High temperature precipitation hardening in a rapidly quenched AlTiNi alloy I. Precipitation hardening response

A significant precipitation-hardening response has been observed in rapidly quenched Al6Ti1.5Ni (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 AlTiNi 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.