Tensile Deformation of a Nickel-base Alloy at Elevated Temperatures

The results of tensile testing involving Waspaloy indicate that the failure strain was gradually reduced at temperatures ranging between ambient and 300 °C. Further, serrations were observed in the engineering stress versus strain diagrams in the temperature range of 300-600 °C. The reduced failure strain and the formation of serrations in these temperature regimes could be the result of dynamic strain aging of this alloy. The extent of work hardening due to plastic deformation was reduced at temperatures above 300 °C. A combination of ductile and intergranular brittle failures was seen at temperatures above 600 °C. γ′ was detected at all tested temperatures.     

Pack Aluminide Coatings Formed at 650 ℃ for Enhancing Oxidation Resistance of Low Alloy Steels

This study aims to investigate the feasibility of forming iron aluminide coatings on a commercial 9Cr-lMo (wt.%)alloy steel by pack cementation at 650 ℃ in an attempt to improve its high temperature oxidation resistance. Pack powders containing Al, Al2O3 and a series of halide salts were used to carry out the coating deposition experiments, which enabled identification of the most suitable activator for the pack aluminising process at the intended temperature. The effect of pack aluminium content on the growth kinetics and microstructure of the coatings was then studied by keeping deposition conditions and pack activator content constant while increasing the pack aluminium content from 1.4 wt.% to 6 wt.%. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the phases and microstructures of the coatings formed and to determine depth profiles of coating elements in the coating layer. Oxidation resistance of the coating was studied at 650 ℃ in air by intermittent weight measurement at room temperature. It was observed that the coating could substantially enhance the oxidation resistance of the steel under these testing conditions, which was attributed to the capability of the iron aluminide phases to form alumina scale on the coating surface through preferential Al oxidation.     

纳米晶Cu-Si合金的抗氧化性能

采用电弧熔炼和机械合金化方法制备了两种晶粒尺寸差别较大的Cu-Si合金,并采用热重分析方法对其在700℃和800℃、1×105Pa流动O2中氧化24 h的抗高温氧化性能进行了研究。结果表明,不是所有合金氧化动力学都遵循抛物线规律,甚至有些合金的动力学曲线十分不规则。总的来说,机械合金化法制备的合金单位面积氧化增重小于相应的熔炼合金。合金氧化后都未形成连续的SiO2保护膜,而形成了相似的复杂的氧化膜,其中都包含由Cu2O和SiO2组成的富含SiO2的混合氧化物层,它限制了铜和氧的进一步扩散。晶粒尺寸的减小促进了活泼组元Si的扩散,加速了SiO2和富含SiO2的Cu2O+SiO2混合氧化物层的形成,提高了Cu-Si合金的抗高温氧化性能。     

Deformation Capacity of a Ternary Magnesium Alloy in a Gas-Forming Process at Elevated Temperatures

JOM - An experimental ternary magnesium alloy, Mg-2Zn-2Ce, has been investigated. The attention was focused on the possibility of using this alloy for an unconventional sheet-forming process based...     

Corrosion of Stainless Steels in Fatty Acid Media at Elevated Temperatures and Pressures

Abstract

Natural fatty acids were separated and purified by steam distillation at 250–300°C. Corrosion rates were determined for three stainless steels 316L, 317L and Ti-stabilised 321, using weight-loss measurements after 14 days in three fatty acid fractions at 260°C. These results for stainless steels were compared with corrosion rates obtained for nickel, molybdenum and manganese by similar weight-loss measurements. Scanning electron microscopy was used to confirm pitting corrosion at the metal surface. The effects of water, low molecular weight fatty acids and impurities such as sodium salts are discussed in relation to the overall corrosion processes in steel steam distillation columns.     

An Advanced Alloy for Elevated Temperatures

While Ti-6242-Si has been used for a variety of gas turbine applications with performance requirements to approximately 540°C (1000°F), a new elevated-temperature titanium alloy has been developed which offers roughly a 55°C (100°F) creep advantage over Ti-6242-Si. The new alloy, designated Ti-1100, is a conventional near-alpha type alloy and is at least as strong as Ti-6242-Si, showing excellent stability after long-term exposures.     

温度对U-5.7Nb合金动态压缩力学性能影响研究

利用Hopkinson杆装置开展了–100~400℃较宽温度范围下U-5.7Nb合金的动态压缩变形行为研究,结果表明:U-5.7Nb合金的压缩性能对温度和应变率较为敏感。随实验温度升高,屈服应力迅速下降,下降速率约为2 MPa/℃,而加工硬化行为基本不变,当温度升高至200℃以上时,静动态应力~应变曲线形态由"双屈服"转变为"单屈服";随应变率提高,屈服应力及去孪生引起的硬化模量均明显增大。最后,结合金相分析结果讨论了400℃时静动态力学曲线分歧的来源。     

Thermal Stability and Oxidation Resistance of Pt–10 Al–4Cr Alloy at Super-High Temperatures

The isothermal-oxidation behavior of refractory superalloy Pt–10Al–4Cr (in at.%) was investigated up to a period of 312 hr in air from 1200 to 1400°C. A comparison of the oxidation behavior of this alloy with a conventional Ni-base superalloy (Inconel 713C) shows an order of magnitude higher oxidation resistance. This experimental alloy oxidizes by forming Al₂O₃ and Cr₂O₃ (and perhaps trace amounts of PtO) with Al₂O₃ as the oxide layer in contact with air. Optical and scanning-electron microscopy (SEM) were used to study the microstructure, morphology, and composition of the scale formed after oxidation. The thermal stability of the alloy after extended periods at 1200, 1300, and 1400°C was studied using transmission-electron microscopy (TEM).     

铝及其合金高温流变应力模型的研究现状

分析铝及其合金的高温热变形特点,重点分析对比铝及其合金高温流变应力模型的研究现状,指出:包含Z参数的函数模型由于忽略了应变量ε的影响,仅可用于一定变形程度下一般规律的对比描述;其它模型虽然反映了整个变形过程中应力与各变形参数之间的数学关系,但仍应根据实际材料热变形特性进行分析和选择.     

不同表面粗糙度镍铬铝合金的高温氧化行为

采用热重法并结合X射线衍射、扫描电子显微镜、能谱仪等分析手段,研究在1000 ℃条件下Ni-10Cr-5Al (质量分数,%) 合金样品表面粗糙度对其氧化行为的影响。结果表明：经1000 ℃氧化215 h后,不同表面粗糙度的合金其氧化动力学均服从抛物线规律,并存在两个抛物线常数,氧化膜主要由Al₂O₃,NiCr₂O₄和NiAl₂O₄组成。随着表面粗糙度的增加,氧化增重越明显,同时氧化膜脱落越严重、氧化膜金属界面越起伏。其中,表面粗糙度对合金氧化的影响主要是在氧化前期阶段促进生成了尖晶石NiCr₂O₄和NiAl₂O₄氧化物。     

Ti-22Al-26Nb合金的高温氧化行为

采用热重法研究Ti-22Al-26Nb合金在800和900 ℃的高温氧化行为.利用X射线衍射分析(XRD)、扫描电镜(SEM)对氧化膜的相组成和形貌进行分析.结果表明:在氧化初期该合金遵循抛物线规律,而20 h以后,合金的氧化动力学符合直线规律.氧化后的产物主要由TiO_2、AlNbO_4和Nb_2TiO_7组成,氧化膜出现了分层结构且氧化温度愈高,分层愈明显.     

原位自生成TiC／Ti基复合材料的高温氧化

利用热重分析(TGA)测量了原位生成的TiC／Ti-6Al复合材料在600，700和800℃下经连续20h的氧化增重特性，利用SEM，EDS和XRD研究了复合材料经氧化试验后表面氧化层的结构，相组成及成分。结果表明，原位生成的Ti基复合材料在高温氧化时遵循抛物线规律在800℃时的氧化增重量远大于600℃和700℃时的，计算获得该复合材料的氧化激活能为255．7kJ／mol。研究发现经20h的600℃或700℃氧化形成的氧化物呈现不连续的岛屿状，主要成分为金红石型的Ti02，而在800℃氧化20h后，氧化物是均匀连续的致密膜，由TiO2，A12O3组成。该复合材料的氧化首先发生在TiC颗粒的表面上，而不是象均质Ti材料一样氧化在整个表面上均匀地发生。     

Mechanical Properties and Phase Stability of WTaMoNbTi Refractory High-Entropy Alloy at Elevated Temperatures

High-temperature structural metals remain in high demand for aerospace aircraft,gas turbine engines,and nuclear power plants.Refractory high-entropy alloys (RHEAs) with superior mechanical properties at elevated temperatures are promising candidates for high-temperature structural materials.In this work,a WTaMoNbTi RHEA with adequate room temperature plasticity and considerable strength at 1600 ℃ was fabricated by vacuum arc-melting.The room temperature fracture strain of the as-cast WTaMoNbTi RHEA was 7.8％,which was about 5.2 times that of the NbMoTaW alloy.The alloy exhibited a strong resistance to high-temperature softening,with a high yield strength of 173 MPa and compressive strength of 218 MPa at 1600 ℃.The WTaMoNbTi RHEA possessed excellent phase stability in the range of room temperature to 2000 ℃.The dendritic grains grew into equiaxed grains after compression test at 1600 ℃ due to the dynamic recrystallization process at high temperature.This work presents a promising high-temperature structural material that can be applied at 1600 ℃.     

Mechanical Properties and Deformation Mechanisms of Mg-Gd-Y-Zr Alloy at Cryogenic and Elevated Temperatures

In this study, mechanical properties and deformation mechanisms of Mg-Gd-Y-Zr alloy at temperatures ranging from 77 K to 523 K have been investigated. The effects of temperature on the mechanical properties, deformation mechanism, and fracture mechanism are discussed. The results show that the strengths of alloy decrease gradually while the elongations increase progressively with increasing temperature. The maximum ultimate tensile strength of the alloy as high as 442 MPa is obtained at 77 K. As the temperature increases from 77 K to 523 K, the ultimate tensile strength of the alloy decreases from 442 MPa to 254 MPa and the elongations increase from 6.3% to 28.9% gradually. The study verifies that the deformation at 77 K is predominated by basal slip and \({{\left\{ {10\bar{1}2} \right\}} \mathord{\left/ {\vphantom {{\left\{ {10\bar{1}2} \right\}} {\left\langle {10\bar{1}1} \right\rangle }}} \right. \kern-0pt} {\left\langle {10\bar{1}1} \right\rangle }}\) deformation twinning system. At 223 K, lots of twins emerge primarily at grain boundaries. At 373 K, all dislocations are proved to be 〈a〉 dislocations. At 523 K, although basal slip is still the dominant deformation mechanism, non-basal slip systems also become activate.     

The Isothermal and Cyclic Oxidation Behavior of a Titanium Aluminide Alloy at Elevated Temperature

The isothermal and cyclic oxidation behavior of Ti-47Al-2Mn-2Nb with 0.8 vol.% TiB₂ particle-reinforced alloy was investigated in air between 700 and 1000 °C. In the study, the kinetics of isothermal and cyclic oxidation were performed by using a continuous thermogravimetric method which permits mass change measurement under oxidation conditions. The oxide scales and substrates were characterized by scanning electron microscopy with energy-dispersive x-ray analysis and x-ray diffraction. At 700 and 800 °C, the alloy showed an excellent oxidation resistance under isothermal and cyclic conditions. After exposure to air above 800 °C, the outer scale of the alloy was dominated by a fast-growing TiO₂ layer. Under the coarse-grained TiO₂ layer was the Al₂O₃-rich scale, which was fine-grained. At 900 and 1000 °C, the extent of oxidation increased clearly. The oxidation rate follows a parabolic law at 700 and 800 °C. However, the alloy, upon isothermal oxidation at 900 °C, can be divided into several stages. During the cyclic oxidation at 900 and 1000 °C, partial scale spallation takes place, leading to a stepwise mass change.     

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe?CCr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe?CCr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite?Cspinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N₂ gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.     

Study of Wear Behavior of Zircon Sand-Reinforced LM13 Alloy Composites at Elevated Temperatures

The present article describes in detail the wear behavior of zircon sand-reinforced LM13 alloy composite at elevated temperatures with variation in load. Zircon sand particles in different amounts were reinforced into LM13 alloy by stir casting route. Dispersion of reinforced particle was examined under optical and scanning electron microscope. The hardness values of the composites were observed to increase with the increasing amount of reinforcement. The coefficients of thermal expansion of LM13 alloy and zircon sand-reinforced composites were recorded in different temperature ranges. Wear behaviors of base alloy and composites have been studied with variation in applied load. Effect of temperature (from 50 to 300 °C) on wear behaviors of both alloy and composites were determined at low (1 kg) and high (5 kg) loads. The improvement in the wear resistance was noticed with the higher amount of reinforcement. A transition from mild-to-severe wear with variations in temperature and load was observed. The results are discussed in light of operative wear mechanisms. Wear track and wear debris of composite materials were also analyzed under SEM to understand the operative wear mechanism under different conditions.     

FeAl基合金的高温抗氧化性能分析

设计了二组化学成分不同的FeAl基合金,进行了1000℃的高温氧化试验,并对其进行了分析.结果表明,含碳量较高的材质较普通FeAl基合金的硬度高;B元素改善品界脆性,Zr使晶界同溶强化,提高了材料的冲击韧度和抗托强度;1000℃下,含碳量较低的FeAl基合金呈完全抗氧化性,其循环氧化动力学曲线均服从抛物线规律.