C.M. Gourlay

Dilatant shear bands in solidifying metals

Compacted granular materials expand in response to shear, and can exhibit different behaviour from that of the solids, liquids and gases of which they are composed. Application of the physics of granular materials has increased the understanding of avalanches, geological faults, flow in hoppers and silos, and soil mechanics. During the equiaxed solidification of metallic alloys, there exists a range of solid fractions where the microstructure consists of a geometrically crowded disordered assembly of crystals saturated with liquid. It is therefore natural to ask if such a microstructure deforms as a granular material and what relevance this might have to solidification processing. Here we show that partially solidified alloys can exhibit the characteristics of a cohesionless granular material, including Reynolds’ dilatancy and strain localization in dilatant shear bands 7–18 mean crystals wide. We show that this behaviour is important in defect formation during high pressure die casting of Al and Mg alloys, a global industry that contributes over

Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography

7.3 billion to the USA’s economy alone and is used in the manufacture of products that include mobile-phone covers and steering wheels. More broadly, these findings highlight the potential to apply the principles and modelling approaches developed in granular mechanics to the field of solidification processing, and also indicate the possible benefits that might be gained from exploring and exploiting further synergies between these fields.

Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions

The behaviour of granular solid–liquid mixtures is key when deforming a wide range of materials from cornstarch slurries to soils, rock and magma flows. Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting. Using synchrotron X-ray tomography, we directly measure the discrete grain response during uniaxial compression. We show that the stress–strain response at 64–93% solid is due to the shear-induced dilation of discrete rearranging grains. This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking. A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying

The mechanisms of Al-Si eutectic refinement due to scandium (Sc) additions have been studied in an Al-7Si-0.3Mg foundry alloy. The evolution of eutectic microstructure is studied by thermal analysis and interrupted solidification, and the distribution of Sc is studied by synchrotron micro-XRF mapping. Sc is shown to cause significant refinement of the eutectic silicon. The results show that Sc additions strongly suppress the nucleation of eutectic silicon due to the formation of ScP instead of AlP. Sc additions change the macroscopic eutectic growth mode to the propagation of a defined eutectic front from the mold walls opposite to the heat flux direction similar to past work with Na, Ca, and Y additions. It is found that Sc segregates to the eutectic aluminum and AlSi2Sc2 phases and not to eutectic silicon, suggesting that impurity-induced twinning does not operate. The results suggest that Sc refinement is mostly caused by the significantly reduced silicon nucleation frequency and the resulting increase in mean interface growth rate.

NiSn4 Formation in As-Soldered Ni-Sn and ENIG-Sn Couples

We show that dilute Al additions can control the size of primary Cu6Sn5 rods in Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, the number of primary Cu6Sn5 per mm2 is ∼7 times higher and the mean three-dimensional length of rods is ∼4 times smaller than in Al-free Sn-0.7Cu/Cu joints, while the area fraction of primary Cu6Sn5 is similar. It is shown that epitaxial nucleation of primary Cu6Sn5 occurs on δ-Cu33Al17 or γ1-Cu9Al4 particles, which are stable␣in the Sn-0.7Cu-0.05Al melt during holding at 250°C. The observed facet relationships agree well with previously determined orientation relationships between δ-Cu33Al17 and Cu6Sn5 in hypereutectic Sn-Cu-Al alloys and result in a good lattice match with <∼2.5% lattice mismatch on two different interfacial planes.

XRD study of the kinetics of β ↔ α transformations in tin

Most research on Sn-Ni solder reactions has focused on the interfacial reactions with the substrate, whereas the microstructure which develops above the intermetallic layers has not been studied in detail. This paper shows that nonequilibrium NiSn4 forms during solidification of the bulk solder in Sn-Ni and Sn-electroless nickel immersion gold (ENIG) solder reactions. With both substrates, the bulk solder solidified to contain Sn-NiSn4 eutectic and primary Ni3Sn4 crystals, and the interfacial layers contained a Ni3Sn4 reaction layer on the Sn side. It is found that Cu, present from dissolution of Cu through cracks in the ENIG layer, promotes the formation of Sn-Ni3Sn4 eutectic. Thus, Sn-ENIG couples contained both Sn-NiSn4 and Sn-Ni3Sn4 eutectic. It is further shown that NiSn4 is not stable at soldering temperatures and that, during isothermal holding at 270°C to 220°C, NiSn4 transforms into Ni3Sn4 and liquid or β-Sn.

Solidification of Sn-0.7Cu-0.15Zn Solder: In Situ Observation

The transformation kinetics of βSn (white tin) to and from αSn (grey tin) are studied by synchrotron X-ray diffraction of seeded powder samples of 99.99% Sn. An analysis of thermal expansion behaviour revealed that the volume change of transformation increases as the temperature decreases. The βSn → αSn transformation was well described by Johnson–Mehl–Avrami kinetics with an Avrami exponent of 3, which was confirmed to be due to three-dimesional growth from pre-existing nuclei by a microstructural study. The αSn → βSn transformation exhibited a decreasing Avrami exponent from ∼4 to ∼1 during the transformation. Time–Temperature–Transformation curves are plotted from the isothermal transformation data and are compared with past work.

Synchrotron radiography of direct-shear in semi-solid alloys

High resolution time-resolved X-ray imaging with synchrotron radiation was used for in situ observation of four distinct events during solidification of a Sn-0.7Cu-0.15Zn solder despite small composition and density differences. These included βSn dendrite growth, Sn-Cu6Sn5 univariant eutectic growth, microporosity formation, and a polyphase reaction in the last stages of freezing. The development of microstructure was described quantitatively by tracking the loci of dendrite tips during grain growth. The results have implications for microstructure control and the understanding of structure–property relationships in Sn-Cu-Zn lead-free solders.

The Influence of Primary Cu6Sn5 Size on the Shear Impact Properties of Sn-Cu/Cu BGA Joints

Understanding phenomena occurring at the scale of the crystals during the deformation of semi-solid alloys is important for the development of physically-based rheological models. A range of deformation mechanisms have been proposed including agglomeration and disagglomeration, viscoplastic deformation of the solid skeleton, and granular phenomena such as jamming and dilatancy. This paper overviews in-situ experiments that directly image crystal-scale deformation mechanisms in equiaxed Al alloys at solid fractions shortly after the crystals have impinged to form a loose crystal network. Direct evidence is presented for granular deformation mechanisms including shear-induced dilation in both equiaxed-dendritic and globular microstructures. Modelling approaches suitable for capturing this behaviour are then discussed.

Growth of Al8Mn5 Intermetallic in AZ91

A method is presented to control the size of primary Cu6Sn5 in ball grid array (BGA) joints while keeping all other microstructural features near-constant, enabling a direct study of the size of primary Cu6Sn5 on impact properties. For Sn-2Cu/Cu BGA joints, it is shown that larger primary Cu6Sn5 particles have a clear negative effect on the shear impact properties. Macroscopic fracture occurred by a combination of the brittle fracture of embedded primary Cu6Sn5 rods and ductile fracture of the matrix βSn. Cleavage of the Cu6Sn5 rods occurred mostly along (0001) or perpendicular to (0001) with some crack deflection between the two. The deterioration of shear impact properties with increasing Cu6Sn5 size is attributed to (1) the larger microcracks introduced by the brittle fracture of larger embedded Cu6Sn5 crystals, and (2) the less numerous and more widely spaced rods when the Cu6Sn5 crystals are larger, which makes them poor strengtheners.