The hot corrosion of Co-25Cr-10Ni-5Ta-3Al-0.5Y alloy (S-57)

A cobalt-base alloy, Co-25 Cr-10 Ni-5 Ta-3 Al-0.5 Y (S-57), was subjected to hot corrosion in Mach 0.3 burner rig combustion gases at maximum alloy temperatures of 900 and 1000°C. Various salt concentrations were injected into the burner; 0.5, 2, 5 and 10 parts per million synthetic sea salt and 4 parts per million sodium sulphate (Na₂SO₄). The extent of corrosion was determined by measuring the maximum depth of corrosion in the alloy and the corrosion process was studied by metallography, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis. While S-57 was found to possess only moderate oxidation resistance at these temperatures, this alloy resisted significant hot corrosion attack under all but the most severe test conditions. The process of the hot corrosion attack under the most severe conditions of this study was primarily sulphidation.     

Co-Ni-Cr-W与Ni-Cr-B-Si合金涂层对几种耐热钢抗热腐蚀疲劳性能的影响

本文以喷射盐水溶液的“HCF”全自动热腐蚀疲劳直接对比试验法,研究了4 Cr10Si2Mo.18SR、20—14三种耐热钢与Co—50、Ni—81两种合金涂层的抗热腐蚀疲劳性能。腐蚀程度的测量由合金的最大腐蚀深度来决定,并以金相、电子探针和X-光衍射技术研究了各钢种的热腐蚀疲劳破坏过程和两种涂层有效地抗热腐蚀疲劳的原因。在“HCF”热腐蚀疲劳试验条件下我们发现:①耐热钢具有三带侵蚀构造。疏松的腐蚀产物层层开裂脱落,毫无保护作用,硫与氧直接向合金内部侵入,氧甚至比硫侵入得更深。②耐热钢热腐蚀疲劳破坏的程度主要与合金内部侵蚀带的侵蚀类型有关。③无论Ni—基或Co—基合金涂层均能生成致密的抗热腐蚀疲劳表面保护膜。本试验结果已成功地指导了延长内燃机排气阀使用寿命的涂层研究工作。     

AZ31铸造镁合金的塑性流动特征及物理概念的本构关系

利用准静态试验机以及Hopkinson压杆装置对AZ31铸造镁合金在不同应变速率和不同温度下的塑性流动性能进行研究,结合金相显微技术对试验后的试样进行微观分析。结果表明:在低应变速率下,随着温度的升高,AZ31镁合金发生明显的由脆性到韧性的转化,其转化温度为473 K左右;当应变速率增加到1.2×104 s-1时,会发生脆化现象,塑性变形能力变差。基于微观分析,低应变速率下晶体中孪晶的存在是促进材料塑性变形增加的主要因素。而在高应变速率下,动态再结晶和第二相粒子沉淀硬化显著地影响金属的塑性变形。结合系统的试验结果,基于热激活位错机制,建立一种物理概念的塑性流动本构模型,对较高应变速率和不同温度下的流动应力进行模型预测。通过对比,模型预测结果与试验数据吻合较好。     

Sn含量对Zr-Sn-Nb-Fe-Cr合金相变温度和沉淀析出相的影响

以Zr-1.0Sn-0.3Nb-0.3Fe-0.1Cr与Zr-0.8Sn-0.3Nb-0.3Fe-0.1Cr2种不同Sn含量锆合金为研究对象,采用差示扫描量热法(DSC)测量2种锆合金的相变温度,探讨Sn含量变化对锆合金相变点的影响。同时,采用SEM对热轧态、热轧退火态、冷轧态、冷轧退火态、终轧态、终轧退火态6种加工工序样品分别进行详细的微观组织观察,随后通过定量金相法对2种锆合金在不同加工工序下的第二相大小及分布等特点进行对比分析,得出2种锆合金定量的统计结果。降低Sn含量对α→β相变点有明显影响,而沉淀析出相的颗粒形状基本不变,但是第二相颗粒平均尺寸减小。扫描电镜的能谱(EDS)分析半定量地说明Sn在锆合金中存在不同程度的成分偏析。     

镍基合金叶片的热腐蚀与防护

对已产生热腐蚀的镍基合金叶片失效分析。用金相,电子探针、扫描电镜、Ｘ－荧光等方法,确定腐蚀产物的形貌和组成,以及铝化物防护层的成分与相结构。证实热腐蚀试样的腐蚀产物形态和分布与实际失效叶片的相似性。在镍基合金表面施加铝、铝－硅、铝－铬防护层能有效地提高叶的抗腐能力。     

Hot isostatic pressing of plasma sprayed Ni-base alloys

This article reports the effects of hot isostatic pressing (HIPing) on the microstructure and properties of plasma sprayed Ni-based alloy coatings. Hot isostatic pressing was used as a post- spray treatment on plasma sprayed Ni-5Al, Ni-20Al, and NiCrAl coatings. The aim was to densify the coatings and modify physical properties such as strength, amount of porosity, and hardness. The coatings were HIPed at 750 to 950 ‡C at pressures of 50 to 200 MPa and held for 1 h. The treated coatings were examined by optical microscopy and scanning electron microscopy (SEM). Coating porosity was determined using a combination of an image analyzer and SEM. Near- zero porosity levels could be obtained, and HIP treatment at increasing temperatures and pressures changed the microstructure and increased the microhardness of the coatings. Mechanical testing of the coatings was performed on a Dynamic Mechanical Analyzer (DMA) from ambient to — 1000 ‡C. The results showed that the elastic modulus of HIPed coatings was greater than as-sprayed coatings up to — 750 ‡C. These changes can be related to plastic flow, interlamellar diffusion, and creep that occur at increased temperatures and pressures.     

Hot ductility behaviour of Al–Li alloy AA2091–T3

Abstract

The hot ductility behaviour of the Al–Li alloy 2091–T3 was investigated using hot ductility testing of the simulated heat affected zone (HAZ) using a Gleeble 1500 machine. Alloy 2091 displayed lower ductility, greater susceptibility to HAZ cracking, and poor ductility recovery on cooling from the nil strength temperature, compared with alloy 2219. Optical metallography and electron microscopy revealed that the ductility loss was accompanied by fracture transition from ductile transgranular mode to brittle intergranular mode. It was found that the HAZ intergranular liquation cracking was caused by non-equilibrium eutectic melting of T₂ phase precipitated at grain boundaries.     

Effect of Zr addition on phase transformation and precipitation in B-added hot stamping steel

The effect of Zr addition on phase transformation and precipitation in B-added hot stamping steels was investigated. First, a thermodynamic calculation was conducted to compare the thermodynamic stability of nitride precipitates for various nitride-forming elements. The equilibrium temperatures for phase transformation and nitride precipitation in B-added hot stamping steels containing Zr were also calculated. The phase transformation kinetics of B-added hot stamping steels with various Zr contents were evaluated by both dilatometry and metallography. It was verified that Zr addition can provide protection of the B-hardenability effect, which produce a fully martensitic microstructure after hot stamping. To confirm the existence of Zr precipitation in the hot stamping steels, transmission electron microscopy and energy dispersive spectroscopy were used. A large number of precipitates were observed in the form of (Ti, Zr)N. The effect of Zr addition in B-added hot stamping steel to provide effective B-protection resulting in an increase of hardenability was identified and discussed.     

Detection of thermally grown oxides in thermal barrier coatings by nondestructive evaluation

The thermal-barrier coatings (TBC) sprayed on hot-section components of aircraft turbine engines commonly consist of a partially stabilized zirconia top-coat and an intermediate bond-coat applied on the metallic substrate. The bond-coat is made of an aluminide alloy that at high engine temperatures forms thermally grown oxides (TGO). Although formation of a thin layer of aluminum oxide at the interface between the ceramic top-coat and the bond-coat has the beneficial effect of protecting the metallic substrate from hot gases, oxide formation at splat boundaries or pores within the bond-coat is a source of weakness. In this study, plasma-sprayed TBC specimens are manufactured from two types of bond-coat powders and exposed to elevated temperatures to form oxides at the ceramic-bond-coat boundary and within the bond-coat. The specimens are then tested using nondestructive evaluation (NDE) and destructive metallography and compared with the as-manufactured samples. The objective is to determine if NDE can identify the oxidation within the bond-coat and give indication of its severity. While ultrasonic testing can provide some indication of the degree of bond-coat oxidation, the eddy current (EC) technique clearly identifies severe oxide formation within the bond-coat. Imaging of the EC signals as the function of probe location provides information on the spatial variations in the degree of oxidation, and thereby identifies which components or areas are prone to premature damage.     

铜合金型材热挤压模失效分析及改进措施

通过扫描电镜电子能谱、X衍射及金相试验等一系列方法,对一副铜合金型材热挤压模的失效部分进行了成分、断口、形貌、能谱及金相分析,判断出了模具失效的原因,并在材料成分和热加工工艺上提出了改进措施。     

含Ti的合金化热镀锌钢板的镀层结构及相变规律的研究

对汽车用基板含微Ｔｉ的热浸镀锌板进行合金化处理，用光学显微镜和扫描电镜观察了合金化镀层的组织形貌，并用Ｘ射线衍射方法和电子探针分析了合金层的相组成。研究表明，合金层的组织结构与合金化温度有关，并提出了合金化过程中的三种相变模式。     

Microstructure and stresses in HVOF sprayed iron aluminide coatings

The microstructure and state of stress present in Fe₃Al coatings produced by high velocity oxygen fuel (HVOF) thermal spraying in air at varying particle velocities were characterized using metallography, curvature measurements, x-ray analysis, and microhardness measurements. Sound coatings were produced for all conditions. The microstructures of coatings prepared at higher velocities showed fewer unmelted particles and a greater extent of deformation. Residual stresses in the coatings were compressive and varied from nearly zero at the lowest velocity to approximately −450 MPa at the highest velocity. X-ray line broadening analyses revealed a corresponding increase in the extent of cold work present in the coating, which was also reflected in increased microhardness. Values of mean coefficient of thermal expansion obtained for assprayed coatings using x-ray analysis were significantly lower than those for powder and bulk alloy.     

等离子弧堆焊TiB2-金属陶瓷涂层的组织及性能

采用等离子弧堆焊设备，通过钛合金和硼合金粉末之间的高温冶金反应，在堆焊过程中原位合成了TiB2，在普通碳钢表面制备了含TiB2的金属陶瓷层。并对堆焊层进行了显微组织、X射线衍射分析、硬度及抗裂性能试验。试验结果表明，这种一步性原位合成工艺所获得的堆焊层主要由针状TiB2晶须与Fe及其硼、碳化合物组成，陶瓷层与基体间实现了冶金结合；堆焊层抗裂性能优于不含TiB2的铁基B4C堆焊层。     

Corrosion Behaviors of Nitride Coatings on Titanium Alloy in NaCl-Induced Hot Corrosion

TiN and TiAlN coatings were deposited by arc ion plating on titanium alloys to study their hot corrosion resistance when they were exposed to NaCl at 600 ℃.The microstructure and corrosion behaviors of nitride coatings were studied using scanning electron microscope,X-ray diffraction,electro-probe microanalyzer and X-ray photoelectron spectroscopy.The results showed that nitride coatings with the different compositions and the ones with the same composition but different thicknesses presented different hot corrosion resistance.TiN and thin TiAlN coatings showed poor corrosion resistance.Serious internal oxidation attacked the alloy substrate.Their corrosion products were mainly consisted of non-protective TiO2 and sodium salt.By contrast,the thick TiAlN coating presented outstanding corrosion resistance.Besides sodium salt,the corrosion products were composed of protective Al2O3.The increasing thickness of TiAlN significantly enhanced the hot corrosion resistance.The corrosion mechanisms of alloy,TiN and TiAlN coatings were discussed in detail.     

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A EXTRUDED RAPIDLY SOLIDIFIED AIFeVSi ALLOY

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Structure of Quenched Ti–Ru Alloys

Quenched titanium–ruthenium alloys containing 0.25–4 at % ruthenium have been studied using X-ray diffraction analysis, optical metallography, transmission electron microscopy, and microhardness measurements. It has been found that, during the quenching of the alloys containing 0.25, 0.5, and 1 at % ruthenium, a polymorphic β → α transformation occurs with the formation of a two-phase (α + β) structure. In Ti–1.5 at % Ru and Ti–2 at % Ru alloys, a martensitic β → α″ transformation occurs. The quenched Ti–3 at % Ru alloy has a β + ω structure. The complete stabilization of the β phase takes place in the alloy with 4 at % ruthenium. In the electron-diffraction patterns of alloy containing 4 at % ruthenium, diffuse scattering that indicates the formation of ω-phase-related displacements in the locations of atoms has been observed.     

Hot corrosion resistance of high-velocity oxyfuel sprayed coatings on a nickel-base superalloy in molten salt environment

No alloy is immune to hot corrosion attack indefinitely. Coatings can extend the lives of substrate materials used at higher temperatures in corrosive environments by forming protective oxides layers that are reasonably effective for long-term applications. This article is concerned with studying the performance of high-velocity oxyfuel (HVOF) sprayed NiCrBSi, Cr₃C₂−NiCr, Ni−20Cr, and Stellite-6 coatings on a nickel-base superalloy at 900 °C in the molten salt (Na₂SO₄-60% V₂O₅) environment under cyclic oxidation conditions. The thermogravimetric technique was used to establish kinetics of corrosion. Optical microscope, x-ray diffraction, scanning electron microscopy/electron dispersive analysis by x-ray (SEM/EDAX), and electron probe microanalysis (EPMA) techniques were used to characterize the as-sprayed coatings and corrosion products. The bare superalloy suffered somewhat accelerated corrosion in the given environmental conditions. whereas hot corrosion resistance of all the coated superalloys was found to be better. Among the coating studied, Ni−20Cr coated superalloy imparted maximum hot corrosion resistance, whereas Stellite-6 coated indicated minimum resistance. The hot corrosion resistance of all the coatings may be attributed to the formation of oxides and spinels of nickel, chromium, or cobalt.     

Oxide scales formation in nano-crystalline TiAlCrSiYN PVD coatings at elevated temperature

Nano-crystalline TiAlCrSiYN plasma vapor deposited (PVD) coatings were developed for oxidation and wear protection at elevated temperatures. Compositional tuning of the coatings was performed to enhance oxidation protection at elevated temperatures.The oxidation kinetics of the coatings has been studied over 180 h at 900 °C in air. Post-oxidation microstructural examinations of specimens were performed using transmission electron microscopy (TEM), secondary electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and glow discharge optical emission spectroscopy (GDOES). Micro mechanical characteristics of the coating were studied using a micro materials nanotest system. Wear resistance of the coatings were studied during turning of Inconel 718.Experimental results clearly indicate that the aluminum-rich PVD TiAlCrSiYN coatings with 60 at.% of Al can improve oxidation resistance of titanium aluminide alloy at the temperature 900 °C as well as wear resistance during machining of Inconel 718. It was shown that during oxidation, continuous protective alumina-based oxide films form on the surface. These oxides are predominantly (Al,Cr)₂O₃-based films. Self-healing behavior of the TiAlCrSiYN coatings was observed in its ultra-fine nano-crystalline structure.     

Oxidation and hot corrosion of coated and bare oxide dispersion strengthened superalloy MA-755E

Cyclic hot corrosion and oxidation testing of an experimental oxide dispersion strengthened (ODS) superalloy MA-755E were conducted in a hot gas stream at Mach 0.3. The response of the ODS alloy, bare or with protective coatings, was similar to that of a conventional cast alloy, IN-792, in hot corrosion at 900°C. However, during oxidation at 1100 and 1150°C the ODS alloy differed from the cast alloy by developing a greater amount of subsurface porosity. Compared with a diffused aluminide coating, an electron beam vapor deposited NiCrAlY coating offered superior oxidation protection and decreased porosity formation. In additional testing, the tendency to form porosity was associated with the large grains of recrystallized powder metallurgy alloys but was independent of the presence of an oxide dispersion.     

Hot corrosion behavior of MCrAlY coatings on IN738LC

The hot corrosion behavior of CoNiCrAlY coatings deposited on IN738LC super alloy using low pressure plasma spray (LPPS) was investigated using samples immersed in a solution of Na₂SO₄-10 wt.% NaCl and dried as to be covered with a 2.5 mg/cm² costing. Specimens were heat-treated in furnace at 850 °C and after 24 h in the furnace were accurately weighed. Microstructural characterizations were carried out by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) indicated interactions within microstructure of the coating with clearly diminished thickness of MCrAlY coating. Phase transformation and β-NiAl phase depletion in the MCrAlY coatings were shown to be directly related to the thermal cycles experienced by the samples and revealed outward diffusion of Al in the coating and the inward migration of Ni toward the coating causing β → γ′ phase transformation as the main cause of instability of the β-NiAl.