The Effect of Composition on the Formation of a BCC Approximant in Ti–Cr–Al–Si–O Alloys

The microstructure, phase composition, and mechanical properties of Ti–Cr–Al–Si–O alloys in ascast state and after annealing at 800°C have been studied. The as-cast alloys with 50 and 60 at.% Ti have two phases: a 1/1 α(TiCrSi) cubic approximant of the quasicrystalline phase and an intermetallic Cr2Ti(C14) Laves phase in different ratios. The as-cast alloy with a high titanium amount (72 at.%) contains three phases: a 1/1α(TiCrSi) approximant and two α-Ti and β-Ti solid solutions. The effect of annealing on the phase composition of the alloys has been studied. After annealing, the alloys with the lowest (50 at.%) and highest (72 at.%) Ti contents change their phase composition with the formation of Ti5Si3 silicide, while the alloy with 60 at.% Ti exhibits high thermal stability—its phase composition remains unchanged. The changes in hardness of the starting and annealed alloys have been examined by automatic indentation. The highest hardness, 11.7 GPa, is shown by the as-cast Ti 72 Cr 20 Al 2 Si 2 (SiO 2 ) 4 alloy. The hardness of the annealed Ti 50 Cr 30 Al 13 Si 2 (SiO 2 ) 5 alloy increases to 14.8 GPa.