] direction, agreeing with the experimental results of Ir single crystal prepared by EBFZM in this study. Besides, the effects of interface curvature undercooling and kinetic undercooling on the growth behavior of single-crystal metals prepared by EBFZM have been discussed. It demonstrates that when the grain size in the specimen is in the order of about millimeter or less, the curvature undercooling would dominate the grain orientation competitive growth. As the grain size becomes in the order of about centimeter or larger, the kinetic undercooling would prefer the grain competitive growth process.

Ni60 alloy has been widely used in many application fields due to its excellent wear resistance, corrosion resistance and high temperature oxidation resistance. However, uneven microstructure was easily formed due to the effect of heat shock and heat accumulation during laser multi-track overlap process. Moreover, Ni60 alloy powder was composed of a variety of elements. The composition segregation and high content CrB, (Cr, Fe)₂₃C₆ were easily present in the coating during the laser cladding process, which can easily lead to the cracking of Ni60 alloy coating. In this work, multi-layer Ni60 alloy coating was prepared by electric-magnetic compound field assisted laser cladding. Synthesis of Ni60 alloy coating was analyzed by coloring agent, OM, SEM, EDS, XRD and microhardness tester. The results showed that cracks and large pores were to appear at the coating when the electric-magnetic compound field was not applied, and the molding quality was also poor. When the electric-magnetic compound field was applied, the surface cracks of Ni60 alloy coating were suppressed, the pores were eliminated, and the molding quality of the coating was also improved. Meanwhile, the particle size of the brittle phase (CrB, (Cr, Fe)₂₃C₆) was decreased from 4~6 μm to 2~4 μm by the aid of the electric-magnetic compound field, and the degree of particle cluster was also reduced, which was beneficial to the elimination of the internal crack. XRD, microstructure and microhardness analysis results showed that the brittle phase content, particle segregation, lattice distortion and hardness were reduced under the condition of electric-magnetic compound field, leading to the decrease of crack initiation probability, so the crack of Ni60 alloy coating was remarkably reduced.

Al-Mg series alloy plays an important role in offshore manufacturing, transportation and aerospace industries for its high strength-to-weight ratio, high corrosion resistance and good welding performance. For high magnesium Al-Mg alloy, β phase always acts as a restraint condition to the whole thermal mechanical processing (TMP) procedure. Its positive effect is still unclear. In this work, the effect of coarse β(Al₃Mg₂) phase of Al-10Mg alloy annealed at 573 K for 24 h and applied double passes uniaxial compression on microstructure evolution was studied by using OM, XRD, EPMA and EBSD. The result shows that discrete coarse β phase was precipitated in the interior of grains after 573 K and 24 h annealing treatment. The true stress-true strain curve of annealed sample was lower than that of solution treated one. Hardening rate of annealed sample was lower in first compression pass, and conversely higher than that of solution treated one in the second pass. Solution Mg atoms play an important role in strain hardening during dynamic recovery. Dislocation slipping was obstructed by coarse β phase, and then low angle boundary (LAB) was stimulated near coarse β phase. Not just bulging nucleation mechanism working, dynamic recrystallization nucleation was stimulated and microstructure was refined. Since part of deformation-stored energy was lured away by LAB, lattice rotation of deformed grains were weakened possessing {001} and {101} textures simultaneously. Schmid factors of three blocks with different lattice orientations were calculated, which suggested that alloy can load more plastic deformation after annealing treatment. Texture of recrystallized new grains was weakened at the same time. Microstructure anisotropy could be controlled by coarse β phase in TMP.

Some NiTi alloys, generally known as shape memory alloys or smart materials, exhibit larger negative thermal expansion (NTE) strain than that of traditional nonmetallic NTE materials. Furthermore, high strength and better ductility of NiTi alloy make it more advantageous compared to nonmetallic materials. The NTE response of NiTi alloy may be attributed to the transformation strain that originates from the volume change accompanying the B2-B19' martensitic transformation in the alloy. Therefore, it is of great importance and interests to study the martensitic transformation crystallography in NiTi alloy. In this work, the martensitic transformation crystallography in an equiatomic NiTi alloy was investigated by using the topological model, as well as the optimum twist criterion developed lately. The optimum twist angle, ω_o, in NiTi alloy was determined to be -0.969°, and the so-obtained transformation crystallography results, including the habit plane index and parent-martensite orientation relationship, agree well with the corresponding experimental data and theoretical calculations in the literature. Furthermore, the martensitic transformation strain of NiTi alloy was calculated based on the analysis of interfacial dislocation movement, and the total transformation strain can be resolved into an in-habit-plane shear strain and an axial strain perpendicular to the habit plane. In particular, the value of the axial strain that represents the volume change due to the B19' to B2 transformation was found to be negative, indicating that the NiTi alloy shrinks during the reverse martensitic phase transformation when heated, which might shed some light on the relationship between the NTE mechanism and martensitic transformation in NiTi alloy. The measured NTE strain is much smaller than the theoretical calculated phase transformation strain in NiTi alloy, due to the self-accommodation effect of martensite variants and the compensation of transformation strains in polycrystalline materials.