Our ambition is to probe the strength of “second” phases in multiphase metal alloys and composites, meaning of hard particles added to strengthen a metal, or alternatively of brittle inclusions that weaken it. Such phases are ubiquitous in structural metals; yet not much is known of the microstructural features that govern their strength.The underlying hypothesis of this project is that defects that limit the strength of such hard second phases can be identified and then altered by processing. Motivations for this enquiry stem from our previous research on metal composites, coupled with the fact that modern methods of nanoscale mechanical characterization now make such a quest feasible.Operationally, we plan to apply and extend nanomechanical testing to probe the strength of micrometric, irregularly shaped, hard particles currently used to strengthen metals. We aim to test such particles whole, and also for their local internal properties. Testing will rely on focused ion beam machining and adapted mechanical nanoprobing. These techniques will be combined to probe, using nanoindentation and original testing procedures, local and global strength values for hard second phase particles. Materials systems to be investigated are: (i) ceramic particles for the reinforcement of metal composites; (ii) silicon in aluminium, (iii) cementite and MC carbides in steel. Defects limiting the strength of these hard brittle phases suggested by nanoscopic mechanical testing will be identified using in-depth microstructural characterization, by electron microscopy notably, of both virgin and tested particles. The data will be supplemented by mechanical testing of macroscopic samples containing the hard particles in question. Processing routes will be explored, towards identification of strategies by which the strength of such second phases can be improved to improve, in turn, the performance of several important engineering materials.