Strengths of Sapphire Crystals
THE strength of single crystals of entirely brittle materials should depend on the presence of stress raising flaws. Internally these could be formed by static dislocation arrays, voids or inclusions of foreign material. On the surface the flaws could be formed by mechanical damage or by surface steps as suggested by Marsh1.
Mechanical Behavior of Sapphire Whiskers at Elevated Temperatures
The tensile behavior of sapphire whiskers was measured at temperatures ranging from 25–2030°C. At room temperature, the tensile strength of the whiskers is size dependent and varies from several hundred thousand psi to 1 500 000 psi. With increasing temperature the strength decreases and becomes less size dependent. The whiskers exhibit delayed fracture above 630°C, the time to fracture being exponentionally related to the applied stress. Delayed fracture occurs in both hydrogen and oxygen atmospheres and hence is not believed to be due to stress corrosion. The mechanical behavior of the whiskers has been rationalized by assuming that they are essentially dislocation free and that failure occurs by thermally activated crack propagation preceded, at high temperatures, by localized dislocation nucleation.
Detection of Screw Dislocations in α‐Al2O3 Whiskers
An x‐ray method has been worked out for the detection of screw dislocations in whiskers and has been proved satisfactory on α‐Al2O3 whiskers in which the screw dislocation is identified by an axial pore and the twist independently measured by optical methods.