Hard second phases in alloys and composites

Probing at the Nanoscale the Structure and Properties of Hard Second Phases in Alloys and Composites

Lionel Michelet, Marta Fornabaio and Andreas Mortensen with former contributions from Vaclav Pejchal, Martin Mueller, Aparna Singh, Goran Zagar, Andreas Rossoll, Cyril Dénéréaz, Raphael Charvet  

Our ambition in this new project funded by an ERC Advanced Grant 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. 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 combined with nanoindentation and original testing procedures, to access 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 mechanical testing are to be identified by means of microstructural characterization, electron microscopy notably, of both virgin and tested particles.

The project, which started in May 2012, is currently in its start-up phase, involving purchase, installation and set-up of nanoindentation equipment, and the design of appropriate focused ion beam machining strategies.