Intermediate temperature grain boundary phenomena in copper alloys

Copper and copper alloys typically experience a sharp loss in ductility at intermediate temperature, induced by grain boundary damage and fracture processes. The susceptibility of the material to intermediate temperature embrittlement is, as a result, strongly affected by the presence, along the grain boundary, of segregated atoms or low-melting phase inclusions, the former potentially weakening atomic bonding and the second causing liquid metal embrittlement along the boundaries. This thesis is an exploration of underlying phenomena that govern the intermediate temperature embrittlement of Cu-Pb alloys, addressing in turn capillary phenomena and then their relation to the tensile strength and ductility of leaded copper alloys. The dihedral angle shown by intergranular lead inclusions in Cu-1Pb alloys is measured varying the purity of the metal and the temperature. Several measurement methods are used and compared, namely classical 2D methods based on metallurgical cross section analysis, and a 3D stereoscopic method that yields the true three-dimensional angle value for individual inclusions straddling a flat grain boundary (first presented by Felberbaum et al., J. Mat. Sci., vol. 40 (2005) p. 3121). Data confirm and extend earlier measurements using the newer 3D method. It is shown that a discrepancy found between literature data and stereoscopic 3D dihedral angle measurements is not caused by impurity effects, but the data indicate that its origin lies in a difference in average dihedral angle values measured between inclusions straddling two grains, and values found at inclusions located where three or more grains meet. The method is then extended towards the simultaneous measurement of relative grain boundary energy and all five macroscopic grain boundary degrees of freedom in a copper polycrystal containing quenched lead inclusions. Data are compared with a model for grain boundary energy in pure fcc metals. The comparison suggests (i) that relaxation can lower the grain boundary energy significantly when equal numbers of broken bonds are present on either side of the grain boundary and (ii) that lead segregation to grain boundaries affects the orientation-dependence of large-angle grain boundary energies in copper. A study of the tensile strength of two Cu-1Pb alloys of different purity and of different microstructure tested at 400 °C and at a strain rate of 10 s-1 is then presented. Data show that the alloy fracture stress is consistent with a Griffith-type fracture criterion that results from assimilation of the inclusions to a void. Measured fracture stresses suggest a 25-fold increase in fracture energy dissipation over surface tension contributions, likely caused by plastic deformation of the ductile copper phase. Tensile tests conducted at 400 °C and at a strain rate of 10-2 s-1 are then presented for a Cu-Ni-Sn-Pb alloy, which exhibits high mechanical strength at room temperature coupled with good machinability. Additions to this alloy of less than 1 wt% are investigated, namely Al, Mn, Zr, B or P. It is found that, unlike the other alloying additions explored, B and P improve the strength and ductility of the leaded alloy and also that these form second phase particles in the alloy; it is proposed that the improvement results from a capillary and mechanical interaction between these particles and molten lead inclusions in the alloy, consistent with the theory advanced to explain results found for the Cu-1Pb alloy. The thesis concludes with a preliminary exploration of grain boundary engineering strategies towards the improvement of the 400 °C strength of Cu-1%Pb, showing that marginal improvements in both strength and ductility can be achieved.