Induced Plastic Flow Research Articles

Acoustic Properties of Metals and Magnetostrictive Materials

The process of inducing plastic flow in metals under the combined action of static and large vibratory stresses using a high-powered horn driver is discussed. In particular, where the metal is ferromagnetic, one has the situation that many properties of the material, including magnetic properties, change under the combined stress action. This necessarily affects the end uses of the material as, for example, in magnetostrictive transducers. Experiments are described in which the properties of nickel are investigated, for example, and the plastic flow threshold curve under combined stress action is determined. It is found that vibratory stress is much more effective in producing flow than static stress. In addition, it is found that, subsequent to the occurrence of plastic flow, the mechanical Q of a sample is altered but partially recovers over a period of hours at room temperature, and the material is hardened against further flow. The separation of the total energy loss into macroeddy, elastic, and microeddy loss is discussed. The alteration of the magnetic properties of plastically flowed samples is quite marked at points near the flow maxima in the samples containing standing intense acoustic waves, and the change in internal friction provides a possible means for predicting impending fatigue in metals without destruction of the materials. [This work was supported by the Office of Naval Research.]

X-Ray-Induced Dislocation Generation in Sodium Chloride

The strain distribution, induced by subjecting one-half of a sodium chloride single crystal to x rays while shielding the other half, has been investigated with a dislocation etch pit technique. Due to the expansion of the irradiated half, a strain gradient arises antisymmetrically about the plane of the interface that is sufficient in magnitude to cause plastic deformation in the protected half, but not in the irradiated half of the crystal. The plastic deformation takes the form of a number of high-density dislocation bands which tend to align in a [100] direction. The bands increase in number but not appreciably in size as irradiation proceeds. The absence of plastic deformation in the irradiated half can be explained by the fact that irradiation raises the yield strength to such a level that the strain is not sufficient to induce plastic flow. In the protected region, however, dislocations are generated and glide until they impinge, causing discrete bands to form roughly parallel to (100) planes. From an analysis of the data, an estimate of the magnitude of the strain produced at the interface and the relationship between the strain and the number of F centers generated can be obtained. The results, although in some ways unexpected, are consistent with previous experiments performed on x-irradiated alkali halides.