Gold Innovations Blog

What is the mechanical strength of pure gold?

Well, it appears to depend on size when one reaches the micro-scale, according to Professor William Nix of Stanford University. He gave the Von Hippel Award talk at the MRS Fall Meeting in Boston in November 2007 (transcript in MRS Bulletin, February 2009 vol 34 (2), 82-91) on “Exploiting New Opportunities in Materials Research by Remembering and Applying Old Lessons”. He reports on how the strength of single crystal gold substantially increases at the sub-micron scale. Surprised? I was!

Material Scientists all know that there is a grain size dependency on the strength and ductility of metals and alloys according to the Hall-Petch equation, but this does not apply in the case of single crystal test specimens. Nix reports on the work of Julia Greer and co-workers (J R Greer et al, Acta Mater. 53, 1821, 2005) on simple uniaxial compression testing of small sub-micron pillars, 400nm diameter and 1960 nm high, of single crystal gold, oriented in the <001> direction, which are made by focused-ion beam machining.  These tests show very high levels of flow stress – over 500 MPa compared to around 30MPa for bulk gold. Reducing the pillar diameter to 290 nm increases the flow stress to around 750 MPa. Truly amazing values in my opinion.

The reason for the jerky stress-strain curves and high flow stress values seen in these experiments were explained in terms of old literature reports from the 1950s/60s (i.e. ‘learning from old lessons’) on how dislocations multiply, i.e. from other dislocations. Nix comments that if the pillar diameter is smaller than the characteristic dislocation length for multiplication, we can expect moving dislocations to exit the crystal more frequently than they multiply. Nix argues that in such situations, the crystals harden by a ‘dislocation starvation’ mechanism; here, plastic deformation is controlled by dislocation nucleation, which requires very high stresses. He also states that this mechanistic theory is hotly debated and disputed by others. Nonetheless, the high strength values are not in dispute!

It is interesting to speculate how one might make use of this phenomenon in real engineering materials. Anyone got any bright ideas for nano-engineered gold?