Contribution to ternary system Al-Ti-B Part 1: Study of diborides present in the alulminium corner

Diborides present in Al-Ti-B alloys with a weight ratio TilB < 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 scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. In Al- Ti-B alloys with composition close to the AlB2-TiB2 tieline; apparently pure spheroidal and platelike TiB2 particles are present. In the as cast condition of all other investigated alloys the formation of AlB2 on primary TiB2 particles is observed regardless of the applied cooling rate. In the investigated alloys apparently pure TiB2 and AlB2 coexist even after 1000 h exposure at 800° C, and the formation of mixed diboride (Al, Ti) B2 was never observed. The results of this work are discussed together with the results of previous work in this area and it seems very likely that the mixed diboride (Al, Ti)B2 is not a thermodynamically stable phase in the aluminium rich corner of the Al-Ti-B system, but onlyap...