THE EFFECT OF SUBSTRATE ON THE MICROSTRUCTURE AND CREEP OF EUTECTIC IN-SN

This study was conducted in order to determine and understand the effe ct of substrate on the behavior of eutectic In-Sn. Samples for mechani cal testing were produced with either bare Cu or Ni on Cu substrates. Both the microstructure and the mechanical behavior are strongly depen dent on substrate. When eutectic In-Sn is joined to bare Cu, Cu diffus ion into the joint causes the alloy to become off-eutectic, giving a n onuniform and irregular microstructure. The addition of a layer of Ni acts as a diffusion barrier, preventing Cu diffusion sufficiently such that a uniform, normal colony-based eutectic forms. Deformation is mo re uniform in the In-Sn on Ni, while it is concentrated along the leng th of the joint in the In-Sn on Cu. This distinction is reflected in t he different shapes of shear stress-strain curves between In-Sn on Cu and In-Sn on Ni. The stress exponents and activation energies for cree p also vary with substrate. Creep deformation is governed by the In-ri ch beta phase for In-Sn on Cu and by the Sn-rich gamma phase for In-Sn on Ni. If In-Sn on Ni samples are aged, the microstructure coarsens a nd the mechanical behavior changes to resemble that of the as-cast In- Sn on Cu.