PHASE-EQUILIBRIA IN THE ALUMINUM-RICH PORTION OF THE BINARY-SYSTEM CO-AL AND IN THE COBALT ALUMINIUM-RICH PORTION OF THE TERNARY-SYSTEM CO-NI-AL/

Phase equilibria were investigated in the aluminium-rich portion of th e binary system Co-Al in the concentration range x(Co) = 0.04 to 0.28. A phase bundle of intermetallic phases Co4Al13 (h), o-Co4Al13 and m-C o4Al13 has been observed between x(Co) = 0.240 and x(Co) = 0.247. The binary high-temperature phase Co4Al13 (h) is formed by a peritectic re action at 1127 degrees C. It cannot be quenched without lattice distor sion causing significant multiple twinning. By means of slow-cooling C o4Al13 (h) decomposes eutectoidaly at 1083 degrees C into the structur al close-related phases: orthorhombic o-Co4Al13 (o-Co4Al13 type) and m onoclinic m-Co4Al13 (m-Co4Al13 type). Microstructural as well as X-ray diffraction investigations show that the high-temperature phase Co4Al 13 (h) can be stabilized by slight nickel addition. Liquid-quenching o f binary Co-Al alloys in the concentration range x(Co) = 0.240 to 0.27 5 yields the metastable decagonal phase d-CoAl3 (m). In the cobalt/alu minium-rich portion of the ternary system Co-Ni-Al, phase equilibria f or x(Ni) = 0 to 0.09 and x(Al) = 0.72 to 0.87 at 1050 degrees C as wel l as phase equilibria for the partial ternary section Co25-xNixAl75 (0 less than or equal to x less than or equal to 10) have also been inve stigated. An addition of nickel to the aluminium-rich Co-Al alloys dec reases the binary-peritectic temperatures downward to the transition p lanes.