Origin of porosity in oxide-dispersion-strengthened alloys produced by mechanical alloying

Oxide-dispersion-strengthened (ODS) alloys produced by mechanical alloying (MA) are very prone to the occurrence of porosity. The reason for this has not been fully understood so far. In this work, a model ferritic ODS alloy has been examined by optical microscopy and scanning electron microscopy in both the powder form and the consolidated condition. Submicron-sized pores have been found in the as-mechanically alloyed powder and in the consolidated alloy. Moreover, pores can be observed in loose powder particles annealed at 1000 °C and 1100 °C. Based on these results, it has been suggested that the MA process might be largely responsible for the porosity problem in ODS alloys.     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute     

Dynamic compaction of titanium Aluminides by Explosively Generated Shock Waves: Microstructure and Mechanical Properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[131] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.     

Dynamic compaction of titanium aluminides by explosively generated shock waves: Microstructure and mechanical properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[13] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.     

Effect of small concentrations of zirconium on high temperature oxidation behaviour of Fe-15 Cr-4 Al

Effect of 1% Zr on oxidation behaviour of Fe-15 Cr-4 Al alloy under isothermal conditions in air, O₂ and O₂-10% H₂O environments in the temperature range 1000–1150°C was investigated. The effect of zirconium concentration was studied at 1 200°C in air. Oxidation rate increases with increase in zirconium concentration. Parabolic rate of oxidation was observed. Limited study on cyclic oxidation was carried out at 1150°C in air. The cycle consisted of one hour holding at isothermal temperature followed by half an hour air cooling. The oxidised samples were examined by X-Ray diffractometry, SEM, EDAX. Extensive spalling was observed in the base alloy, Fe-15 Cr-4 Al in all environments. Zirconium additions eliminated the spalling of the scale. The EDAX analysis of a spalled region shows the presence of iron and chromium while the unspalled region is aluminium rich. A common structural feature, localised formation of granules/nodules was observed in the scale of zirconium containing alloys in all the environments. The number of granules increased with increase in zirconium concentration and was observed to be a maximum in 1% Zr and also increases with increasing temperature. The observations reveal that 1% Zr alloy shows lower oxidation rate in O₂-H₂O environment under isothermal conditions. X-Ray diffraction analysis shows the additional presence of Fe₂O₃ and Cr₂O₃ in α-Al₂O₃ scale which have not been detected in the α-Al₂O₃ scale formed in other environments, air and O₂. 0.2% Zr is most effective in increasing oxidation resistance of Fe-15 Cr-4 Al alloy both under isothermal and cyclic oxidation conditions.     

High Temperature Oxidation Behavior of Conventional and Nb-Modified MAR-M246 Ni-Based Superalloy

The present investigations focused on the thermal oxidation of two variants of MAR-M246 alloy having the same contents of Ta and Nb in at. pct, considering the effects of total replacement of Ta by Nb. The alloys were produced by investment casting using high purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally at 800 °C, 900 °C and 1000 °C up to 1000 hours under lab air. Protective oxidation products growing on the surface of the oxidized samples were mainly Al₂O₃, Cr₂O₃. Other less protective oxide such as spinels (NiCr₂O₄ and CoCr₂O₄) and TiO₂ were also detected as oxidation products. The conventional alloy exhibited slight internal oxidation at 800 °C and an enhanced resistance at 900 °C and 1000 °C. The Nb-modified alloy presented an exacerbated internal oxidation and nitridation at 900 °C and 1000 °C and an enhanced resistance at 800 °C. At 1000 °C, Nb-modified alloy was particularly affected by excessive spalling as the main damage mechanisms. From a kinetic point of view, both alloys exhibit the same behavior at 800 °C and 900 °C, with k_p values typical of alumina forming alloys (2 × 10⁻¹⁴ to 3.6 × 10⁻¹³ g² cm⁻⁴ s⁻¹). However, Ta modified alloys exhibited superior oxidation resistance at 1000 °C when compared to the Nb modified alloy due to better adherence of the protective oxide scale.

     

The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700 °C

The high-temperature corrosion behavior of monolithic Fe-Al alloys, with 0 to 20 wt pct Al, was investigated at 700 °C in a reducing atmosphere (p(S₂) = 10⁻⁴ atm, p(O₂) = 10⁻²⁵ atm) for up to 100 hours. Postexposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy, in combination with energy dispersive spectroscopy, electron probe microanalysis, and quantitative image analysis. From the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duration of each stage directly related to the aluminum content of the alloy. The first stage, labeled the inhibition stage, was characterized by low weight gains and the absence of rapid degradation of the alloy. Typically observed for compositions with 10 to 20 wt pct Al, protection was afforded due to the development of a thin, continuous alumina scale. For alloys with 7.5 wt pct A1, the ability to maintain the initially formed alumina scale was not observed, resulting in the breakdown stage. Localized corrosion product nodules, containing iron sulfide (Fe_1-x S) and the spinel-type tau phase (FeAl₂S₄), developed through the alumina scale due to sulfur short-circuit diffusion. These growths were accompanied by relatively high corrosion rates. Further decreasing the aluminum content to 5 wt pct and below lead to the formation of a continuous sulfide scale whose growth was controlled by iron and sulfur diffusion through the previously formed product. The alloy wastage rates in the steady-state stage were relatively high when compared to the previous two regions.     

Isothermal oxidation of TiAl alloy

Isothermal oxidation behavior of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range 850 °C to 1000 °C. The oxidation was essentially parabolic at all temperatures with significant increase in the rate at 1000 °C. Effective activation energy of 404 kJ/mol was deduced. The oxidation products were a mixture of TiO₂ (rutile) and α-Al₂O₃ at all temperatures. An external protective layer of alumina was not observed on this alloy at any of the temperatures studied. A layered structure of oxides was formed on the alloy at 1000 °C.     

Enhancing the High Temperature Capability of Ti‐Alloys

Titanium is a widely used structural material for applications below approximately 500°C but right now it cannot be used at higher temperatures. Titanium forms a fast growing rutile layer under these conditions. Furthermore enhanced oxygen uptake into the metal subsurface zone leads to embrittlement which deteriorates the mechanical properties. To overcome this problem a combined Al‐ plus F‐treatment was developed. The combination of Al‐enrichment in the surface zone so that intermetallic Ti_xAl_y‐layers are produced which form a protective alumina layer during high temperature exposure plus stabilization of the Al₂O₃‐scale by the fluorine effect led to significantly improved resistance against increased oxidation and embrittlement in high temperature exposure tests of several Ti‐alloys. In this paper, the experimental procedures and achieved improvements are described. The results will be discussed for the use of Ti‐alloys at elevated temperatures.     

Oxidation mechanism of three Fe-Al alloys with and without addition of 0.1 at% yttrium at 800 °C

The isothermal oxidation behavior of Fe-10Al, Fe-15Al, Fe-20Al alloys with and without the addition of 0.1 at% Y was studied at 800 °C under 1 × 10⁵ Pa of flowing pure O₂ for 24 h. The oxidation of three Fe-Al alloys can be divided into transient state and steady state oxidation stages. The oxidation of each stage is approximately in accordance with the parabolic law. The addition of 0.1 at% Y changes the oxidation behavior obviously and leads to a significant increase of the weight gain of Fe-10Al and Fe-15Al. The scale grown on Fe-10Al is much thicker and more complicated than that grown on Fe-20Al, which is composed of an exclusive thin layer of Al₂O₃ protective film. Due to the formation of a large number of nodules, the scales grown on Fe-15Al cannot provide full protection for the alloy. Scale microstructure of the three Fe-Al-0.1Y alloys is similar to their corresponding Fe-Al alloys. However, nodules with very small size still appear on the surface of Fe-20Al-0.1Y alloy. The critical Al concentration to form an exclusive Al₂O₃ protective layer for Fe-Al binary alloy is on the borderline between 15 at%–20 at%. For Fe-Al-0.1Y alloy, the presence of 20 at% Al is not enough to inhibit the growth of nodules.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Fabrication and characterisation of bulk Al2O3/Mo nanocomposite by mechanical milling and sintering

Abstract

In this study, fabrication and mechanical properties of alumina based ceramic matrix nanocomposite reinforced with 15 and 26·6 vol.-%Mo particles were investigated. Alumina–molybdenum nanocomposite powders were prepared by ball milling of Al and MoO₃ in an SPEX8000 type ball mill. The powder particles were consolidated by cold uniaxial pressing followed by sintering in vacuum atmosphere at 1300 and 1400°C. The structural evaluation of as milled and sintered samples was studied by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. Sintered samples were examined by hardness measurements and three-point flexural strength. Results show a significant improvement in flexural strength of Al₂O₃–Mo nanocomposites in comparison to monolithic alumina and increases by Mo content. During sintering, grain growth and α-Al₂O₃ to γ-Al₂O₃ transformation occurred. In addition, an increase in temperature of sintering resulted in higher density and hardness of consolidated nanocomposites.     

Some water vapor effects during the oxidation of alloys that are <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> formers

Oxidation studies were performed at 1100 °C in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form α-Al₂O₃ scales under oxidizing conditions. The alloys investigated included RENé N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENé N5, and a Ni-8 wt pct Cr-6 wt pct Al model alloy. The water vapor affected the oxidation of the alloys in three important ways: (1) The scales spalled more profusely during cyclic oxidation in wet air than in dry air, particularly for those alloys with alumina scales, which are only moderately adherent under dry conditions. The results were consistent with the mechanism previously proposed (Reference 1), whereby the water molecules decrease the fracture toughness of the alumina/alloy interface. (2) Thicker oxides are formed during oxidation in wet air than dry air. This effect comes primarily from accelerated transient oxidation during exposure in wet air. (3) Spinel was found to form on top of the alumina scales during long-term exposure. This phenomenon occurred in all atmospheres but was much more pronounced for exposures in wet atmospheres. Mechanisms for the preceding observations are proposed.     

On development of press and sinter Al–Ni–Mg powder metallurgy alloys

Abstract

The objective of this research was to initiate the development of powder metallurgy alloys based on the Al–Ni–Mg system. In doing so, binary (Al–Mg) and ternary (Al–Ni–Mg) blends were prepared, compacted and sintered using elemental and master alloy feedstock powders. Research began with fundamental studies on the sintering response of the base aluminium powder with additions of magnesium. This element proved essential to the development of a well sintered microstructure while promoting the formation of a small nodular phase that appeared to be AlN. In Al–Ni–Mg systems a well sintered structure comprised of α aluminium plus NiAl₃ was produced at the higher sintering temperatures investigated. Of these ternary alloys studied, Al–15Ni–1Mg exhibited mechanical properties that were comparable with existing commercial 'press and sinter' alloys. The processing, reaction sintering and tensile properties of this alloy were also found to be reproducible in an industrial production environment.     

The influence of sulfides on the oxidation behavior of nickel-base alloys

The oxidation of presulfidized chromium, Ni?Cr, and Ni?Al alloys, and complex nickel base alloys was studied at 1000°C in 1.0 atm of oxygen. Sulfur-rich surface layers were produced in the pretreatment by using H₂S?H₂ mixtures. Presulfidized chromium oxidized at a rate similar to that of sulfur-free chromium. The oxidation rate of presulfidized Ni?Cr alloys was affected by sulfur only when liquid nickel sulfide was present which accelerated the oxidation rate by creating rapid diffusion paths through the Cr₂O₃ scale. The oxidation behavior of presulfidized Ni?Al alloys, with aluminum contents sufficient for the formation of a protective Al₂O₃ layer in the sulfur-free condition, was influenced by sulfur only when aluminum sulfide was formed in the presulfidation treatment which caused the Al₂O₃ scale to be porous. The oxidation behavior of nickel-base alloys containing both chromium and aluminum was insensitive to the presence of sulfides when the concentration of aluminum in the alloy was such that a protective Al₂O₃ scale was formed during oxidation of the sulfur-free alloy and aluminum sulfide was not formed in the presulfidizing treatment.     

Influence of hot isostatic pressing on microstructure and mechanical behaviour of nanostructured Al alloy

Abstract

Aluminium alloy AA 5083 [Al–4·4Mg–0·7Mn–0·15Cr (wt-%)], powder was ball milled in liquid nitrogen via the cryomilling method to obtain a nanocrystalline (NC) structure. Samples of the powder were hot vacuum degassed to remove interstitial contaminants, then consolidated by hot isostatic pressing (HIPing) at six temperatures (from 0·46T_m to 0·89T_m), before being high strain rate forged (HSRF) to produce plate material. The microstructure was analysed at the different processing stages. The compressive properties of the as HIPed material, plus tensile properties of the final product were studied. Despite grain growth during HIPing, an ultrafine grain (UFG) structure was retained in the consolidated material, which consequently had increased strength over conventionally processed AA 5083. As the HIP temperature was increased, the density increased. Strength changes were minimal in compression and tension with varying HIP temperature, once near full density was attained at 275°C (～0·64T_M). Yield strength data indicate negligible variation in the grain size of the materials.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

Effect of Temperature on Scale Morphology of Fe-1.5Si Alloy

Because of the effect of silicon on the formation of oxide scale, red scale is the main surface defect of hot-rolled Fe-Si plate, making the scale difficult for descaling compared with carbon steel. Thermogravimetric analyzer (TGA) is used to simulate isothermal oxidation process of Fe-1.5Si alloy for 60 min under air condition, and the temperature range is from 700 to 1200 °C. Electron probe microanalysis (EPMA) is used to observe cross-sectional scale morphology and analyze elemental distribution of the scale. Relational graph of temperature, scale thickness and scale structure is obtained. It is found that scale structure (outer Fe oxide layer+inner FeO/Fe₂SiO₄ layer+internal Si oxide precipitates) is almost unchanged with temperature except at 1000 and 1200 °C. At 1000 °C internal Si oxide precipitates cannot be found at the subsurface of the alloy, and at 1200 °C FeO/Fe₂ SiO₄ not only forms a layer as usual but also penetrates into the outer Fe oxide layer deeply.     

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.     

The oxidation behavior of some Ni-Cr-Al alloys at high temperatures

Oxidation of ternary Ni-Cr-Al alloys containing different Cr/Al ratios has been studied in the temperature range 800° to 1300°C. Most of the studies were performed in 1 atm oxygen or air, but the oxygen pressure dependence for one of the alloys was also investigated. The experimental methods included thermogravimetric measurements of oxidation rates and studies on reacted specimens by means of X-ray diffraction, metallographic techniques, electron microprobe analysis, and electron microscopy. In general, the oxidation rates decrease faster with time than that for an ideal parabolic behavior. The major reaction products were NiO, Cr₂O₃,α-Al₂O₃, and Ni(Cr,Al)₂O₄. The relative amounts of these were a function of composition, temperature, oxygen pressure, and reaction time. The Ni-9Cr-6Al alloy has the best oxidation resistance due to the formation ofα-Al₂O₃ at all temperatures investigated. The oxidation mechanism of the alloy is discussed.