Formation and characterization of an Al‐rich metastable phase in the Al–B phase diagram

Vacuum heat treatment of mechanically alloyed powders of boron and aluminium leads to the formation of a metastable Al‐rich phase, which can be quenched. Its structure, composition and thermal stability are established. With the chemical formula Al1.28B the rhombohedral phase is unusually rich in Al. The parameters of the unit cell determined from X‐ray powder diffraction are a = 18.3464 (19), c = 8.9241 (9) Å, V = 2601.3 (6) Å3, space group R3. It is stable on heating to 630°C. It is suggested that this phase is an important intermediate step in the formation of AlB2 and, eventually, of other borides; its nucleation and thermal stability are explained by high elastic energy hindering the formation of equilibrium phases at low temperatures. A new metastable Al‐rich phase (chemical formula Al1.28B) stable up to 630°C is synthesized by vacuum heating of mechanically alloyed Al and B powders. Its structure is solved from X‐ray powder diffraction.