Alloys having composition similar to commercial precipitation-hardenable stainless steels, but modified by the addition of 1.4 to 2.4 wt % boron, are disclosed. The alloys are subjected to a rapid solidification processing (RSP) technique which produces cooling rates between .about.10.sup.5 -10.sup.7 .degree.C./sec. The as-quenched RSP ribbon or powder, etc., consists primarily of a metastable crystalline solid solution phase. The metastable crystalline phases are subjected to suitable heat treatments so as to produce a transformation to a stable multiphase microstructure, which includes borides; this heat treated alloy exhibits superior mechanical properties and thermal stability in conjunction with good corrosion and oxidation resistance.