CONTRIBUTION TO TERNARY-SYSTEM AL-TI-B PART-1 - STUDY OF DIBORIDES PRESENT IN THE ALUMINUM CORNER

Diborides present in Al-Ti-B alloys with a weight ratio Ti/B < 2.2 are investigated in the arc melted and annealed conditions as well as in the controlled heated and cooled specimens by means of light and scann ing electron microscopy, energy dispersive spectrometry and X-ray diff raction. In Al-Ti-B alloys with composition close to the AlB2-TiB2 tie line; apparently pure spheroidal and platelike TiB2 particles are pres ent. bz the as cast condition of all other investigated alloys the for mation of AlB2 on primary TiB2 particles is observed regardless of the applied cooling late. In the investigated alloys apparently pure TlB2 and AlB2 coexist even after 1000 h exposure at 800 degrees C, and the formation of mixed mixed diboride (Al, Ti) B-2 was never observed. Th e results of this work are discussed together with the results of prev ious work in this area and it seems very likely that the mixed diborid e (Al, Ti)B-2 is not a thermodynamically stable phase in the aluminium rich corner of the Al-Ti-B system, but only apparently pure AlB2 and TiB2 are present. MST/3822.