Surface phase transition in Cu-Fe solid solutions

Particles of a second phase on grain boundaries and free surface in copper-based solid solutions with iron were observed. Previously the samples were tested on diffusion creep for surface free energy measurements. The test method is based on measuring of time dependences of stress in foils. The stresses were low (up to 300 kPa) and most of time the foils were balancing around zero stress. Experiments were performed in a dry hydrogen atmosphere. The temperature range was from about 900 °C to 1070 °C. The volume concentration of iron for second phase formation on surfaces was about 0.4 at% Fe. The particles had uniform distribution on the grain boundaries and free surface. The average sizes are from 20 nm at 0.4 at% to about 1 µm at 0.74 at% Fe. The surface phase transition is confirmed by measurements of viscosity and surface energy. Surface free energy isotherm has extremum near 0.4 at% Fe and temperature dependence of viscosity has a gap at Fe concentration higher 0.4 at%.