The growth and mechanical properties of metal whiskers

Abstract

It was demonstrated that single crystal filaments or whiskers of many metals can be grown by the chemical or thermal reduction of halides. The conditions for their growth and some of their physical properties such as crystal orientation, purity, shape, etc. were determined.

When the stress and strain were measured simultaneously, it was found that iron no longer obeys Hooke's law above two percent strain i.e. the stress is no longer proportional to the strain. Deviations from linearity were not observed in copper at least up to 2.8 percent strain.

The tensile properties of the whiskers were determined both in the elastic and plastic region. The elastic strengths of the thinnest copper, iron and silver whiskers were found to be close to those predicted for perfect crystals. As the size of the whiskers increased, their strength decreased with considerable scatter. The whiskers maintained their high elastic strength, at least in bending, at temperatures as high as 900°C. The state of perfection of the whiskers and the cause of the size effect are discussed.

When dislocations were introduced by means of plastic deformation, the whiskers became weak. The copper and silver whiskers exhibited extremely sharp yield points and an "easy glide" region which was characterized by the propagation of Lüders bands.

The mechanism of growth is not the same for all whiskers. Some grow by the addition of material at their tips while others grow at their base. It has been concluded that the whiskers do not grow by the direct condensation of metal vapor. The kinetics of the whisker growth were examined and mechanisms for their growth employing dislocation models were proposed.