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.

Microstructure and embrittlement of leaded copper alloys

Laurent Felberbaum

Mechanisms responsible for the embrittlement of leaded "free-machining" copper alloys at intermediate temperatures (i.e. around 400 °C) are examined using a combination of microstructural and mechanical characterization. Tensile tests are conducted on pure copper and on industrial high-strength Cu-Ni-Sn alloys, comparing leaded and unleaded versions, at strain rates ranging from 10-4 to 10 s-1, and various temperatures between 25 °C and 800 °C. It is found that, in the leaded samples, both liquid metal embrittlement and grain boundary embrittlement operate at intermediate temperature,i.e., near the melting point of lead. Their respective importance depends mainly on the strain rate: fracture at high strain rate (10 s-1) displays characteristic signatures of liquid metal embrittlement, while at low strain rate (10-4 s-1) the ductility trough behaviour can be attributed to grain boundary embrittlement. In leaded copper and copper alloys, lead is mostly present as discrete inclusions, frequently located along grain boundaries. The dihedral angle of intergranular lead inclusions is then the main microstructural parameter besides their size. A new technique is proposed and demonstrated for the measurement of inclusion dihedral angle in a high-purity Cu-1 wt.% Pb alloy, based on room temperature characterization of samples that were held at elevated temperature at times sufficiently long for shape equilibration of the inclusions and then rapidly quenched. Quantitative scanning electron microscopic analysis of metallographic surfaces along which individual inclusions have been dissolved is used to deduce the dihedral angle using a mathematical fit of their solid/liquid interface. For a specific temperature, we show that the dihedral angle is not unique; this reflects the fact that high-angle grain boundary energies are not constant in copper. Existing micromechanical analyses of shape equilibrium for intergranular inclusions in the presence of external stress are adapted to produce an improved description at low stress and to take into account the inclusion bulk compressibility. The derivation is based on minimization of the global capillary and elastic strain energy associated with such inclusions. An adimensional parameter that depends on the applied stress, the inclusion volume, the elastic properties of the solid, and the relevant interfacial energies emerges from the analysis as the sole factor governing the shape of the inclusion. If void nucleation occurs easily within the inclusion, such that its apparent bulk modulus is nil, the inclusion become unstable, degenerating to a crack when this adimensional parameter exceeds a critical value. Measurements of the dihedral angle on samples previously subjected to a remote stress at 400 °C show that applied stress causes a reduction in the apparent dihedral angle of liquid lead inclusions. Calculated critical stresses for both leaded pure copper and leaded copper-nickel-tin alloys that fail at high strain rate by liquid metal embrittlement are calculated for individual inclusions in the metal. These calculated critical stresses correlate relatively well with the measured fracture stress, suggesting that shape instability of the inclusions cause fracture of the alloys. This result is applied towards the suggestion of general strategies for reduction of the susceptibility of leaded copper alloys to liquid metal embrittlement. A case study addressing the problem of quench cracking of leaded Cu-Ni-Sn alloys on the basis of experimentation and finite element thermomechanical simulation, showing how the problem can be alleviated in production, concludes the thesis.

EPFL2005

Characterization of capillary forces during liquid metal infiltration

Maryam Bahraini Hasani

This thesis addresses the link between capillarity parameters, i.e. surface tension and contact angle, and the infiltration of porous preforms with molten metal by pressure infiltration The method used is to conduct pressure infiltration experiments of selected preforms with molten aluminum or copper based alloys, and characterize the level and, where relevant, kinetics of metal ingress into the preforms. Several systems are investigated in turn, comprising chemically inert and reactive systems. Capillarity in the infiltration of alumina particulate preforms with copper are investigated by means of infiltration experiments conducted at 1200°C under argon with a low oxygen partial pressure, characteristic of equilibrium with carbon at that temperature. The experiments were run in a gas pressure driven liquid metal infiltration apparatus capable of melting copper under gas pressures up to 20 MPa. Capillarity parameters are extracted from drainage curves that plot the volume fraction of metal in the porous medium vs. the applied pressure. Mercury porosimetry, which is based on the same principle, is also used on similar preforms for comparison. The effect of volume fraction, particle geometry and capillary parameters on the drainage curve are studied and compared with the expression proposed by Brooks and Corey. The influence of the particle volume fraction and capillary parameters characterizing wetting in the two systems is discussed to derive an effective contact angle for wetting of alumina particles by molten metal. The value thus derived agrees with literature data from sessile drop experiments. A new technique is proposed for the direct measurement of capillary forces during infiltration in systems of relevance to the processing of metal matrix composites. A custom-designed device enables dynamic tracking of the molten metal surface while the infiltration apparatus is pressurized. It comprises an LVDT (Linear Variable Differential Transformer) that is fixed to the top of the infiltration machine and whose core is connected via an alumina tube to a graphite plunger that floats atop the liquid metal. Upon pressurization, the floater movement caused by flow of the metal into the preform is tracked by the LVDT. Capable of handling melt temperatures up to 1250°C and infiltration pressures up to 20 MPa, the method is essentially a high-temperature analogue of mercury porosimetry. Its accuracy is demonstrated by comparison with data obtained using other techniques and its use is illustrated with the infiltration of diamond particle preforms by Al and Al-Si alloys at 700°C. The present method for direct measurement of drainage curves during the infiltration process is then used for the characterization of wetting of SiC particle performs by molten aluminum and Al-12.2at%Si. The ceramic particle diameter is varied from 7 µm to 40 µm and infiltration was characterized at 750°C, varying the applied pressure from 0.1 to 10 MPa. From these data, the relevant work of immersion is calculated by comparison to the work exerted by the external gas pressure over the whole saturation range. The thus determined work of immersion is translated in an apparent contact angle during infiltration, which is compared with the values determined by other techniques reported in literature to find good agreement. Kinetics effects in reactive systems such as SiC/Al and graphite/Cu-Cr were studied by direct measurement of the drainage curves varying the pressurization rate and conducting measurements interrupted at a given pressure. In addition to the analysis of the capillarity parameters already used for the non-reactive systems, the data are analyzed by a simple model based on the Brooks-Corey relation allowing to link the triple-line velocity in liquid metal infiltration with the saturation given one characteristic distance. It is found that alloying elements in systems investigated drive infiltration at fixed pressure, but do not assist pressure infiltration. Quite to the contrary, they may even hamper efficient infiltration by blocking narrow channels via formation of reaction products with the preform material.

EPFL2007

Probing the local strength and toughness of microscopic hard second phases in metals and composites

Marco Cantoni, Raphaël Charvet, Cyril Dénéréaz, Willy Dufour, Marta Fornabaio, Lionel Michelet, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Goran Zagar

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases, e.g. silicon particles in aluminium alloys, carbides in steels or ceramic particles in metal matrix composites. It is well understood that the mechanical properties of such alloys and composites are strongly influenced by those of their reinforcing particles; yet not much is known of the nature and size of strength-limiting flaws within such microscopic second phases. To investigate the strength and fracture toughness of these microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting techniques and bespoke finite element simulation. Local strength is probed by micromachining individual reinforcing particles in such a way that tensile stresses are produced in a volume of material free of FIB and polishing artifacts upon the application of a load via nanoindenter. Fracture toughness is measured by the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are currently being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, with the objective to measure their local mechanical properties and identify microstructural flaws that govern the strength of such particles, using in-depth electron microscopy characterization combined with microtesting.