Effect of Yttrium Alloying on Intermediate to High-Temperature Oxidation Behavior of Mo-Si-B Alloys

The oxidation behavior of 0.2 Y-alloyed Mo-9Si-8B (at. pct) was investigated in a wide temperature range from 923 K to 1673 K (650 °C to 1400 °C). Formation of a thin yttrium-silicate scale at the outer layer along with the thick silica-rich inner layer containing Y-rich oxide inclusions was detected beyond 1573 K (1300 °C). A substantial improvement in the oxidation resistance of the alloy could be realized at 1073 K to 1273 K (800 °C to 1000 °C) with the addition of yttrium. The formation of a viscous silica-rich protective scale could prevent the permeation of MoO₃ at the initial stages of oxidation at this temperature regime. The growth of the internal oxidation zone followed a parabolic rate at 1273 K to 1673 K (1000 °C to 1400 °C), and the activation energy values calculated for both the outer oxide scale and internal oxidation zone formation indicated the inward diffusion of oxygen as the dominant rate controlling mechanism. The microstructural and kinetic data obtained for internal and external oxidation indicate that yttrium-silicate scale reduces the inward diffusion of oxygen, thereby improving the oxidation resistance of the alloy at high temperatures in any oxidizing environment.     

Nonisothermal and Cyclic Oxidation Behavior of Mo-Si-B and Mo-Si-B-Al Alloys

A study on nonisothermal and cyclic oxidation behavior of the reaction-hot-pressed 76Mo-14Si-10B, 77Mo-12Si-8B-3Al, and 73.4Mo-11.2Si-8.1B-7.3Al alloys has been carried out in dry air, and the results have been compared with those of isothermal tests. Nonisothermal studies by thermogravimetric (TG) analysis up to 1300 °C have shown a transient mass gain between 700 °C and 860 °C, followed by a sharp mass loss with increased temperature, with the amount of mass change dependent on the heating rate (5 °C/min to 35 °C/min). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of oxide scales formed on the alloys held at 790 °C and 820 °C for 10 or 20 minutes suggest that the oxidation of α-Mo and Mo₃Si precedes that of Mo₅SiB₂. Thermal cyclic tests involving exposure at 1150 °C for 1 hour, followed by either air cooling to room temperature (RT) or furnace cooling to 700 °C, 800 °C, or 900 °C, and the subsequent examination of oxidation products, have confirmed that the formation of B₂O₃-SiO₂ scale provides complete and partial protection for the Mo-Si-B and Mo-Si-B-Al alloys, respectively. The results of this study show that oxidation resistance is deteriorated upon Al addition. Residual stress measured by XRD is found to be largely compressive in Mo and in mullite phases of oxide scales. Thermal shock and the mismatch in the coefficients of thermal expansion (CTEs) between the constituent phases of the oxide scale appear to be the main causes of damage.     

Ti40阻燃钛合金的高温氧化行为

研究了高稳定β型阻燃钛合金Ti40合金在900℃-1050℃温区内氧化时的氧化行为。结果表明：随着温度的升高及时间的延长，试样表面氧化皮由较为平整变化为表面氧化皮严重脱落且基体表面呈现为淡绿色。对该物质进行能谱分析后显示淡绿色物质为TiO2和Cr2O3的混合物。900℃～1000℃的氧化动力学曲线显示，随温度升高氧化增重急剧增加，在温度1000℃～1050℃范围内，试样氧化增重虽仍随时间的增加而增加，但总量有所下降。     

Phase Equilibria,Microstructure, and High-Temperature Strength of TiC-Added Mo-Si-B Alloys

TiC was added to Mo-Si-B alloys using a conventional Ar arc-melting technique, and the phase equilibria, microstructure evolution, and high-temperature strength at 1673 K (1400 °C) were investigated. The primary phase changed to Mo solid solution (Mo_ss), Mo₅SiB₂ (T₂), or TiC depending on the composition. Following the primary phase solidification, a Mo_ss + TiC, Mo_ss + T₂, or Mo_ss + T₂ + TiC + Mo₂C eutectic reaction took place as the secondary solidification step. In some alloys, Mo_ss + T₂ + TiC and Mo_ss + T₂ + Mo₂C eutectic reactions were present as higher-order solidification steps. After annealing at 2073 K (1800 °C) for 24 hours, Mo_ss, T₂, TiC, and Mo₂C coexisted stably with microstructural coarsening. The coarsening rate was much faster in an alloy with no TiC dispersion, suggesting that TiC has a strong pinning effect on the grain boundary and interface migration. Compression tests conducted at 1673 K (1400 °C) revealed strength properties of almost all the alloys that were better than those of the Mo-Hf-C alloy (MHC). Alloy densities were 9 g/cm³ or less, which is lighter than pure Mo and MHC (≥10 g/cm³) and competitive with Ni-base superalloys. TiC-added Mo-Si-B alloys are promising candidates for ultrahigh-temperature materials beyond Ni-base superalloys.     

Nb-Ti-Al高温铌合金氧化行为研究

研究了Nb-35Ti-6Al、Nb-15Ti—llAl以及Nb-30Ti-15Al三组合金于900℃和1000℃在空气中的氧化行为，建立了Nb-Ti—A1合金高温氧化动力学模型。研究表明，元素Ti和Al的加入能有效改善合金的抗氧化性能，合金中占相的存在降低了氧的溶解度。同时抑制氧的扩散，因而两相合金Nb-15Ti-llAl和Nb-30Ti-15Al（β＋δ相）抗氧化性能优于单相合金Nb-35Ti-6Al（β相）。     

Development of Ultra-high Purity (UHP) Fe-Based Alloys with High Creep and Oxidation Resistance for A-USC Technology

Metallurgical and Materials Transactions A - The design of ultra-high purity (UHP) Fe-based model alloys for advanced ultra-supercritical (A-USC) technology is attempted in this work. Creep testing...     

Nb-Based Nb-Al-Fe Alloys: Solidification Behavior and High-Temperature Phase Equilibria

High-melting Nb-based alloys hold significant promise for the development of novel high-temperature materials for structural applications. In order to understand the effect of alloying elements Al and Fe, the Nb-rich part of the ternary Nb-Al-Fe system was investigated. A series of Nb-rich ternary alloys were synthesized from high-purity Nb, Al, and Fe metals by arc melting. Solidification paths were identified and the liquidus surface of the Nb corner of the ternary system was established by analysis of the as-melted microstructures and thermal analysis. Complementary analysis of heat-treated samples yielded isothermal sections at 1723 K and 1873 K (1450 °C and 1600 °C).     

低铬合金化TiAl的高温氧化

用热重分析法（TGA）研究了Ti-48Al-(0～4）Cr合金在各种条件下100h内的高温氧化行为,氧化增重△M随TiAl中铬元素添加量的增大而先升后降。Ti-48Al-Cr在各种条件下均表现为耐氧化性最差,原因在于适量铬固溶于TiAl基体中,有助于氧化时钛离子的扩散;铬含量进一步增高时,将有可能引入第三相Ti2CrAl(β2）,易形成较具保护性的Cr2O3,从而抑制氧化反应速度。一般地,低铬合金化后TiAl的耐氧化性下降,但氧化膜粘附性、致密性均有明显提高。TiAl表面氧化物呈分层结构,主相为TiO2与Al2O3。     

High-Temperature Oxidation Behavior of Vanadium, Titanium-Bearing Magnetite Pellet

 By means of isothermal oxidation and chemical analysis, great importance was attached to the parameters that made effects on the oxidation degree of vanadium, titanium-bearing magnetite pellet in high-temperature processing (1073-1323 K). Based on the experimental data, oxidation kinetics of pellet was analyzed according to shrinking unreacted-core model subsequently. Experiment results display that the oxidation degree of pellet increases with increasing of oxidation time, oxidation temperature and oxygen content, as well as shrinking of pellet diameter. Under the condition of oxidation time 20 min, oxidation temperature 1223 K, oxygen content 15%, and pellet diameter 12 mm, oxidation degree of pellet reaches 92.92%. The analysis of oxidation kinetics indicates that oxidation process of pellet is controlled by chemical reaction with activation energy 68.64 kJ/mol at a relatively lower temperature (1073-1173 K). Oxidation process of pellet is mixed-controlled by chemistry reaction and diffusion with activation energy 39.66 kJ/mol in the temperature range of 1173-1273 K. When oxidation temperature is higher than 1273 K, the limited link of oxidation reaction is the diffusion control with the activation energy 20.85 kJ/mol. These results can serve as a reference to the production of vanadium, titanium-bearing magnetite pellet.     

Overview of Strategies for High-Temperature Creep and Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels

A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the ~923 K to 1173 K (650 °C to 900 °C) temperature range due to the formation of a protective Al₂O₃ scale rather than the Cr₂O₃ scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe₂(Mo,Nb)-Laves, Ni₃Al-L1₂, etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA alloys are discussed relative to compositional and microstructural factors.     

Resistance to High-Temperature Oxidation and Wear of Various Ferrous Alloys Used in Rolling Mills

Various materials are commonly used to manufacture work rolls for hot rolling mills, such as ICDP (Indefinite Chill Double Pour) cast irons, high-chromium white cast irons, and high speed steels (HSS). Various chemical compositions and microstructures are studied in order to optimize the in-use behavior of those grades of rolls. In this paper, six grades of ferrous alloys (an ICDP cast iron; an ICDP cast iron enriched in vanadium, niobium, and molybdenum; a HSS; a graphitic HSS; a high-chromium white cast iron (Hi-Cr); and a niobium-molybdenum-doped high-chromium white cast iron) were investigated. High-temperature oxidation tests with gravimetric means at 575 °C in water vapor atmosphere and sliding wear tests were carried out. The oxidation kinetics was followed during oxidation test. The microstructure was observed by optical and scanning electron microscopies. The oxides formed on the surface of the samples were analyzed by XRD and EDS. The thickness of the oxide scales and the mass gain were measured after oxidation test. The results showed that the behavior of all the grades differed. The oxide scale of HSS and HSS-G grades was fine and their friction coefficient was low. The weight gain after oxidation test of HSS was high. Hi-Cr and M-Hi-Cr grades presented highly porous oxide layer and an important increase of the friction coefficient during wear test. ICDP and M-ICDP had intermediate behavior.     

High-Temperature Creep Deformation and Fracture Behavior of a Directionally Solidified Ni-Base Superalloy DZ951

The high-temperature creep deformation and fracture behavior of a directionally solidified Ni-base superalloy DZ951 have been investigated over a wide stress range of 110 to 880 MPa at high temperatures (700 °C to 1000 °C). In this article, the detailed creep deformation and fracture mechanism have been studied. The results show that the creep curves exhibit strong temperature dependence. From transmission election microscopy (TEM) observations, it is suggested that the deformation mechanism is temperature dependent and mainly consists of three dislocation-controlling mechanisms: stacking faults and dislocation-pair shearing, dislocation bowing, and dislocation climbing. It is found that the fracture mode of DZ951 alloy changes from cleavagelike fracture at low temperature to ductile fracture at high temperature. At 700 °C, the creep cracks mainly initiate at the surface and propagate along the cleavagelike facets. With increasing temperature, cracks can initiate at the surface, carbide/matrix interface, and cast pore. The growth of microcrack has a direction perpendicular to the stress direction. The creep-rupture data follow the Monkman–Grant relationship in different temperature regions.     

Nb和Ti对444超纯铁素体不锈钢高温氧化行为的影响

研究了444Nb和444NbTi不锈钢在1100℃下短时(4h)高温氧化行为,利用X射线衍射、扫描电子显微镜和能谱仪分析氧化膜物相、微观形貌及化学成分分布.结果表明,444NbTi钢每cm2氧化增重为444Nb钢两倍,这与Ti改变Cr2O3氧化膜缺陷结构相关.444Nb钢存在氧化膜剥落现象,原因可能为Mn的快速扩散促进...     

High Temperature Oxidation of Copper and Copper Aluminum Alloys

Abstract

Small amounts of aluminum (1 to 3 wt% Al) were alloyed with copper to study oxidation resistance at high temperatures. Pure copper and the alloyed samples were oxidized from 700 to 1000 °C in air for various lengths of time. The oxidation rates of the samples increased with temperature. The oxidation rates of pure copper and the 1 wt% Al sample were found to follow parabolic rate law kinetics in the temperature range studied indicating diffusion controlled growth. The oxidation resistance of the 2 wt% Al alloy disappeared between 800 and 900 °C. It was found that the 3% Al-Cu samples oxidized at a rate 100 times lower than that of pure copper. The activation energy of copper oxidation at 122 kJ/mol was found to be very close to reported values.

On a allié de petites quantités d’aluminium (1 à 3% Al en poids) avec du cuivre afin d’étudier la résistance à l’oxydation à des températures élevées. On a oxydé le cuivre pur et les échantillons alliés entre 700 et 1000 °C, à l’air, sur des durées variées. Le taux d’oxydation des échantillons augmentait avec la température. On a trouvé que le taux d’oxydation du cuivre pur et celui de l’échantillon de 1% en poids d’Al suivaient une loi cinétique à taux parabolique dans le domaine de température étudié, indiquant une croissance contrôlée par la diffusion. La résistance à l’oxydation de l’alliage à 2% en poids d’Al disparaissait entre 800 et 900 °C. On a trouvé que les échantillons de 3% Al-Cu s’oxydaient à un taux 100 fois plus faible que celui du cuivre pur. L’énergie d’activation de l’oxydation du cuivre, à 122 kJ/mol, se rapproche des valeurs rapportées.     

Irradiation Behavior in High Entropy Alloys

As an increasing demand of advanced nuclear fission reactors and fusion facilities,the key requirements for the materials used in advanced nuclear systems should encompass superior high temperature property,good behavior in corrosive environment,and high irradiation resistance,etc.Recently,it was found that some selected high entropy alloys(HEAs) possess excellent mechanical properties at high temperature,high corrosion resistance,and no grain coarsening and self-healing ability under irradiation,especially,the exceptional structural stability and lower irradiation-induced volume swelling,compared with other conventional materials.Thus,HEAs have been considered as the potential nuclear materials used for future fission or fusion reactors,which are designed to operate at higher temperatures and higher radiation doses up to several hundreds of displacement per atom(dpa).An insight into the irradiation behavior of HEAs was given,including fundamental researches to investigate the irradiation-induced phase crystal structure change and volume swelling in HEAs.In summary,a brief overview of the irradiation behavior in HEAs was made and the irradiation-induced structural change in HEAs may be relatively insensitive because of their special structures.     

Cu-Cr合金高温氧化行为分析

该文用热分析天平，结合金相、X射线衍射(XRD)、扫描电镜(SEM)和能谱(EXD)研究了Cu—Cr合金在不同温度下的氧化行为。结果表明，Cu—Cr合金在700℃-900℃氧化符合抛物线规律，其最外层氧化膜为CuO，内层为Cu2O和Cr2O3，铬有助于提高合金的抗氧化能力。     

Prediction of Thermodynamic Properties of Mo-Si-B Alloys from First-Principles Calculations

Metallurgical and Materials Transactions A - Many technological applications such as heat treatment processes and their computational modeling and simulation require knowledge of the thermodynamic...     

织构对先进锆合金蠕变性能各向异性的影响

研究了两种组织形貌相似的先进锆合金M5TM和N36锆合金核燃料包壳管材的单轴拉伸和内压蠕变性能.利用x射线衍射仪分析了它们的织构.试验发现两种先进锆合金包壳材料的蠕变性能表现出明显的各向异性.根据试验条件下的蠕变机理,结合弹性粘塑性自洽模型定性地分析了织构对锆合金管材蠕变各向异性的影响,解释了先进锫合金各向异性随应力指数变化的共性规律.揭示了织构与先进锆合金管材蠕变各向异性的定性关系.由于成分和织构因素的共同作用,在研究的试验条件下,N36合金的初始蠕变应变、稳态蠕变速率低于M5合金.织构是合金蠕变行为产生各向异性的主要原因,对于再结晶状态的先进锆合金包壳管,具有(0002)织构特征时,应力指数越高(即施加的应力水平越高),其蠕变的各向异性值越大.