Microstructures and mechanical properties of the sputter deposited copper(x) tantalum(1-x) films

Abstract

Copper-refractory metal composites/alloys are of interest for aerospace and related applications requiring good thermal conductivity and high strength at elevated temperatures. These materials, due to generally very low mutual solubilities, may allow development of high strength microstructures which retain their physical properties at temperatures for exceeding those suitable for precipitation strengthened alloys.

A series of Cu$\sb{\rm 1-x}{\rm Ta}\sb{\rm x}$ alloys (0$<{\rm x}<$0.2) were created by RF sputter deposition. Cu-Ta films deposited at-120$\sp\circ$C contained nanoscale FCC Ta particles in a Cu matrix, with some Ta in solid solution. The amount of Ta in solution increased with increasing overall Ta content. Films deposited at 100$\sp\circ$C contained discrete, 2-3 nm diameter FCC Ta particles oriented isomorphically with the Cu matrix. Some of the FCC Ta particles are self-aligned onto Cu $\{111\}$ and $\{100\}$ habit planes. No detectable Ta was found in solid solution in Cu.

The films were annealed (900$\sp\circ$C, 1 hour) and uniaxially hot-pressed (35MPa, 900$\sp\circ$C, 1 hour). The hot-pressed and annealed Cu-16%Ta films form 50-100nm diameter, randomly oriented BCC Ta particles which are randomly distributed in the Cu matrix and co-exist with the isomorphically oriented nanoscale FCC Ta particles. Annealing the Cu-6%Ta film at 900$\sp\circ$C up to 100 hours does not change the size or distribution of the self-aligned Ta particles. In addition the Cu grain size remains sub-micron.

Lattice imaging revealed that the Cu/Ta interphase boundaries are partially coherent, with the mismatch accommodated by a/2$\langle 100\rangle$ misfit dislocations spaced $\sim$10A apart. Amorphous regions were found to often surround the FCC Ta particles. FCC particles are believed to nucleate and grow from the amorphous regions.

Tensile, Knoop hardness and nanoindentation testing showed that Ta effectively strengthened the Cu matrix and the mechanical strengths do not decrease even after annealing at 900$\sp\circ$C for 100 hours.