The constitution and phase stability of overlapping melt trails in Ag- Cu alloys produced by continuous laser melt quenching

Coatings consisting of overlapping trails melted with a scanning CW CO₂ laser have been produced on Ag-Cu alloys with the following compositions: Cu 17 at. pct Ag; Cu 37 at. pct Ag; Cu 61.7 at. pct Ag; Cu 71.8 at. pct Ag; and Cu 82 at. pct Ag. The laser beam was scanned at a velocity of 34 cm s¹ and with an intensity of 3.6 MW cm^-2. Selected trails were examined by X-ray diffractometry, optical microscopy, and scanning electron microscopy in the as-irradiated condition and after annealing for various periods of time in the temperature range 100 to 450 ° C. Time-temperature-transformation diagrams based on the annealing studies are presented. Significant amounts of the metastable extended solid solution (γ) were observed in the Ag-rich alloy trails. The silver rich terminal solid solution (α) was also detected, formed probably by solid state precipitation. An α’ phase with lattice parameter lying between that ofy and α was also observed in the Cu 61.7 at. pct Ag alloy. A metastable equilibrium diagram has been constructed and is employed to interpret these observations. The most striking microstructural feature of the trails are bands marking sequential positions of the melt-solid interface. We propose that these bands are evidence for planar, oscillating - steady-state, interface motion. The observation of a periodic cellular breakdown of the planar interface in the Cu 61.7 at. pct Ag alloy is attributed to a diffusional instability previously predicted by Baker and Cahn.