Hot working characteristics of HEXed PM nickel-based superalloy during hot compression

To study the hot deformation behavior of a new powder metallurgy nickel-based superalloy,hot compression tests were conducted in the temperature range of 1020−1110℃ with the strain rates of 0.001−1 s^−1.It is found that the flow stress of the superalloy decreases with increasing temperature and decreasing strain rate.An accurate constitutive equation is established using a hyperbolic-sine type expression.Moreover,processing map of the alloy is constructed to optimize its hot forging parameters.Three domains of dynamic recrystallization stability and instability regions are identified from the processing map at a strain of 0.7,respectively.The adiabatic shear band,intergranular crack and a combination of intergranular crack and wedge crack are demonstrated to be responsible for the instabilities.Comprehensively analyzing the processing map and microstructure,the optimal isothermal forging conditions for the superalloy is determined to be t=1075−1105℃ andε&=10^−3−10−2.8 s^−1.

Crack initiation and propagation induced by inclusions in a nickel-base P/M superalloy under fatigue load

In situ fatigue tests in special designed SEM were conducted to trace the whole process of crack initiation and propagation fill to fracture in mckel-base P/M superalloy seeded inclusions. The experimental results show that non-metallic inclusions can induce crack initiation. When the inclusion size is larger than the critical one, the crack can propagate as the main crack that induces the specimen to fracture. As a result, the LCF life of the specimen decreases.

THE ROLE OF NIOBIUM IN NICKEL-BASED SUPERALLOYS AND CHARACTERIZATION OF PM ALLOY EP741NP

The role of niobium in nickel-based superalloys is reviewed. The importance of niobium as a strengthener is discussed. New developments in nickel-based superalloys are also briefly mentioned, including some results that show improved resistance to sulfidation by niobium. Research results from a current program on the role of niobium in the Russian powder metallurgy alloy EP741NP are presented. Future research plans on the role of niobium in superalloys are also discussed.

Cooling γ′ precipitation behavior and strengthening in powder metallurgy su-peralloy FGH4096

Two cooling schemes （continuous cooling and interrupted cooling tests） were applied to investigate the cooling γ precipitation behavior in powder metallurgy superalloy FGH4096. The effect of cooling rate on cooling γ precipitation and the development of γ precipitates during cooling process were involved in this study. The ultimate tensile strength （ErrS） of the specimens in various cooling circumstances was tested. The experiential equations were obtained between the average sizes of secondary and tertiary γ precipitates, the strength, and cooling rate. The results show that they are inversely correlated with the cooling rate as well as the grain boundary changes from serrated to straight, the shape of secondary γ precipitates changes from irregular cuboidal to spherical, while the formed tertiary γ precipitates are always spherical. The interrupted cooling tests show that the average size of secondary γ precipitates increases as a linear function of interrupt temperature for a fixed cooling rate of 24℃/min. The strength first decreases and then increases against interrupt temperature, which is fundamentally caused by the multistage nucleation of γ precipitates during cooling process.

Oxidation during the production of FGH4095 superalloy powders by electrode induction-melt inert gas atomization

Super-clean and super-spherical FGH4095 superalloy powder is produced by the ceramic-free electrode inductionmelt inert gas atomization（EIGA） technique.A continuous and steady-state liquid metal flow is achieved at high-frequency（350 k Hz） alternating current and high electric power（100 k W）.The superalloy is immersed in a high-frequency induction coil,and the liquid metal falling into a supersonic nozzle is atomized by an Ar gas of high kinetic gas energy.Numerical calculations are performed to optimize the structure parameters for the nozzle tip.The undesired oxidation reaction of alloying elements starts at 1000℃ with the reaction originating from the active sites on the powder surfaces,leading to the formation of oxides,MexOy.The role of active sites and kinetic factors associated with the diffusion of oxygen present in the atomization gas streams are also examined.The observed results reveal that the oxidation process occurring at the surface of the produced powders gradually moves toward the core,and that there exists a clear interface between the product layer and the reactant.The present study lays a theoretical foundation for controlling the oxidation of nickel-based superalloy powders from the powder process step.

Microstructural evolution in a powder metallurgical Ti-7Mo alloy with continuous oxygen gradient

A titanium alloy containing continuous oxygen gradient was prepared by powder metallurgy(P/M) and the composition–property relationship was studied on a single sample. The alloy was sintered with layered powder of different oxygen contents via vacuum sintering and spark plasma sintering(SPS), respectively. After subsequent heat treatments, high-throughput characterizations of the microstructures and mechanical properties by localized measurements were conducted. The Ti-7% Mo(molar fraction) alloy with an oxygen content ranging from 1.3×10^(-3) to 6.2×10^(-5)(mass fraction) was obtained, and the effects of oxygen on the microstructural evolution and mechanical properties were studied. The results show that SPS is an effective way for fabricating fully dense Ti alloy with a compositional gradient. The average width of α′ phase coarsens with the increase of the content of oxygen. The content of α″ martensitic phase also increases with the content of oxygen. At oxygen contents of 3×10^(-3) and 4×10^(-3)(mass fraction), the Ti alloys present the lowest microhardness and the lowest elastic modulus, respectively. The results also indicate that the martensitic phases actually decrease the hardness of Ti-7Mo alloy, and oxygen effectively hardens the alloy by solid solution strengthening. Therefore, the high-throughput characterization on a microstructure with a gradient content of oxygen is an effective method for rapidly evaluating the composition–property relationship of titanium alloys.

CRITICAL ISSUES OF POWDER METALLURGY TURBINE DISKS

P/M superalloy disks obtain their final strength by appropriate heat treatments; the maximum attainable strength depends on the rapid cooling rate from the solution annealing. A rapid quench of a large disk forging can cause two problems, surface cracking and shape distortion.In the past,many attempts employ the finite element code to model and to predict temperature evolution and induced stress distribution in a large turbine disk. The major difficulty was the correct description of alloy behavior; particularly the thermomechanical properties and the failure criteria of material during the cooling. High temperature fatigue resistance is always the key requirement for disk materials. New methodology of residual life management emphasizes the initiation as well as the propagation of the cracks developed under the service conditions. One of major challenges to P/M superalloys is the time-dependent behavior of fatigue cracking, which relates to the well-known SAGBO (stress-assisted grain boundary oxidation) phenomenon.A great effort has been done to understand the micro-mechanism of time-dependent fatigue crack propagation resulted in the second generation of P/M superalloys. Further improvement on temperature capability of disk alloys at rim area may lead to the idea of dual-property disks.Different grain structures at different portions of a large disk are possible,as the property requirements for different locations are different. This goal is achievable if the thermal history at specific disk locations can be controlled to develop desirable microstructures and properties.Some suggestions on the future direction of research efforts will be discused.

Prediction on grinding force during grinding powder metallurgy nickel-based superalloy FGH96 with electroplated CBN abrasive wheel

In this article,a grinding force model,which is on the basis of cutting process of single abrasive grains combined with the method of theoretical derivation and empirical formula by analyzing the formation mechanism of grinding force,was established.Three key factors have been taken into accounts in this model,such as the contact friction force between abrasive grains and materials,the plastic deformation of material in the process of abrasive plowing,and the shear strain effect of material during the process of cutting chips formation.The model was finally validated by the orthogonal grinding experiment of powder metallurgy nickel-based superalloy FGH96 by using the electroplated CBN abrasive wheel.Grinding force values of prediction and experiment were in good consistency.The errors of tangential grinding force and normal grinding force were 9.8%and 13.6%,respectively.The contributions of sliding force,plowing force and chip formation force were also analyzed.In addition,the tangential forces of sliding,plowing and chip formation are 14%,19%and 11%of the normal forces on average,respectively.The pro-posed grinding forcemodel is not only in favor of optimizing the grinding parameters and improving grinding efficiency,but also contributes to study some other grinding subjects(e.g.abrasive wheel wear,grinding heat,residual stress).

An investigation on machined surface quality and tool wear during creep feed grinding of powder metallurgy nickel-based superalloy FGH96 with alumina abrasive wheels

In this study,the machined surface quality of powder metallurgy nickel-based superalloy FGH96(similar to Rene88DT)and the grinding characteristics of brown alumina(BA)and microcrystalline alumina(MA)abrasive wheels were comparatively analyzed during creep feed grinding.The infuences of the grinding parameters(abrasive wheel speed,workpiece infeed speed,and depth of cut)on the grinding force,grinding temperature,surface roughness,surface morphology,tool wear,and grinding ratio were analyzed comprehensively.The experimental results showed that there was no significant difference in terms of the machined surface quality and grinding characteristics of FGH96 during grinding with the two types of abrasive wheels.This was mainly because the grinding advantages of the MA wheel were weakened for the difficult-to-cut FGH96 material.Moreover,both the BA and MA abrasive wheeIs exhibited severe tool wear in the form of wheel clogging and workpiece material adhesion.Finally,an analytical model for prediction of the grinding ratio was established by combining the tool wear volume,grinding force,and grinding length.The acceptable errors between the predicted and experimental grinding ratios(ranging from 0.6 to 1.8)were 7.56%and 6.31%for the BA and MA abrasive wheels,respectively.This model can be used to evaluate quantitatively the grinding performance of an alumina abrasive wheel,and is therefore helpful for optimizing the grinding parameters in the creep feed grinding process.

Systematic analysis of distinct flow characteristics and underlying microstructural evolution mechanisms of a novel fine-grained P/M nickel-based superalloy during isothermal compression

Isothermal forging(IF)is an effective method for forming difficult-to-deform materials like P/M superalloys.Understanding the isothermal compression microstructural evolution mechanism of a novel P/M s-peralloy provides the basis for its optimized IF planning.In this study,the isothermal compression tests of a novel fine-grained P/M nickel-based superalloy were carried out at 1000-1150℃with strain rates of 0.001-0.01 s^(−1).The results indicated that the alloy exhibits three distinct flow characteristics:continuous softening after reaching the peak stress,near-steady superplastic flow,and discontinuous hardening,corresponding to different strain rate sensitivity exponent(m)values.Varied microstructural evolution mechanisms,including grain boundary sliding(GBS),dynamic recrystallization(DRX),and grain growth,are dominated in different m-value domains.Meanwhile,different roles of primaryγ’play in microstruc-tural evolution were clarified.A moderate fraction of primaryγ’with 8.5%-14.2%can well coordinate the GBS and hinder excessive grain growth at a high m value domain(m>0.4).When 0.2<m<0.4,the role of the primaryγ’is changed to promote dislocation accumulation,accelerating the nucleation of DRXed grains.As the primaryγ’is dissolved at 1150℃,obvious grain growth was observed after compression.Work hardening effect by overgrown grains competed with DRX softening results in the discontinuous rising stress.

Evolution of grain size and grain shape during thermomechanical processing in a powder metallurgical nickel-based superalloy

With a strain rate range of 0.01–10 s^(−1) and a deformation temperature range of 1110–1200℃,the isothermal compression test was performed on one powder metallurgy superalloy which is macroscopic segregation free.Using electron backscatter diffraction,the effect of strain rate and deformation temperature on grain shape and grain size of superalloys during thermal deformation was studied.The results established that exquisite and equiaxed dynamic recrystallization(DRX)grains are procured at supernal deformation temperature and high strain rate because of the high dislocation density.At the same time,the interaction between high DRX nucleation rate and low grain growth rate at high strain rate is favorable in making finer DRX grains.The equivalent medial grain size expanded with lowering strain rate and elevating proof temperature.Moreover,the grain shape was researched by the effective method of aspect ratio.Most aspect ratio of original grains is 0.61,and the aspect ratio has important implications for DRX and grain growth process.The average aspect ratio increases slightly when deformation temperature rises from 1110 to 1140℃,while the average aspect ratio increases memorably as the deformation temperature is higher than 1140℃.

不同粒度的镍基高温合金粉末及其对P/M成形件组织性能影响的研究

研究了用PREP法制造的不同粒度范围的FGH95合金粉末的物理特征及其HIP成形件的组织性能。结果表明 ,使用 5 0～ 10 0 μm和 5 0～ 15 0 μm粒度范围的粉末是降低成本、简化工序、保证产品质量的最佳选择。

Effect of Processing and Composition on the Structure and Properties of P/M EP741NP Type Alloys

A study was carried out on the effects of processing and composition on the structure and properties of P/M EP741NP type alloys. The objectives of this study were to understand the role of Hf in a P/M superalloy containing high niobium used in aircraft engines and to determine the effects of extrusion and forging the powders as contrasted to HIPing （hot isostatic pressing） only. Two alloys of the P/M EP741NP composition were atomized： one alloy contained 0.26%Hfand the other was Hf free. After the as-atomized powders from both alloys were characterized, the powders were extruded into billets, forged and heat treated. After each process, the microstructures were characterized by SEM and the phases were extracted and identified by X-ray diffraction. The presence of Hf in the residues was probed by EDS （energy dispersive spectroscopy）. The alloys were given the published Russian heat treatment as well as a more conventional heat treatment more typical of western powder alloys. Tensile, creep and stress rupture mechanical property tests were run. Results of the structural behavior of the alloys after each processing step will be presented and discussed. The role of the Hfon the mechanical properties will be discussed.

Effect of Counterface Material on Friction and Wear Behavior of Ni-Cr-W-Al-Ti-MoS_2 Composites

Ni-Cr-W-Al-Ti-MoS2 self-lubricating composites were prepared through the powder metallurgy （P/M） method. Their friction properties were investigated by a pin-on-disk tribometer in the range from the room temperature to 600 ℃. Alumina, silicon nitride and nickel-iron-sulfide alloys were selected as the counterface materials. Results indicate that the lowest friction coefficients under 0.22 can be obtained at 600℃ when rubbed against alumina. When rubbed against nickel-iron-sulfide alloys, are presented the lowest wear rates in the magnitude of 10^-6 mm^3/N-m, one order of magnitude lower than those when rubbed against ceramics. In the case of three rubbing pairs, the wear rates of the composite containing MoS2 present themselves inversely proportional to friction coefficients. With alumina ceramics used as a counterface, transfer films and glaze layers will form on the contact surface playing a main role in lubrication at high temperatures. However, when silicon nitride and nickel-iron-sulfide alloy are used, the lubricating transfer films appear not to be prominent.

SEM in-situ investigation on fatigue cracking behavior of P/M Rene95 alloy with surface inclusions

The low-cycle fatigue behavior of powder metallurgy Rene95 alloy containing surface inclusions was investigated by in-situ observation with scanning electron microscopy （SEM）. The process of fatigue crack initiation and early stage of propagation behavior indicates that fatigue crack mainly occurs at the interface between the inclusion and the matrix. The effect of inclusion on the fatigue crack initiation and the early stage of crack growth was very obvious. The fatigue crack growth path in the matrix is similar to the shape of inclusion made on the basis of fatigue fracture image analysis. The empiric relation between the surface and inside crack growth length, near a surface inclusion, can be expressed. Therefore, the fatigue crack growth rate or life of P/M Rene95 alloy including the inclusions can be evaluated on the basis of the measurable surface crack length parameter. In addition, the effect of two inclusions on the fatigue crack initiation behavior was investigated by the in-situ observation with SEM.

采用氢化-脱氢(HDH)钛粉和氢化钛粉制备MIMTi-6Al-4V合金

以HDH钛粉制备MIMTi-6Al-4V合金的过程为参照,对使用氢化钛粉为原料制备P/M模压钛合金以及MIM钛合金的工艺过程进行了探索。使用HDH钛粉制备的MIMTi-6Al-4V合金抗拉强度为819MPa,拉伸样延伸率为7%;使用氢化钛粉制备的P/M模压Ti-6Al-4V合金拉伸样抗拉性能高于850MPa;使用氢化钛粉为原料制备MIMTi-6Al-4V合金,顺利实现了脱氢-脱脂过程。

粉末冶金法制备SiC_p/6061 Al复合材料的热释放现象

利用机械合金化 粉末冶金制备了SiCp 6 0 6 1Al复合材料。对实验中发现的一种热释放现象进行了计算机在线测量和初步分析。实验表明 ,经机械合金化后制备的SiCp 6 0 6 1Al复合材料中增强体分布均匀 ,在固溶水淬后有明显的热释放现象 ,而且热释放的程度与粉末的尺寸、试样的状态有关。试样在室温自然状态下 ,温度可升高至 80℃左右。

Influence of Rare Earth on Carbide in Weld Metal

The physical characteristics of Ni based superalloy powder with different particle sizes produced by plasma rotation electrode process (PREP) and the microstructure and mechanical properties of P/M superalloy products were investigated. The experimental results show that the optimum powder particle sizes should be in the range of 50-100 μm or 50-150 μm, which can reduce production cost, simplify process and guarantee P/M product quality.

FGH96合金锻造盘坯热处理过程中的晶粒长大行为

研究了等温锻造后的FGH96合金盘坯件在热处理过程中的晶粒长大规律。结果表明,当热处理温度低于1050℃时,盘坯轮心部位的晶粒几乎不长大;当热处理温度高于1120℃时,轮缘部位晶粒长大迅速;在单相奥氏体区,晶粒的长大规律满足Beck方程,其生长指数随温度升高而增加。当温度一定,初始晶粒越细,其长大速率越快并且生长指数值越高。建立了轮缘部位的晶粒长大动力学方程。

粉末高温合金涡轮盘不同部位冷却γ′相的析出和强化

通过模拟热处理实验研究了新型镍基粉末高温合金涡轮盘在过固溶线温度热处理过程中盘件不同部位冷却γ′相的析出特征,并采用中断冷却方式考察了γ′相在冷却过程中的发展以及合金强度的变化。结果表明:与盘缘部位析出的γ′相比较,盘心部位的平均尺寸和尺寸分布范围更大,形态更为复杂,晶界宽化更明显,而强度较低。在冷却过程中,盘心和盘缘部位的二次γ′相平均尺寸随中断温度的降低均线性增加,但盘心部位的长大速度更大。其中合金强度随冷却温度的减低呈非单调性变化(先降低后升高),这与γ′相在冷却过程的多阶段形核有关。