Microstructure and mechanical behaviour of Nb–Al–V alloys with 10–25 at.% Al and 20–40 at.% V—I: microstructural observations

The microstructures of three Nb–Al–V alloys with nominal compositions Nb–10Al–20V, Nb–15Al–20V and Nb–25Al–40V (in at.%) have been investigated. It is shown that the alloys each exhibit an A2 or B2 matrix and often contain A15 and/or σ phase precipitates depending on thermal history. Both the A15 and σ phase precipitates exhibit two different well-defined orientation relationships and for the former these correspond to minimisation of elastic strain energy. ALCHEMI data from the B2 phase indicate that this is more stable for higher Al concentrations, and this is consistent with measurements of A2/B2 order-disorder transformation temperatures. In the alloy Nb–15Al–20V, the precipitation of the A15 phase in a supersaturated B2 matrix is preceded by the separation of the B2 phase into Al-rich domains in an Al-lean matrix.     

Selective dissolution of nanolamellar Ti–41 at.% Al alloy single crystals

The selective dissolution behavior of Ti–41 at.% Al single crystals having a α₂/γ-lamellar structure with a lamellar thickness in the range 20 nm to 1 μm have been investigated in 0.5 M NaCl aqueous solution focusing on the effect of lamellar thickness on the dissolution of γ-lamellae. In the case where γ-lamellae were thicker than ～100 nm on average, γ-lamellae were selectively dissolved and, as a result, crevasses whose widths were close to the γ-lamellae thicknesses were formed. However, not all γ-lamellae were dissolved and the distribution of crevasses was much less uniform compared with that of the γ-lamellae. On the other hand, when the average γ-lamellae thickness was <100 nm relatively thick γ-lamellae were preferentially dissolved, but the distribution of the crevasses was relatively uniform compared with that obtained from the coarse lamellar structure. The reasons for this difference are discussed from the viewpoint of the absence of misfit dislocations in nanoscaled lamellae and the difference in ion transport through crevasses of different width formed by dissolution of γ-lamellae.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Phases and evolution of microstructures in Ti–60 at.% Al

The formation and stability of Al-rich Ti–Al phases is reviewed and the kinetics of the phase transformations and evolution of lamellar TiAl + r-TiAl₂ microstructures is discussed. For this a couple of Ti–60 at.% Al alloys were processed by different techniques to generate different initial microstructures. The kinetics were studied by annealing the differently processed alloys for 1, 10, 100 and 1000 h at temperatures between 800 and 1000 °C and then analysing the quenched microstructures by optical, scanning electron, and transmission electron microscopy. In addition, in situ heating and cooling experiments using differential thermal analysis and transmission electron microscopy were performed to verify the results obtained for the quenched samples. The results conclusively show why the metastable phases h-TiAl₂ and Ti₃Al₅ form. The stability and transformation of the metastable phases have been determined in dependence on time and temperature and the kinetics of the two different mechanisms by which the stable phase r-TiAl₂ forms have been established. The effects of differing initial microstructures on the evolution of the microstructure with time and temperature are discussed.     

Microstructure and mechanical behaviour of Nb–Al–V alloys with 10–25 at.% Al and 20–40 at.% V—II: mechanical behaviour and deformation mechanisms

The mechanical behaviour of three Nb–Al–V alloys with nominal compositions Nb–10Al–20V, Nb–15Al–20V and Nb–25Al–40V (in at.%) have been investigated. Both conventional constant strain rate deformation and compressive creep tests have been performed and the deformation microstructures have been examined by transmission electron microscopy (TEM). At room temperature all three alloys deform by planar slip, with dislocation/particle interactions giving significant strengthening for the two phase alloys. Deformation at higher temperatures occurs by a combination of dislocation glide and climb processes, giving more homogeneous microstructures. All of the dislocations in the B2 phase of these alloys are uncoupled superpartial dislocations with b=1/2<111>. The influence of dislocation/domain boundary interactions on the formation of slip bands and uncoupled superpartials is discussed.     

Effect of Nb on the high temperature oxidation of Ti–(0–50 at.%)Al

The oxidation behavior of Ti–Nb, Ti₃Al–Nb and TiAl–Nb (Nb: 0–30 at.%) has been investigated at 1173 K in air. When Nb is in solid solution with TiO₂, the addition of Nb can improve the oxidation resistance of the alloys by impeding mass transfer in TiO₂. However, Nb decreases the oxidation resistance when the amount of Nb is too high and forms TiNb₂O₇ or AlNbO₄ phases in the scale.     

Oxidation behaviour of Fe–40 at.-% Al intermetallics with Li or Cu additions at high temperature

Fe–40?at.-% Al intermetallics with the addition of Li (1 and 3?at.-%) and Cu (3 and 5?at.-%) were produced using standard casting techniques. The oxidation behaviour was obtained using thermogravimetric analyses in an atmosphere of 99.99% oxygen at 800, 900, 1000, and 1100°C. The oxidation product layers on the top and cross-sectional views were characterised using SEM and energy-dispersive X-ray spectroscopy mapping. The different alumina phases can produce a double layer with different mechanical and chemical properties. The results showed that the addition of 1% Li assisted the activation of the oxide production at 1000 and 1100°C, while the rest of the third element additions slightly modified the oxidation resistance.     

X-ray study of the short-range order structure in Cu–24.3 at.% Mn alloy

The diffuse X-ray scattering from a single crystal of Cu–24.3 at.% Mn aged within the fcc phase at 770 K was measured at room temperature. Diffuse peaks were found at 2n₁+0.2, 2n₂+0.2, 0 and equivalent positions (n₁ and n₂ are integers) in reciprocal space. The Warren–Cowley short-range order parameters were derived from the separated short-range order scattering and used to generate the corresponding local atomic arrangements on a computer. The generated arrangements indicated that Mn atoms form clusters. A set of effective pair interaction energies obtained using an inverse Monte Carlo method showed that the interaction between the first nearest neighbors mainly affects the clustering of like atoms.     

Influence of Al Addition on Oxidation Behavior of Cu–Ni–Cr Alloys at 973–1073 K in 1.01 × 10<Superscript>2</Superscript> kPa Pure Oxygen

The oxidation behavior of Cu–20Ni–15Cr–2.5Al and Cu–20Ni–20Cr–2.5Al alloys was studied at 973–1073 K in 1.01 × 10² kPa pure oxygen. The oxidation kinetics exhibited large deviations from the parabolic rate law and were comprised of three or four quasi-parabolic stages. Oxidation rates of the present alloys were much lower than those previously reported for a Cu–20Ni–20Cr alloy. Cu–20Ni–15Cr–2.5Al alloy formed a continuous scale of chromia in contact with the alloy, while at other locations, the scale formed deep protrusions into alloy along β phases. Cu–20Ni–20Cr–2.5Al alloy formed a continuous scale of chromia with a small quantity of light and unoxidized precipitates of α phase, especially at 1073 K. There was a thin layer depleted in Cr beneath the continuous scales of chromia. The addition of 2.5 at.% Al to Cu–Ni–Cr alloy made the diffusion of reactive component Cr become much faster and facilitated the formation of a continuous external scale of chromia for a lower Cr content.     

Anomalies of the plastic yield stress in the intermetallic compound Fe–30 at.% Al

The plastic properties of Fe–30 at.% Al were investigated in compression, cyclic tension/compression and shear tests between room temperature and 870 K. At elevated temperatures in all tests a positive temperature dependence of the yield stress was observed. At room temperature an asymmetry of the flow stress with respect to the deformation direction (tension or compression) was found, which disappeared for temperatures T⩾570 K. The results are discussed on the basis of the present state of knowledge about the decoupling of D0₃-superdislocations at elevated temperatures and the deformation behaviour of b.c.c. metals at low temperatures.     

Miscibility gap of B2 phase in NiAl to Cu3Al section of the Cu–Al–Ni system

The phase separation in the bcc phase of the Cu–Al–Ni system at 600–700 °C was investigated mainly by energy dispersion X-ray spectrometry (EDS) and differential scanning calorimetry (DSC). The compositions of the β₁ (A2 or B2: Cu-rich), β₂ (B2: NiAl-rich) and γ (γ-brass type) phases in equilibrium were determined. It was found that there is a β₁+β₂ miscibility gap in the β phase region as previously reported by Alexander. It was confirmed by means of high temperature in situ TEM observation that this miscibility gap consists of the B2+B2 phases but not the A2+B2 phases which is sometimes observed in many other Ni–Al and Co–Al base ternary bcc alloys. Thermodynamic calculation was performed which indicates that this characteristic feature suggests that the β₁ (B2)+β₂ (B2) miscibility gap is a part of a Cu-rich B2+NiAl-rich B2 miscibility gap island formed around the center of the composition triangle of the isothermal section. The phase separation in the β phase region and the stability of the ordered bcc aluminide are presented and discussed.     

Oxidation of Two Ternary Cu–Ni–20 wt.% Cr Alloys at 700–800°C in 1 atm O2

Two ternary Cu–Ni–Cr alloys containing approximately 20 wt.% chromium, but with a different Cu and Ni content, have been oxidized in 1 atm of pure oxygen at 700–800°C. The alloy containing about 60 wt.% nickel (Cu–60Ni–20Cr) was composed of a single solid-solution phase and formed external scales of chromium ocide with an outermost layer containing a mixture of copper and nickel oxides. The alloy comprised of about 40 wt.% nickel (Cu–40Ni–20Cr) contained a mixture of two metal phases and formed complex external scales, containing copper oxide and a nickel–chromium spinel plus a region where islands of the metallic phase richer in chromium surrounded by a thin chromia layer were mixed with oxidized islands rich in copper and nickel, producing a situation out of equilibrium. With time, a very irregular and thin but essentially continuous layer of chromia formed at the base of the mixed internal region for this alloy, producing a gradual decrease of the corrosion rate down to very low values. The oxidation behavior of the two alloys is interpreted in terms of their different microstructure. In particular, the fast initial oxidation of Cu–40Ni–20Cr, associated with the formation of large amounts of copper oxides, is attributed to restrictions in chromium diffusion in the alloy due to the simultaneous presence of two metal phases.     

Formation of nanoquasicrystalline Al–Cu–Fe coatings at electron beam physical vapour deposition

Coatings with a quasicrystalline Al–Cu–Fe structure were formed during high rate (∼100 nm/s) electron beam deposition of the vapour phase at substrate temperatures in a range of 100–800 °C. Features of the structure and some mechanical properties of quasicrystalline coatings obtained at the reducing of substrate temperature were studied.     

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H₂–HCl–H₂S–CO₂ gas mixture providing 3.7 × 10⁻²² atm O₂, 2.4 × 10⁻¹⁴ atm Cl₂ and 3.9 × 10⁻⁹ atm S₂. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H₂–CO₂ mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Phase decomposition of the γ phase in a Mn–30 at.% Cu alloy during aging

The phase decomposition process of γ phase in a Mn–30 at.% Cu alloy, when aged at 723 K from 2 to 50 h, is investigated with electrical resistivity and magnetic susceptibility measurement. In conjunction with the antiferro-magnetic transition of the Mn-rich regions during cooling to room temperature from the aging temperature, the temperature coefficient of electrical resistivity shows a continuous increase in a certain temperature range. The temperature where the coefficient has the maximum increasing rate is defined as the T_N temperature of the Mn-rich regions. It was found that the T_N temperature was 20–30 K higher than the concomitant f.c.c.–f.c.t. transformation temperature T_t, determined with the minima of Young’s modulus in the aged samples. The increment of temperature coefficient of electrical resistivity involved in the magnetic transition is used to estimate the changes of volume fraction for Mn-rich regions vs aging time. At the same time, the paramagnetic feature above the spin-freezing transition temperature for quenched Cu-rich alloys is summarized, and the Mn concentration in Cu-rich regions of aged samples is calculated. It should be noted that Mn and Cu-rich regions had already formed in the 2 h-aged Mn–30 at.% Cu sample, and longer aging further enriched Mn or Cu, while the volume fraction of Mn-rich regions decreased slightly with aging time. Electrical resistivity measurement sensitive to Mn-rich regions and the magnetic susceptibility measurement for Cu-rich regions have shown the compositional heterogeneity in decomposed phases. TEM observation confirms the interconnectivity of the two regions in the aged microstructure. All the results support the hypothesis that the decomposition of γ phase in Mn–Cu alloys proceeds in the spinodal manner.     

Internal Oxidation of an Ag–1.3 at.% Te Alloy

The internal oxidation of Ag–1.3 at.% Te was studied at 750, 800, and 830°C in pure oxygen (1 atm). The internal oxidation under such high oxygen pressure resulted in formation of two different types of oxide particles and two different fronts of internal oxidation in the internal oxidation zone. The coarser Ag₂TeO₃ particles were formed through the in situ internal oxidation of Ag₂Te particles and the tiny oxide precipitates (most probably also Ag₂TeO₃) were formed through internal oxidation of tellurium from solid solution. Considering the mechanism of internal oxidation, both diffusionless and diffusive modes were found to be present simultaneously in the oxidation of Ag–1.3 at.% Te alloy. These results were examined with regard to the solubility of tellurium in silver, which was found to be 0.1 at.% Te at 750°C and 0.26 at.% Te at 830°C, as well as the presence and dissolution of Ag₂Te particles.     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

Microstructures and mechanical properties of Ti–24Al–17Nb (at.%) laser beam welding joints

Laser beam welding of Ti–24Al–17Nb (at.%) alloy was conducted to investigate the microstructures and the mechanical properties of its joints. The results indicated that the weld microstructure consisted primarily of retained ordered β phase (namely B2 phase) and was independent of the laser welding parameters, while the size and the orientation of the weld solidification structures and then the bend ductility of the joints were related to the welding conditions. The microstructures became coarser and the strains of inducing crack and fracturing decreased as the heat input increased. The fracture occurred in the base metal when the transverse tensile test of the joints was conducted. The tensile strength of the joints was equal to that of the base material and the tensile ductility could reach 12∼17%, which was near to that of the base material.