[en] A model of PPV type for the anomalous increase of the yield stress with temperature in L1₂ compounds is presented. It is based on two thermally activated processes that describe respectively, the pinning and the unpinning of [101] screw dislocations moving in the (111) plane. The unpinning mechanism invokes the concept of the major breakaway of the dislocation. A steady state involving a balance of the pinning and unpinning rates is formulated. The model explains many of the macroscopic features associated with the yield stress anomaly in the entire temperature regime

[en] An overview of the fatigue and fracture mechanisms in lamellar TiAl alloys is presented. Investigations of tensile fracture, fatigue crack initiation, growth of small and large fatigue cracks, and fracture toughness properties are summarized. Pertinent crack-tip micromechanics measurements are utilized to illustrate the various processes by which a crack extends in lamellar TiAl alloys. Based on the results, the influence of microstructure, temperature, and loading rate on tensile properties, fatigue crack initiation and growth, and fracture toughness are elucidated and related to the controlling fracture mechanisms

[en] The intermetallic compound gamma-TiAl has attracted a great deal of research interest because of its potential for high-temperature structural applications. The interest has recently intensified, because multi-phased alloys based on this compound are found to have improved processability and mechanical properties suitable for several uses. The improvements are largely a result of certain alloying considerations, such as the control of Ti/Al ratio and the addition of selected alloying elements. The purpose of this paper is to review those alloying considerations that are critical to the development of gamma-based alloys. 64 refs., 13 figs., 1 tab

[en] The shaping methods of Nb-Al intermetallic compounds, especially melting and casting, have considerably different characteristics as compared with those for other metals and alloys. The authors have investigated melting and casting processes for Nb-Al compounds to develop precision casting processes for these intermetallics. Fundamental properties of Nb-Al compound castings have been also investigated for high temperature structural use in this work. An advanced Induction Skull Melting (ISM) furnace has been developed and the advantages of ISM have been recognized as a result of this study. The mechanical properties, such as hardness and compression strength, are dependent upon the Al content in Nb-Al binary compounds

[en] Studies on a Super Alpha-2 sheet alloy with a microduplex structure have shown it to have a considerable potential for SPF at temperatures of 940--980 C, and to remain free from cavitation after large tensile strains. The material could be readily isostatically diffusion bonded in this temperature range but the pressures required were appreciably higher than those for the bonding of Ti-6Al-4V. The different pressure requirements reflected, at least partially, the higher flow stresses of Super Alpha-2 for comparable deformation conditions. The isostatic DB model of Pilling gave reasonable agreement with experiment within the limits of reliability of the materials property data available to test the model. The present investigation shows that microduplex Super Alpha-2 has a considerable potential for fabrication using a DB/SPF route

[en] The status and usefulness of various processing methods for structural intermetallics and comparable Nb-Ti-Al alloys are important considerations in bringing these materials into service. This paper describes the results of a progression of experiments that produced increasing quantities of these materials via plasma arc melting. Results on direct withdrawal ingots of gamma Ti-48Al-2Cr-2Nb and TiAl serve as an introduction to the upscale melting and processing of two Nb-Ti-Al alloys which contain the same basic elements and the same chemical uniformity problem (segregation tendency) in larger size ingots. The developed practice serves as a model for producing homogeneous ingots and wrought processing to mill product forms

[en] The measurement of low, medium and high stacking fault energies is reviewed. Two methods appropriate to high fault energies are illustrated: (i) using a configuration which gives a specially large partial separation (ii) measuring the minimum size of screw dipoles. Other aspects of faults are mentioned: the importance of supplementary displacements to APB contrast and the possible implications of a strain field to ± g weak beam image asymmetry. Finally, the influences of temperature and composition on fault energies are discussed

[en] The high temperature flexural properties of MoSi₂ and discontinuously reinforced MoSi₂ composites have been determined over the range of 1,000 to 1,400 C using four-point bend tests. Tests were performed on in situ processed composites containing 30 volume percent of either SiC, TiB₂, or HfB₂. These composites were fabricated through a combined XD trademark synthesis and powder processing approach. The yield and flow stresses of the composites were found to be up to 75% greater than those of the unreinforced sample. The apparent ductile-to-brittle transition temperature for the reinforced and unreinforced samples was approximately 1,300 C. This article examines the influence of reinforcement type, morphology, size, and distribution on the high temperature strength and fracture behavior of MoSi₂ matrix composites. In addition, the possible influence of matrix grain size and any glassy-phase contaminates are discussed

[en] The present work is an overview of research carried out in the authors' laboratory on L1₂ trialuminides based on Al₃Ti and Al₃Zr which were macroalloyed with third element (Cu, Fe, Mn and Cr), to render the L1₂ crystallographic structure. The results of microstructural investigation, including the long-range order parameter S and the results of mechanical testing, are reported. Observed differences in ductility between L1₂ Al₃Ti+X and L1₂ Al₃Zr+X alloys are discussed in terms of the strain-induced microcracking and the ductile-to-brittle transition theory

[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