A. V. Samelyuk

Microstructure, Growth Kinetics, and Abrasive Wear Resistance of Boride Layers on Fe–30% Cr Alloy

Two boride layers are found to form at the interface between reacting phases in the course of boriding a Fe–30% Cr alloy in boron powder with KBF4 (activator) in the temperature range of 850–950°C and reaction times 3600–43200 sec (1–12 h). Each of these layers is single-phase structurally (crystallographically) and two-phase compositionally (chemically). The outer boride layer bordering boron consists of the crystals of the (Fe, Cr)B and (Cr, Fe)B compounds, while the inner layer adjacent to the alloy base comprises the crystals of the (Fe, Cr)2B and (Cr, Fe)2B compounds. The characteristic feature of both layers is a profound texture. Diffusional layer-growth kinetics are close to parabolic and can alternatively be described by a system of two non-linear differential equations dx/dt = (kB/x) – (rgkFe/py), dy/dt = (kFe/y) – (qkB/sgx), where x is the outer FeB layer thickness (m), y is the inner Fe2B layer thickness (m), kB is the FeB layer growth-rate constant (m2⋅sec–1), kFe is the Fe2B layer growth-rate constant (m2⋅sec–1), g is the ratio of the FeB and Fe2B molar volumes, p = q = r = 1, and s = 2 (factors from the chemical formulae of FeB and Fe2B). The temperature dependence of the layer growth-rate constants obeys a relation of the Arrhenius type K = Aexp (–E/RT), where K stands for any constant, A is the frequency factor, E is the activation energy, R is the gas constant, and T is the absolute temperature. Application of the least-squares fit method yielded the following equations: kB = 3.42⋅10–8⋅exp(–175.4 kJ × × mol–1/RT) m+⋅sec–1, kFe = 7.45⋅10–9 exp(–144.6 kJ⋅mol–1/RT) m2⋅sec–1. Microhardness values are 18.1 GPa for the outer boride layer, 15.2 GPa for the inner layer, and 1.75 GPa for the alloy base. The dry abrasive wear resistance of the outer boride layer, found from mass loss measurements, is more than 300 times greater than that of the Fe–30% Cr alloy base. Such a huge increase in wear resistance is due to the microstructure of boride layers having a peculiar regular arrangement of enhanced rigidity.

Effect of Microwave Heating on the Mass Transfer, Phase Formation, and Microstructural Transformations in the Y2O3–Al2O3 Diffusion Couple

The phase composition, phase growth kinetics, and structures of diffusion zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer Al2O3–Y2O3 samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusion annealing was conducted at 1700°C for 5 h in vacuum, the heating rate being 10°C/min in all experiments. The diffusion couples included alumina layers, such as coarse-grained polycore or sintered Al2O3–5 vol.% ZrO2 layers, and yttria layers, such as sintered coarse-grained samples or fine Y2O3 powder layers on the Al2O3 surface. It is shown that the phases formed during reactive diffusion do not uniquely correspond to the phase diagram, but depend on the initial structure of contacting layers and the type of heating. This is attributed to the contribution of kinetic factors to the competitive phase growth, particularly to the structural sensitiveness of diffusion coefficients whereby the diffusive phases grow and the stresses appearing when new phases form. It is found that MWH influences the competitive phase growth in the Al2O3–Y2O3 system, which involves both the change in the phase composition of the diffusion zone compared to that formed under CH and the acceleration of phase growth. The maximum effect of the phase growth acceleration under MWH is observed for the YAG phase, which is 30 times as fast as that under CH. It is suggested that the structure of grain boundaries changes and, accordingly, their permeability increases under MWH. The accelerated GB diffusion under MWH promotes the YAG phase growth in both oxides as a result of opposite diffusion flows of Al and Y ions along GBs. Under TH the YAG phase is formed only in Y2O3 oxide because of the unipolar diffusion of Al3+ ions to Y2O3. The validity of the proposed mechanism is confirmed by numerical evaluations.

Influence of Heat Treatment on High-Temperature Mechanical Properties of Ti-5Dy-5Si-Sn Alloys

By the methods of X-ray diffraction, metallography, TEM, SEM and mechanical testing for compression, the influence of heat-treatment condition on the structure and properties of Ti-bL)y-5Si-Sn alloys was studied. High dispersity and stability of the microstructure were observed, predetermining a high level of mechanical properties. Annealing by the regime 1350°C, 31.5 h + 1250°C, 32 h + 1200°C, 32 h + 1100°C, 32 h was shown to provide optimum combination of high-temperature strength and roomtemperature plasticity.

Constitution of Alloys and Phase Diagram of the Hf–Ru–Rh System. IV. Liquidus Surface and Melting Diagram of the Partial Ru–HfRu–HfRh–Rh System

Using the data obtained in study of the as-cast Hf–Ru–Rh alloys in the range 0–50 at.% Hf and considering the solidus surface constitution, physicochemical analysis techniques are employed to construct for the first time the liquidus surface of the Ru–HfRu–HfRh–Rh partial system onto the composition triangle, the melting diagram, and the reaction scheme reflecting processes during crystallization of the alloys. It is shown that five surfaces of primary crystallization of a continuous series of solid solutions between isostructural (CsCl-type) phases formed by compound HfRu and its high-temperature modification (δ phase), ruthenium and rhodium solid solutions, as well as the ε phase based on compound HfRh3 (AuCu3-type structure) and θ phase based on compound Hf3Rh5 (Hf3Rh5-type structure) are parts of the liquidus surface. Three invariant four-phase processes involving liquid take place at 1900, 1810, and 1720°C.

Alloy Constitution and Phase Equilibria in the Hf–Ru–Rh System. II. Liquidus Surface, Melting Diagram, and Vertical Sections of the Partial Hf–HfRu–HfRh System

According to the constitution of the solidus surface in the Hf–Ru–Rh system over the composition range 50−100 at.% Hf and the data obtained in studying the as-cast alloys by physicochemical analysis techniques, we constructed, for the first time, the liquidus surface of the partial Hf–HfRu–HfRh system on the composition triangle and its melting diagram. The vertical sections at 5 at.% Ru, 10 at.% Rh, and 75 and 80 at.% Hf at the ratio Ru : Rh = 1 : 1 are presented. The processes that occur when the alloys are crystallized are shown in the reaction scheme. The primary crystallization regions for a continuous series of solid solutions between isostructural (CsCl type) phases formed by the HfRu compound and its high-temperature modification (δ phase) as well as β-Hf and γ-Hf2Rh (Ti2Ni type) solid solutions are parts of the liquidus surface. An invariant four-phase equilibrium involving liquid, LU + δ ⇆ γ + , is observed at 1373°C in the system.

Phase Equilibria During Solidification in the Ti–TiAl–DyAl2–Dy Region of the Ti–Dy–Al System

Phase equilibria during solidification in the Ti–TiAl–DyAl2–Dy region of the Ti–Dy–Al system are studied by differential thermal analysis, X-ray diffraction, metallography, and electron microprobe analysis. The liquidus and solidus surfaces, vertical sections, and reaction scheme in the solidification range are presented. No ternary compounds are found in the studied composition range. It is shown that DyAl2 undergoes polymorphic transformation at ~1200°C. The αl and α2 phases that coexist only with solid phases in the binary Ti–Al system participate in equilibria with the liquid phase in the ternary Ti–Dy–Al system. The liquidus surface is characterized by the primary solidification fields of the phases based on βTi (β), high-temperature αTi (αh), lowtemperature αTi (αl), Ti3Al (α2), TiAl (γ), Dy2Al, Dy3Al2, DyAl, βDyAl2, αDyAl2, βDy, and αDy. The solidus surface has elven three-phase fields: β + (βDyAl2) + αl, (βDyAl2) + αl + α2, β + αh + (βDyAl2), αh + γ + (βDyAl2), (βDyAl2) + α2 + (αDyAl2), (DyAl) + (Dy3Al2) + (αDyAl2), (αDy) + β + αl, (DyAl2) + α2 + (Dy3Al2), (Dy3Al2) + α2 + (Dy2Al), α2 + αl + (Dy2Al), and αl + (αDy) + (Dy2Al). The first two fields result from invariant four-phase peritectic reactions, LP1 + β + (DyAl2) ⇄ αl and LP2 + αl + (βDyAl2) ⇄ α2 proceeding at 1130 ± 5°C and 1180 ± 7°C, respectively. The next eight three-phase fields result from invariant four-phase transition reactions: LU1 + β ⇄ αh + (βDyAl2) at 1325 ± 8°C, LU2 + αh ⇄ γ + (βDyAl2) at 1260°C, LU3 + (βDyAl2) ⇄ α2 + (αDyAl2) at 1060 ± 4°C, LU4 + (DyAl) ⇄ (Dy3Al2) + (αDyAl2) at 1010 ± 9°C, LU5 + (αDy) ⇄ β + αl at 970 ± 4°C, LU6 + (αDyAl2) ⇄ α2 + (Dy3Al2) at 960 ± 8°C, LU7 + (Dy3Al2) ⇄ α2 + (Dy2Al) at 955 ± 16°C, and LU8 + α2 ⇄ αl + (Dy2Al) at ~930°C. The three-phase αl + (αDy) + (Dy2Al) field results from an invariant eutectic process, LE ⇄ αl + (αDy) + (Dy2Al), at 910 ± 15°C. The two-phase region in the solidus surface has a temperature maximum at 1343 ± 5°C, corresponding to the invariant three-phase le1 ⇄ β + (βDyAl2) reaction.

Interdiffusion and Structural Changes in the Cr2O2–Al2O3(ZrO2) Diffusion Couple under Microwave Heating

The interdiffusion and microstructural evolution of the Cr2O3–Al2O3 (5 vol.% ZrO2) diffusion couple are studied in the temperature range 1600–1800°C under microwave heating (24 Hz) and, for comparison, under traditional heating using electron microprobe analysis and microscopic analysis. It is found that the concentration of chromium is distributed differently in Al2O3 in diffusion zones under microwave and traditional heating. This is due to greater contribution of grain-boundary diffusion to the effective diffusion flux under microwave heating. Bulk diffusion and average grain-boundary diffusion coefficients are calculated. The grain size in the diffusion zone toward Al2O3 is smaller after microwave heating. Traditional heating induces grain growth by recrystallization, whereas two processes, recrystallization and polygonization, are superimposed during microwave heating. The polygonization is due to the generation of dislocations under thermal stresses originating from nonuniform temperature distribution in the diffusion zone with variable concentrations of the components. The calculated bulk and grain-boundary diffusion coefficients can be used to predict the kinetics of various diffusion mass-transfer processes in Al2O3 and Cr2O3 oxides and their mixtures.

Features of High-Temperature Oxidation of High-Entropy AlCrFe3CoNiCu Alloy

The paper examines how the scale forms on the AlCrFe3CoNiCu alloy when oxidized at 900°C for 50 h and how high-temperature annealing influences the structure and phase transformations and mechanical properties of the alloy matrix. It is found that long-term annealing at 900°C leads to the formation of a three-phase alloy consisting of a solid solution with a BCC lattice of B2 structural type and two solid solutions with FCC crystal lattices (one contains a higher amount of Ni and Co, and the other is a solid solution enriched with 66 wt.% Cu). A two-phase scale containing Al2O3 and CuO forms on the alloy in the oxidation process. Indentation method has shown that the mechanical properties of the AlCrFe3CoNiCu alloy remain stable after long-term high-temperature annealing.

Electrochemical Corrosion Behavior of Air-Exposed Zr–Mn–Cr–Ni–V Alloy

Scanning electron microscopy and electron microprobe analysis show that the ZrMnCrNiV alloy has a dendritic structure and is chemically inhomogeneous. The corrosion mechanism for the unexposed alloy and the alloy exposed in air for 7 and 15 days, followed by aging in a 30% KOH solution, is the same: corrosion originates at the interphase boundary and propagates along it, which is typical of pitting corrosion. If the alloy is preliminary exposed in air, its surface has a greater number of pittings, but all of them are smaller in area and depth, making the corrosion process more uniform. In hydrogenation–dehydrogenation of this alloy, even more uniform distribution of smaller corrosion areas is observed. Studies of the corrosion resistance of this alloy in a KOH solution carried out by atomic adsorption spectrometry show that the alloy powder exposed in air has higher corrosion resistance compared to the unexposed powder. Electrochemical corrosion studies of the alloy conducted in the anodic region using the method of polarization curves indicate that the corrosion rate for the unexposed and exposed alloys is controlled by the rate at which passivating films form. The most extensive passivation region is observed in the alloy exposed in air for 15 days. It shows adequate corrosion resistance in a 30% KOH solution. The cyclic resistance studies for the electrodes produced from the alloy powder exposed for 10 days, at a discharge to potential difference E = –1.0 V and E = –0.8 V, demonstrate that oxidation in the hydrogenation-dehydrogenation process affects the cyclic resistance. It is found that there is liming time for exposing the alloy in air (as an ingot and/or powder) after which the cyclic resistance deteriorates.

Mechanism for Improving the Mechanical Properties of Sintered Iron–Copper Composites Alloyed with Molybdenum

The structure of Fe–Cu composites after solid-phase and liquid-phase sintering was studied. It is shown that 2–10 wt.% molybdenum additions have an activating effect on the diffusion processes in densification, grain growth, and recrystallization, as well as on the amount and composition of copper and iron solid solutions. Molybdenum additions to 70 wt.% Fe–30 wt.% Cu composites simultaneously influence their strength and ductility properties. With increasing molybdenum content, the solubility of iron in copper decreases, promoting higher ductility of the composites, and the solid solutions of copper and molybdenum in iron preserve their strength characteristics. Solidphase sintering results in fine-grained FeCuMo samples with high relative density (up to 98.8%) and high ductility.