1997 TMS Annual Meeting: Thursday Abstracts

INTERNATIONAL SYMPOSIUM ON RHENIUM AND RHENIUM ALLOYS: Session X: "Rhenium Effect" and Alloy Development

Session Chairpersons: R.W. (Bill) Buckman Jr., Refractory Metals Technology, P.O. Box 100551, Pittsburgh, PA 15236; Dr. Lynn B. Lundberg, Materials Consultant, 2832 W. 33rd N., Idaho Falls, ID

8:30 am

RHENIUM AS AN ALLOY ADDITION TO THE GROUP VA METALS: R.W. (Bill) Buckman Jr., Refractory Metals Technology, P.O. Box 100551, Pittsburgh, PA 15236

The anomalous effect rhenium has on the ductility of the Group VIA elements Mo, W, and Cr is not observed in the Group VA metals Ta, Nb, and V. The low temperature ductility of the Group VA elements is significantly impaired at rhenium additions greater than about 3 atom percent. However, rhenium has been shown to significantly improve the high temperature creep strength of tantalum alloys and the effect is saturated at about 1.5 atom percent rhenium, a level which does not alter the ductile-to-brittle transition temperature of tantalum alloys significantly.

8:50 am

UNDERCOOLING EXPERIMENTS ON RHENIUM AND RHENIUM ALLOYS BY DROP-TUBE PROCESSING: Bernard Vinet and Sandrine Tournier, Commissariat á l' Énergie Atomique, DTA/CEREM-Département d'Étude des Matériaux, 17 rue des Martyrs, 38054 Grenoble cédex 9, France

Solidification of deeply undercooled melts can lead to the same variety of solid state metastabilities as do methods of rapid quenching of the liquid onto cold substrates. Since undercooling is realized at slow cooling rates, containerless processing is often needed to avoid external nucleation sources. In such a way, refractory materials are studied by letting single droplets fall through an evacuated high drop-tube as for instance the 48 metre high facility built in Grenoble. In this contribution, an overview is proposed on the most interesting results obtained so far for rhenium and rhenium alloys. Undercooling experiments have been realized on refractory binary alloys showing complex (D8b, type CrFe, 30 atoms per cell) and X (A12, type -Mn, 58 atoms per cell) phases in the equilibrium phase diagram. The studied systems associate a b.c.c. (W, Ta, Mo, Nb) transition metal with a h.c.p. metal, rhenium in the event (Tc, Os and Ru should also be considered if only from a phenomenological point of view). A favoured primarily formation of the metastable bcc-structure instead of the direct formation of the stable phase is obtained in the case of the Re-W system. Moreover, a double recalescence phenomenon is obtained in a composition range between 65 and 82 at% W. This result is discussed in connection with the appearance of the metastable A15 phase. In the case of the Re-Ta system, it is shown that the% phase nucleates from the melt.

9:10 am

PHYSIKO AND CHEMICAL PRINCIPLES OF RHENIUM ALLOYS DEVELOPMENT: K.B. Povarova, M.A. Tylkina, Baikov Institute of Metallurgy, Russian Academy of Sciences, Leninsky Pr. 49, 117911 Moscow, Russia

Physiko-chemical principles of rhenium alloys development are considered and examples are shown of application of these principles to the development of conventional and advanced rhenium alloys. The features of physiko-chemical interaction of rhenium with elements of Periodic table in binary, temary and more complicated alloy systems are analysed. Special attention is given to the theoretical analysis of experimental "composition-property" diagrams. The role of electronic structure is discussed as concerns the effect of the chemical composition on some physical and mechanical properties of Cr- Moe, or WRe-based solid solutions. The stability is estimated of solid solutions, intermetallic compounds, and interstitual phases in rhenium alloys, as well as the tendencies and rates of diffusion processes in multi component heterogenious rhenium alloys. Special rhenium effects responsible for decreasing of cold brittleness phenomena in IV group metals are discussed as well as high strain-hardening rates of W-and Mo-alloys with Re. Application of Re-contained alloys are observed.

9:30 am

THE RHENIUM EFFECT IN W- AND Mo- BASE ALLOYS: THE EXPERIMENTAL REGULARITIES AND THE PHYSICAL NATURE: A.D. Korotaev, A.N. Tyumentsev, Yu. I. Pochivalov, Siberian Physical & Technical Institute, Revolution Sq. 1, 634050 Tomsk, Russia

The experimental data on the rhenium effect and related phenomena, such as the solution softening, the elastic twinning and superelasticity, the high strengthening capacity of Mo-Re alloys, allowing a strength of 9000 Mpa, etc., are critically reviewed. Particular attention is given to the authors' results on the high ( 4 at.%) solubility of oxygen in Mo-Re alloys, to the segregation of the interstitial on the structure defects, and to the contribution of these factors to the increase of the plasticity. Some considerations are advanced on the reason for the increase in plasticity for alloys with a rhenium content of 5-10 at.%. The effect of the rhenium alloying on the decrease in the mobility of screw dislocations and the contributions of the Peierls forces to the increase in plasticity are discussed. The problem of the formation of local composition irregularities with an atomic short-range order and a type A15 lattice and the relevant experimental data are considered. It is supposed that the grain bulk-to-boundary strength ratio is important for high temperature (1300 K) strength and plasticity of Mo-Re alloys to be attained.

9:50 am BREAK

10:10 am

THE SOLUBILITY OF OXYGEN IN RHENIUM-ALLOYED MOLYBDENUM: A.D. Korotaev, A.N. Tyumentsev, V.V. Manako, Siberian Physical & Technical Institute, Revolution Sq. 1, 634050 Tomsk, Russia

The regularities of the phase transformations and the variation of the composition of Mo-Re-base alloys in the process of oxygen diffusion alloying were investigated. It has been found that the alloying with rhenium to a concentration of 47 wt.% increases (to 4 at.% and more) the solubility of oxygen, reduces (by 2-3 orders) its diffusion coefficient in molybdenum, and prevents the increase in the oxygen solubility at dislocations and small-angle boundaries of the substructure and the segregated enriching of intergrain boundaries with oxygen. For the Mo-Re alloys subjected to solid-solution hardening with oxygen it has been found that the phenomenon of mechanical twinning is suppressed and the characteristics of the dislocation structure are changed on plastic deformation. Moreover, in the solid solutions of the Mo 47% Re alloys, local regions of the short-range order or type Mo3ReOx complexes with the A15 structure, stabilized as a result of the diffusion of oxygen atoms, are observed. The role played by the regularities revealed in the realization of the rhenium effect is discussed.

10:30 am

THE ROLE OF PRECIPITATES OF CLOSE PACKED PHASES IN THE "RHENIUM EFFECT": Yu. N. Gornostyrev , M.I. Katsnelson, Institute of Metal Physics, 620219 Ekaterinburg, Russia; A.V. Trefilov, Kurchatov Institute, 123182 Moscow, Russia

The connection of "rhenium effect" (simultaneous increase of the strength and plasticity in W, Mo, Cr by adding rhenium at a concentration close to the solubility limit) with the peculiar structural state of corresponding alloys is discussed. The hypothesis is suggested, according to which the main factor for the appearance of the rhenium effect is the formation of dispersed close packed (of Frank-Kasper type) phases such as W3Re with A15 structure or distorted -phase. The general problem of the appearance of dispersed Frank-Kasper phases near the instability boundary of BCC lattice in transition metal alloys is considered in the framework of a simple crystallogeometrical model. It is shown that the precipitates of the phases in BCC host may provide the increase of the solubility of interstitial impurities. The replacement of carbide phases by the precipitates of the Frank-Kasper phases may lead to the improvement of mechanical properties of the alloys.

10:50 am

LOW- AND HIGH-RHENIUM TUNGSTEN ALLOYS: PROPERTIES, PRODUCTION, AND TREATMENT: K.B. Povarova, O.A. Bannych, E.K. Zavarzina, Baikov Institute of Metallurgy, Russian Academy of Sciences, Leninsky Pr. 49, 117334 Moscow, Russia

Author's experimental data on the effect of rhenium on the microstructure and some physical and mechanical properties of W-Re alloys, including molybdenum-containing alloys and alloys strengthened by interstitial phases are discussed. On the base of these data as well as on literature data, low-alloyed and high alloyed W-Re-alloys for different operating conditions are developed. Advantages and disadvantages of the vacuum melting and powder metallurgy processes for production of these alloys are discussed. The ways for increase in purity and improve of homogeneous distribution of rhenium in sintered PM-alloys are discussed too. Analysis of numerous experimental data on the effect of plastic deformation and heat treatment on microstructure, fracture mode and mechanical properties of the rhenium alloyed materials shows the possibility to predict mechanical properties of deformed alloys during their thermo-meohanical treatment and for operating in the wide temperature range.

11:10 am

DISPERSION AND SUBSTRUCTURE HARDENING OF Mo-Re BASE ALLOYS: A.N. Tyumentsev, A.D. Korotaev, Yu. P. Pinzhin, Siberian Physical & Technical Institute, Revolution Sq. 1, 634050 Tomsk, Russia

Using the literature and original data on the phenomenon of internal oxidation (IO) in low alloys based on Mo and Mo-47% Re, the basic mechanisms of this phenomenon and the scientifc principles for controlling the parameters of the heterophase structure formed on IO are discussed. On this basis, new methods for dispersion and combined dispersion and substructure hardening of type Mo-47% Re - Zr alloys have been developed and used. These methods provide a depth-uniform distribution of the ZrO2 oxide stable until 2500K and a high recrystallization temperature (2300K). The resulting high-strength state is stable to heat with the large plasticity margin retained. Based on the study of the regularities and mechanisms of the plastic deformation and destruction of internally oxidized Mo-Re-base alloys, the hardening mechanisms with IO and the potentialities of the methods developed for increasing the heat resistance of the Mo-Re-base alloys have been analyzed.

11:30 am

"RHENIUM EFFECT" ON THE IMPROVING OF MECHANICAL PROPERTIES IN Mo, W, Cr AND THEIR ALLOYS: Yu. V. Milman, G.G. Kurdumova, Institute for Problems of Material Science, 3 Krzhizhanovsky Str., Kiev 252180,Ukraine

The current state of science and mechanical properties improving for Cr, Mo and W alloyed by Re (so-called "rhenium effect") is reviewed. The main possibilities for increasing low-temperature plasticity of Cr, Mo and W by Re additions are closely connected with electron structure change during alloying; that is, plasticity is increased when alloying essentially changes the electron structure by disturbing the optimum conditions of the resonance covalent bond and changing the filling of energy bands. The observed growth of the density of electron states at the Fermi level N(EF) is accompanied by lowering of Peierls stress, decrease of staking fault energy ( SFE), increase of activation volume and interstitial solubility, and the involvement of an additional deformation mechanism - twinning. The increase of N(EF) should not be very sharp for SFE and interstitial solubility to have the optimum value. SFE has to be lowered so that twinning can occur, but the mobility of dislocation screw components should not be impeded. Lowering of SFE is accompanied by growth of activation volume and increase of stress relaxation rate at concentrators. The increase of interstitial solubility should be sufficient to diminish the tendency to impurity segregation on dislocations as well as on grain and subgrain boundaries, but not so great as cause the abrupt hardening of a solid solution . The experimental results devoted to the influence of Re additions on the dislocation structure, mechanical properties and fracture mechanisms are discussed.