Stress-assisted corrosion mechanism of 3Ni steel by using gradient boosting decision tree machining learning method

Traditional 3Ni weathering steel cannot completely meet the requirements for offshore engineering development,resulting in the design of novel 3Ni steel with the addition of microalloy elements such as Mn or Nb for strength enhancement becoming a trend.The stress-assisted corrosion behavior of a novel designed high-strength 3Ni steel was investigated in the current study using the corrosion big data method.The information on the corrosion process was recorded using the galvanic corrosion current monitoring method.The gradi-ent boosting decision tree(GBDT)machine learning method was used to mine the corrosion mechanism,and the importance of the struc-ture factor was investigated.Field exposure tests were conducted to verify the calculated results using the GBDT method.Results indic-ated that the GBDT method can be effectively used to study the influence of structural factors on the corrosion process of 3Ni steel.Dif-ferent mechanisms for the addition of Mn and Cu to the stress-assisted corrosion of 3Ni steel suggested that Mn and Cu have no obvious effect on the corrosion rate of non-stressed 3Ni steel during the early stage of corrosion.When the corrosion reached a stable state,the in-crease in Mn element content increased the corrosion rate of 3Ni steel,while Cu reduced this rate.In the presence of stress,the increase in Mn element content and Cu addition can inhibit the corrosion process.The corrosion law of outdoor-exposed 3Ni steel is consistent with the law based on corrosion big data technology,verifying the reliability of the big data evaluation method and data prediction model selection.

Kinetic Laws of Heating Initiated Reactions for Materials in Aerospace Applications

Cure and decomposition reaction kinetics of typical organic materials in aerospace applications are introduced.From the data of dynamic differential scanning calorimetry(DSC)experiments,and based on changes of the peak temperatures(T_(p))with different heating rates(β),a linear equation,T_(p)=T_(1)+△Tlnβ,has been obtained more reasonably.The above equation can be used to explain some laws of higher or lower of apparent activation energies(E_(a)),by which the apparent activation energy(E_(a))is nearly equal to RT^(2)_(1)/△T.A number of kinetic investigations of typical thermosetting resins and energetic materials in aerospace applications were chosen to validate the above equations.

Effects of microstructure characteristics on the tensile properties and fracture toughness of TA15 alloy fabricated by hot isostatic pressing

Powder hot isostatic pressing(HIP) is an effective method to achieve near-net-shape manufacturing of high-quality complex thinwalled titanium alloy parts, and it has received extensive attention in recent years. However, there are few reports about the microstructure characteristics on the strengthening and toughening mechanisms of powder hot isostatic pressed(HIPed) titanium alloys. Therefore, TA15powder was prepared into alloy by HIP approach, which was used to explore the microstructure characteristics at different HIP temperatures and the corresponding tensile properties and fracture toughness. Results show that the fabricated alloy has a “basket-like structure” when the HIP temperature is below 950℃, consisting of lath clusters and surrounding small equiaxed grains belts. When the HIP temperature is higher than 950℃, the microstructure gradually transforms into the Widmanstatten structure, accompanied by a significant increase in grain size. The tensile strength and elongation are reduced from 948 MPa and 17.3% for the 910℃ specimen to 861 MPa and 10% for the 970℃ specimen.The corresponding tensile fracture mode changes from transcrystalline plastic fracture to mixed fracture including intercrystalline cleavage.The fracture toughness of the specimens increases from 82.64 MPa·m^(1/2)for the 910℃ specimen to 140.18 MPa·m^(1/2)for the 970℃ specimen.Specimens below 950℃ tend to form holes due to the prior particle boundaries(PPBs), which is not conducive to toughening. Specimens above 950℃ have high fracture toughness due to the crack deflection, crack branching, and shear plastic deformation of the Widmanstatten structure. This study provides a valid reference for the development of powder HIPed titanium alloy.

Effect of Cooling Rates on Solidification Microstructures and Tensile Property of a Novel Wrought Superalloy

The effects of cooling rates on solidification behaviors,segregation characteristics and tensile property of GH4151 alloy were investigated using microstructure characterization and tensile test.Firstly,a relationship between the secondary dendrite arm spacing and cooling rate was determined and it was confirmed to be valid.Secondly,it can be found from microstructure observations that the morphology of(Nb,Ti)C carbides transits from blocky and script type to fine script type and spotty type,and the refinedγ'phase was observed due to decrease of segregation with increasing cooling rates.Thirdly,the solidification microstructures of the industrial-scale samples were analyzed.The morphology ofηphase changes from indistinguishable shape,fine needle-like shape to large block-like shape with increasing ingot diameter.As a result,the mechanical properties of alloy decrease due to increase of brittle precipitations.The experimental results show that the precipitation behavior of GH4151 is affected by segregation degree of elements,and the segregation degree is determined by solute distribution process and solid back-diffusion process.

Numerical Simulation of Low Cycle Fatigue Behavior of Ti_(2)AlNb Alloy Subcomponents

Many titanium alloy subcomponents are subjected to fatigue loading in aerospace engineering,resulting in fatigue failure.The fatigue behavior of Ti_(2)AlNb alloy subcomponents was investigated based on the Seeger fatigue life theory and the improved Lemaitre damage evolution theory.Firstly,the finite element models of the standard openhole specimen and Y-section subcomponents have been established by ABAQUS.The damage model parameters were determined by fatigue tests,and the reliability of fatigue life simulation results of the Ti_(2)AlNb alloy standard open-hole specimen was verified.Meanwhile,the fatigue life of Ti_(2)AlNb alloy Y-section subcomponents was predicted.Under the same initial conditions,the average error of fatigue life predicted by two different models was 20.6%.Finally,the effects of loading amplitude,temperature,and Y-interface angle on fatigue properties of Ti_(2)AlNb Y-section subcomponents were investigated.These results provide a new idea for evaluating the fatigue life of various Ti_(2)AlNb alloy subcomponents.

Influence of Ru Content on Microstructural Stability and Stress Rupture Property of DD15 Alloy

A fourth generation single crystal(SC)nickel based cast superalloy DD15 with 1%Ru,3%Ru,5%Ru was prepared using vacuum induction single crystal furnace in order to optimize the properties and cost of DD15 alloy.The exposure experiment of three alloys was conducted at 1100℃for 1000 h.The stress rupture properties tests were performed at 1100℃temperature and 137 MPa pressure.The composition optimization of Ru element in DD15 alloy had been studied.It was found that the alloys with different Ru contents all consist of cuboidalγ′phase embedded coherent inγphase.Theγ′phase of the alloy all has a size of about 300-500 nm and a volume content of more than sixty percent.The dimension ofγ′precipitates is reduced and uniform with increase of Ru content.Ru element can reduce the distribution ratio of high melting point element,so the microstructural stability is enhanced with Ru content increasing.No topologically close-packed(TCP)phase precipitated in the 5%Ru alloy even after 1000 h exposure.The stress rupture life of the alloy is significantly improved as Ru content rising.The raft breadth decreases slightly as Ru content increases.The specimen with 1%Ru and 3%Ru exhibits the presence of TCP phases and without TCP phases precipitated in fracture specimen with 5%Ru.The density and integrity ofγ/γ′interfacial dislocation network increase as Ru content of the alloy rises.

Microstructure and mechanical properties of forged Al-7.1Zn-1.1Mg-1.6Cu-0.14Zr alloy after two-step ageing treatment at 120 and 170℃

The tensile properties, electrical conductivity, and microstructure of the forged A1-7.1Zn-1.1Mg-1.6Cu-0.14Zr alloy were investigated after a two-step ageing treatment at 120 and 170℃. The results indicate that the strength of the alloy reaches the peak value at 170~C for 1 h during the second step ageing and then decreases sharply. However, the electrical conductivity value increases continuously with the second ageing time increasing. The fracture mechanism of the alloy is intergranular fracture for 1 h and then changes to dimple transgranular fracture later, and the toughness of the alloy is improved significantly. The phases of rl＇ and 1＂1 are major precipitates in the alloy under the two-step ageing condition. Discontinuous grain boundary precipitates and precipitate-flee zones along the grain boundary are clearly observed.

Deformation behavior and mechanisms of Ti-1023 alloy

The deformation behavior and mechanisms of Ti-1023 alloy were studied in the temperature range of 650-900 ℃ and strain rate range of 0.001-10 s-1 by compression and tensile tests. The results show that in a limited strain rate range of 0.001-0.1 s-1, the kinetic rate equation is obeyed and a linear fit is obtained at all the temperatures. The apparent activation energy is 322 kJ/mol in the α-β region and 160 kJ/mol in the β region, respectively. Power dissipation maps of this alloy developed by using Gleeble test data show three domains in the tested range. Superplasticity, marked by abnormal elongation at 700 ℃, occurs in the temperature range of 650-750 ℃ and at strain rates below about 0.03 s-1. Large grain superplasticity takes place in the temperature range of 750-850 ℃ and strain rates range of 0.001-0.03 s-1. Dynamic recrystallization occurs in the temperature range of 850-900 ℃ and at strain rates below about 1 s-1. The instability maps of this alloy were also developed.

Development and application of titanium alloy casting technology in China

The development and research of titanium cast alloy and its casting technology, especially its application in aeronautical industry in China are presented. The technology of molding, melting and casting of titanium alloy, casting quality control are introduced. The existing problems and development trend in titanium alloy casting technology are also discussed.

Effects of Ru on the microstructure and phase stability of a single crystal superalloy

Two experimental single crystal superalloys, the Ru-free alloy and the Ru-containing alloy with [001 ] orientation, were cast in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Ru on the microstructure and phase stability of the single crystal superalloy were investigated, y＇ directional coarsening and rafting were observed in the Ru-free alloy and Ru-containing alloy after long-term aging at 1070~C for 800 h. Needle-shaped o topologically close packed （TCP） phases precipitated and grew along the fixed direction in both the alloys. The precipitating rate and volume fraction of TCP phases decreased significantly by adding Ru. The compositions ofy and y＇ phases measured using an energy-dispersive X-ray spectroscope （EDS） in transmission electron microscopy （TEM） analysis showed that the addition of Ru lessened the partition ratio of TCP forming elements, Re, W and Mo, and decreased the satu- ration degrees of these elements in y phase, which can enable the Ru-containing alloy to be more resistant to the formation of TCP phases. It is indicated that the addition of Ru to the Ni-based single crystal superalloy with high content of the refractory alloying element can enhance phase stability.

Effect of surface recrystallization on the creep rupture property of a single-crystal superalloy

The effect of surface recrystallization by heating after shot-peening on the creep rapture property and fracture behavior of a single-crystal superalloy was investigated. The results show that the creep rupture property of the single-crystal superalloy was greatly influenced by surface recrystallization. A recrystallized surface layer with a depth of 101 ~m resulted in a decrease in creep rupture life by nearly 50%, and an almost linear reduction of creep rupture life was observed with the increase of recrystallization depth. A lower strength of the recrystal- lized layer, inhomogeneous deformation between the recrystallized layer and the matrix, and stress concentration caused by notch effect resuited in the decrease in creep rupture life of the single-crystal superalloy.

Effects of chromium on the corrosion and electrochemical behaviors of ultra high strength steels

The effects of chromium on the corrosion and the electrochemical behaviors of ultra high strength steels were studied by the salt spray test and electrochemical methods. The results show that ultra high strength steels remain martensite structures and have anodic dissolution characteristic with an increase of chromium content. There is no typical passive region on the polarization curves of an ultra high strength stainless steel, AerMet 100 steel, and 300M steel. However, chromium improves the corrosion resistance of the stainless steel remarkably. It has the slowest corrosion rate in the salt spray test, one order of magnitude less than that of AerMet 100 and 300M steels. With the increase of chromium content, the polarization resistance becomes larger, the corrosion potential shifts towards the positive direction with a value of 545 mV, and the corrosion current density decreases in electrochemical measures in 3.5wt% NaCl solutions. Because of the higher content of chromium, the ultra high strength stainless steel has a better corrosion resistance than AerMet 100 and 300M steels.

Effects of RRA Treatments on Microstructures and Properties of a New High-strength Aluminum-Lithium Alloy-2A97

A new high strength 2A97 Al-Cu-Li-X alloy was subjected to triple-aging of retrogression and re-aging treatments （RRA）. Transmission electron microscopy （TEM）, differential scanning calorimetry （DSC）, and tensile tests were used to investigate the effects of RRA treatment on the microstructures and properties. DSC test reveals the reversion temperature range of the strengthening δ＇ （Al3Li） phase. The results show that the microstructure consists of δ＇ （Al3Li） phase, T1 （Al2CuLi） phase and θ″/θ′（Al2Cu） phase for 2A97 alloy treated by a triple-aging of a retrogression and re-aging treatment in the following order： （1） at 165℃×30 min, （2） at 220 ℃ or 240℃ × 15 min, （3） at 165℃×24 h. The plastic deformation, incorporated into the treatment after secondary high temperature aging, promotes the T1 precipitation during final re-aging. The tensile properties of the alloy treated by the retrogression and re-aging treatment reach the peak level of alloy single-aged at 165℃ in T6 temper.

Effect of directional solidification process on microstructure and stress rupture property of a hot corrosion resistant single crystal superalloy

The influences of different directional solidification processes, i.e., the high rate solidification(HRS) and liquid metal cooling(LMC), on microstructure and stress rupture property of DD488 alloy were investigated. The DD488 alloy was directional solidified by both HRS and LMC processes. The microstructure and stress rupture properties at 980 ℃/250 MPa were investigated by using optical microscopy(OM), scanning electron microscopy(SEM), electron microprobe analyzer(EPMA), transmission electron microscopy(TEM) and stress rupture testing. The results indicated that the LMC process refined the primary dendrite arm and decreased the microporosity volume fraction and solidification segregations of Cr and Co in as-cast DD488 alloy. After standard heat treatment of 1,260 ℃/4 h, AC(air cooling) + 1,080 ℃/4 h, AC + 870 ℃/24 h, AC, the γ′ morphology in LMC alloy was more cuboidal than that in HRS alloy, and the γ′ volume fraction of LMC alloy was higher than that of HRS alloy. The stress rupture life at 980 ℃/250 MPa of HRS alloy was 76.8 h, and it increased to 110.0 h in LMC al oy. The LMC process increased the stress rupture life due to the higher γ′ volume fraction, more perfect rafting structure and finer interfacial dislocation networks.

Effects of Y and Ce on Microstructures and Properties of Nb-Si System Composites

The effects of yttrium and cerium on microstructures and properties of Nb-Si system composites were investigated by scanning electron microscopy( SEM), energy dispersive spectrum(EDS), X-ray diffraction(XRD) and high temperature oxidation experiments.It is found that the coarse primary silicide phase became finer and more homogeneous with Y and Ce addition.The results of high temperature oxidation experiments show that the oxidation rates of NbSi system composites with an appropriate amount of Y and Ce decrease compared with those of alloys without Y or Ce addition, and oxidation products mainly distribute along the phase boundaries between the Nb solid solution and silicide.

Research on investment casting of TiAl alloy agitator treated by HIP and HT

因为它的低密度，高提高的温度力量，和反氧化能力，用 TiAl，代替 superalloy 的合金是在飞机引擎工业的一个热话题。在这研究， Ti-47.5AL-2Cr-2Nb-0.2B 合金被用作测试材料。由使用做的陶器的壳模子和核心的一个联合过程，融化的真空弧并且离心的流，和热均衡说的紧迫(臀部) 并且热处理(HT ) 等等，为飞机引擎扔的 TiAl 旋涡煽动者成功地被做。，在与投资扔做 TiAl 旋涡煽动者的介绍关键技术处理的这篇论文包括机械性质和 machinability 的一些试验性的结果，和能影响 TiAl 铸造物的应用的解释一些担心。

Research Progress of Refractory High Entropy Alloys:A Review

Owing to superior comprehensive performance than conventional superalloys at high temperature,refractory high entropy alloy(RHEA)is becoming a promising candidate for the next generation high-temperature material.Herein,contemporary aspects of corresponding development of RHEAs are reviewed to discuss various factors affecting the organization structure and service performance.It mainly covers alloying system and strengthening mechanism,the preparation method,plastic deformation and the related mechanism,as well as microstructure control by heat treatment.Firstly,the alloy systems and strengthening mechanism are introduced.This is followed by different preparation methods and the comparison of strengths and shortcomings based on different RHEAs.Then,hot deformation behavior and plastic deformation under different loadings are analyzed.Based on this,the influence of heat treatment on microstructures prior to and after the deformation is further summarized.Finally,some important research areas to be carried out in future are pointed out.This review will give a deep understanding of the effects of different factors on the service performance and provide scientific guide in designing RHEAs with improved performance.

High-temperature mechanical properties and deformation behavior of high Nb containing TiAl alloys fabricated by spark plasma sintering

A high Nb containing TiA1 alloy was prepared from the pre-alloyed powder of Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y （at%） by spark plasma sintering （SPS）. Its high-temperature mechanical properties and compressive deformation behavior were investigated in a temperature range of 700 to 1050℃ and a strain rate range of 0.002 to 0.2 s 1. The results show that the high-temperature mechanical properties of the high Nb containing TiA1 alloy are sensitive to deformation temperature and strain rate, and the sensitivity to strain rate tends to rise with the deformation temperature increasing. The hot workability of the alloy is good at temperatures higher than 900℃, while fracture occurs at lower temperatures. The flow curves of the samples compressed at or above 900℃ exhibit obvious flow softening after the peak stress. Un- der the deformation condition of 900-1050℃ and 0.002-0.2 s 1, the interrelations of peak flow stress, strain rate, and deformation tempera- ture follow the Arrhenius＇ equation modified by a hyperbolic sine function with a stress exponent of 5.99 and an apparent activation energy of 441.2 kJ.mol-1.

Experimental Study and Numerical Simulation of Directionally Solidified Turbine Blade Casting

汽轮机片样品铸造物的方向性的团结过程在工作被调查。可变退却率用一致 rate.A 在一个退却过程并且与其它相比被使用为热放射转移和方向性的团结过程的微观结构模拟的数学模型基于 CA-FD 被开发方法。温度分发和微观结构被模仿并且与试验性的结果相比。迷路的谷物被预言并且与试验性的结果相比。站台的不平的温度分发是迷路的谷物的形成的主要原因。

Constitutive behavior and processing maps of low-expansion GH909 superalloy

The hot deformation behavior of GH909 superalloy was studied systematically using isothermal hot compression tests in a temperature range of 960 to 1040°C and at strain rates from 0.02 to 10 s^(-1) with a height reduction as large as 70%. The relations considering flow stress,temperature,and strain rate were evaluated via power-law,hyperbolic sine,and exponential constitutive equations under different strain conditions. An exponential equation was found to be the most appropriate for process modeling. The processing maps for the superalloy were constructed for strains of 0.2,0.4,0.6,and 0.8 on the basis of the dynamic material model,and a total processing map that includes all the investigated strains was proposed. Metallurgical instabilities in the instability domain mainly located at higher strain rates manifested as adiabatic shear bands and cracking. The stability domain occurred at 960–1040°C and at strain rates less than 0.2 s^(-1); these conditions are recommended for optimum hot working of GH909 superalloy.