The stacking fault energy in noble metal alloys

A model for the stacking fault energy, γ, in noble metals and their alloys is developed which qualitatively explains many features of experimental observations. The model is based on bothd band and free electron contributions to the cohesion. An important prediction of the model is that γ can be decreased in noble metal alloys by the addition of a solute with an emptyd band withZ = 1 (i.e., lithium in silver). Measurements of γ were made as a function of lithium concentration in a series of Ag-Li alloys up to 25 at. pct Li using the node method. The results show that γ decreases with the addition of lithium in a manner predicted by the model.     

A theory for solute impurity diffusion, which considers engel-brewer valences, balancing the fermi energy levels of solvent and solute, and differences in zero point energy

A theory that assumes the Engel-Brewer valence of elements (one for bcc structures, two for cph structures, and three for fcc structures) and considers the effects of balancing the solute and solvent Fermi energy levels and differences in zero point energy between solvent and solute atoms to calculate an “effective” relative valence for solute impurities is presented. The calculated values of relative valence and the experimental values of the differences in diffusional activation energy between solute and solvent atoms, ΔQ, are compared to the values of ΔH ₂ + ΔE calculated from the Lazarus-LeClaire theory for several solute impurities in ten solvent metals. The calculated results agree very well with the experimental values for the large majority of solutes. The theory presented adequately describes solute impurity diffusion in both α-Fe and γ-Fe, Al, Ni, and the noble metals. In particular, the low activation energies for impurity diffusion of the alkali metals (ground state valence of one) in Al (ground state valence of three) are accounted for by the theory. It is shown that the diffusion of the electronegative solute impurities (Cr, Mn, Fe, and Co) in Al is not anomalous when the relative valence is calculated by the proposed theory. The diffusion of electronegative solute impurities in the noble metals, which has been problematic in the past, is also well described by the proposed theory. The proposed theory introduces a simple method of estimating the effective electron densities of solute impurities and illustrates that the Lazarus-LeClaire theory adequately describes solute impurity diffusion in the ten solvent metals studied. It is expected that more accurate calculations of effective electron density for solute impurities would result in even better agreement between experimental and calculated results.     

On the energy criterion for the erosion resistance of metals

The possibility of attaining critical transitions in metals depending on the nature, intensity, and time of action of a concentrated energy flux (laser radiation, spark discharge) is analyzed. A criterion of erosion resistance based on critical constants is proposed. Metals Science Institute of the Far Eastern Division of the Russian Academy of Sciences, Khabarovsk. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 111–116, January–February, 1998.     

Surface Tension of Liquid Alkali,Alkaline, and Main Group Metals: Theoretical Treatment and Relationship Investigations

An improved theoretical method for calculating the surface tension of liquid metals is proposed. A recently derived equation that allows an accurate estimate of surface tension to be made for the large number of elements, based on statistical thermodynamics, is used for a means of calculating reliable values for the surface tension of pure liquid alkali, alkaline earth, and main group metals at the melting point, In order to increase the validity of the model, the surface tension of liquid lithium was calculated in the temperature range 454 K to 1300 K (181 °C to 1027 °C), where the calculated surface tension values follow a straight line behavior given by γ = 441 – 0.15 (T-Tm) (mJ m⁻²). The calculated surface excess entropy of liquid Li (–dγ/dT) was found to be 0.15 mJ m⁻² K⁻¹, which agrees well with the reported experimental value (0.147 mJ/m² K). Moreover, the relations of the calculated surface tension of alkali metals to atomic radius, heat of fusion, and specific heat capacity are described. The results are in excellent agreement with the existing experimental data.     

Thermochemistry of binary liquid gold alloys: The systems Au-Ni,Au-Co,Au-Fe,and Au-Mn

In this paper we report enthalpy of mixing data for the liquid alloys of gold with manganese, iron, cobalt, and nickel obtained by a Calvet-type calorimeter at 1378 K. The enthalpies of mixing are compared with Gibbs energies calculated from earlier emf and vapor pressure studies to yield information on the excess entropies of mixing. The limiting enthalpies of solution of the liquid transition metals in liquid gold are compared with values predicted from the semi-empirical model of Miedemaet al. and with earlier data for the same transition metals in liquid copper. The calculated values of the excess entropy of solution in liquid gold are compared with the corresponding values in liquid copper near 1400 K. For Ni, Co, and Fe as solutes we observepositive shifts of 5 to 9 J K⁻¹ mol⁻¹ which are attributed to vibrational entropy terms. For Mn there is a strongnegative shift of about 35 J K⁻¹ mol⁻¹. This shift probably is due to “complex” or “associate” formation between gold and manganese atoms.     

Energy states andPV-diagrams of borides or transition metals

The paired-potential model of atomic interactions in diborides and hexaborides of transition metals is considered. The parameters of the Mie—Grüneisen, Morse, Buckingham, and Rose potentials are determined and the E(V)-and P(V)-diagrams of these compounds are calculated. The critical characteristics of the pressure and volume of the P(V)-diagrams, which determine the ultimate strength in uniform tension and the corresponding critical elastic strain, are determined. Special features of the potentials and P(V)-diagrams of the borides are discussed. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 111–117, March–April, 1999.     

Imperfection of carbon sublattice: Effect on the properties of refractory carbides of transition metals

The paper outlines comprehensive research of the physical and technical properties possessed by monocarbides of transition metals from groups IV and V of the Periodic Table within their homogeneity region. Experimental results are interpreted. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 20–28, 2008.     

Configuration model of matter and its role in theoretical science of materials

The major principles of Samsonov’s quasichemical configuration model are discussed in terms of the many-electron theory of condensed systems. It is shown that the interrelation between the order of the s-and d-levels in isolated atoms of transition elements and the occupation of the corresponding bands in metals can be interpreted with quasiparticle theory that takes the d-d-and s-d-electronic correlations into account more accurately than ordinary one-electron theory does. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 29–36, 2008.     

First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part I. Iron (Fe)-Solute (B,C, P,and S) Systems

The microscopic mechanism of grain boundary (GB) embrittlement in metals by solute segregation has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy (=2·surface energy − GB energy) of bcc Fe Σ3(111) symmetrical tilt grain boundary (STGB) is reduced by the segregation of sulfur (S) and phosphorous (P) while it is increased by the segregation of boron (B) and carbon (C). The rate of the decrease/increase in the cohesive energy is proportional to the experimental shift in the DBTT of high-purity iron with increasing segregation. This indicates that the change in the cohesive energy of GB plays a key role in the GB embrittlement of metals.     

Nanocrystalline metals prepared by high-energy ball milling

This is a first systematic report on the synthesis of completely nanocrystalline metals by high-energy deformation processes. Pure metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) structures are subjected to ball milling, resulting in a decrease of the average grain size to ≈9 nm for metals with bcc and to ≈13 nm for metals with hcp crystal structures. This new class of metastable materials exhibits an increase of the specific heat up to 15 pct at room temperature and a mechanically stored energy determined as up to 30 pct of the heat of fusion after 24 hours of high-energy ball milling. The grain boundary energy as determined by calorimetry is higher than the energy for fully equilibrated high-angle grain boundaries. E. Hellstern, formerly Research Associate, California Institute of Technology This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Interface effects in superplastic deformation of alumina containing zirconia,titania or hafnia as a second phase

The superplastic properties of alumina can be changed in interesting ways by small additions of transition metals zirconium, hafnium, or titanium. All three introduce a “leaky” threshold stress of 5–20 MPa. Above the threshold the flow rate is linearly related to the applied stress. A similar threshold is observed in sinterforging experiments. The additions also have the effect of enhancing the superplastic ductility and the fracture strength, presumably by increasing interfacial cohesion. This higher interfacial cohesion appears to correlate to the higher activation energy for boundary diffusion and to a lower interfacial energy. The threshold is explained in terms of a roughening transition of interfaces that are faceted. Below the transition the flow rate is interface limited, while above it, it is diffusion limited. The stress for the roughening transition is obtained from a model that describes the transition in terms of a barrier for the nucleation of interface dislocations. The barrier is shown to depend on the change of the interface energy with misorientation. It is estimated that dγ/dθ = 1 mJ m⁻²/degree can give rise to a threshold stress of 10 MPa.     

A New Paradigm for Designing High-Fracture-Energy Steels

The steels used for structural and other applications ideally should have both high strength and high toughness. Most high-strength steels contain substantial carbon content that gives poor weldability and toughness. A theoretical study is presented that was inspired by the early work of Weertman on the effect that single or clusters of solute atoms with slightly different atom sizes have on dislocation configurations in metals. This is of particular interest for metals with high Peierls stress. Misfit centers that are coherent and coplanar in body-centered cubic (bcc) metals can provide sufficient twisting of nearby screw dislocations to reduce the Peierls stress locally and to give improved dislocation mobility and hence better toughness at low temperatures. Therefore, the theory predicts that such nanoscale misfit centers in low-carbon steels can give both precipitation hardening and improved ductility and fracture toughness. To explore the validity of this theory, we measured the Charpy impact fracture energy as a function of temperature for a series of low-carbon Cu-precipitation-strengthened steels. Results show that an addition of 0.94 to 1.49 wt pct Cu and other accompanying elements results in steels with high Charpy impact energies down to cryogenic temperatures (198 K [–75 °C]) with no distinct ductile-to-brittle transition. The addition of 0.1 wt pct Ti results in an additional increase in impact toughness, with Charpy impact fracture energies ranging from 358 J (machine limit) at 248 K (–25 °C) to almost 200 J at 198 K (–75 °C). Extending this concept of using coherent and coplanar misfit centers to decrease the Peierls stress locally to other than bcc iron-based systems suggests an intriguing possibility of developing ductile hexagonal close-packed alloys and intermetallics.     

High-temperature friction of refractory compounds

The paper overviews long-term studies into the behavior of metallic (carbides, borides, and nitrides of transition metals), and nonmetallic (boron and silicon carbides, aluminum nitride) refractory compounds as well as composite materials based on them in high-temperature friction in vacuum and air. The friction characteristics (wear rate and friction coefficient) are indicated as a function of temperature in the range from room temperature up to 1000–1400 °C. Data of x-ray examination and electron microscopy of friction surfaces are cited. The fracture mechanism for contacting surfaces of materials in friction is considered. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 167–178, 2008.     

Alloys with the shape memory effect in systems of d-transition metals containing metals of the platinum group

Our own and literature data on the reactions in binary and ternary systems of transition metals, one component of which is a platinum group metal, and which contain phases which undergo a martensitic transformation, are reviewed. The transformation is responsible for the shape memory effect, which occurs in the alloys under consideration at temperatures above 300°C. In all equiatomic phases of the binary systems discussed (including quasibinary sections in the ternary systems Ti(Sc)–Ni(Co)–Me; Me=platinum group metal), in which the shape memory effect occurs, the transformation takes place from a matrix phase with a CsCl-type crystal structure. The properties of the parent and transformation phases, phase equilibria in the region of phase formation, and the effect of composition on these are described. Institute for Materials Science Problems, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 47–59, July–August, 1997.     

Relaxation time effects in the stacking fault energy of noble metal alloys

The stacking fault energy, γ, in noble metal alloys can be expressed as a sum of two terms: δu_fe, the contribution due to the conduction electrons; and δu_c, the contribution due to thed electrons. For noble metals and alloys δu_c 〉 δu_fe while for multivalent normal metals δu_fe 〉 δu_c. The theory is first discussed in terms of recent calculations of the pseudopotential form factors of the noble metals (Moriarty) and some of the typical solutes (Shaw). The theory is then extended in a phenomenological fashion to include the effects of a finite relaxation time, τ, of the conduction electrons. It is shown that, for concentrated noble metal alloys with the electron-to-atom ratio,Z 〉 1.14 and multivalent normal metals, δuf_e and hence γ will be dependent on both temperature and deformation through their effects on τ. An increase in τ results in a decrease in the magnitude of δu_fe. In the case of concentrated noble metal alloys this results in an increase in γ with increasing t while for multivalent normal metals γ decreases with increasing τ.     

Interconnection between short-term strength, static stress-rupture strength, and creep resistance of tungsten, molybdenum, and alloys based on them

Comprehensive analysis of experimental data for the high-temperature mechanical properties of commercial purity tungsten, molybdenum, and their alloys with solid solution, dispersion, and mixed hardening prepared by powder metallurgy with different forms of uniaxial tension is provided. It was established that for materials of this class at high temperature (0.5–0.8 T_m) there is a close correlation between short-term and static stress-rupture strength, and creep resistance, which is described by unified functional dependences that are common for all of the metals and alloys studied. Institute of Strength Problems, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 93–99, July–August, 1999.     

Structural Relaxation of La55Al25Ni10Cu10 Bulk Metallic Glass

The relaxation process and glass transition kinetics of the La₅₅Al₂₅Ni₁₀Cu₁₀ bulk metallic glass (BMG) below the calorimetric glass transition were investigated by differential scanning calorimetry. Enthalpy relaxation was observed in isothermal annealing experiments below the glass transition temperature T _g . The time-dependent enthalpy relaxations were well described by a stretched exponential relaxation function, with an exponential index 0.78. An apparent activation energy E = 60.8 kJ/mol was derived from the temperature dependence of the relaxation time. The equilibrium free volume concentration at a temperature about 30 K below T _g is 10⁻¹⁷ to 10⁻¹³, calculated using an empirical Vogel–Fulcher–Tammann (VFT) type equation. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Interaction of nitrides of group IV–V transition metals with chromium

The interaction of the nitrides TiN, ZrN, HfN, VN, and TaN with chromium is studied in the temperature range 1600–1900°C in pure nitrogen and argon by x-ray, metallographic, and microdurometric analyses. The temperature T of the liquid phase that forms at the nitride-chromium interface is found to be below the melting point of pure chromium coressponds to the melting point of the eutectic composition of the solid solution and intermetallic compounds of the nitride-forming metal with chromium (with the exception of vanadium). The wetting temperature and the angles of contact of nitrides of group V transition metals are less than those of nitrides of group IV metals; they increase with the number of the nitride-forming metal in the group. The growth of the lattice parameter of most of the nitrides obtained after interaction with chromium is attributable to Cr atoms occupying vacant sites of the metalloid sublattices of the nitride. Intermetallic compounds such as MeCr₂, the nitride Cr₂N, and solid solutions of chromium in nitrides as well as solid solutions of the nitride-forming metal in chromium are formed in the liquid-phase interaction. Titanium nitride is the most stable of the nitrides studied. Institute for Materials Science Problems. Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 3–4, pp. 90–96, March–April, 1997.     

Initial-stage Sintering Kinetics of Nanocrystalline Tungsten

Initial-stage sintering kinetics of nanocrystalline tungsten has been studied in the temperature range of 1273–1473 K (1000–1200 °C). Nanocrystalline tungsten sinters initially through a grain boundary diffusion mechanism. The calculated activation energy was 388 ± 11 kJ/mol at low temperatures (1273–1373 K (1000–1100 °C)) and 409 ± 7 kJ/mol at high temperatures (1373–1473 K (1100–1200 °C)), which are close to the experimentally measured activation energy for grain boundary diffusion (385 kJ/mol).     

Reaction of rare earth metals with VI- and VII-group transition metals and boron

The results of experimental investigations of reactions of rare-earth (Ln) and transition (Me^{VI, VII}) metals with boron were reviewed. An analysis of the phase equilibrium diagrams of 50 Ln−Me−B systems was carried out. The laws of formation of ternary borides were determined, and the characteristics of 12 structural types in which they crystallize described. The coordination and types of boron atom linkage, as well as the valence state of the Ln atoms were considered. I. Franko L’vovsk Institute. Translated from Poroshkovaya Metallurgiya. Nos. 1–2, pp. 116–125. January–February, 1998.