Optical Investigation of the Effects of Substrate Orientation on Oxidation of Single Crystal β-NiAl

The residual stresses in -Al₂O₃ scales formed on high-purity FeCrAl and FeCrAlY during oxidation at 1200°C and subsequent cooling and reheating were studied using photostimulated luminescence spectroscopy. Systematic measurements of the stress in the oxide were made as a function of both alloy thickness and cooling rate and the development of stresses in the oxide scale during thermal cycling was investigated as a function of the temperature. Depending on the ratio of the alloy and oxide-scale thicknesses, substantial stress relaxation was observed to occur during cooling at a rate of 8 K/min. Compressive growth stresses of 1 GPa were determined by comparing the residual-stress measurements at room temperature with those calculated based on thermal-expansion mismatch. No significant differences in the growth stresses, the total residual stresses, or the stress relaxation during thermal cycling were observed between the oxides formed on the Y-doped and undoped FeCrAl samples. Stress relaxation is also shown to lead to the development of tensile stresses in the oxide scale under certain conditions, in particular, those associated with slow cooling followed by rapid heating.     

The effect of Y-ion implantation on the oxidation of β-NiAl

The influence of Y-ion implantation on the oxidation of -NiAl single crystals has been investigated using SEM, TEM, and STEM. Y ions having an energy of 70 keV were implanted with a concentration of 5×10¹⁶ ions/cm². The oxidation experiments were performed in air at 1223 K. Y-ion implantation resulted in a 45-nm disordered layer. Oxidation of Y-implanted -NiAl leads to the formation of a fine-grain layer, consisting of -Al₂O₃ containing Y and a -Al₂O₃ layer. After further oxidation the metastable Al₂O₃ transformed into -Al₂O₃, which started at the metal-oxide interface. Y-Al-garnet (YAG) particles were observed and Y segregation to -Al₂O₃ grain boundaries has been detected.     

Effects of Ti and La Additions on the Microstructures and Mechanical Properties of B-Refined and Sr-Modified Al–11Si Alloys

The effects of Ti and La additions on the microstructures and mechanical properties of B-refined and Sr-modified Al–11Si alloys were investigated in the present work. The interactions among Ti, La, B and Sr elements were discussed employing microstructure observation, thermal analysis and tensile test, respectively. It was found that the addition of 0.05 wt% B induces a transformation of eutectic Si from finely fibrous to coarsely plate-like morphology in the Al–11Si alloy with 0.02 wt%Sr modification, owing to the poisoning of IIT mechanism, and the eutectic Si grows only with TPRE mechanism. Both titanium and lanthanum can neutralize the co-poisoning effect between Sr and B in the Al–11Si alloy, but the neutralizing effect of La is dependent on the addition sequence. The combinative addition of La and B elements promotes the effective refinement of α-Al grains, but an inhomogeneous modification of eutectic Si phases is also observed, leading to a slightly decrease in the elongation.     

Effect of Sub-T_g Annealing on the Corrosion Resistance of the Cu–Zr Amorphous Alloys

Despite the economy of material cost and excellent toughness of Cu-based amorphous alloys, especially Cu_(50)Zr_(50), their poor corrosion resistance to a chloride medium limits their widespread applications. In this study, corrosion tests were performed on the Cu_(50)Zr_(50) amorphous alloy with different degrees of short-range order, which were prepared by annealing below the glass transition temperature(T_g). It was found that the corrosion resistance of amorphous alloys is improved to a significant level when the alloys were heated below T_g. Calorimetric studies showed that thermally activated relaxation process of created disorder, which occurs during sub-T_gannealing, is responsible for the improvement in the corrosion resistance. Molecular dynamics simulations performed on the Cu–Zr amorphous alloys demonstrated that the relaxation process of the alloys is associated with the formation of energetically stable icosahedra and icosahedron-like structures. Our study highlights the effects of sub-T_gannealing on the improvement in the corrosion resistance of the amorphous alloys from the viewpoint the relaxation process of the short-range orders.     

Effects of Chromium on the Oxidation Performance of β-FeAlCr Coatings

-FeAl coatings containing various Cr contents of 6.5–45 wt.%were produced with a closed-field, unbalanced magnetron sputter (CFUMS)deposition technique. Cyclic oxidation tests at 1100°C in air for100 1-hr cycles and isothermal exposures at 1000°C in pure O₂ for100 hr were carried out with the coatings and an as-cast FeAlspecimen. All of the coatings showed good scale-spallation resistanceduring cyclic oxidation and the coating with 6.5 wt.% Cr exhibited thelowest oxidation rates in both cyclic and isothermal oxidationexposures. After oxidation, fine-grain ridge-type oxide scales formed onthe coatings, while the oxide scale formed on the cast FeAl showed alarge quantity of -Al₂O₃ blades and large interfacial voids on thebase–alloy surface. The transformation from to -Al₂O₃was accelerated due to the presence of Cr in the coatings. The fasttransformation considerably reduced oxidation rates, suppressed fastoutward Al diffusion for the growth of a -Al₂O₃ scale, and preventedthe formation of interfacial voids that played a major role in causing thescale spallation.     

The effect of platinum on defect formation energies in β-NiAl

First-principles density functional theory is used to examine the effect of Pt on point defects and defect clusters in NiAl. It is found that Pt promotes the formation of Ni and Al vacancies and Ni and Al antisite atoms. Defect clusters that are minima in postulated Ni diffusion mechanisms in NiAl are also found to be stabilized by the presence of Pt. By decreasing defect formation energies, Pt may decrease the overall activation barrier to the diffusion of Ni and Al in NiAl. The results provide clues as to how Pt enhances thermal barrier coating lifetime.     

Effect of Strontium and Magnesium Additions on the Microstructure and Mechanical Properties of Al–12Si Alloys

Metals and Materials International - In this study, a binary Al–12Si, eight ternary Al–12Si–Sr, and six quaternary Al–12Si–0.1Sr–(0.2–1)Mg alloys were...     

Impacts of Microalloying Elements on the Hardening β-Phase in Automotive AlMgSi Alloys

Microalloying elements play a crucial role in mechanical properties and phase stability of metallic alloys. In this work, we employ first-principles calculations and atomic-scale high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to find promising microalloying elements that will improve the stability and properties of β/Al interface and β phase in Al-Mg-Si alloys. First, we define a substitution energy for evaluating the stability of β phase and β/Al interface with microalloying elements doped. Then, experiments of HAADF-STEM imaging are carried out to verify the calculational results. Next, using the most stable structures doped with microalloying elements, the mechanical properties of the β bulk and the β/Al interface were calculated and analyzed. At last, we have figured out the effects of all considered microalloying elements and obtained a rule that the stable occupancy of solute atoms is related to their own radius and the radius of Mg, Si, and Al. These findings will provide some theoretical basis for future microalloying strategies of Al-Mg-Si alloys.     

Effect of theθ-α-Al2O3 transformation on the oxidation behavior ofβ-NiAl + Zr

Isothermal oxidation of NiAl + Zr has been performed over the temperature range of 800–1200°C and studied by TGA, XRD, and SEM. A discontinuous decrease in growth rate of two orders of magnitude was observed at 1000° C due to the formation of -Al₂O₃ from -Al₂O₃. This transformation also resulted in a dramatic change in the surface morphology of the scales, as a whisker topography was changed into a weblike network of oxide ridges and radial transformation cracks. It is believed that the ridges are evidence for a shortcircuit outward aluminum diffusion growth mechanism that has been documented in a number of¹⁸O tracer studies.     

Effect of Constituent Elements on the Corrosion Resistance of Single-Phase CoCrFeNi High-Entropy Alloys in NaCl SolutionEI北大核心CSCD

High entropy alloys (HEAs) origin from a new alloy design concept with multi-principal elements, which have attracted significant interests in the past decade. The high configurational entropy in HEAs results in simple solid solutions with fcc and bcc structures. Especially, the single solid solution CoCrFeNi alloy exhibits excellent properties in many aspects, such as mechanical properties, thermal stability, radiation resistance and corrosion resistance. The excellent corrosion resistance of CoCrFeNi alloy is ascribed to the single-phase structure and uniform element distribution coupled with much higher Cr content than stainless steel. The single-phase structure and uniform element distribution can prevent the occurrence of localized corrosion, and higher Cr content can protect the alloy surface better with the form of oxidation film. Moreover, the corrosion resistance of CoCrFeNi-based HEAs, such as CoCrFeNiAlx, CoCrFeNiCux, CoCrFeNiTix, have also been extensively investigated. In most CoCrFeNi-based HEAs, the elements of Co, Cr, Fe and Ni are with equal-atomic ratio. However, the equal-atomic ratio is not necessary to obtain satisfactory properties and to ensure the single fcc structure in Co-Cr-Fe-Ni system. Accordingly, it is essential to further consider the effect of alloying elements on the corrosion resistance in Co-Cr-Fe-Ni HEA. In this work, the effect of Co, Fe and Ni elements on the corrosion resistance of single fcc Co-Cr-Fe-Ni system with concentrated constitution but different atomic ratios in 3.5% NaCl solution are investigated by using LSCM and EIS. The potentiodynamic polarization results indicate that the increase of Fe and the decrease of Ni will decrease the passivation current density of the alloys when the Co and Cr contents are equal. With the increase of Co and the decrease of Ni, the alloys show smaller passivation current density and better corrosion resistance when the Fe and Cr contents are equal. With the decrease of Co and the increase of Fe and Ni, the alloys show higher corrosion potential and smaller corrosion tendency when the Cr content is constant. These results will be helpful for the design of corrosion resistant HEAs in NaCl aqueous solution.     

The effect of various oxide dispersions on the phase composition and morphology of Al2O3 scales grown on β-NiAl

A series of oxide-dispersed-NiAl alloys were oxidized in order to explore the effect of various cation dopants on the - phase transformation in the Al₂O₃ scale and the effect of phase composition on the scale microstructure. Larger ions such as Y, Zr, La, and Hf appeared to slow the- to-Al₂O₃ phase transformation, while a smaller ion, Ti, appeared to accelerate the transformation.     

Effect of Solution Heat Treatment in α+β Phase Region on Shape Memory Characteristics of Cu-Al-Ni-Mn-Ti Alloys

The effect of aluminium content and solution heat treatment in α+β phase region on the shape memorycharacteristics and mechanical properties of cold wrought Cu-Al-Ni-Mn-Ti alloy are studied in this paper. Re-sults indicate that the transformation temperature (Tt) of Cu-Al-Ni- Mn-Ti alloy reduces obviously with the in-crease of the amount of α-phase. During aging at 623K, T_l increases at first up to a peak value, then decreaseswith prolongation of aging time. Life time of heat resistance of the alloy at high temperatures is improved withincrease of the amount of α-phase, this life time becomes poor with Bainite precipitation. When the amount ofα-phase is less than 5% the ratio of shape recovery brought about by the solution heat treatment in α+β phaseregion is almost not effected. However, plasticity of the alloy increases obviously as aluminium content de-creases. We believe that improving cold workability of Cu-Al-Ni-Mn-Ti alloy and keeping good heat resistantproperty and shape memory effects are possible by means of reducing the content of aluminium and solulionheat treatment in α+β phase region.     

Peculiarities of a Unified Environmentally Clean System of Etching and Bright Pickling α- and α + β-Titanium Alloys

The kinetics of dissolution, hydrogenation, and bright pickling of titanium alloys in either still or stirred solutions, or under the effect of ultrasound is investigated. In solutions containing H₂O₂ or K₂S₂O₈ as an oxidizer, under ultrasonic treatment, the hydrogenation is practically absent due to stable positive electrode potential, while the dissolution rate can be increased by a factor of 2 to 2.5. A unified environmentally clean solution system for etching and bright pickling of 1-0, 4200, and T6 alloys is developed instead of nitric acid solutions.     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.     

Effect of Zr Addition on Resistance to High Temperature Softening and Resistivity of 6082 Al-Mg-Si Alloys

The influence of zirconium on the resistance to high temperature softening of 6082 Al-Mg-Si alloys has been researched. The softening process of 6082 alloys with Zr and without Zr, isothermally conditioned at 250℃, has been investigated. The results show the inclusion of Zr inhibits the decrease of HB in hardness(HB) compared with the alloys without Zr. This is due to the Zr and Al forming coherent dispersoid-Al3Zr, and Al3Zr particles within an Al matrix, which effectively restricts grain boundary movement. A study is also carried out to investigate the changes in resistivity, which occurs during the ageing of alloys containing Zr, compared to those without Zr. It is found that in both cases there is an initial rapid rise in resistivity followed by a slower rise in resistivity as ageing proceeds. The value of resistivity is lower for 6082 alloys with Zr compared with 6082 alloys without Zr; this is explained in terms of the role of Zr in the 6082 alloys.     

Effects of Platinum on the Hot Corrosion Behavior of Hf-Modified �á�-Ni3Al?+?��-Ni-Based Alloys

The establishment of a protective ??-Al₂O₃ scale is critical for providing high temperature protection from oxidation and hot corrosion, thereby improving lifetimes of advanced gas turbine engine components. Recent work by our group has shown that a wide range of Pt + Hf-modified ?á?-Ni₃Al + ??-Ni alloy compositions form a very adherent and slow-growing Al₂O₃ scale and exhibit excellent oxidation resistance. The main thrust of the present study was to understand the effects of Pt addition on the Type I (900 °C) and Type II (705 °C) hot corrosion (HC) behavior of model Hf-modified ?á? + ?? alloy compositions. The salt used to bring about hot corrosion was Na₂SO₄. It was found that the Type I HC resistance of ?á? + ?? alloys improved with up to about 10 at.% Pt addition, but then decreased significantly with increasing Pt content up to 30 at.% (the maximum level studied); however, under Type II HC conditions the resistance of ?á? + ?? alloys progressively improved with increasing Pt content up to 30 at.%. The effect of pre-oxidation on hot corrosion resistance was also examined, and the results indicated that pre-oxidation generally improved Type II HC resistance for the test duration studied.     

Effects of Cooling Rate and Component on the Microstructure and Mechanical Properties of Mg–Zn–Y Alloys

The microstructure and mechanical properties of Mg–6Zn–1Y and Mg–6Zn–3Y(wt%) alloys under different cooling rates were investigated. The results show that the second dendrite arm spacing(SDAS) of Mg–6Zn–1Y and Mg–6Zn–3Y is reduced by 32 and 30% with increasing cooling rates(Rc) from 10.2 to 23 K/s, which can be predicted using a empirical model of SDAS=68 R 0:45:45cand SDAS=73 R 0c, respectively. The compressive strength of both alloys increases with increasing the cooling rate, which is attributed to the increase of volume fraction(Vf) of secondary phases under high cooling rate. The interaction of the cooling rate and component with SDAS has been theoretically analyzed using interdependence theory.