The possible scaling modes in the high-temperature oxidation of two-phase binary alloys. Part I: High oxidant pressures

The main possible modes of the high-temperature corrosion of binary twophase alloys by a single oxidant under gas-phase pressures sufficient to corrode both alloy components are examined to highlight the differences in their behavior with respect to single-phase alloys. It is shown that in the absence of important diffusion processes of the metal components in the alloy the expected scale structures are significantly different from those typical of single-phase alloys. The effects due to the existence of different degrees of deviation from equilibrium as a result of kinetics hindrances for the formation of the most stable oxide and in the absence of alloy diffusion are then examined. It is also shown that when diffusion in the alloy becomes important the alloy may develop an outer single-phase layer depleted in the most-reactive component, which may lead to various possible scale structures. The conditions for the transition between the various oxidation modes as well as the effect of the various parameters of kinetics, thermodynamic and structural nature over the corrosion behavior of two-phase alloys are also examined.     

The internal oxidation of two-phase binary alloys under low oxidant pressures

The main features of the internal oxidation in two-phase binary alloys are examined for insignificant and important diffusion of the most-reactive component and are compared with the behavior of corresponding single-phase systems. It is shown that two-phase alloys may have two different types of internal oxidation, one of which is similar to that of the single-phase alloys (classical type), producing a uniform distribution of small oxide particles in the zone of internal oxidation, while another is typical of two-phase systems and involves the in situ conversion of the most-reactive component into its oxide. It is also shown that, under the same values of all the relevant parameters, the classical internal oxidation of two-phase alloys involves faster kinetics and smaller degrees of enrichment of the most-reactive component in the zone of internal oxidation than for single-phase alloys. As a consequence of this, the transition to the external oxidation of the most-reactive component in these systems involves higher overall concentrations of the most-reactive component than in corresponding single-phase alloys.     

Oxidation of multicomponent two-phase alloys

The high-temperature corrosion behavior of two-phase alloys presents a number of differences compared to that of single-phase alloys. These differences are mainly a consequence of the limitations that the presence of two phases impose on the diffusion of the alloy components. In this review, it is shown that the exclusive scale formation of the more stable, slow-growing oxide is more difficult on a two-phase alloy, requiring a higher concentration of the more reactive alloy component than for a corresponding single-phase alloy. The main types of corrosion behavior for binary two-phase alloys are also considered, showing that if diffusion in the alloy is slow the scale structure will closely reflect that of the starting material. When diffusion in the alloy is not negligible, the scale structure becomes similar to what forms on single-phase alloys. The oxidation of two-phase ternary alloys is shown to be even more complex than the two-phase binary alloys. The principal added complexity compared to the binary alloys is that diffusion in the ternary alloys may also occur in the presence of two metal phases, as a result of an extra degree of freedom in the ternary system. The oxidation behavior of two-phase ternary alloys is discussed in the context of a number of recent experimental results.     

The internal oxidation of two-phase binary alloys beneath an external scale of the less-stable oxide

The internal oxidation of two phase binary A-B alloys by a single oxidant at high temperatures, under partial pressures sufficient to also form external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of single-phase alloys. It is shown that, depending on various factors, particularly on the solubility and diffusivity of the most-reactive component B in the most-noble component A, this process may or may not involve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an increase of the rate of internal oxidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to single-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.     

The Criteria for the Transitions Between the Various Oxidation Modes of Binary Solid-Solution Alloys Forming Immiscible Oxides at High Oxidant Pressures

The possible high-temperature corrosion modes ofbinary solid-solution alloys forming two immisciblecompounds by a single oxidant include (1) the exclusivegrowth of external scales of the most-noble component, which may or may not be associated with theinternal oxidation of the most-reactive component, (2)the formation of composite external scales containing amixture of the two compounds, or finally (3) the exclusive growth of the most-stable compound asan external scale. The conditions for the stability ofeach scale structure depend on a number of thermodynamicand kinetics parameters, whose effects are examined quantitatively in this paper. Theconditions for the stability of the various structuresand the criteria for the transitions among them are alsoexamined. The maximum number of possible scale structures is four, but it can reduce to threeand, in some cases, only to two. In particular, theinternal oxidation of the most-reactive component maynot occur if the stabilities of the two oxides are not sufficiently different from eachother.     

An improved treatment of the conditions for the exclusive oxidation of the most-reactive component in the corrosion of two-phase alloys

The conditions for the exclusive oxidation of the most-reactive component during the corrosion of binary, two-phase alloys by a single oxidant are reexamined by using a more correct form of the mass balance for this component. Moreover, the previous treatment is extended to include the case in which the transition falls in the range of alloy compositions corresponding to the stability of the single phase rich in the most-reactive component. The limiting conditions for the transition in the single and two-phase fields are examined and discussed.     

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

Ternary A–B–C alloys, where A is the most-noble and C the most-reactive component, exposed to oxygen pressures above the stability of the least stable oxide AO (high-oxidant pressures) may present two different types of internal oxidation, i.e. either a coupled internal oxidation of both B and C beneath an external AO scale or a single internal oxidation of C beneath an external scale of the oxide of B. This paper examines the conditions required to avoid the coupled internal oxidation of B plus C beneath external AO scales, considering both the case of formation of a single and a double front of internal oxidation. The analysis, based on an extension to ternary alloys of the criterion defined by Wagner for the transition between the internal and external oxidation of the most-reactive component of binary alloys, shows that the addition of B to binary A–C alloys is very effective in reducing the C content needed for this transition in comparison with binary A–C alloys. This results provides a basis for a possible explanation of the third-element effect. However, the actual possibility of its occurrence depends also on the ability to avoid other oxidation modes, not examined here.     

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.     

The steady-state corrosion kinetics of single-phase binary alloys with a solubility gap forming the most-stable oxide

The steady-state, high-temperature oxidation kinetics of single phase alloys rich in a most-reactive componentB in binaryA-B systems presenting a limited solubility of the two components (beta phase alloys) have been examined assuming the exclusive formation of the most-stable oxideBO _v. Alloys sufficiently rich inB can form externalBO _v scales directly in contact with the beta phase, while below a criticalB content the growth ofBO _v involves also the appearance of an intermediate layer ofB-depleted solid solution ofB inA (alpha phase). The parabolic rate constants for the oxidation of single-phase beta alloys are lower than those of alloys of identicalB content which are single-phase over the whole range of composition (solid-solution alloys) but higher than for two-phase alpha + beta alloys under the same values of all the relevant parameters. Moreover, the tendency of single-phase beta alloys to form the most-stable oxide simultaneously as an external scale and internally to the alloy is greater than for solid-solution alloys but smaller than for two-phase alloys.     

The transition from the formation of mixed scales to the selective oxidation of the most-reactive component in the corrosion of single and two-phase binary alloys

The conditions for the transition from the formation of mixed scales to the exclusive oxidation of the component B, forming the most stable oxide, are examined for both single-phase and two-phase binary A-B alloys by taking into account the displacement of the alloy-scale interface due to the growth of the protective oxide. This procedure eliminates the inconsistencies arising from Wagner's classical treatment for single-phase alloys when the interdiffusion coefficient in the alloy is small with respect to the parabolic rate constant for outer-scale growth; but the same procedure leads to a significantly-improved treatment also for two-phase alloys. For the latter systems, the transition is shown to depend also on the solubility of B in the A-rich phase.Moreover, the exclusive growth of the most-stable oxide is more difficult than for single-phase alloys because it requires higher average concentrations of B in the alloy and may even become impossible if the parabolic rate constant of oxidation is large with respect to the interdiffusion coefficient in the alloy.     

The steady-State corrosion kinetics of two-phase binary alloys forming the most-stable oxide

The steady-state kinetics in the high-temperature oxidation of binary A-B alloys containing a mixture of the conjugated solid solutions of B in A (alpha phase) and A in B (beta phase) with exclusive formation of the most-stable oxide BO_v have been examined, assuming that the external scale grows on top of a subsurface layer of alpha phase. The results obtained are compared with the corresponding behavior of alloys which are single phase in the whole range of composition. Under identical values of all the parameters involved the concentration of B at the alloy-scale interface is smaller for two-phase than for single-phase alloys under the same concentration of B in the alloy as a result of the restricted flux of B through the alpha-phase layer. As a consequence of this, the two-phase alloys corrode more slowly than single-phase alloys and this difference increases as the solubility of B in the alpha phase decreases. Finally, the simultaneous formation of BO_v both externally and as internal oxide is more likely for two-phase than for single-phase materials.     

An approximate treatment of the transient state in the corrosion of binary alloys forming the most-stable oxide. Part I: Solid-solution alloys

The initial transient stage in the oxidation of binary alloys forming scales exclusively composed of the most stable oxide is examined by means of a simplified approach which avoids the numerical integration of the diffusion equation for the transport of the metal components in the alloy. At variance with previous solutions to this problem obtained by means of numerical methods, this treatment takes into account also the effect of the gas-scale reaction at the outer surface of the oxide. The concentration of the most-reactive component at the alloy surface changes gradually with time from the initial bulk value towards the corresponding steady-state value without involving any minimum, while the overall rate of the reaction presents a gradual transition from an initial nearly linear towards final parabolic behavior.     

The Internal Oxidation of Ternary Alloys. IV: The Internal Oxidation of the Most-Reactive Component Beneath External Scales of the Component Having Intermediate Reactivity

The kinetics of internal oxidation of the most-reactive component C of ternary A–B–C alloys in the presence of external scales of the oxide of the component of intermediate reactivity B, BO, are examined using convenient approximations. The precipitation of the most-stable oxide leaves behind a matrix composed of a mixture of the two most-noble components, A and B, whose composition changes with depth due to the consumption of B to form the external scale. For the calculation of the parabolic rate constant use is made of approximate expressions for the concentration of oxygen dissolved in the binary A–B metal matrix within the zone of internal oxidation and for the diffusion coefficient of oxygen as functions of the alloy composition. Numerical calculations of the parabolic rate constant of internal oxidation 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 the same conditions.     

The transient state in the oxidation of binary alloys forming the most-stable oxide: A numerical solution

The initial transient high-temperature oxidation stage for binary alloys forming the most-stable oxide has been examined by means of a numerical procedure based on the finite-difference method. At variance with previous models, the present treatment takes into account the effect of the rate of the reaction at the scale/gas interface over the corrosion kinetics. The calculations concerning the transient stage are developed either using the general parabolic rate law to represent the overall scaling kinetics or using the rate law of the reaction at the scale/gas interface as a boundary condition without imposing any particular rate law to the overall process. A correct analysis of the oxidation behavior of binary alloys during the transient stage must take into account the kinetics effect of the rate of the surface reaction. The concentration of the most-reactive element at the alloy/scale interface changes regularly with time, decreasing gradually from the initial bulk value to its final steady-state value. The present results are in good agreement with those obtained by means of an approximate analytical model developed previously.     

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.     

Corrosion of Ni-Ti alloys in the molten (Li,K)2CO3 eutectic mixture

The corrosion of pure Ni and of binary Ni-Ti alloys containing 5, 10, and 15 wt.% Ti respectively in molten (0.62Li,0.38K)₂CO₃ at 650°C under air has been studied. The corrosion of the single-phase Ni-5Ti alloy was slower than that of pure Ni, forming an external scale composed of NiO and TiO₂. The two-phase Ni-10Ti and Ni-15Ti alloys underwent much faster corrosion than pure Ni, producing an external scale containing NiO and TiO₂, and a thick internal oxidation zone of titanium mainly involving the intermetallic compound TiNi₃ in the original alloys. The rates of growth of the external scales for the Ni-Ti alloys were reduced with the increase of their titanium content, while the internal oxidation was significantly enhanced. The corrosion mechanism of the alloys is also discussed.     

Grain size effects on the oxidation of two-phase Cu-Fe alloys

The oxidation behavior of thin layers of two Cu-Fe alloys containing 25 and 50 wt.% Fe, respectively, prepared by magnetron sputtering deposition on cast alloys of the same composition (Cu-Fe coatings) and presenting grain sizes in the nanometer range, was studied at 600-800 °C in air to examine the influence of the reduction in the grain size on the selective oxidation of the most reactive component in two-phase binary systems. A continuous Fe₃O₄ layer formed beneath an external region of copper oxide on the Cu-25Fe coating, whereas an external iron oxide scale mostly composed of Fe₃O₄ free from copper oxides formed on the Fe-50Cu coating. In both cases, an iron-depleted region was present in a subsurface alloy layer. These results differ remarkably from the oxidation behavior of cast Cu-Fe alloys of similar composition but with a large grain size, which formed mixed external scales of iron and copper oxides in air and simultaneous internal and external oxidation of Fe under both high and low oxygen pressures. Therefore, a grain size reduction can effectively promote the selective external oxidation of the more reactive component in binary two-phase alloys due to an increase in the mutual solubility of the two components associated with the method of alloy preparation as well as to the presence of a large density of grain boundaries in the coatings which may act as short-circuit diffusion paths, allowing a faster outward diffusion of iron during 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.     

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.     

Recovery,recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys

The equiatomic high-entropy alloy FeNiCoCrMn is known to crystallize as a single phase with the face-centered cubic (FCC) crystal structure. To better understand this quinary solid solution alloy, we investigate various binary, ternary and quaternary alloys made from its constituent elements. Our goals are twofold: (i) to investigate which of these lower order systems also form solid solution alloys consisting of a single FCC phase, and (ii) to characterize their phase stability and recovery, recrystallization, and grain growth behaviors. X-ray diffraction (XRD) and scanning electron microscopy with backscattered electron images showed that three of the five possible quaternaries (FeNiCoCr, FeNiCoMn and NiCoCrMn), five of the ten possible ternaries (FeNiCo, FeNiCr, FeNiMn, NiCoCr, and NiCoMn), and two of the ten possible binaries (FeNi and NiCo) were single-phase FCC solid solutions in the cast and homogenized condition, whereas the others either had different crystal structures or were multi-phase. The single-phase FCC quaternary, FeNiCoCr, along with its equiatomic ternary and binary subsidiaries, were selected for further investigations of phase stability and the thermomechanical processing needed to obtain equiaxed grain structures. Only four of these subsidiary alloys—two binaries (FeNi and NiCo) and two ternaries (FeNiCo and NiCoCr)—were found to be single-phase FCC after rolling at room temperature followed by annealing for 1 h at temperatures of 300–1100 °C. Pure Ni, which is FCC and one of the constituents of the quinary high-entropy alloy (FeNiCoCrMn), was also investigated for comparison with the higher order alloys. Among the materials investigated after thermomechanical processing (FeNiCoCr, FeNiCo, NiCoCr, FeNi, NiCo, and Ni), FeNiCo and Ni showed abnormal grain growth at relatively low annealing temperatures, while the other four showed normal grain growth behavior. The grain growth exponents for all five of the equiatomic alloys were found to be ∼0.25 (compared to ∼0.5 for unalloyed Ni), suggesting that solute drag may control grain growth in the alloys. For all five alloys, as well as for pure Ni, microhardness increases as the grain size decreases in a Hall-Petch type way. The ternary alloy NiCoCr was the hardest of the alloys investigated in this study, even when compared to the quaternary FeNiCoCr alloy. This suggests that solute hardening in equiatomic alloys depends not just on the number of alloying elements but also their type.