The phase formation sequence during the thermally induced solid-state reaction of a 10-nm-thick magnetron-sputtered Ni film with a Si(001) substrate amorphized by ion implantation (a-Si) is investigated using a combination of in situ temperature-resolved x-ray diffraction and ex situ x-ray pole figure analyses, transmission electron microscopy, and Rutherford backscattering spectrometry. Our results reveal (1) that the metastable θ-phase grows from δ-Ni2Si and directly transforms into NiSi and (2) that cleaving the as-prepared, amorphized samples are often subject to a spontaneous reaction possibly resulting from cleaving. In the spontaneously reacted samples, a mixture of θ and NiSi is observed from the beginning of the thermal treatment. The θ phase exhibits a (110) fiber texture with a ±10° misorientation, which is then inherited by NiSi. The rest of the phase sequence is the same as for nonreacted samples, the NiSi2 growing from 390 °C if the amorphized Si is not completely consumed. © 2013 American Vacuum Society.