HIGH-TEMPERATURE PRECIPITATION HARDENING IN A RAPIDLY QUENCHED AL-TI-NI ALLOY .1. PRECIPITATION HARDENING RESPONSE

A significant precipitation-hardening response has been observed in ra pidly quenched Al-6Ti-1.5Ni (wt.%) alloy aged isothermally in the temp erature range 300-500 degrees C, and the underlying precipitate micros tructures characterised using transmission electron microscopy (TEM). Primary intermetallic dispersoids of cubic ternary phase in as-quenche d alloy decompose rapidly during heat treatment and are replaced by un iform precipitation of fine-scale, coherent particles of a metastable L1(2) phase. These metastable precipitates evolve into a transitional, three-dimensional cross-like morphology and eventually into nano-scal e (< 100 nm) spheroidal particles of equilibrium D0(22) phase delta-Al -3(Ti,Ni). The changes in form are accompanied by the development of a series of one-dimensional long period superlattices, culminating in f ormation of equilibrium b.c.t. phase. Maximum hardness (175 kg mm(-2)) , which is associated with a dispersion of coherent intermediate preci pitates and a minor fraction of delta-Al-3(Ti,Ni), is comparable with that of conventional high strength precipitation-hardening alloys (150 -200 VHN). The temperatures of this ageing response, together with the thermal stability of the precipitate phase(s), suggest that low densi ty, rapidly quenched Al-Ti-Ni alloys, with weight ratio Ti:Ni in the r ange 3:1-4:1, may have potential for applications involving elevated t emperatures (150-200 degrees C), where the creep resistance of convent ional precipitation-hardened alloys declines rapidly.