In situ measurement of instantaneous dislocation velocities in iron single crystals at low temperatures ∼4.2 K

Date of Completion

January 1999

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

In situ measurements of instantaneous dislocation velocities were carried out in a iron single crystals, plastically deformed at ∼4.2 K and below, greatly enhancing the current understanding of plasticity in iron. These are the first measurements of this kind in iron. ^ The motion of dislocations governs the plasticity of materials.(1, 2, 3, 4, 5, 6) Despite the importance of dislocations and the extensive work which has been conducted concerning their behavior their motion is not yet completely understood. This results from the complexity of their motion and lack of in situ measurement techniques. This research exploits the effect which a magnetic field has upon an electron dislocation interaction. This interaction was successfully used to measure the velocities of dislocations in iron single crystals in situ as the crystal is deforming. This technique allowed for the determination of instantaneous dislocation velocities as a function of temperature, strain rate, and stress. ^ In the experiments a magnetic field was swept through the slip plane of a plastically deforming iron crystal. When the field was within the slip plane the electron-dislocation interaction was enhanced resulting in an increase in the flow stress, a requirement for maintaining a constant strain rate.(7, 8) Applying electron - dislocation interaction theory the magnitude of the stress increase was used to determine the in situ instantaneous dislocation velocities. ^ Dislocation velocities of 40 m/sec to 160 m/sec were measured as temperature, strain rate, and stress were varied. It was determined that the dislocation velocity was independent of temperature from 4.2 to 2.0 K. This result shows that in this temperature range either viscous drag or quantum tunneling is controlling the dislocation motion, rather than thermal activation. Stress level and strain rate were shown to have little effect on dislocation velocities. Further, chaos theory has been used to explain observed peak widths in excess of classical predictions. Beyond the knowledge gained from this velocity data, the establishment of an electron - dislocation interaction in iron has greatly added to the basic understanding of plasticity in iron, something which was not previously available. ^

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