[en] A topological model is proposed for metallic glass formation through destabilization of the host crystalline lattice by substitutional and/or interstitial solute elements. A solute element may partition between substitutional and interstitial sites and the model calculates relative site frequency as a function of the strain energy associated with each site. The strain energy, in turn, depends upon solute and solvent elastic properties and relative sizes, and upon temperature. The crystalline lattice is destabilized leading to amorphization when solute elements produce a critical internal strain required to change local coordination numbers. Fractions of solute atoms in interstitial and substitutional sites and the internal strain introduced by these atoms are calculated as functions of atomic radii and elastic moduli of solvent and solute elements and the absolute temperature. The critical concentration of a solute element required to destabilize the crystalline lattice of a binary alloy is also calculated as a function of the radius ratio R=R_B/R_A of the solute and solvent elements. In the range of 0.5< R<1, the critical concentration decreases, reaches a minimum at R∼0.8 and then increases as the size of the solute element decreases relative to the solvent

[en] A qualitative examination of the toughness behavior of the Cr-Cr2Zr, Cr-Cr2Hf, Cr-Cr2Ta, Cr-Cr3Si, Ta-Cr2Ta, and Ta-Ta5Si eutectics is presented. The evaluation is based on the microcracking from microhardness indentations and fracture appearance. Relative oxidation resistance at 800 and 1200 C is also presented. It is shown that, with the exception of the Ta-Cr2Ta and Cr-Cr2Ta eutectics, each of the eutectic alloys exhibits potential room temperature metallic phase toughening of a brittle matrix. At 1000 C and above, where data are available, the eutectic alloys including the Ta-Cr2Ta and Cr-Cr2Ta exhibit potential metallic phase toughening of a brittle matrix. 5 refs

[en] Phase equilibria in the α/α₂ phase region of the Ti-Al-Si-Nb system at Nb content 2.5, 3.5 and 5 at.% were studied in alloys as-cast and heat-treated at 800 deg. C. Samples were prepared by arc-melting technique, homogenized at 1350 deg. C and then heat-treated at 800 deg. C, followed by ice water cooling. The structure of the alloys was characterized by means of X-Ray diffraction, differential thermal analysis, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy. The continuous solid solutions with variable compositions (Ti_1-x,Nb_x)₃(Si_1-y,Al_y) (η) (0.05≤x≤0.07, 10^-3≤y≤0.02) was detected at 800 deg. C for the first time in the multi-component alloys based upon the Ti-Si system. It was stabilized by Nb additions in the alloys with low Al content. A peritectoid reaction β+α→η was observed. Additions of Al neutralized the stabilizing effect of Nb resulting in an α + Ti₅Si₃ (z) equilibrium

[en] The purpose of this research is to establish structure-forming principles that govern the atomic structures of metallic glasses. A structural model based on efficient atomic packing will be discussed and applied to the topological systems that represent most metallic glass alloys. The concept of efficient atomic packing has direct and specific implications regarding the local structure and composition of metallic glasses. Specific solute-to-solvent atomic radius ratios and specific solute concentrations related to these ratios are shown to be preferred in this model, and analysis of a wide range of metallic glass systems shows a very strong correlation with these predicted values. Relationships between atomic size and concentration are discussed, and new insights are proposed based on the current structural model. Possible local atomic configurations (i.e., atomic clusters) are defined based on topological constraints that are derived from the requirement of efficient atomic packing. Experimental observations drawn from the literature that provide support for this model are presented

[en] Al₈₅Ni₁₀Y_2.5La_2.5 alloy powder produced by gas atomization was compacted using equal channel angular extrusion (ECAE). The powder particle size was below 40 μm (-325 mesh grade), the powder was partially (∼60%) amorphous and it contained intermetallic phases. Differential scanning calorimetry showed that crystallization of the amorphous phase starts at about 220 deg. C by precipitation of Al-based f.c.c. particles and continues at temperatures above 300 deg. C by formation of intermetallic phases. Four-pass B_C route compaction of the powder was conducted in a copper can at 280 deg. C, to avoid additional formation of intermetallic phases and retain some level of amorphicity of the compacted material. A non-homogeneous deformation of the compacted material during ECAE process was detected. The powder was completely consolidated and no voids were observed in the regions where powder particles were severely deformed. In other regions, non-deformed round particles, cracks, and high porosity were observed. Microstructural analysis showed that the severely deformed powder particles contained amorphous and/or f.c.c. phases only, while the non-deformed and/or fractured powder particles were fully crystalline and consisted mostly of intermetallic phases

[en] The quasi-binary NiAl-Mo system exhibits a large two-phase field between NiAl and the terminal (Mo) solid solution, and offers the potential for producing in situ eutectic composites for high-temperature structural applications. The phase stability of this composite system was experimentally evaluated, following long-term exposures at elevated temperatures. Bend strengths as a function of temperature and room-temperature fracture toughness data are presented for selected NiAl-Mo alloys, together with results from fractography observations. 10 refs

[en] The intent of the symposium was to provide a detailed and in-depth perspective of the approaches, results and progress toward the structural application of intermetallic compounds and their composites. Longer and insightful presentations which focused on real progress and trends rather than recent results, and extended discussion periods formed the centerpiece of this symposium to achieve these goals. Emphasis was placed on a balance of presentations covering basic research, alloy development and applications. Programs of large magnitudes are being carried out throughout the world to develop intermetallics for structural applications. Programs of large magnitudes are being carried out throughout the world to develop intermetallics for structural applications. The majority of the focus is on the development of microstructure and alloy compositions to solve the poor ductility and toughness of these intermetallics. Considerable progress has been made in understanding as well as solving the ductility and toughness issues. Component and engine tests are being initiated with Ti₃Al, TiAl, Ni₃Al and NiAl alloys. Separate abstracts were prepared for 94 papers in this symposium

[en] A project involving the development of a new family of turbine-blade materials with the increased temperature capabilities necessary for advanced high-performance engines was initiated in the mid-1970's. An extensive investigation of alloys based on the Ni-Al-Mo system was conducted in connection with this project. The investigation included the task to produce an experimentally validated ternary equilibrium phase diagram for the Ni-rich corner of the Ni-Al-Mo system in the temperature range from 1260 to 927 C, and to study the occurrence of class II four-phase reaction. At 1171 and 1260 C the measured isotherms were found to agree well with the literature. However, at 1093 C and below the measured isotherms reflect the occurrence of the class II four-phase reaction given by the relation gamma + alpha cooling/heating gamma-prime + delta. 12 references

[en] Recent work on a new class of titanium aluminide composites with (O + β₀) or (O + β₀ + α₂) matrices containing the ordered β₀ phase (CsCl structure based on β Ti) and the ordered O phase (orthorhombic structure based on Ti₂AlNb) has shown significant improvements over the (α₂ + β) matrix composites including reduced reactivity with the fiber, higher strength, and greater resistance to thermal fatigue and thermomechanical fatigue. With regard to the matrix alloys, the transition from Ti-24Al-11Nb to compositions near the orthorhombic phase such as Ti-22Al-23Nb and Ti-22Al-27Nb involves a large increase in the Nb content which leads to an increase in the density. The objective of this study was to evaluate the attributes of Ti-24.5Al-17Nb, an alloy with an intermediate Nb concentration, as a composite matrix material. It should be noted that this composition was previously developed as a monolithic alloy with improved toughness and it has shown interesting properties following thermomechanical processing

[en] An interfacial region (i.e., an interface coating or a reaction product) between the fiber and matrix is known to exist in most metal-matrix composites. Since the load transfer between the fiber and matrix depends on the properties and conditions of this interfacial region, the mechanical behavior of the composites is strongly affected by its characteristics. In this study, finite element analysis has been used to investigate the distribution of residual thermal stresses in the interfacial region, characteristics of interfacial crack initiation and propagation, and mechanical response of the composites under transverse tensile loading. The matrix material is taken to be Ti-6Al-4V (wt%) and behaves elasto-plastically, while the SiC-fiber is assumed to be elastic. The interface is treated as a thin layer with a finite thickness between the fiber and matrix. Three separate interfacial conditions (i.e., a graded carbon coating, a Y₂O₃ coating and an uncoated interface) have been considered to evaluate the influence of the (independent) thermal and mechanical properties of the interfacial region. For comparison, an infinitely strong bond at the interface is also assessed. The results indicate that the properties of the interfacial region affect the stress distribution, the interfacial crack initiation and propagation, and the mechanical response of the composites. Based on these results, the thermal and mechanical properties of the coating for improved performance of the composites under transverse loading conditions are proposed