  Vacuum heat treatment of mechanically-alloyed powders of boron and aluminum
leads to formation of a metastable Al-rich phase, which can be quenched. Its
structure, composition, and thermal stability are established. With chemical
formula Al1.28B the rhombohedral phase is unusually rich in Al. Parameters of
the unit cell determined from X-ray powder diffraction are: a=18.3464(19)
{\AA}, c=8.9241(9) {\AA}, 2601.3(6) {\AA}3, space group R-3. It is stable at
heating up to 630 {\deg}C. It is suggested that this phase is an important
intermediate step in formation of AlB2 and, eventually, of other borides and
its nucleation and thermal stability are explained by high elastic energy
hindering formation of equilibrium phases at low temperatures.
