Discussion of “fabrication of composites by diffusion processing”

Abstract

Although solid state bonding is an attractive method for the fabrication of composite materials, Darroudi, Vedula, and Heckel have reported in a recent paper, I~ the limitations of the technique for Ni-AI system. They have prepared the composites by hot pressing alternate layers of nickel wire and aluminum foil at a temperature of 500~ and then carrying out the diffusion reactions at 600~ The ultimate tensile strength values of their composites containing 7 and 13 vol pct. 0.127 mm nickel wires were very low (generally in the range of 8.45 to 9.16 kg per sq ram) and almost independent of reaction time and initial nickel content. These strength values are about one-half to one-third of those reported ~3'14 for unidirectionally solidified ALNiAl3 composites containing only 10 vol pct of NiAl3. Though they could not make composites with acceptable mechanical properties by using the solid state reactions on Ni-AI system, they believed that the technique is useful in other alloy systems. The purpose of the present investigation is to show how the solid state reactions can be used for Ni-Al composites by controlling the interface reactions. Composites were prepared by using 0.31 mm nickel wire and aluminum powder by using the hot pressing technique. Desired quantities of the nickel wire and aluminum powder are introduced alternatively into a rectangular hot pressing die, as carefully as possible to ensure uniform distribution and unidirectional orientation of the wire. Hot pressing was carried out under a pressure of 11 kg per sq mm for temperatures ranging from 500 ~ to 600~ and time varying from 9 • 10 z to 3.6 x I0 ~ sec. Composites prepared at 525~ and 1.8 • 103 sec were found to have maximum values for tensile strength. Tensile strength values of the order of 20.7 kg per sq mm and 29 kg per sq mm were obtained for composites containing 12 vol pct and 20 vol pct of the nickel wire respectively. These values are in agreement with the values predicted by the rule of mixtures. ~5 Fabrication conditions such as lower temperatures and time durations resulted in a composite with poor mechanical properties, mainly because of inadequate bonding between the matrix and the wire. Higher temperatures and longer time durations also deteriorated the properties of the composite, because of excessive interface reaction. Fig. 1 shows the interface microstructure resulting in the maximum strength of the composites while Fig. 2 shows a typical interface structure formed as a result of excessive interface reaction. The deterioration of the properties of the composites formed by exces-