Harry A. Lipsitt

The deformation and fracture of TiAl at elevated temperatures

The tensile properties of the intermetallic compound TiAl have been determined at several temperatures in the range 25 to 1000°C. Additional variables studied were the influence of strain rate and the effect of exposure to oxidizing conditions prior to testing. The modes of deformation under the various testing conditions were studied in the electron microscope, the modes of fracture were studied in the scanning electron microscope, and these data were correlated with the mechanical properties. The results indicate that the ductilebrittle transition behavior of TiAl at about 700°C is controlled by the trailinga/6 [112] partial dislocation components of thea [011] superdislocations overcoming their pinning barriers. It was also shown that prior exposure to oxidizing conditions does not markedly influence the mechanical properties of TiAl.

Thermal stability of SiC fibres (Nicalon

The degradation behaviour of Nippon Carbon Co. SiC fibres (Nicalon®) after heat treatment in various environments was studied. Regardless of the heat-treatment conditions, the Nicalon® fibre strength degraded when the fibres were subjected to temperatures higher than 1200° C (temperatures below 1200° C were not investigated). This degradation is associated with the evaporation of CO from the fibres as well as withβ-SiC grain growth in the fibres.

Titanium Aluminides - An Overview

The object of this paper is to describe a coordinated research and development program which has been pursued by an Air Force-Industry-University team for more than twelve years. The focus of our attention has been on the development, processing, and engine testing of alloys based on intermetallic compounds, specifically on the aluminides of titanium, iron, and nickel. The titanium aluminides, Ti 3 Al and TiAl, are the materials with which we have been working the longest and on which development has proceeded the furthest. This Symposium has provided the first opportunity to review the progress of the titanium aluminide development programs sponsored by the Air Force and some of the engine testing efforts undertaken by the engine manufacturers.

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti3Al and Ti3Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m−2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m−2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m−2, a transition regime with a lower stress-exponent value was obtained.

Nickel-aluminum-molybdenum phase equilibria

Recent work on alloys based on the Ni-Al-Mo system has brought to light several inconsistencies with published equilibrium phase diagrams for this system. Published diagrams have been based on theoretical computer models and on data gathered ostensibly before equilibrium was achieved, especially at temperatures below 1100 °C. The intent of this effort was to produce an experimentally validated ternary equilibrium phase diagram for the Ni-Al-Mo system. Specimens for this task were produced by both conventional casting and powder metallurgy techniques. The temperatures studied included 1260, 1171, 1093, 1038, and 927 °C (2300, 2140, 2000, 1900, and 1700 °F) for times up to 2500 hours. Phases were identified using an electron probe microanalyzer and X-ray diffraction. The results show significant deviations from the proposed phase diagrams published in the literature in the temperature range investigated. In particular, a class II four-phase equilibrium reaction γ + α cooling // heating γ′+ δ has been shown to occur at 1127 ± 2 °C (2060 ± 4 °F).

Deformation behaviour of single crystals of titanium carbide

Single crystals of titanium carbide were deformed in compression over a wide range of temperature, and the operative slip systems were determined by etch-pitting and electron microscopy. Around the brittle-ductile transition temperature, the slip system undergoes a gradual change from {1 1 0} 〈1 ¯1 O〉 to {1 1 1} 〈1 ¯1 0〉; this is interpreted to be the mechanism governing the brittle-ductile transition in titanium carbide.

Mechanical behaviour of polycrystalline TiC

The mechanical behaviour of hot-pressed TiC was studied. Compacts were fabricated from commercially available TiC powders, which were found to contain impurities including iron, cobalt, silicon and free carbon. Those impurities segregated to the grain boundaries and formed low melting-point phases which degraded the high-temperature strength. Differences in the temperature at which strength began to fall sharply were correlated with differences in the impurity chemistry of the TiC starting powders. It was found that a high-temperature vacuum heat treatment of the as-hot-pressed TiC significantly reduced the levels of impurities at the grain boundaries. This, in turn, caused a dramatic improvement in high-temperature strength. The TiC, so treated, possessed a grain size of approximately 25μm and exhibited a D-B transition in bending at about 1425° C.

Electron irradiation damage in TiC

Electron irradiation experiments were carried out on (1 0 0) TiC0.93 thin foils inside an electron microscope operated at 100 kV at room temperature. Both graphite rings and extra diffraction spots were observed as irradiation continued. The extra diffraction spots correspond to an ordered cubic superstructure having a composition near TiC0.5, as determined by Goretzki. These extra spots were disrupted with continued irradiation. These results are discussed in the light of displacement damage in TiC.

Observations on the early stages of oxidation of titanium carbide

Thin sections of a TiC single crystal having the {100} orientation were oxidized at 900 °C in four different partial pressures of oxygen, and the oxidation products were examined using transmission electron microscopy. At a higher partial pressure of oxygen, complete oxidation of TiC to TiO2 was observed; whereas at lower partial pressures, epitaxial oxides were formed. These epitaxial regions were characterized, and the epitaxial growth relationships of these oxides with respect to the TiC matrix were established.

Die Forming Doubly Focusing X‐Ray Monochromators

Reliable forming techniques for the preparation of doubly curved LiF x‐ray monochromators are described. These techniques are applicable to the formation of doubly focusing one piece monochromating crystals with geometries suited to a wide range of experimental requirements. The resulting dislocation structures were analyzed and utilized in the development of the techniques and are presented as a guide for the preparation of similar focusing crystals.