Internal-Nitriding Behavior of Ni-V and Ni-3Nb Alloys

Ni-2V, Ni-5V, Ni-12V, and Ni-3Nb alloys (w/o)were nitrided in 10 v/o NH₃(bal.H₂) over the range of 700-1000°C. Thegrowth rates of the reaction zones followed parabolicbehavior for all of the alloys from 700 to 900°C. At 1000°C, Ni-2V andNi-3Nb formed nitride scales, whereas Ni-5V and Ni12Vformed internal-nitride zones. Nitridation ratesdecreased with increasing vanadium content for the Ni-Valloys. VN precipitated in the Ni-V alloys and NbNprecipitated in Ni-3Nb for all exposure conditions inwhich internal nitridation occurred. The precipitatemorphology changed with temperature and distance from the gas-metal surface. The VN and NbNprecipitates were generally small and spheroidal nearthe surface, increasing in size with distance andtemperature. The NbN precipitates became Widmanstattenat higher temperatures and/or increasing distance withinthe nitrided zone. The solubility of nitrogen in pure Niwas determined and found to decrease with increasingtemperature from 700 to 1000°C. Expressions for the diffusion coefficient of nitrogen in nickelwere determined from the measured permeabilities of eachalloy and the nitrogen solubilities in nickel.     

Internal-Carburizing Behavior of Ni-V,Ni-Cr,and Ni-3Nb Alloys

Ni-2V, Ni-5V, Ni-12V, Ni-10Cr, Ni-20Cr, andNi-3Nb alloys were carburized in 1.5 v/oC₃H₆ (bal. H₂) over therange 700-1000°C. Carburization of Ni-5V, Ni-12V,Ni-20Cr, and Ni-3Nb obeyed the parabolic rate law. Ni2V and Ni-10Cr, however, formed onlythin carbide scales upon carburizing. Carburizationrates decreased with increased vanadium content fromNi-5V to Ni-12V for all exposure conditions. V4C3 formed throughout the reaction zones of Ni-12V.Cr₃C₂ formed in the surfaceregions and Cr₇C₃ formed withinthe interior of Ni-20Cr. NbC precipitated in Ni-3Nbunder all conditions. The precipitate morphology changed with temperature and distance from thegas-metal surface. V₄C₃ andCr₃C₂ particles were generallysmall and spheroidal near the surface of Ni-12V andNi-20Cr, respectively, increasing slightly in size with distance from the surface and withincreasing temperature. The vanadium and chromiumcarbides formed intergranular networks toward thereaction fronts. The NbC precipitates were generallylarge and became Widmanstatten at increasing distancewithin the carburized zone of Ni-3Nb. Expressions forthe diffusion coefficient of carbon in nickel from themeasured permeabilities and carbon solubility data were determined. Solubility products weredetermined for all of the carbides formed and found tobe large in comparison with the product of theactivities of the precipitate elements. Wagner's theory of internal oxidation was shown to be anapproximation to the carburization kinetics attemperatures of 900°C or higher.     

The Effect of Supersaturation on the Internal Oxidation of Binary Alloys

According to the theory of Bohm and Kahlweit ofthe internal oxidation of binary A-B alloys, theparabolic rate constant for the formation of reasonablystable internal BO oxides as well as theconcentrations of O and B at the oxidation front arecontrolled only by the degree of supersaturationnecessary for the nucleation of new oxide particles. Theeffects of this factor on the previous parameters arecalculated for various values of the solubility product ofthe oxide and of the diffusion coefficients of O and B.Moreover, an alternative procedure for the calculationof the critical degree of supersaturation behind the precipitation front required for oxideprecipitation, which is a function of the concentrationof the reactants at the internal oxidation front, isproposed. A simple modification of Wagner's theory of internal oxidation is also presented, andits results are compared with those of the treatment byBohm and Kahlweit. Finally, the limitations of the twomethods are examined.     

The Formation of Two Layers in the Internal Oxidation of Binary Alloys by Two Oxidants in the Absence of External Scales

An improved treatment of the simultaneousinternal oxidation of binary solid-solution alloys bytwo different oxidants (double internal oxidation)producing two subsequent layers divided by a sharpplanar interface in the absence of external scales ofthe most-noble component has been developed by removingsome of the approximations involved in previoustheories. The kinetics of internal oxidation arecalculated, based on the assumption that all the partialprocesses follow the parabolic rate law, as expected fordiffusion-controlled processes. The overall thickness ofinternal oxidation for dual oxidants is found to be equal to or smaller than the sum of thoseproduced by the two oxidants taken separately under thesame experimental conditions. The ratio between thethickness of the two regions depends critically on the ratio between the diffusion coefficientsof the two oxidants, so that under appropriate limitingconditions, one of the two layers becomes negligible.The degree of enrichment of the most-reactive component in the region of internal oxidationby two oxidants is found to be uniform over the wholezone, but lower than what would be produced by each ofthe oxidants taken singularly. The theoretical predictions are then compared to someexperimental results. It is finally shown that theexternal layer will not contain the compound most stableon an absolute scale, but that most stable with respect to the actual composition of the gas phase,while scaling structures different from that assumedhere are also possible.     

On the Influence of Non-Protective CuO on High-Temperature Oxidation of Cu-Rich Cu-Ni Based Alloys

A number of copper-rich Cu-Ni alloys wereoxidized from 750 to 1000^°C at differentoxygen pressures. The oxide scales formed consist of anouter copper oxide layer and an inner porous layer where internal oxide-derived NiO particles aredispersed in a copper oxide matrix. The copper oxide maybe both single-phase CuO and a two-phase(CuO+Cu₂O). At the lower part of thetemperature range, the oxidation kinetics and oxidemorphology depend strongly upon the formation of CuO.The CuO layer is nonprotective and further oxidation ofCu₂O, forming CuO, therefore changes the oxidation from being approximately parabolic tohaving a breakaway-like behavior. The relative thicknessof nonprotective CuO increases with increasing NiO andreflects that the solid-state flux of copper across the Cu₂O decreases due to abarrier effect of the NiO particles and porosity in theoxide and NiO particles in the alloy. The beneficialeffect of Ni in reducing the oxidation rate is lost due to the extensive formation ofnonprotective CuO.     

The Long-Term,Cyclic-Oxidation Behavior of Selected Chromia-Forming Alloys

Long-term, cyclic-oxidation testing in still airfor about 2 years (720 days) at 982°C and 1 year(360 days) at 1093, 1149, and 1204°C has beenconducted on the commercial, high-temperaturechromia-forming HR-120®, HR160®, and 230® alloys(all trademarks of Haynes International, Inc.). Eachthermal cycle consisted of 30 days at temperaturefollowed by about 4 hr at ambient. The resultsdemonstrated the significant effects of alloy composition on long-term,cyclic-oxidation resistance. Each of the alloysexhibited scale spallation; however, the manner by whichspallation occurred varied between the alloys. The 230 alloy, which contains 0.02 wt.% La, exhibitedpartial scale spallation, thus allowing for the easierformation of a protective or semiprotectiveCr₂O₃-rich scale during subsequentoxidation. The HR-160 alloy exhibited complete spallation owinglargely to its relatively high silicon content (2.75wt.%). However, the silicon was also beneficial inpromoting protective or semiprotective scale formationwhen the exposed alloy was subsequently oxidized.The HR-120 alloy showed the poorest cyclic-oxidationresistance, due in part to poor scale adhesion and thetendency of the iron in this alloy (33 wt.%) toeventually oxidize and result in the formation of aless-protective scale. All of the alloys underwentinternal attack in the form of internal oxidation andvoid formation. In most cases, the extent of internalattack was significantly greater than that of metalloss.     

Composition Dependence of the Kinetics and Mechanisms of Thermal Oxidation of Titanium-Tantalum Alloys

The oxidation behavior of titanium-tantalumalloys was investigated with respective concentrationsof each element ranging from 0 to 100 wt.%. Alloys wereexposed to argon-20% oxygen at 800 to 1400°C. Theslowest oxidation rates were observed in alloys with5-20% Ta. The oxidation kinetics of alloys containingless than approximately 40% Ta were approximatelyparabolic. Pure Ta exhibited nearly linear kinetics. Alloys containing 50% or more Taexhibited paralinear kinetics. The activation energiesfor oxidation ranged between 232 kJ/mole for pure Ti and119 kJ/mole for pure Ta, with the activation energies of the alloys falling between these values andgenerally decreasing with increasing Ta content. Theactivation energies for oxidation of the end members, Tiand Ta, agree well with published values for the activation energies for diffusion of oxygenin -Ti and Ta. Scale formation in the alloys wasfound to be complex exhibiting various layers of Ti-,Ta-, and TiTa-oxides. The outermost layer of the oxidized alloys was predominately rutile(TiO₂). Beneath the TiO₂ grew avariety of other oxides with the Ta content generallyincreasing with proximity to the metal-oxide interface.It was found that the most oxidation-resistant alloys hadcompositions falling between Ti₅Ta andTi-15Ta. Although Ta stabilizes the -phase of Ti,the kinetics of oxidation appeared to be rate limited byoxygen transport through the oxygen-stabilized -phase.However, the kinetics are complicated by the formationof a complex oxide, which cracks periodically. Tantalumappears to increase the compositional range ofoxygen-stabilized -phase and reduces both the solubilityof oxygen and diffusivity of Ti in the - and-phases.     

Internal Nitridation of Nickel-Base Alloys. Part I. Behavior of Binary and Ternary Alloys of the Ni-Cr-Al-Ti System

The internal-nitriding behavior of several modelalloys of the Ni-Cr-Al-Ti system in an oxygen-freenitrogen atmosphere at 800-1100°C was studied.Thermogravimetry as well as various metallographic techniques (SEM and TEM) were used. It wasshown that both the nitrogen solubility and the nitrogendiffusion coefficient are strongly affected by the Crcontent of the Ni alloy. Hence, in Ni-Cr-Ti alloys a higher chromium content leads to an increaseddepth of the internal precipitation of TiN. Nitridationof the alloying element Cr takes place only at highconcentrations of Cr. In general, the nitridation rate was found to obey Wagner's parabolic ratelaw of internal oxidation. Changes in the parabolic rateconstant with alloy composition can be understood bymeans of thermodynamic calculations in combination with microstructural observations.     

The Oxidation Behavior of Several Ti-Al Alloys at 900°C in Air

Ti-23Al, Ti-50Al and Ti-50Al-2Nb (at.%) wereoxidized in air at 900°C for times up to 1130 hr.The resulting oxide scale structures were analyzed ingreat detail by metallographic and microprobe investigations and theAl₂O₃ structures in the complexoxide scales were correlated with the course of thethermogravimetric curves. It appears that in order toachieve long-term protective behavior of the scales, it is necessary to stimulate theformation of a thin Al₂O₃ barrierat the scale-metal interface and not at a position inthe outer part of the scale. The Nb effect seems to bemostly due to this stimulation of anAl₂O₃ layer at theinterface.     

The Oxidation of Two Fe-Y Alloys Under Low Oxygen Pressures at 600-800°C

The corrosion of pure yttrium and of two Fe-basealloys containing approximately 15 and 30 wt.% Y wasstudied at 600-800°C in H₂-CO₂mixtures providing an equilibrium oxygen pressure of10^-24 atm at 600°C and 10^-20 atm at 700 and800°C. The corrosion of yttrium under these lowoxygen pressures resulted in the growth ofY₂O₃ scales and presented twoapproximately parabolic stages at 800°C, while at 600-700°C it was faster andnonprotective. The corrosion of the two alloys followedapproximately the parabolic rate law, except for Fe-15Yat 600°C which oxidized nearly linearly. At 600 and700°C, when the gas-phase oxygen pressure was in thefield of stability of iron oxide, the alloys formed alsoa thin external Fe₃O₄ layer, whileat 800°C, when the oxygen pressure was below thestability of FeO, a thin outermost layer of pure iron wasobserved to form. Under all conditions a region ofinternal oxidation formed in the alloy, in which theyttrium-rich phases were transformed into a mixture ofiron metal and oxides, which included double Fe-Yoxides as well as Y₂O₃. Themicrostructure of the internal-oxidation region followedclosely that of the original alloys, which moreover didnot undergo any yttrium depletion. These results are examinedby taking into account the low solubility of yttrium iniron and the presence of intermetallic compounds in thealloys.     

Oxidation Behavior of ODS Alloy MA 6000

MA 6000 is a mechanically alloyed, Ni-base ODSalloy. Excellent high-temperature strength makes it astrong candidate for application in high-temperatureindustrial processes. In order to assess its usefulness for high-temperature structural components,in-depth knowledge of its oxidation behavior,particularly long-term exposure, is necessary. Thepresent work deals with studies of the cyclic andisothermal oxidation of MA 6000 in the temperature range900-1050°C, with emphasis at 1050°C. A fewcomplementary studies have been carried out on the oxideintegrity under creep conditions for exposure times of up to 11,000 hr. The results have shown thatoxidation of MA 6000 involves rather complex mechanismsand alterations of the oxidation behavior still occurafter long-term exposure. Excellent oxidation resistance is based on the formation of an internalcontinuous Al₂O₃ layer.     

The Morphology of Thermally Grown α-Al2O3 Scales on Fe-Cr-Al Alloys

The morphology and growth kinetics of protective-Al₂O₃ scales formed duringhigh-temperature oxidation of the Fe-Cr-Al andFe-Cr-Al-Y alloys were studied. Scanning electronmicroscopy observations were focused on the oxidewrinkling phenomenonand formation of interfacial cavities in the course ofoxidation of the yttrium-free alloy. In this paper,emphasis is also placed on the effects of surface preparation and yttrium additions on the scaleand scalealloy interface structures. Several features ofalumina morphology, which have not received sufficientattention previously, are discussed in the context of existing models; some criticalissues are outlined.     

The Response of Alloys to Erosion-Corrosion at High Temperatures

Pure nickel, Ni-20Cr and Ni-30Cr alloys wereexposed to conditions of erosion and corrosionsimultaneously at 700°C and 800°C. The exposureswere made using normal impact of an air stream loadedwith 20-m alumina. The alumina particles flowed at therate of 400 mg per min and the velocities used were 75m/s and 125 m/s. The reaction kinetics were measureddiscontinuously by interrupting the exposure and measuring the weight loss. The specimens wereexamined using SEM-EDAX both on the surface and incross-section. Under simple oxidation, the alloyspecimens developed a thin protective layer of chromia. Under erosion-corrosion conditions, thisprotective scale was prevented from forming and thealloys were found to undergo aggressive attack at a ratethat was the same as that experienced for pure nickel, the surface oxides were identified asCr₂O₃ and NiO. It is proposedthat, under erosion corrosion, the erosive streamprevents the formation of a continuous layer of chromiaby removing the oxide faster than it can spread laterally. The specimen issaid to be in a state of erosion-maintained transientoxidation. This mechanism implies that it would bedifficult for protective scales to form in the presence of erosion and the oxidation behavior of analloy cannot be used as a guide to its resistance toerosion-corrosion.     

The Use of Oxygen Isotopic Labeling to Understand Oxidation Mechanisms

Oxidation of Metals - Isotopic labeling is a powerful tool to evaluatetransport and reaction mechanisms of oxidation. Theevaluation can answer the essential question inhigh-temperature oxidation:...     

Oxidation Behavior of Nanocrystalline Al Alloys Containing 5 and 10 at.% Ti

The oxidation behavior of mechanically alloyed(MA) Al-Ti alloys containing 5 and 10 at.% Ti wereinvestigated at 500-600°C under 1 atm of oxygen. Ateach temperature, alloys oxidized linearly during the initial stage and later followed the parabolicrate law. During the initial stage, the oxidation ratesof nanocrystalline (50 nm) Al-Ti alloys were fasterthan those of conventional (200 nm) alloys. It is suggested that more grain boundaries innanocrystalline alloys provide more nucleation sites foroxides, so that the oxide scales grew faster as denseprotective layers. During the parabolic stage, the nanocrystalline alloys had greater oxidationresistance than conventional alloys because of the denseprotective layer. Oxide scales on both alloys consistedof a mixture of -Al₂O₃ andTiO₂ in the outer layer and-Al₂O₃ near the alloy as aprotective layer.     

The Influence of Erosion-Induced Roughness on the Oxidation Kinetics of Ni and Ni-20Cr Alloys

Surface roughness plays a dominant role inincreasing the oxidation rate of metals and alloysduring erosion compared to the oxidation rate in staticair. Ni and Ni-20Cr were eroded at two different impact velocities (35 and 65 m/s) and for twodifferent impact angles (90° and 30°). Theeroded samples were subsequently isothermally oxidizedin static air at three different test temperatures. Theincreased oxidation kinetics in the case of Ni could beexplained on the basis of increased roughness caused byerosion prior to oxidation. In the case of Ni-20Cr, theeffect of increased roughness on oxidation was largely offset by the fact that the number ofgrain-boundary diffusion paths decreased due tocoarsening of the grains of the oxide scale.     

Sulfidation Behavior of Rhenium and Cobalt-Rhenium Alloys

The sulfidation behavior of Re and three Co-Realloys, 15, 30, and 45 w/o, was studied over thetemperature range 700-800^°C at sulfurpressures of 10^-4 and 10^-2 atm. The kinetics of sulfidation followed theparabolic rate law and the activation energies for allalloys were similar to that of pure cobalt. A positiverate dependency on sulfur pressure was observed and Pt markers were located at the metal-scaleinterface, both observations clearly suggesting thatoutward cation diffusion through a P-type sulfide scaleoccurred. Two dominant sulfides,Co₉S₈ and ReS₂, formed. Weight gainsdecreased for a given set of conditions with increasingrhenium content. An order of magnitude decrease in thesulfidation rate occurred as the rhenium content increased from 15 to 45 w/o. Preferentialsulfidation of cobalt initially occurred, causing arhenium-enriched zone to form in the substrate beneaththe cobalt-sulfide scale. Steep Re-concentrationgradients developed, the zone depth increasing withrhenium content. Significant sulfur diffusion into thesubstrate also occurred, with greater sulfur penetrationtaking place as the rhenium content increased. Sulfides formed at all temperatures were the same on thethree alloys, but the scales were denser on thehigher-rhenium alloys. The initial sulfide to form wasCo₃S₄, but, subsequently,Co₉S₈ became the dominant sulfide, forming beneaththe outer Co₃S₄ layer.ReS₂ formed at lower cobalt levels. Pure Rewas also studied, the sulfidation rate being about10⁴ times slower than that of cobalt. Thedecreasing rate of sulfidation with increasing Recontent is attributed primarily to slower cobaltdiffusion outward through the Reenriched substrate, aphenomenon similar to that observed by C. Wagner for theoxidation of Ni-Pt alloys.     

Thermal analysis applied to estimation of solidification kinetics of Al–Si aluminium alloys

Abstract

Evaluation of solidification kinetics by thermal analysis is a useful tool for quality control of Al–Si melts before pouring provided it is rapid and highly reproducible. Series of thermal analysis records made with standard cups are presented that show good reproducibility. They are evaluated using a Newton's like approach to get the instantaneous heat evolution and from it solidification kinetics. An alternative way of calculating the zero line is proposed which is validated by the fact that the latent heat of solidification thus evaluated is within 5% of the value calculated from thermodynamic data. Solidification kinetics was found highly reproducible provided appropriate experimental conditions were achieved: high enough casting temperature for the cup to heat up to the metal temperature well before solidification starts; and equal and homogeneous temperatures of the metal and of the cup at any time in the temperature range used for integration.     

Duplex surface layer treatment of Al alloy: electron beam alloying and plasma nitriding

Abstract

Al alloys offer a high potential as lightweight construction materials due to their low density, specific strength and processing properties. However, the field of application is limited by their low hardness and poor wear properties. Duplex surface treatment combining electron beam (EB) alloying and plasma nitriding offers one possibility to produce hard and wear resistant surface layers on Al alloys. The EB alloyed surface layer acts as supporting layer for the hard AlN coating so that the load bearing capacity can be enhanced. In the present study duplex treatment of Al-5083 (AlMg4·5Mn0·7) Al alloy has been investigated. Before the EB treatment alloying material deposition was carried out by atmospherically plasma spraying. Various sandwich layers based on Al and Fe respectively, have been applied. Different beam deflection techniques have been tested and their effect on surface deformation, microstructure and hardness was evaluated. Plasma nitriding was carried out in order to evaluate the nitriding behaviour of the surfaces modified by EB. Applying the EB meander technique results in smooth surfaces, good microstructural connection to the matrix material and a homogeneous distribution of the alloying elements together with an increase in hardness of ～300 HV0·1. Plasma nitriding leads to the formation of AlN layers of ～5 μm thickness.     

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.