Powder Metallurgy and Metal Ceramics (v.49, #7-8)

Physical nature of the temperature dependence of yield stress by Yu. V. Mil’man; V. I. Trefilov (374-385).
Below is one of the most significant works by V. I. Trefilov and Yu. V. Mil’man. The idea that the temperature dependence of the yield stress is determined by the thermally activated escape of potential barriers by dislocations is used to derive an equation that includes, as a particular case, the Seeger equation that describes the linear dependence of the yield stress on low temperatures and the Haasen equation that represents the exponential temperature dependence of the yield stress. The equation derived was later used to describe brittle-ductile transition and introduce the so-called deformation temperature at which the yield stress begins to strongly depend on temperature and strain rate and the ductile and brittle ranges are separated. These ideas were widely used to examine the deformation and fracture mechanisms for a range of promising materials with contained plasticity (refractory metals, ceramic materials, covalent crystals, quasicrystals, sintered materials).
Keywords: yield stress; potential barriers; dislocations; rate of thermally activated process; effective stress

Trefilov and strength physics today by S. A. Firstov (386-396).
The paper discusses the main topics of the plenary report given at the 49th International Conference “Relevant Problems of Strength” (Kiev, June 14–17, 2010) dedicated to the 80th anniversary of Academician Trefilov’s birth. Special attention is paid to strengthening in transition from micro- to nanosizes. It is shown that S-functions can be used to obtain a generalized equation for the dependence of the yield strength on grain size with regard to changes in mechanisms of strengthening in polycrystals in the vicinity of grains with critical sizes dcr1 and dcr2. This equation can describe softening in transition to nanostructures (inverse Hall–Petch relation) and abrupt strengthening under increased intergranular cohesion.
Keywords: strength; nanostructured state; strengthening mechanism; polycrystals; critical size

The paper examines the effect of chemical and structural inhomogeneity formed in the rapid crystallization of aluminum powders on the structure and mechanical properties of alloys produced from these powders. The powders are obtained by high-pressure water atomization. High-strength and high-temperature Al–Zn–Mg, Al–Zn–Mg–Cu, Al–Fe–Cr, Al–Cr–Zr, and Al–Fe–Ce alloys additionally doped with transition metals and scandium are studied. Auger electron spectroscopy, secondary ion mass spectroscopy, and scanning and transmission electron microscopy are used. To examine mechanical properties, tensile tests at room and elevated temperatures are used. The contribution of different doping elements to the segregation and formation of an oxide film on the powders and their effect on the phase composition and mechanical properties are studied. It is shown that the cooling rate determines characteristics of the alloys.
Keywords: aluminum alloys; water-atomized powders; rapid crystallization; surface segregation; oxide film; doping with transition metals

High-speed x-ray photography permits a quantitative assessment of the probability density α of stacking faults in austenite. The determination of α using high-speed x-ray photography is peculiar in that the same sample serves as the test and reference material during heating and cooling. Changes in the relative position of the (111) and (311) line angles rather than their absolute values are measured. The method permits a quantitative assessment of the probability density α of stacking faults in austenite during phase transformations under continuous heating and cooling of pure iron and steels 30 and U8 in annealed, deformed, and tempered states. The formation and disintegration of austenite in iron and steel involves broadening and relative shift of the (311) and (111) lines.
Keywords: austenite; iron; steel; probability density of stacking faults; high-speed x-ray photography; heating; cooling

Mechanical properties of porous Ti3SiC2/TiC, Ti3AlC2/TiC, and Ti4AlN3/TiN nanolaminates at 20 to 1300°C by S. A. Firstov; V. F. Gorban’; I. I. Ivanova; E. P. Pechkovskii (414-423).
The microindentation, macrohardness, and uniaxial compression methods are used to examine the effect of porosity (5–35%), content of the other phase (TiC or TiN, 5–70 vol.%), and loading temperature (20–1300°C) on the mechanical properties of Ti3SiC2/TiC, Ti3AlC2/TiC, and Ti4AlN3/TiN nanolaminate composites produced by reaction sintering of powder mixtures. A comparative analysis of the mechanical properties shows that the strength of the materials increases in the following sequence: Ti3AlC2/TiC, Ti4AlN3/TiN, and Ti3SiC2/TiC. Temperature–strain and force boundaries of their existence in the deformed state are established. Among all porous nanolaminate composites investigated, Ti3SiC2/TiC is the most optimal in respect to porosity, content of the other phase, and strength: 24% porosity and 30 vol.% titanium carbide.
Keywords: nanolaminate composites; porosity; second phase; strength; plasticity

Instrumented indentation for determining the structural state of materials by V. F. Gorban’; É. P. Pechkovskii (424-429).
Crystalline and noncrystalline materials with different compositions and phase and structural states are examined by the instrumented indentation method. Based on the previously obtained fundamental equation of indentation HIT/E* = 0.687ctgα(hs/hc), it is shown that the ratio of beyond-contact and contact indentation depths (hs/hc) characterizes the mechanical properties of a specific material under loading of this type. With the new technique for processing and analyzing the results from instrumented indentation, structural states of materials can be identified on small samples.
Keywords: indentation; hardness; elasticity; strength; structure

Structure and properties of Al–Mg alloys depending on scandium and zirconium additions and production methods by Yu. V. Mil’man; O. D. Neikov; A. I. Sirko; N. I. Danilenko; A. V. Samelyuk; N. P. Zakharova; A. I. Sharovskii; R. K. Ivashchenko; V. A. Goncharuk; N. G. Chaikina (430-437).
The paper examines how scandium and zirconium doping of Al–Mg alloys produced by conventional casting and powder metallurgy methods influences their structure and mechanical properties. It is shown that Al–Mg powder alloys are not recrystallized in the plastic deformation process and subsequent heating to 550°C. In cast alloys, recrystallization begins even in the extrusion process. Additions of scandium, zirconium, and chromium restrain the recrystallization of alloys when heated to 550°C. Doping of Al–Mg alloys with scandium and zirconium increases their strength by 80−120%. Doped powder alloys have much greater strength than cast alloys and quite high plasticity.
Keywords: aluminum alloys; casting; powder metallurgy methods; structure; mechanical properties; fine particles

Contact formation in iron powder compacts during electric sintering by S. A. Firstov; Yu. N. Podrezov; N. V. Minakov; V. A. Nazarenko; O. S. Koryak (438-445).
A high-speed x-ray diffraction real-time unit (XRD-RTU) is used for electric sintering of iron powder. Variation in the resistivity, temperature, and γ-line intensity is analyzed. The samples produced by electric sintering in different conditions are tested by bending. It is shown that physical and mechanical contacts form simultaneously after heating above the phase transition point.
Keywords: electric sintering; contact formation; iron powder; mechanical properties

Precipitation-hardened ferritic steels for fast neutron reactors by I. I. Ivanova; A. N. Demidik (446-453).
The paper describes precipitation-hardened ferritic steel developed for the core of fast neutron reactors. The effect of the type and amount of hardening oxide phase on the stress-rupture strength and structure of the material at 700°C is examined. The data are extrapolated to 10,000-hour holding to show a stress-rupture strength of 70 to 80 MPa. The radiation resistance of the steel is determined under ion and neutron irradiation to 100 dpa (neutron fluence 2.6 ⋅ 1023 n/cm2). The steel has high volume stability and low sensitivity of its mechanical characteristics to radiation. The material does not lose its plasticity after the maximum exposure. The corrosion behavior of the precipitation-hardened steel in Na, Li, Pb, and eutectic Pb–Li melts, Pb–Bi alloys, and Cs and Te vapors is investigated. The corrosion resistance of the steel is practically commensurable with or somewhat higher than that of the best reactor steels ÉP172 and ÉP450.
Keywords: precipitation-hardened steels; radiation-resistant materials; creep resistance; corrosion resistance in metal melts

Resistance of titanium– chromium and zirconium–chromium alloys to air oxidation by N. P. Brodnikovskii; I. V. Oryshich; N. E. Poryadchenko; T. L. Kuznetsova; N. D. Khmelyuk; E. A. Rokitskaya (454-459).
The influence of chromium (0.5-8 at.%) on the oxidation resistance of binary Ti and Zr alloys at 500–700°C and 50 h holding is investigated. It is shown that the oxidation of these alloys is described by a parabolic-law function at 500–600°C and a linear function at 700°C. It is established that 8 at.% Cr decreases the high-temperature oxidation resistance of titanium by 50–67%, but the chromium content up to 4 at.% hardly influences that of zirconium. Higher chromium concentrations also decrease the oxidation resistance. The results for titanium are attributed to specific chemical interaction of titanium and zirconium oxides and for zirconium to the structural factor associated with the formation of brittle ZrCr2 intermetallide.
Keywords: titanium; zirconium; chromium; alloys; oxidation; kinetics; scale

Effect of vanadium on hydrogenation of mechanically treated zirconium-containing alloys by Yu. M. Solonin; V. A. Lavrenko; O. Z. Galii (460-464).
Mechanical treatment of powders with additions of nickel to serve as a catalyst of the chemical adsorption of hydrogen is used to activate the hydrogenation of zirconium-containing alloys. Polarization curves are plotted to study the kinetics of electrochemical release and adsorption of hydrogen on zirconium alloys in which aluminum is replaced by vanadium. It is shown that the replacement is efficient and the maximum hydrogen capacity depends on the optimum concentration of the catalyst in the alloy.
Keywords: zirconium alloy; catalyst; vanadium; aluminum; hydrogenation

Gradient nanoporous structures based on silicon carbide and boron nitride in self-propagating high-temperature synthesis by V. I. Uvarov; I. P. Borovinskaya; V. V. Zakorzhevskii; I. G. Malevannaya (465-467).
The paper examines high-temperature corrosion-resistant strong nanoporous structures based on silicon carbide and boron nitride and effective membranes produced from them. The samples made of silicon carbide and boron nitride have the following characteristics: 40–48% and 38% open porosity; 0.05 μm and 0.045 μm pore size; 29 MPa and 19 MPa bending strength; and 0.119 μm2 and 0.0819 μm2 gas permeability. Anisotropy is revealed in membranes. It permits using the membranes as gas diodes and as a peculiar storage of gas molecules in the membrane (in particular, hydrogen molecules).
Keywords: microstructure; temperature; combustion; gradient; mixture; intermetallide; powder; particle

Good-quality bilayer coatings with a carbon interlayer and outer carbonate hydroxyapatite layer are obtained on AISI 316L stainless steel substrate using RF PACVD and sol-gel methods. It is found that protective properties of both composite C/HAP coating and HAP coating depend on the temperature of heat treatment. Hydroxyapatite with a carbon interlayer coating annealed at 500°C significantly improves corrosion resistance of the stainless-steel substrate as well as of unary carbon and HAP coatings. The composite C/HAP coating annealed at 500°C exhibits better corrosion resistance in comparison with the HAP coating heated at the same temperature. Stainless-steel substrates with a HAP and C interlayer heated at 700°C have worse corrosion features in comparison with uncoated AISI 316L. The weak corrosion resistance of this sample is due to the formation of metal carbides at this temperature.
Keywords: sol–gel method; hydroxyapatite; stainless steel; coating

Experimental temperature dependences of the enthalpies of R5Ge3 (R=La, Pr, Gd, Dy, Ho, Lu) are used to calculate the melting enthalpies and temperature dependences of the heat capacity for other germanides with this structure.
Keywords: enthalpy; heat capacity; thermodynamics

Thermodynamic properties of Ni–Hf melts by V. V. Sudavtsova; N. V. Podoprigora; M. A. Shevchenko (478-483).
Isoperibolic calorimetry is used to determine the integral and partial mixing enthalpies of Ni–Hf melts at 1770 ± 5 K. It is established that Ni–Hf melts form with release of a great amount of heat. Temperature dependence of mixing enthalpies for binary Ni–Hf melts is established. Activities of the melt components are determined from coordinates of the liquidus lines in the phase diagram of this system. The calculated and experimental activities of the Ni–Hf melt components agree well. These data and the mixing enthalpies are used to calculate the Gibbs energies and mixing entropies of Ni–Hf melts. It is shown that they are negative and their absolute value is high.
Keywords: calorimetry; nickel–hafnium; thermodynamic properties of alloys; calculation from phase diagram

It is shown that α2-Ti3Al alloy and hydrogen interact at 298–1533 K in accordance with the destructive hydrogenation reaction whose products are titanium hydride and intermetallic compounds rich in aluminum. The structural mechanism of α2-Ti3Al destructive hydrogenation is as follows. Hydrogen dissolves in Ti3Al, thus leading to disordering of its crystalline structure. Then hydrogen interacts with titanium, but not with the disordered alloy, according to the following scheme: α-Ti (hcp) → α-TiHx (hcp) → β-TiHy (bcc) → TiH1.9 (fcc) → TiH2 (bct). Intermetallic compounds rich in aluminum show up when new stoichiometric concentrations of titanium and aluminum form in accordance with the Ti–Al phase diagram. Solid composites with nanosized components result from the destructive hydrogenation of Ti3A at 973 K for 1 and 4 h. Their phase compositions are Ti3Al–(α-TiH–Ti3–xAl) and ((α-Ti–Hx)–(β-Ti-Hy)–TiH1.9 (fcc)–TiH2 (bct))–(Ti3Al–TiAl–TiAl2), respectively.
Keywords: α2-Ti3Al; destructive hydrogenation; solid nanocomposite