Prof Bill Gerberich, Brittleness Transitions: Strength, Strain Rate, Structure and Scale
Date: Tue 15 Oct 2013, 13:00 - 14:00
Location: Nanoforce Seminar Room (Joseph Priestley Building)
Brittleness Transitions: Strength, Strain Rate, Structure and Scale
W.Gerberich, E. Hintsala and P. Suri
With increasing temperature, almost all classes of materials with cubic crystal structures undergo brittle to ductile transitions. This includes (BCC) body-centered cubic (Fe,W) metals, to “BCC” like (CsCl, CsI) halides, to many FCC ceramics to diamond cubic or “FCC” like (Si, GaAS) semiconductors among others. The commonality is at what temperature crystal plasticity, most generally through dislocation mobility, prevents or retards brittle fracture. Dislocation mobility is the chosen term here since it strongly depends on all of the above Ss in the title. This evolves through the thermal component of the flow stress, the dynamics of imposed strain rates and the microstructure through its relation to the size scale. Details of thermally-activated parameters are shown to be related to all of these parameters. This gives the brittleness transition dependence a common base set for all of the above materials. Details are shown for single crystals of Fe-3% Si a metal, for Cesium Iodide a halide, and Silicon a semiconductor. A proposed but incomplete model demonstrates how shifts in the brittleness transition due to strain rate or scale can depend on dislocation shielding of pre-existing cracks.