三元合金高温理论氧化图 Ⅱ.低氧压下的近似分析

在假设三元Ａ－Ｂ－Ｃ固溶合金中三组无氧化物不互溶且不形成复合氧化物,并忽略合金内氧化的条件下,计算并建立了合金在单一氧化剂、低氧压下形成各氧化物的动力学氧化图并考虑组元的扩散作用,描述了变氧压条件下该四元系合金与各氧化物之间的平衡关系以及合金的成膜规律．     

三元合金的高温理论氧化图Ⅰ.高氧压下的近似分析

通过对三元合金系高温氧化行为的分析,把Ｗａｇｎｅｒ关于形成不互溶氧化物的二元合金经典理论扩展到三元合金体系,忽略活泼组元内氧化,以热力学稳定相图为参考,建立了相应的动力学氧化图并分析了影响动力学氧化图的因素,为进上步研究三元合金高温氧化行为提供了重要的理论依据。     

低氧压下三元合金最活泼组元单一内氧化的理论分析

对含单一氧化剂的气氛中氧分压低于A与B两组元氧化物分解压的情况，三元A-B-C合金系中最活泼组元C内氧化过程进行了理论分析.由于最活泼组元C内氧化生成氧化物的沉淀析出，令内氧化带的基体变成富集A、B两组元的合金.该合金的组成将影响氧在其 中的溶解度和扩散，进而影响内氧化反应动力学过程.对于内氧化遵循抛物线律的情况，我们提出了描述这种影响的一个近似关系式，并据此预测组元C的内氧化动力学.同时，对三元合金的预测结果与含有等量组元C的多种二元A-C 和 B-C合金的氧化行为进行了比较，提出并定义了“三元合金中A与B的浓度比” 的新参数.对于任何给定氧压和组元C含量的情况，借助该参数可以方便地界定三元A-B-C与二元A-C或B-C合金的内氧化行为间的差别.  〖HT5”H〗中图分类号：〖HT5”SS〗〓〓     

在600～800℃低氧压下Fe—Nb合金的氧化

研究了Ｆｅ－１５Ｎｂ和Ｆｅ－３０Ｎｂ（ｗｔ％）合金在６００～８００℃低氧压气氛的化行为，实验所用的氧压处于煤的所气化气氛下的氧分压范围，氧化后合金仅发生了Ｎｂ的氧化，生成ＮｂＯ２与Ｎｂ２Ｏ５，有些情况下还生成ＦｅＮｂ２Ｏ６。在６００℃时氧化速度很低，但在７００～８００℃时则颇高。这些合金内氧化的重要特征之一，是其内氧化物的分布向乎完全与合金的高含Ｎｂ相一致。此外，合金氧化后均不出现外氧化层，这些坷     

在600～800℃低氧压下Fe－Nb合金的氧化

研究了Ｆｅ－１５Ｎｂ和Ｆｅ－３０Ｎｂ（Ｗｔ％）合金在６００～８００℃低氧压气氛中的氧化行为，实验所用的氧压处于煤的气化气氛下的氧分压范围。氧化后合金仅发生了Ｎｂ的内氧化，生成Ｎｂ２与Ｎｂ２Ｏ５，有些情况下还生成ＦｅＮｂ２Ｏ６。在６００℃时氧化速度很低，但在７００～８００℃时则颇高。这些合金内氧化的重要特征之一，是其内氧化物的分布几乎完全与合金中的高含Ｎｂ相一致。此外，合金氧化后均不出现外氧化层。这些可能都是由于Ｎｂ在Ｆｅ中溶解度很低且合金呈两相组织所致。     

Ag—Y两相合金在高—低氧压下的高温氧化

研究了含8mass%Y的Ag-Y合金在10^5Pa和10^-15Pa两个氧压于700℃的氧化。结果表明,在10^5Pa的氧化很快,并遵循近似的抛物线规律;10^15Pa的氧化则很慢且偏离抛物线规律。在高氧压仅发生Y的原位内氧化,这主要是氧快速向内渗通而没有Y明显地外扩散所致,在低氧压则生成含Ag颗粒的Y2O3外氧化层,而其Y的内氧化层则远较高氧压者薄。。内氧化Y2O3颗粒沿深度而变化,近表面处小并呈圆球状,随深度增加逐渐变大并拉长;在深处呈针状且垂直合金表面。此外,在两种氧压下合金最表面有Ag层或分散的Ag颗粒生成。     

纯Ce及Fe—Ce合金在低氧压下的氧化

研究了纯Ｃｅ和含１５％Ｃｅ（质量分数,下同）的Ｆｅ－Ｃｅ合金在６００￣８００℃低氧分压下的氧化。纯Ｃｅ氧化后形成ＣｅＯ２的外氧化。而Ｆｅ－１５Ｃｅ则产生快速Ｃｅ的内氧化,且内氧化区的显微组织保持了原始合金的显微组织,在内氧化区的前沿没有观察到Ｃｅ的贫化。     

在600-800℃低氧压下Co-Nb合金的氧化

研究了含15与30wt%Nb的两种Co-Nb合金在600-800℃低氧压F的氧化性能选择的氧压低于相应温度的氧化钴分解压,由H2-CO2混合气获得,在600℃为10-24atm,而700与800℃为10-20atm.两种合金在600与700℃氧化结果仅产生由α-CO与氧化铌(NbO2与Nb2O5)混合物组成的内氧化带在700℃尚可能生成双元氧化物CONb2O6在内氧化带界而无贫Nb层可见.两会金,尤其是Co-30Nb在800℃时发生了由内氧化向外氧化的转变,伴随有贫Nb的单相区出现于合金的表层.从Co中N     

低氧分压系统及其在二元合金选择性氧化中的应用

根据氧化锆陶瓷管材在高温（〉８７３Ｋ）时，因管壁内外的氧分压下同形成氧浓差电池（Ｐｔ，Ｏ２（Ｐ＾ＩＯ２）│ＺｒＯ２│空气（Ｐ＾ＩＯ２），Ｐｔ的原理，研制了可控氧分子（Ｏ２）发射量的低氧分压发生系统。用该系统对Ｎｉ２０Ｃｒ合金进行低氧分压预氧处理后，促进了Ｃｒ在１２７３Ｋ时的选择性氧化，提高了合金的抗氧化性能，在低氧分压下，Ｃｕ１．５Ａｌ合金发生了Ａｌ的选择性内氧化。     

铸造镍基高温合金K35的高温氧化行为

测定了铸造镍基高温合金K35在850-1000℃温度范围内的氧化动力学曲线；并计算出其氧化激活能Qp1=274kJ／mol，Qp2=315kJ／mo1．其氧化动力学曲线都符合抛物线规律．900℃以下，K35合金属于完全抗氧化级；900-1000℃为抗氧化级．X射线衍射、扫描电镜和能谱分析表明，K35合金的氧化膜分为3个区域：外层是性质疏松的Ti及Cr氧化物混合层，并含有少量尖晶石Nicr2O4与NiAl2O4；中间层是性质致密的Cr2O3氧化层；内层(过渡层)是A12O3．     

Complete Maps for the Internal Oxidation of Ideal Ternary Alloys Forming Insoluble Oxides under High Oxidant Pressures

This paper presents an analysis of the conditions of stability of the different forms of internal oxidation of ideal ternary A-B-C alloys, where A is the most noble and C the most reactive component, forming insoluble oxide and exposed to high pressures of a single oxidant. The treatment, based on an extension to ternary alloys of Wagner's criterion for the transition from internal to external oxidation in binary alloys, allows to predict the existence of three different forms of internal oxidation. In fact, in addition to the most common kinds of internal attack, involving the coupled internal oxidation of B+C beneath external AO scales and the internal oxidation of C beneath external BO scales, a third mode, involving the internal oxidation of C beneath external scales composed of mixtures of AO+BO, becomes also possible under special conditions. A combination of the boundary conditions for the existence of these different types of internal oxidation allows to predict three different kinds of complete maps for the internal oxidation in these systems, one of which involves only two modes, while the other two involve all the three possible modes of internal oxidation.     

The Internal Oxidation of Ternary Alloys I: The Single Oxidation of the Most-Reactive Component Under Low Oxidant Pressures

The internal oxidation of the most-reactive component C of ternary A–B–C alloys by a single oxidant is examined assuming a gas-phase oxidant pressure below the stability of the oxides of the other two components. The precipitation of the most-stable oxide leaves behind a matrix composed of a binary alloy of the two less-reactive components, whose composition affects the solubility and diffusivity of the oxidant within the region of internal oxidation, with an effect on the reaction kinetics. Approximate relations between these properties are proposed and used to predict the kinetics of internal oxidation of C under the assumption of parabolic rate law. The results obtained for the ternary alloys are compared with the behavior of binary A–C and B–C alloys with the same C content. A new important factor in establishing the difference between the internal oxidation in ternary A–B–C alloys and in binary A–C and B–C alloys under a fixed gas-phase oxygen pressure and C content is the ratio between the concentrations of A and B in the bulk ternary alloy.     

An Approximate Analysis of the External Oxidation of Ternary Alloys Forming Insoluble Oxides. II: Low Oxidant Pressures

The construction and the properties of three-dimensional diagrams showing the regions of stability of the various compounds, which can form as a result of the oxidation of ideal ternary A–B–C alloys by a single oxidant at a constant temperature (kinetics diagrams) are examined for oxidant pressures insufficient to oxidize all possible alloys within the system (low oxidant pressures). For the calculation it is assumed that the various oxides do not dissolve into each other and do not form double oxides and that the alloy has an ideal behavior, while internal oxidation of the most-reactive components is disregarded. The range of meaningful oxidant pressures is divided into six intervals, which correspond to the formation of different types of scales. The simplified two-dimensional (2D) kinetics diagrams presented are obtained by projecting the appropriate three-dimensional (3D) lines of equilibrium between the alloy and the various oxides on the base triangle, which gives the composition of the system in terms of the three metal components only. The kinetics diagrams are correlated with the corresponding equilibrium phase diagrams for the same quaternary A–B–C–O systems.     

The Internal Oxidation of Ternary Alloys II: The Coupled Internal Oxidation of the Two Most-Reactive Components Under Intermediate Oxidant Pressures

The kinetics of the coupled internal oxidation of the two most-reactive components in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most noble component are examined using a number of simplifying conditions which allow to develop an approximate analytical treatment. The precipitation of the two oxides may occur either at a single front or at two different fronts of internal oxidation. The former case corresponds to a unique solution for all the parameters involved in the process. On the contrary, the existence of two fronts of internal oxidation yields a finite range of possible solutions for the oxidation kinetics as well as for all the other relevant parameters. Even though the present treatment does not allow to predict which solution will be adopted by a real system, it is possible to set limits to the values of the parameters yielding physically-acceptable solutions. After considering a general case, the treatment is applied to a real system already examined experimentally.     

An Approximate Analysis of the External Oxidation of Ternary Alloys Forming Insoluble Oxides. I: High Oxidant Pressures

The general properties of the isothermal thermodynamic phase diagrams for ternary A–B–C alloys exposed to a single oxidant at high temperatures are examined, assuming, for simplicity, the formation of insoluble p-type oxides, an ideal behavior of the alloy, and a common value of the self-diffusion coefficient for the alloy components, independent of the alloy composition, but disregarding the possibility of internal oxidation. Simplified two-dimensional phase diagrams are produced by projecting the lines and points of equilibrium of the complete three-dimensional diagrams on the base triangle, which gives the composition of the system in terms of the metal components only. Similar kinetics diagrams concerning the nature and the growth kinetics of the scales formed on ternary alloys as functions of the bulk substrate composition are also calculated for oxidant pressures above the stability of the oxides of all the alloy components and some of their general features are examined. The kinetics diagrams for ternary alloys exposed to a single oxidant contain seven regions, three of which correspond to the areas of stability of each single oxide, three to the possible mixtures of two oxides, and one to a mixture of all three oxides.     

The Internal Oxidation of Ternary Alloys. III: The Coupled Internal Oxidation of the Two Most-Reactive Components under High Oxidant Pressures

The kinetics of the simultaneous internal oxidation of the two most-reactive components B and C in ternary A–B–C alloys, where A is the most-noble and C the most-reactive component, in the presence of an external layer of the A oxide, AO, (high oxidant pressures) are examined assuming the existence of either a single or a double front of internal oxidation. For a single front a unique solution is obtained for the parabolic rate constant of internal oxidation under assigned values of all the parameters involved. When the front is double, a finite range of solutions is allowed with an upper limit equal to the single-front solution and a lower limit equal to the rate constant for the growth of external AO scales. In the case of a single front of internal oxidation, an increase of the rate constant for the growth of the external scales produces an increase of the rate constant for the internal oxidation but a decrease of the degrees of enrichment of the components being oxidized internally within the region of internal oxidation. The behavior in the case of a double front is more complex because it depends also on the actual value of the ratio between the rate constants of internal oxidation for the two fronts.     

The Internal Oxidation of Ternary Alloys. V: The Transition from Internal to External Oxidation of the Most-Reactive Component under Low Oxidant Pressures

This paper examines the conditions for the transition from internal to external oxidation of the most-reactive component C of ternary A–B–C alloys by a single oxidant under gas-phase oxidant activities below the stability of the oxide of the two most-reactive components using Wagners criterion. For this, approximate relations between the solubility and diffusivity of oxygen and the composition of the binary A–B alloy matrix in the zone of internal oxidation, already developed previously, are used. The critical C content needed for the transition in ternary alloys is calculated as a function of the many parameters involved. At variance with the behavior of binary alloys, for ternary alloys this critical C content depends also on the ratio between the concentrations of A and B in the bulk alloy. The results calculated for ternary alloys are compared with those obtained for binary A–C and B–C alloys under the same values of all the relevant parameters. Finally, complete oxidation maps for ternary alloys under low oxidant pressures,including the condition for the stability of external scales of the C oxide, are also presented.     

The Internal Oxidation of Ternary Alloys. VII: The Transition from the Internal to the External Oxidation of the Two Most-Reactive Components Under Intermediate Oxidant Pressures

The conditions for the transition between the coupled internal oxidation of two most-reactive components and the formation of external scales in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most-noble component, denoted as a situation of intermediate oxidant pressures, are examined under a number of simplifying conditions which allow to develop an approximate analytical treatment. If the precipitation of the two oxides occurs at the same front of internal oxidation, the kinetics of internal oxidation as well as the critical B and C contents needed for the transition have a single solution under fixed conditions of all the parameters involved. On the contrary, in the presence of two different fronts, when the most-stable oxide forms at the innermost front, a whole range of possible solutions is predicted. In both cases, the critical-C content needed to avoid the simultaneous internal oxidation of B plus C is progressively reduced by the addition of B. This behavior provides the basis for a possible interpretation of the “third-element effect”. However, the existence and the magnitude of this effect are complicated by the occurrence of other modes of oxidation for these systems. Thus, a general treatment of the third-element effect under intermediate oxidant pressures requires an exhaustive analysis of all the oxidation modes permitted for ternary alloys under these conditions.     

The Internal Oxidation of Ternary Alloys. VI: The Transition from Internal to External Oxidation of the Most-Reactive Component under Intermediate Oxidant Pressures

The conditions for the transition from internal to external oxidation of the most-reactive component C of ternary A–B–C alloys are examined, assuming the presence of external scales of the oxide of the component of intermediate reactivity B. For this, approximate expressions for the diffusion coefficient of oxygen and for the concentration of oxygen dissolved in the binary A–B alloy matrix within the zone of internal oxidation as functions of the composition of the metal matrix within the zone of internal oxidation are used. Numerical calculations of the critical content of C needed for this transition are carried out for different combinations of values of the various parameters involved. The results obtained for the ternary alloys are compared with the corresponding data calculated for the binary A–C and B–C alloys under oxygen pressures insufficient to oxidize the most-noble alloy component. This allows to predict the possibility of existence of a third-element effect under intermediate oxidant pressures.     

Complete Kinetic Maps for Binary Alloys Forming Two Insoluble Oxides Under High Oxygen Pressures

The oxidation under high pressures of a single oxidant of binary A–B alloys forming two insoluble oxides involves the internal oxidation of the most reactive component B when its concentration falls below an upper critical concentration defined by means of a criterion due to Wagner. However, this oxidation mode is no longer stable for alloys sufficiently dilute in B. The paper predicts the nature of the oxidation modes stable in this alloy region and calculates the critical contents of B required for the corresponding transitions.