The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M₂C eutectic ledeburite. Excessive amount of aluminum, 1.2%, favors the formation of ferrite and needle-like carbides. After the addition of aluminum, eutectic carbides are distributed more homogeneously. Additionally, the morphology of M₂C eutectic carbides transforms from the fibrous to the plate-like, and their microstructure also changes significantly. The plate-like M₂C has crystal defects, such as micro-twins and stacking faults, and different growing orientation between adjacent plates whereas the fibrous carbides have few defects and single crystal orientation. Compared to fibrous carbides, the plate-like carbides are much difficult to get spheroidized at high temperature, which is unfavorable for carbide refinement. The ferrite, formed by adding excessive amount of aluminum, cannot be eliminated by ordinary heat treatments, decreasing remarkably the hardness of high speed steel after quenching.

以核电汽轮机缸体用G18CrMo2-6耐热钢为研究对象, 分析了显微组织、第二相类型、形貌、尺寸和分布随回火温度的变化及其对冲击韧性的影响. 结果表明, G18CrMo2-6钢正火经不同冷速冷却后得到不同的基体组织, 经680 ℃回火后, 冲击韧性均远高于指标要求, 因此基体组织差异不是导致冲击韧性急剧恶化的决定性因素. 经炉冷正火后在560~710 ℃区间回火, 显微组织均为铁素体+回火贝氏体, 随回火温度上升, 室温冲击韧性增加. 经560和600 ℃回火后, 块状马氏体/奥氏体(M/A)岛、条状颗粒不均匀分布于贝氏体铁素体基体上, 平均冲击韧性分别为17和29 J. 710 ℃回火后块状M/A岛分解, 条状颗粒转变为细小的颗粒状呈弥散分布, 冲击韧性达到峰值93 J. 除了基体组织的软化效应外, 第二相的类型、形貌、尺寸和分布能够明显改变诱发裂纹萌生的临界断裂应力, 是影响G18CrMo2-6钢冲击性能的一个关键因素.

分别采用传统铸造与喷射成形工艺制备了H13钢并对其进行了锻造和传统热处理. 利用OM, SEM和XRD研究了铸造与喷射成形H13钢在不同工艺状态下组织的差异, 并且在相同的热处理制度下测试了两者的常温与高温力学性能. 结果表明: 与传统铸造H13钢相比, 喷射成形H13钢具有更好的回火稳定性、更高的室温与高温拉伸强度, 室温冲击韧性提高了2倍, 并且消除了带状偏析, 提高了组织的等向性. 喷射成形H13钢力学性能的提高主要归因于沉积态H13钢组织均匀细小, 消除了宏观偏析, 更没有粗大的一次碳化物, 这使得淬火后基体含有更多的合金元素, 分布也更均匀, 从而在回火时析出的二次碳化物更弥散并且晶粒也更细.

通过熔融玻璃净化与循环过热相结合的方法获得过冷度ΔT=6~280 K范围内的Fe₈₂B₁₇Si₁共晶合金的凝固组织演变; 结合突变方程和JMAK模型拟合凝固过程的冷却曲线, 拟合结果符合Fe₈₂B₁₇Si₁共晶合金的组织类型及形态随过冷度的变化规律. 结果表明, 当6 K≤ΔT<75 K时, Fe₈₂B₁₇Si₁合金中形成了复杂规则共晶及准规则共晶组成的混合共晶组织; 当75 K≤ΔT<180 K时, 凝固组织由混合共晶组织和深过冷非规则共晶组织组成; 当180 K≤ΔT<250 K时, 凝固组织由不同含量的初生a-Fe相和枝晶间深过冷非规则共晶组织组成; 当ΔT >250 K时, 凝固组织为完全非规则共晶组织.

采用循环干/湿模拟腐蚀增重实验、动电位极化曲线、电化学阻抗谱和XRD方法, 研究了模拟海岸-工业大气中SO₂对Q235B钢腐蚀行为的影响. 结果表明, 在腐蚀初期SO₂抑制了Q235B钢的腐蚀; 在腐蚀后期, SO₂促进了Q235B钢的腐蚀. 当SO₂的浓度较低时, 腐蚀速率随SO₂浓度的升高而增大; 当SO₂的浓度较高时, 腐蚀速率随SO₂浓度的升高而减小. 工业-海岸大气中的SO₂组分可以抑制腐蚀产物中g -FeOOH和b -FeOOH的生成, 而促进a -FeOOH生成. 低碳钢腐蚀速率随SO₂浓度变化出现的极值现象与SO₂导致的锈层相组分变化密切相关.

The supercritical CO₂ corrosion problems of oil country tubular goods (OCTG) become increasingly prominent along with the application of CO₂ flooding enhanced oil recovery (EOR) technique and the exploitation of deep oil wells under high temperature and high pressure. Actually, OCTG steel often suffers from multiphase fluid corrosion of crude oil, water and supercritical CO₂ at different stages of oil and gas production. However, studies about CO₂ corrosion of carbon steel used for oil and gas production generally are carried out considering only the aqueous phase without proper consideration of the oil phase that may be present. The crude oil in crude oil/water production environments is a key factor affected the corrosion behavior of carbon steel. Corrosion rate and corrosion type of J55 steel were investigated under the conditions of different oil/water ratios saturated with supercritical CO₂. SEM, EDS and XRD were employed to analyze the morphology and characteristic of corrosion scale on the steel. The corrosion models were developed to understand the corrosion mechanism with consideration of a variation of oil/water ratio in reality. The results show that uniform corrosion occurs along with the lower corrosion rate due to the protection of crude oil when the water cut of crude oil is within the range, i.e., 0~30% and the water-in-oil fluid is formed. But the local corrosion rate of the steel increases rapidly due to the inhomogeneous adsorption of crude oil with the fluid changing from water-in-oil to oil-in-water emulsion when the water cut is between 30% and 75%. The corrosion products deposited on the steel surface change the wettability of oil and water phase, therefore, water phase can preferentially wet the localized deposited scale, leading to the development of pitting corrosion under the scale. However, when the water cut is higher than 75% and the oil-in-water fluid is formed, water phase infiltrates the metal surface that blocks the corrosion inhibition of crude oil for the steel, hence, the corrosion rate increases dramatically. The localized failure of corrosion scale due to scouring action of the fluid and the dissolution of aggressive medium leads to mesa corrosion on the steel. When the water cut is 100%, serious uniform corrosion occurs as a result of the strong corrosiveness of supercritical CO₂ dissolved in the water phase. Furthermore, crude oil can weaken the dissolution of corrosion scale in the supercritical corrosive medium, which modifies the grain size, morphology and chemical composition of corrosion scale and improves the protection performance of the scale.

In order to control the grain size of forged turbine disk of wrought superalloy like GH4738 more effectively, constitutive equations and grain structure evolution models of GH4738 alloy are used in Deform 3D^TM for achieving integrated simulation of whole forging process of GH4738 alloy turbine disk (from preheating billet for upsetting to die forging). By using of integrated simulation, the variation of temperature, average grain size, etc., during the whole forging process has been explored, making it possible to control these parameters quantitatively. Comparing with traditional simple stage simulation, results of integrated simulation are more consistent with corresponding experimental results of forged turbine disk (300 mm in diameter). Therefore, the reliability of the integrated simulation is verified. Finally, with the application of integrated simulation, GH4738 alloy turbine disk with a diameter of 1450 mm has been successfully forged by 8×10⁴ t forging press. This work provides a more practical simulation method for helping the process design of forging large turbine disk.

As promising light-weight high-temperature materials, g-TiAl base alloys are considered as prospective candidates for automobile and aerospace application due to their high specific yield strength. Adding Nb to TiAl alloys increases the liquidus temperature and results in improvents of creep resistance, high temperature strength and oxidation resistance. High Nb-containing TiAl alloys have attracted much attention during past decades. With the addition of carbon in Ti-46Al-8Nb-xC alloys (x=0, 0.7, 1.4, 2.5, atomic fraction, %), the formation of precipitates, the orientation relationship between precipitates and the TiAl matrix and the evolution of the precipitates during heat treatments have been investigated in this work by XRD, SEM and TEM. The results show that lath-shaped precipitates of Ti₂AlC can be formed during the preparation of ingots with the addition of 1.4% and 2.5% of C. With good thermal stability, the size, amount and distribution of Ti₂AlC precipitates remain almost stable during the aging process. Needle-shaped precipitates of Ti₃AlC are formed in the aged alloys with 0.7%, 1.4% and 2.5% of C. And the precipitates are preferentially formed in g grains. The orientation relationship between Ti₃AlC precipitates and g phase is found to be {100}_Ti3AlC //{100}_γ and <001> _Ti3AlC //<001>_γ. Meanwhile, the precipitation behavior and morphology of Ti₃AlC are also discussed. Ti₃AlC precipitates grow slightly after prolonged aging, while the amount of the precipitates remains small. With a higher aging temperature, the size of Ti₃AlC precipitates increases significantly and an increasing amount of the precipitates is observed.

A new Ni-based superalloy with the refined grains is to be used in industrial and aircraft turbines because of its high strength and excellent fatigue resistance at lower and medium temperatures (500~800 ℃). The grains with six different sizes have been made by decreasing the pouring temperature from 1460 to 1480 ℃ then 1500 ℃ and adding refiner to alloy and planting seed on the surface of mold. The size of equiaxed crystal grain is reduced to 0.5 mm in the center part of specimen with the columnar crystals in the outside of specimen made by the refining process which is finer than those of traditional process. It has been found that γ' phase and carbide are finer in refined grains than those in the coarse grains made by decreasing the pouring temperature. The room-temperature tensile properties and high cycle fatigue properties of tested alloy are improved with decreasing grain size. The stress-rupture properties are increased under the conditions of 760 ℃ and 662 MPa while are decreased with decreasing the grain size. The grain structure and size are refined by the refining process that dominated the excellent mechanical properties of tested alloy at lower and medium temperatures. However, it is not good for the mechanical properties at high temperatures.

The influences of Ni on the phase transformation temperatures, γ /γ′ two-phase microstructural evolution, γ′ dissolution behavior and microhardness have been investigated in four Co-Al-W base alloys containing various Ni contents (15%~45%, atomic fraction). The results show that the γ′ solvus temperatures continuously increase and the solidus temperatures are nearly unchanged with increasing the Ni content. The γ/γ′ two-phase microstructure is generated in four experimental alloys after the heat treatment at 900 ℃ for 50 h, whereas the γ′ morphology changes from cuboidal to nearly spherical and the γ′ volume fraction reduces as the Ni content increases. When prolonged heat treatment at 900 ℃ for 300 h is employed, no significant change in the γ′ morphology is observed in four experimental alloys but the γ′ volume fraction decreases to different degree as a function of Ni concentration. High temperature treatments at 970~1060 ℃ are conducted after experimental alloys are heat treated at 900 ℃ for 300 h. In the high temperature range, the dissolution of the γ′ phase is more pronounced as the temperature elevates, whilst the γ′ morphology becomes spherical and cuboidal in alloys containing the low and high levels of Ni, respectively. The microhardness results of the experimental alloys after heat treatment at 900 ℃ for 50 h and 300 h indicate that the microhardness is lowered in alloys with higher Ni content, but it increases as the heat treatment time is prolonged.

The effects of carbon addition on the solidification microstructure and rupture life were investigated in five different carbon level single crystal superalloys. With the increasing of carbon level, the volume fraction of eutectic decreased markedly and the volume fraction of carbide increased. The carbides mainly distributed in interdendrite zone, when the carbon level was high, there were little carbides in the dendrite core. After heat treatment, coarse γ/γ′ eutectics in interdendrte zone mainly were dissolved, a little γ/γ′ eutectic was not dissolved. Morphologies of carbide became much simpler, the size of carbide decreased, the volume fraction of carbide decreased, and the distribution of carbide became much more dispersion, and the type of carbide became much more variety. Grain and chainlike M₂₃C₆ appeared after heat treatment. With the increasing of carbon level, the rupture life of single crystal superalloy increased at first and then decreased, and the rupture life came up to the maximum when the carbon level was 0.045%. SEM observation indicates that the cracks of alloys mainly originate from shrinkage, carbides and eutectics. The change of rupture life was mainly because the un-dissolved eutectic and carbides of alloy which act as the source of cracks. The variation trend of carbide and eutectic was contrary with the increasing of carbon level; therefore, the carbon content should be controlled in the perfect level.

Retrogression and re-aging (RRA) treatments are divided into pre-aging, retrogression and re-aging. Although peak aging was used as the pre-aging of RRA treatment in the past, some different opinions were reported in recent years. The effects of pre-aging of RRA treatment on microstructure, mechanical properties and conductivity of spray formed 7075 aluminum alloy were investigated by TEM, tensile and conductivity test. The results show that the mechanical properties and conductivity of spray formed 7075 alloy could be improved by the under aging at 120 ℃ for 16 h as the pre-aging of RRA treatment. The properties of the 7xxx series aluminum alloys depend on matrix precipitates (MPt), grain boundary precipitates (GBPs) and precipitate free zones (PFZs). The tiny MPt can increase the tensile strength. The connected GBPs and narrow PFZs will lower the conductivity and the elongation of the alloy. The under aging is more beneficial for the re-dissolution of the MPts at retrogression treatment at 200 ℃ for 10 min, and is more conducive to interrupt distributions of the GBPs than the early aging after RRA treatment. With the under aging as the pre-aging treatment, the growth of the MPts was actively suppressed and the GBPs at grain boundaries are continuous. During retrogression treatment, the MPts were re-dissolved absolutely and the GBPs were transformed from a chain to dissociation. After re-aging treatment, lots of tiny dispersive MPts precipitated out again in matrix, the GBPs were totally separated and the PFZ were widened. After pre-aging at 120 ℃ for 16 h and RRA treatment, the tensile strength and yield strength of the alloy are 782 and 726 MPa, respectively, which are higher than that after peak aging treatment or conventional RRA treatment, the conductivity of the 7075 alloy is excellent with 22.7 MS/m.

The true stress-true strain curves have been attained through the isothermal compression experiment of TC18 titanium alloy. The influence of deformation parameters on the shape of stress-strain curves and peak stress has been analyzed through the working-hardening and dynamic softening effects. The true stress-true strain curves show the similar characteristics under different deformation conditions. However, the peak stress is sensitive to the changes of deformation parameters. The type of dynamic softening mechanisms in hot deformation under certain conditions can be obtained through Poliak-Jonas criterion. The dynamic recrystallization process tends to take place during the deformation with lower strain rates. And the choice of the dynamic softening mechanisms is not sensitive to deformation temperatures. The suitable constitutive models under different softening mechanisms have been constructed based on the traditional Arrhenius equations. With the identification of the dynamic softening mechanisms in hot deformation of TC18 alloy with different conditions, the response of true stress, at strain of 0.7, to the deformation temperatures and strain rates can be described through the proposed models. And the sensitivity coefficient of strain rates and deformation activation energy, of the process with the dynamic recrystallization as the major softening mechanism, are much larger than the ones of process with dynamic recovery.

Hybrid high power impulse magnetron sputtering (HIPIMS) is a new-generation HIPIMS technique with a pulse and dirrect current power supply parallelled connection operation. In this work, the influence of dirrect current from 0 to 4.0 A supplied by the dirrect current power is investigated on hybrid HIPIMS Ti discharge characteristics, plasma parameters (plasma potential, electron temperature and electron density) and Ti film properties in an Ar atmosphere. The results show that target voltage and current are characterized by a peak with variation of time in different dirrect currents. Although the target voltage is barely affected, the target current decreases with increasing the dirrect current during the pulse turn-on stage. The plasma parameters determined by a Langmuir probe have been significantly influenced by the dirrect current. Moreover, the deposition rate and average roughness increase while the hardness and elastic modulus have a slight decrease with the variation of dirrect current from 1.0 to 3.0 A. The samples are selected for comparison with that prepared by conventional direct current magnetron sputtering (DCMS) at the same average target power 650 and 1500 W. The results demonstrate that Ti films using hybrid HIPIMS have a close deposition rate and a superior quality and performance to those prepared using DCMS especially at the low target power 650 W when the direct current is 1.0 A.

The properties of modern materials, especially superplastic, nanocrystalline or composite materials, depend critically on the properties of internal interfaces such as grain boundaries (GBs) and interphase boundaries (IBs). All processes which can change the properties of GBs and IBs affect drastically the behaviour of polycrystalline metals and ceramics. In this work, the annihilation processes of low-angle symmetric tilt GBs and dislocations during plastic deformation in the representative system of these materials near but below the melting point and the temperature at liquid-solid coexistence line were simulated using the phase-field crystal model, respectively. The results show that local premelting occurs around lattice dislocations near the melting point but the dislocation structure in the premelting region does not change, while the region become significantly larger when the system reaches the melting temperature. After premelting, deformation to the system causes dislocations in the premelting GB to begin to glide then annihilate with opposite Burgers vectors via the movement, finally the GB and the premelting region disappear. The annihilation mechanisms of dislocations are similar to those for premelting conditions. The more the temperature is closer to the melting point, the more obvious the atomic lattice around the premelting region is softened leading to the atomic binding strength around the dislocations being lowered. Only at this moment, the lattice atoms enable to reduce the resistance of the dislocation motion and accelerate its velocity during deformation. At the temperature reaching to the liquid-solid coexisting region in the simulation, the original premelting regions are induced to develop into bigger ones by the external strain acting. During this process, it can be seen some interactions including the multiplication dislocation pairs, the rotation of dislocation pairs and their annihilation. Furthermore, the shape of the premelting region changes with the variation of the interaction of dislocations inside the region, it is observed that the premelting regions approach each other and consolidate together, then decompose and segregate from each other. Although the shape of the premelting region changes with the applied strain, these regions do not disappear at the end of the simulation, totally different those in lower premelting temperature.