Ultrafast precipitation kinetics in an ultrafine-grained Al–Cu alloy used for oil drill pipes

In the ultrafine-grained (UFG, <1 μm) Al–Cu alloys having very poor thermal stability, only intergranular equilibrium θ-Al₂Cu precipitates were found after aging treatment, which means that the precipitation was greatly accelerated even at room temperature and has directly transformed to equilibrium θ phases while bypassed the metastable phases GP zones, θ″ and θ′ as usually seen in representative precipitation transition sequence of coarse grained (CG, >10 m) Al–Cu alloy. It was found the precipitation kinetics of θ phases in the UFG Al–Cu alloy occurred at several orders of magnitude faster than that can be predicted by conventional grain boundary precipitation theory. Considering ultrafast solute diffusion at the UFG length-scale, a developed/modified precipitation kinetics model at the UFG length-scale has been established. It yields predictions in satisfied accordance with experimental aging precipitation process/parameters of UFG Al–Cu alloys.