High temperature precipitation hardening in a rapidly quenched Al-Ti-Ni alloy I. Precipitation hardening response

A significant precipitation-hardening response has been observed in rapidly quenched Al-6Ti-1.5Ni (wt.%) alloy aged isothermally in the temperature range 300-500°C, and the underlying precipitate microstructures characterised using transmission electron microscopy (TEM). Primary intermetallic dispersoids of cubic ternary phase in as-quenched alloy decompose rapidly during heat treatment and are replaced by uniform precipitation of fine-scale, coherent particles of a metastable L1₂ phase. These metastable precipitates evolve into a transitional, three-dimensional cross-like morphology and eventually into nano-scale ( < 100 nm) spheroidal particles of equilibrium D0₂₂ phase δ-Al₃(Ti,Ni). The changes in form are accompanied by the development of a series of one-dimensional long period superlattices, culminating in formation of equilibrium b.c.t. phase. Maximum hardness (175 kg mm^-2), which is associated with a dispersion of coherent intermediate precipitates and a minor fraction of δ-Al₃(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 density, rapidly quenched Al-Ti-Ni alloys, with weight ratio Ti:Ni in the range 3:1-4:1, may have potential for applications involving elevated temperatures (150-200°C), where the creep resistance of conventional precipitation-hardened alloys declines rapidly.