31.12.06 - Microstructure and Embrittlement of Leader Copper Alloys. Thèse EPFL, n° 3217 (2005). Dir.: Prof. A. Mortensen.

"For the development of a new approach to the measurement of dihedral angles at intergranular inclusions, leading in turn to an improved understanding of the strength of liquid-containing alloys."

Microstructure and Embrittlement of Leader Copper Alloys.

Mechanisms responsible for the embrittlement of leaded "free-machining" copper alloys at intermediate temperature (i.e. around 400 °C) are examined using a combination of microstructural and mechanical characterisation.

In these alloys, lead is mostly present as discrete intergranular inclusions. Besides the size (in the mm range), the dihedral angle f is the main microstructural parameter.

A new technique is proposed and demonstrated for the measurement of f. The method is based on the selective dissolution of the content of the inclusions visible along a polished section, and, successive 3-D reconstruction of the inclusions envelope. f is therefore deduced from the three-dimensional shape of a single inclusion.

The evolution of f as a function of external stress is theoretically estimated. The derivation is based on the minimization of the global capillary and elastic strain energies. An adimensional parameter emerges from the analysis as the sole factor governing the shape of the inclusion.

This parameter depends on the external stress, the inclusion volume, the elastic properties of the solid, and the relevant interfacial energy.

The inclusion may become unstable, and thus induce fracture, once this parameter exceeds a critical value.

A good correlation is found between predicted and measured fracture stresses. This is true for both a model Cu-Pb alloy and for leaded industrial high-resistance Cu-Ni-Sn alloys. The analysis can therefore be used as a guideline for solving the systematic quench cracking of these alloys.