Search

Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed.

Antimony is attracting interest as an addition to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt.% antimony as the major addition in two ball grid array (BGA) packages. SbSn particles are shown to precipitate on both Cu6Sn5 and as cuboids in β-Sn, with reproducible orientation relationships and a good lattice match. Similar to Sn-Ag-Cu solders, the microstructure and damage evolution were generally localised in the β-Sn near the component side where localised β-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and β-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solid-state precipitation within β-Sn dendrites, which supplements the Ag3Sn that formed in a eutectic reaction between β-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic β-Sn.

Magnesium alloys are commonly melted and held in steel or cast iron crucibles and small but important amounts of Fe and C can dissolve into Mg-rich melts. Here, carbide formation is studied during interface reactions between solid Fe-xC alloys (x = 0–3.6 wt%) and liquid Mg-9Al-0.7Zn-0.2Mn (wt.%, AZ91) at temperatures from 700 to 800 °C. Two ternary carbides, AlFe3C and Al2MgC2, formed in the reaction layers between the Fe-C and AZ91, and T2-Al2MgC2 additionally formed within the AZ91 alloy due to carbon pickup. T2-Al2MgC2 grew in liquid AZ91 as hexagonal plates that were commonly twinned. A reproducible orientation relationship was measured between T2-Al2MgC2 and α-Mg, and the grain refinement of magnesium by heterogeneous nucleation on T2-Al2MgC2 is explored.

Manganese-bearing intermetallic compounds (IMCs) are important for limiting micro-galvanic corrosion of magnesium-aluminum alloys and can initiate cracks under tensile load. Here, we use electron backscatter diffraction (EBSD), deep etching, and focussed ion beam (FIB) tomography to investigate the types of Al-Mn phases present, their faceted growth crystallography, and their three-dimensional distribution at different locations in high-pressure die cast (HPDC) AZ91D. The Al-Mn particle size distributions were well-described by lognormal distributions but with an additional population of externally solidified crystals (ESCs) formed in the shot chamber analogous to α-Mg ESCs. The large Al8Mn5 particles were cyclic twinned. Differences in the particle size distributions and number density in the center compared with the HPDC skin are identified, and the spatial relationship between Mg17Al12 and Al-Mn particles is explored.

The influence of Ni on the size and twinning of primary Cu6Sn5 crystals in Sn-0.7Cu-0.05Ni and Sn-xCu (x = 0.7, 0.9, 1.1) (mass%) solder joints is studied using synchrotron radiography and SEM-based EBSD. It is shown that the Ni addition does not cause significant refinement of primary Cu6Sn5 if the alloy is fully melted. However, for peak temperatures ≤250 °C relevant to industrial soldering, primary Cu6Sn5 are not completely melted in Sn-0.7Cu-0.05Ni and there are 10–100 times more numerous and smaller crystals than in Sn-0.7Cu. X-shaped Cu6Sn5 crystals with an angle of ∼70° commonly formed in Sn-0.7Cu-0.05Ni/Cu joints and are shown to be penetration twins. This type of growth twinning was only found in partially melted samples, both in Sn-0.7Cu-0.05Ni/Cu joints and binary Sn-1.1Cu alloy. The frequency of twinned crystals was significantly higher in Ni-containing solders. The results are discussed in terms of the influence of Ni on the Cu6Sn5 liquidus slope and on the lattice parameters of (Cu,Ni)6Sn5.

Sn-0.7Cu-0.05Ni is a widely used Pb-free solder that solidifies into a near-eutectic microstructure and a small fraction of primary Cu6Sn5. This paper overviews in-situ time-resolved imaging experiments on the solidification of Sn-0.7Cu-0.05Ni solder under three conditions: (i) directional solidification, (ii) continuous cooling in a near-uniform thermal field, and (iii) solder joint solidification on a Cu substrate. Primary Cu6Sn5 grow as rods along [0001] in each case but can also grow as X-shaped crystals in (iii). There are significant differences in eutectic growth due to nucleation difficulties for tin in conditions (ii) and (iii).

Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single βSn grain or twinned grains. With so few and variable βSn orientations, each joint is mechanically unique due to the anisotropy of tetragonal βSn. Here we explore the potential of increasing the number of βSn orientations in solder joints by promoting heterogeneous nucleation during solidification. It is shown that large (60 g) samples can be grain refined effectively by combining growth restricting solute with potent heterogeneous nucleant particles introduced by alloying additions of Zn, Ti, Co, Ir, Pd, or Pt. These mechanisms are discussed with reference to the grain refinement of structural casting alloys. In 500 μm solder balls, these grain refinement approaches were less effective and a relatively high cooling rate of 17 K/s combined with solute and nucleant particles were required to trigger multiple nucleation events. With this approach, up to 12 independent grains formed in 500 μm balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions.

The potential of cobalt substrates to control the microstructure of 550 μm Sn-3Ag-0.5Cu ball grid array (BGA) joints is explored. It is shown that cobalt substrates give a small and reproducible nucleation undercooling for tin and prevent the formation of large primary Ag 3 Sn blades and Cu 6 Sn 5 rods. βSn dendrites grew from the CoSn 3 reaction layer and the βSn grains are shown to inherit their orientation from the CoSn 3 layer due to heterogeneous nucleation with the following orientation relationship: (100) Sn ||(100) CoSn3 with [001] Sn ||[001] CoSn3 . Changes in the dendrite and eutectic microstructure are shown to be caused by the reduced nucleation undercooling for βSn and the altered solidification path due to cobalt substrate dissolution.

The improvement of solder joint mechanical performance due to bismuth additions is of ongoing interest. This paper investigates the influence of 0–14 wt% Bi on microstructure formation in Sn-0.7 wt%Cu-0.05 wt%Ni-xBi solder balls on Cu substrates. It is shown that Bi additions reduce the Ni content of (Cu,Ni)6Sn5, increase the size of primary (Cu,Ni)6Sn5 rods and reduce the thickness of the Cu6Sn5 reaction layer, although these effects are subtle until the Bi content reaches ∼5 wt%Bi and Bi additions of ≤5 wt% did not significantly affect the positive effects of Ni. Higher Bi additions (≥8 wt% Bi) caused a transition of the tin growth texture from columnar grains with a fibre texture towards a single grain with subgrains. Bi additions increased the non-equilibrium freezing range and caused a nonequilibrium ternary eutectic reaction in joints containing ≥2 wt% Bi. Bi contents less than 2 wt% are required to prevent (Bi) phase formation during solidification.

The Sn-0.7Cu-0.05Ni solder composition was first introduced in 1999 as a Pb-free interconnection material for the electronics industry and quickly earned widespread popularity as a cost-effective alternative to the Ag-containing solders then being widely promoted as the best way of meeting the requirement of the impending EU RoHS regulations. Recently there has been ongoing industrial interest in the development of next-generation Pb-free and Ag-free electronic solder alloys with enhanced reliability in a wider range of applications. In the search for improved reliability, first generation Pb-free solders are being modified with alloying additions, such as Bi, Sb and In to improve their performance in harsh environments. The present investigation explores microstructures formed in Sn-0.7Cu-0.05Ni-X/Cu solder joints (where X = 0-5wt%Bi, Sb, In or combinations thereof) and explores the correlation between the resulting solder joint microstructures and their mechanical response during shear impact testing.

When two solid metals are in contact at high temperature, interdi usion occurs leading in some cases to the growth of intermetallic compounds. The study of nucleation, growth and properties of these intermetallic compounds are of interest since it can be critical for many applications in industries. Yet, the e ect of these reactions on the initial surfaces of both metals is not well understood and particularly when surfaces are not perfectly flat and for short contact time. The purpose of the present study is to demonstrate that the growth of an intermetallic compound layer between to solid metals can lead to the surface reconstruction of one of them. The silver–zirconium system will be presented in order to illustrate this new phenomenon. The e ect of contact point on the di usion- reaction process has been modelled by patterning the Zr surface. The nucleation and growth of the intermetallic compounds occur along the contact points which leads to silver surface reconstruction with the growth of the preferential crystal planes f 111 g ad f 100 g . A model explaining this new phenomenon is developed based on the minimisation of Gibbs energy and the di usion rates af both Ag & Zr atoms in the binary system Ag / Zr.

Sn-3Ag-3Bi-3In solder has been investigated to improve the understanding of microstructure formation in this solder during solidification and soldering to Cu and Ni substrates. The as-solidified microstructures of Sn-3Ag-3Bi-3In samples were found to consist of a significant fraction of βSn dendrites with a complex eutectic between the dendrites. In total five phases were observed to form during solidification: βSn, Ag3Sn, Bi, ζAg and a “Sn-In-Bi” ternary compound. Soldering of Sn-3Ag-3Bi-3In to substrates changed the phase equilibria in the system and caused the formation of additional phases: Cu6Sn5 during soldering to Cu and Ni3Sn4 and metastable NiSn4 during soldering to Ni. It is shown that metastable NiSn4 forms as a primary phase in a complex 5-component Sn-3Ag-3Bi-3In-Ni system. In and Bi were detected in solid solution in the βSn matrix in amounts of ~1.5-2at% and ~1.2at% respectively. Bi also existed as fine particles of two distinct types. (i): sub-micron (

The solidification of Sn-3Ag-0.5Cu and Sn-0.7Cu-0.05Ni are overviewed and compared. In joints on Cu substrates, both solders begin solidification with primary Cu6Sn5 growing in the bulk liquid prior to tin nucleation. In freestanding balls and joints, SAC305 generally solidifies with a single tin nucleation event and exhibits a mutually-twinned tin grain structure. In contrast, SN100C BGA balls and joints exhibit multiple independent tin grains that grow as a columnar array in joints.

Sn-Cu-Ni solders were introduced to electronic assembly in 1999 and the Sn-0.7Cu-0.05Ni (wt%) composition was widely accepted as a Pb-free and Ag-free alternative to Sn-Ag-Cu alloys for wave and reflow soldering. The minor Ni addition of 0.05wt% was shown to have several beneficial effects on the Cu6Sn5 intermetallic that form during soldering. In recent years, Bi additions have been identified as a beneficial alloying component that significantly improves the mechanical properties of solder alloys. The present investigation explores BGA joint microstructures that form in Sn-0.7Cu-xNi-yBi solders and seeks to identify the Ni and Bi levels that deliver optimum performance in soldering processes and reliability in service.

This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu6Sn5 in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu6Sn5 intermetallics, making them small. In contrast, without Ni, primary Cu6Sn5 intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu6Sn5. The results provide direct evidence of the sequence of events in the reaction of Ni-containing Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu6Sn5 intermetallic.

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 γ 1-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

The solidification of Pb-free solder joints is overviewed with a focus on the formation of the βSn grain structure and grain orientations. Three solders commonly used in electronics manufacturing, Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu-0.05Ni, are used as case studies to demonstrate that (I) growth competition between primary dendrites and eutectic fronts during growth in undercooled melts is important in Pb-free solders and (II) a metastable eutectic containing NiSn4 forms in Sn-3.5Ag/Ni joints. Additionally, it is shown that the substrate (metallization) has a strong influence on the nucleation and growth of tin. We identify Co, Pd, and Pt substrates as having the potential to control solidification and microstructure formation. In the case of Pd and Pt substrates, βSn is shown to nucleate on the PtSn4 or PdSn4 intermetallic compound (IMC) reaction layer at relatively low undercooling of ~4 K, even for small solder ball diameters down to

Cu6Sn5 is a common intermetallic in Pb-free soldering. We explore the influence of dilute aluminium additions on the heterogeneous nucleation and grain refinement of primary Cu6Sn5 in Sn–4Cu–xAl solders. For cooling rates relevant to soldering, it is found that 0.02 and 0.2 wt%Al cause significant grain refinement of Cu6Sn5 with 0.2 wt%Al increasing the number of Cu6Sn5 grains per unit area by a factor of ∼8. Grain refinement is shown to be due to heterogeneous nucleation of Cu6Sn5 on either delta-Cu33Al17 or gamma1-Cu9Al4, coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Reproducible orientation relationships are measured between the Cu–Al intermetallics and Cu6Sn5, with a planar lattice mismatch of less than 2.5%. The role of potent nuclei and solutal growth restriction are discussed with reference to the grain refinement of structural casting alloys.

The influences of 0–0.65 wt% Sc additions on microstructure evolution during solidification, and on as-cast tensile properties and fracture behavior have been investigated in Al-7Si-0.3Mg foundry alloys. The results show that the addition of Sc significantly refines both the primary α-Al grain size and the eutectic silicon particle size, and Sc also results in the formation of AlSi2Sc2 in up to two eutectic reactions. The as-cast yield strength was improved by the addition of Sc, however, at high Sc level (0.65 wt%) the ultimate tensile strength decreased, owing to the formation of relatively large AlSi2Sc2 crystals in the Al-AlSi2Sc2 eutectic. The fracture path in the tensile samples is shown to pass through the eutectic AlSi2Sc2 intermetallic and eutectic silicon. The Sc level should not be higher than 0.40 wt% Sc to avoid the deleterious effect of AlSi2Sc2.

The directional coefficient of thermal expansion (CTE) of intermetallics in electronic interconnections is a key thermophysical property that is required for microstructure-level modelling of solder joint reliability. Here, CTE ellipsoids are measured for key solder intermetallics using synchrotron x-ray diffraction (XRD). The role of the crystal structure used for refinement on the CTE shape and temperature dependence is investigated. The results are used to discuss the βSn-IMC orientation relationships (ORs) that minimise the in-plane CTE mismatch on IMC growth facets, which are measured with electron backscatter diffraction (EBSD) in solder joints on Cu and Ni substrates. The CTE mismatch in fully-intermetallic joints is discussed, and the relationship between the directional CTE of monoclinic and hexagonal polymorphs of Cu6Sn5 is explored.

Real time X-ray imaging videos of the entire soldering process for preform solders, solder pastes and BGA solders as well as solder alloy solidification and microstructure characterisation has been achieved over numerous experiments at the SPring-8 synchrotron. This paper overviews the developments of this challenging technique and provides key findings related to understanding the reliability of solder joints. There are large difference when comparing the microstructures and solidification pathways between soldered alloys on Cu substrates (Cu-OSP) and bulk alloy solidification.

There is ongoing research seeking solders with improved performance under harsh environmental conditions, elevated operation temperatures, and higher mechanical loads while, at the same time, meeting new emerging requirements for the continuous miniaturization of electronics. Bi has been identified as one of the advantageous alloying elements that significantly improve solder performance. The present investigation explores the influence of Bi on microstructure formation in Sn-Cu-Ni-Bi/Cu joints containing 0–14 wt% Bi. It is shown that Bi additions have no catalytic effect on βSn nucleation, however, they alter βSn growth textures in Sn-Cu-Ni-Bi/Cu joints causing a transition from a columnar grain growth to a virtually single grain structures as the Bi content increases above 8wt%. Bi additions are demonstrated to reduce the thickness of the Cu 6 Sn 5 IMC layer and to cause formation of non-equilibrium grain boundary βSn + (Cu, Ni) 6 Sn 5 + (Bi) eutectic in joints with Bi contents ≥2wt% Bi.

Synchrotron X-ray radiography at 125 frames per second was used to study deformation mechanisms in semi-solid Fe-C alloys at high solid fraction and shear strain rates of 10−1/s. An image correlation approach was also used to quantify the shear strain fields and study shear-induced dilation and the origin of shear cracking. It was shown that, at high solid fraction (90 to 93 pct solid), rearrangement including rotation and translation of solid particles became restricted and shear strain localized into narrow liquid-filled channels/fissures. Shear cracking was shown to originate from inadequate liquid flow into the expanding spaces between solid particles caused by shear-induced dilation. At lower solid fraction (~85 pct solid), solid particles rearranged with a significantly higher component of rotation leading to more shear-induced dilation and a wider shear band.

An improved understanding of the response of solidifying microstructures to load is required to further minimize casting defects and optimize casting processes. This article overviews synchrotron radiography studies that directly measure the micromechanics of semisolid alloy deformation in a thin sample direct-shear cell. It is shown that shear-induced dilation (also known as Reynolds’ dilatancy) occurs in semisolid alloys with morphologies ranging from equiaxed-dendritic to globular, at solid fractions from the dendrite coherency point to ~90% solid, and it occurs in both Al alloys and carbon steels. Discrete-element method simulations that treat solidifying microstructures as granular materials are then used to explore the origins of dilatancy in semisolid 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.

Al8Mn5 is crucial for impurity control and corrosion resistance of AZ91. We study the growth morphology of primary and eutectic Al8Mn5 after the equiaxed solidification of AZ91 cooled at ~1 K/s. Primary Al8Mn5 grew as ~10 μm equiaxed faceted crystals with multiple growth facets. The eutectic Al8Mn5 grew with a complex faceted morphology ranging from rod to sheet and folded plate-like, often with growth steps on the largest facets. The results are compared with the AZ91 solidification path predicted by PanDat with the PanMg8 database.

The study of mushy - zone deformation in steels is importan t for limit ing defect formation in continuous casting. Here, we use in situ synchrotron radiography to quantify the shear deformation mechanisms of a n equiaxed carbon steel at a solid fraction >0.9 and to understand how these mechanisms lead to casting defects . We show that the grain assembly undergoes shear - induced dilation (Rey nolds’ dilatancy) which opens liquid - filled fissures and cracks at high solid fraction . We further show a complex interaction between grain rearrangement, grain deformation and local coarsening, where rearrangement reduces the grain - grain contact area and coordination number which alters the stress network and also drives coarsening as grains move apart.

It is well known that during service the layer of Cu 6 Sn 5 intermetallic at the interface between the solder and a Cu substrate grows but the usual concern has been that if this layer gets too thick it will be the brittleness of this intermetallic that will compromise the reliability of the joint, particularly in impact loading. There is another level of concern when the Cu-rich Cu 3 Sn phase starts to develop at the Cu 6 Sn 5 /Cu interface and an imbalance in the diffusion of atomic species, Sn and Cu, across that interface results in the formation at the Cu 3 Sn/Cu interface of Kirkendall voids, which can also compromise reliability in impact loading. However, when, as is the case in some microelectronics, the copper substrate is thin in relation to the volume of solder in the joint an overriding concern is that all of the Cu will be consumed by reaction with Sn to form these intermetallics. This paper reports an investigation into the kinetics of the growth of the interfacial intermetallic, and the consequent reduction in the thickness of the Cu substrate in solder joints made with three alloys, Sn-3.0Ag-0.5Cu, Sn-0.7Cu-0.05Ni and Sn-1.5Bi-0.7Cu-0.05Ni. A simple model developed for the reduction of the Cu thickness as a result of diffusion controlled reaction with Sn to form Cu 6 Sn 5 was found to fit the experimental data well. The results reported in this paper provide an example of the way in which microstructural features that can affect joint reliability are affected by small alloying additions.

Silver is well known to be thermally etched in an oxygen-rich atmosphere and has been extensively studied in the laboratory to understand thermal etching and to limit its effect when this material is used as a catalyst. Yet, in many industrial applications the surface of rolled silver sheets is used without particular surface preparation. Here, it is shown by combining FIB-tomography, FIB-SIMS and analytical SEM that the kinetics of thermal etch pitting are significantly faster on rolled Ag surfaces than on polished surfaces. This occurs due to range of interacting phenomena including (i) the reaction of subsurface carbon-contamination with dissolved oxygen to form pores that grow to intersect the surface, (ii) surface reconstruction around corrosion pits and surface scratches, and (iii) sublimation at low pressure and high temperature. A method to identify subsurface pores is developed to show that the pores have {111} and {100} internal facets and may be filled with a gas coming from the chemical reaction of oxygen and carbon contamination.

Effects of Bi additions to Ag-containing lead-free solders have been the focus of a considerable amount of past investigation. However, the influence of Bi on Sn-Cu-Ni solders has not been studied extensively. In the present study, we explore the influence of Bi on microstructure formation of Sn-0.7Cu-0.05Ni/Cu solder joints both in the bulk and at the interface. It is shown that (i) Sn-0.7Cu-0.05Ni solidifies to produce a markedly different grain structure to Ag-containing lead-free alloys, with 5–8 independent βSn grains in each joint; (ii) Bi additions to Sn-0.7Cu-0.05Ni maintain this distinct βSn grain structure and had no discernible effect on the (Cu,Ni)6Sn5 interfacial intermetallic layers or primary intermetallic crystals.

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.

The growth of NiSn 4 is studied during the solidification of Sn–Ni alloys containing 0–0.45wt%Ni. Where past research has found metastable NiSn 4 after solid state ageing of Ni–Sn couples, it is shown here that NiSn 4 also forms during solidification as both a primary and a eutectic phase over a wide range of solidification conditions. The NiSn 4 phase has crystal structure isomorphous to PtSn 4 (o C 20) and no evidence is found for NiSn 4 being stable at solidification temperatures. Primary NiSn 4 crystal growth was promoted by high cooling rates and a comparison of growth ledges on the facets of primary NiSn 4 and Ni 3 Sn 4 crystals suggests easier interface attachment kinetics for NiSn 4 . The Sn–NiSn 4 eutectic grew under all solidification conditions used, with an orientation relationship that produces a low lattice disregistry at the interface plane and the same NiSn 4 growth direction as that selected during free growth of primary NiSn 4 . In hyper-eutectic compositions, both the Sn–NiSn 4 and Sn–Ni 3 Sn 4 eutectics grew with the latter growing from primary Ni 3 Sn 4 crystals.

A layer of distinctive microstructure known as the surface layer or the skin is often observed near the casting surface of high-pressure die cast (HPDC) parts. With its different microstructure, the surface layer could influence mechanical performance, corrosion properties and also pressure tightness of the whole cast component. This research aims to develop a better understanding of surface layer formation, which is essential to control the microstructure and therefore properties of HPDC components. In this study, microstructural characterization has been performed on HPDC specimens cast from an HPDC-specific Al alloy for structural applications, AlMg5Si2Mn. Most regions in the samples investigated contain ∼10–30 μm globular-rosette primary α-Al grains and [Al + Mg2Si] eutectic, while very dendritic primary α-Al grains are also present in the surface layer. The surface layer was observed in the region where the alloy did not directly impinge on the die surface during die filling (the so-called indirect impingement zone). In the region between the surface layer and inner regions (the so-called surface-layer edge), the only primary grain population is the very dendritic grains also observed in the surface layer. The surface layer formation is related to shearing at the interface between two parts of material containing different solid fractions (fs) which can occur either during the die-filling or during the intensification stage.

Deformation mechanisms in equiaxed, partially solid Al–15 wt.% Cu are studied in situ by coupling shear-cell experiments with synchrotron X-ray radiography. Direct evidence is presented for granular deformation mechanisms in both globular and equiaxed-dendritic samples at solid fractions shortly after crystal impingement. It is demonstrated that dilatancy, arching and jamming occur at the crystal scale, and that these can cause stick–slip flow due to periodic dilation and compaction at low displacement rate. Granular deformation is found to be similar in globular and equiaxed-dendritic samples if length is scaled by the crystal size and packing is considered to occur among crystal envelopes. Rheological differences between the morphologies are discussed in terms of the competition between crystal rearrangement and crystal deformation.

The rheology of an Al–10Cu alloy with four different level of grain refiner was studied by vane rheometry at solid fractions close to the dendrite coherency solid fraction for each composition. The focus is on the influence of grain size, crystal morphology and solid fraction on the development of strength and Reynolds’ dilatancy. The results show that the dendrite coherency solid fraction marks the onset of dilatancy in all alloys (i.e. the semi-solid alloys expand when sheared after dendrite coherency). The amount of dilatancy increases with increasing solid fraction and grain size. The influence of grain size is linked to the formation of shear bands within the samples where most of the volume change occurs. These bands are approximately 10–15 mean grains wide. The influence of crystal morphology on the development of strength after dendrite coherency and dilatancy is low, but it strongly influences the dendrite coherency solid fractions in the four alloys.

The influence of external mechanical stresses on agglomeration and bending of solidifying crystals has been investigated by microstructural characterisation of hypoeutectic Al cast specimens. The samples were produced by near-static cooling, gravity die casting and high pressure die casting (HPDC), where the solidifying crystals experience different levels of mechanical stresses. Electron backscatter diffraction (EBSD) technique was used to acquire grain misorientation data which can be linked to crystal agglomeration and bending behaviour during solidification. The length fraction of low-energy grain boundaries in HPDC samples was substantially higher than in gravity diecast and ‘statically cooled’ samples. This is related to the high amount of shear applied on the solidifying alloy, which promotes crystal collisions and agglomeration. In-grain misorientations were significant only in branched dendritic crystals which were subjected to significant shear stresses. This is attributed to the increased bending moment acting on long, protruding dendrite arms compared to more compact crystal morphologies.

This work focuses on understanding the feeding behavior during high-pressure die casting (HPDC). The effects of intensification pressure (IP) and gate thickness on the transport of material through the gate during the latter stages of HPDC were investigated using an AlSi3MgMn alloy. Microstructural characterization of the gate region indicated a marked change in feeding mechanism with increasing IP and gate size. Castings produced with a high IP or thick gate contained a relatively low fraction of total porosity, and shear band-like features existed through the gate, suggesting that semisolid strain localization in the gate is involved in feeding during the pressure intensification stage. When a low IP is combined with a thin gate, no shear band is observed in the gate and feeding is less effective, resulting in a higher level of porosity in the HPDC component. Although shear banding through the gate was found to reduce porosity in HPDC parts, if gates are not properly designed, deformation of the mushy zone through the gate can cause severe macrosegregation, large pores, and large cracks, which could severely reduce the performance of the component.

Agglomeration and bending of equiaxed crystals were studied by microstructural characterization of specimens produced by near-static cooling, high-pressure die casting and Thixomolding®, where the solidifying crystals experience different levels of mechanical stresses. EBSD was used to acquire statistical grain misorientation data which is linked to crystal agglomeration and bending behavior during solidification. An aluminum alloy and two magnesium alloys were used to compare grain misorientations for different crystal structures. The length fraction of low-energy grain boundaries in HPDC and Thixomolded samples was substantially higher than in “statically cooled” samples. This is attributed to the high shear stresses and pressure applied on the solidifying alloy, which promote crystal collisions and agglomeration. In-grain misorientations were found to be significant only in branched dendritic crystals which were subjected to significant shear stresses. This is related to the increased bending moment acting on long, protruding dendrite arms compared to more compact crystal morphologies.

Trace elements are increasingly being incorporated into lead-free solder compositions. This paper analyses the distribution of trace elements in solder joints when commercial purity Sn-based alloys are soldered onto Cu substrates. Analysis techniques include μ-XRF (X-ray fluorescence) mapping performed at the SPring-8 synchrotron radiation facility. The mapping results indicate that Ni is present in the Cu6Sn5 intermetallic reaction layer, and is distributed in a relatively homogeneous fashion as (Cu,Ni)6Sn5. In alloys containing trace levels of Ge (60 ppm), this element is comparatively concentrated within the oxide at the solder surface, and a lower concentration is distributed homogeneously in the solder matrix and the intermetallic reaction layer. In Sn–Pb alloys, the Pb was found to segregate to the boundaries between adjacent Cu6Sn5 grains.

Defect bands following the casting contour are often observed in high-pressure die cast (HPDC) components. In this article, suitable methods for measuring the thickness (w) and grain size in the bands (dsb) in HPDCs have been developed. The w/dsb relationship of defect bands has been investigated in HPDC specimens from a range of alloys, casting geometries and band locations within castings. Samples include aluminum alloy, AlSi4MgMn and AlMg5Si2Mn, tensile test bars cast by cold-chamber (cc) HPDC and a magnesium alloy, AM50, hot-chamber (hc) HPDC steering-wheel component. The band thicknesses were measured to be in the range 7–18 mean grains wide. This is substantial evidence that defect bands form due to strain localization in partially solidified alloys during cc and hc HPDC. Additionally, within this range, the w/dsb ratio decreases with increasing distance from the casting center in a cross-section containing multiple bands.

In order to optimise the high pressure die casting (HPDC) process, more understanding of microstructure and defect formation is essential. This article gives an overview of the key microstructural features and the mechanisms of microstructure formation in this process. The incavity solidified grain size in HPDC can be as low as 10 μm, but externally solidified crystals (ESCs) as large as 200 μm are often also present. The eutectic microstructure is very fine with an interlamellar spacing around 500 nm. Bands of positive macrosegregation and sometimes with cracks/porosity, so-called defect bands, are also often observed. Intensification pressure (IP) is one of the major factors governing the porosity level in the casting. At high IP, defect bands form in the gate region and appear to be assisting the feeding during the intensification stage.

The technique of trace element doping to modify the solidification mechanism of faceted/non-faceted eutectics has been applied to the Mg–Mg 2 Ni alloy system. It is demonstrated that the micro- and nano-structure of cast hypoeutectic Mg–Mg 2 Ni alloys can be varied by trace additions of Na, Ca or Eu to the liquid prior to solidification. As a result, the reversible hydrogen absorption capability was in excess of 90% of the theoretical value of 6.8 wt.% under the absorption parameters of 350 °C and 1 MPa for 24 min and subsequent desorption at 0.2 MPa for 24 min after activation. The hydrogen absorption kinetics have been dramatically improved under realistic industrial conditions, and show no sign of reduced capacity over 200 cycles. This processing route results in a non-pyrophoric material that may be produced in large quantities at comparatively low cost.

A method of limiting cracking in the Cu6Sn5 intermetallic compounds (IMCs) at the interface between lead-free solders and copper substrates has been developed. To explore the mechanism of crack inhibition in the nickel-containing IMC reaction layers, detailed synchrotron x-ray powder diffraction with Rietveld analysis and differential scanning calorimetry have been used. The results show that nickel stabilizes the high-temperature hexagonal allotrope of Cu6Sn5, avoiding stresses induced by a volumetric change that would otherwise occur on transformation to the monoclinic phase.

This article focuses on the influence of intensification pressure (I.P.) on the feeding through the gate during high pressure die casting (HPDC). Two values of intensification pressure, the lowest and highest possible for the HPDC machine used, were applied to cast AlSi3MgMn tensile-bar specimens. The castings produced with higher I.P. contained a lower total fraction of porosity, as expected. Microstructural characterisation of the gate region showed markedly different features in and adjacent to the gate at the two levels of I.P. used. The microstructures indicate a change in feeding mechanism with increasing I.P. At high I.P. shear band-like features exist through the gate, suggesting that strain localisation in the gate is involved in the feeding of solidification shrinkage during the I.P. stage. At low I.P. such shear bands were not observed in the gates and feeding was less effective, resulting in a higher level of porosity in the HPDC parts.

The rheology of equiaxed solidifying alloys containing 0-50% solid has been studied by vane rheometry. We focus on the incipient motion and subsequent flow mechanisms of quiescent solidifying microstructures. It is demonstrated that, once the growing crystals have impinged, the rheology can be interpreted within the framework of saturated compacted granular materials with little intercrystal cohesion. The material expands in response to shear and deforms primarily by Reynolds' dilatancy-enabled crystal rearrangement. Dilatancy leads to strain localisation in dilatant shear bands shortly after crystal impingement and is proposed to cause the initiation of cracks in the expanding regions between crystals at higher solid fraction.

Phosphorus is often added to wave-solder baths as an anti-oxidation agent. Despite this practice, there is little information on how phosphorus influences the solidification and flow properties of new lead-free solders such as Sn-0.7Cu-0.05Ni. This paper investigates the effects of phosphorus content on microstructure and maximum fluidity length in Sn-0.7Cu-0.05Ni-xP alloys containing 0-0.08 mass% phosphorous. Ppm levels of phosphorous are found to cause Sn-JrP, Ni-xP-(Sn) and Cu-xP-(Sn) intermetallic compounds to form in the liquid solder. The IMCs are less dense than liquid Sn and float towards the surface of the melt driven by buoyancy. It is shown that P-free Sn-0.7Cu-0.05Ni solidifies with a near-eutectic microstructure whereas, when P is added to this alloy, a significant volume fraction of primary Sn dendrites form once the P content exceeds ∼0.01 mass% P. It is further shown that P additions decrease the ability of Sn-0.7Cu-0.05Ni to flow as it solidifies.

Transitions in the mechanical behaviour of solidifying alloys influence feeding mechanisms and defect formation during many casting processes. This paper explores dendrite coherency and the cracking transition during the equiaxed dendritic solidification of Mg alloy AZ91 using the continuous-torque technique and vane rheometry. The two techniques yielded similar dendrite coherency values of ~17% solid, and the cracking transition occurred at 40% solid in vane rheometry tests. These transition values are similar to those reported for Al alloys with equiaxed dendritic morphology and a similar grain size.

During wave soldering, it is important that a solder is able to flow easily to fill joints and to drain away to leave tidy fillets. The maximum fluidity length (L f) is a simple measure of the flow behavior of solidifying alloys, defined as the distance a cooling and solidifying alloy can flow in a constant cross-section before the developing microstructure arrests flow. This paper explores the influence of alloy composition on L f in Sn-rich Sn-Cu-Ag-Ni alloys with compositions relevant to wave soldering. Significant differences in L f are measured among candidate lead-free solder alloys, which are discussed with respect to the phase diagrams and the mode of solidification.

Compacted granular materials expand in response to shear1, 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 avalanches2, geological faults3,4, flow in hoppers and silos5, and soil mechanics6,7. 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’ dilatancy1 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 $7.3 billion to the USA’s economy alone8 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.

Sn-Ag based solders are a popular Pb-free alternative in consumer electronics and have a relatively long history with a large body of dedicated research. Past research has primarily focused on the formation of interfacial intermetallic (IMC) layers and the growth of sSn, and little attention has been paid to Ni-containing IMCs that form in the bulk solder when Sn-Ag alloys are soldered to Ni-containing metallizations. The present study illustrates a new phenomenon: formation of metastable NiSn4 as a primary and a eutectic phase in solder joints between Sn-Ag solders and Ni-containing surface finishes. The metastable NiSn4 was demonstrated to transform into sSn and equilibrium Ni3Sn4 IMC phase during ageing at 150°C. Impact shear tests suggested a significant drop in solder joint mechanical response when larger than 2μm NiSn4 crystals were present in the solder bulk. Further exploration of the influence of alloying elements on metastable NiSn4 in solder joints yielded three scenarios: (i) Au, Pt, Pd and Co promoted NiSn4 formation and increased its volume fraction by substituting for Ni atoms in NiSn4; (ii) Bi, In, Fe and Pb additions appeared to have little or no discernible effect on NiSn4 and (iii) Cu was found to promote equilibrium Ni3Sn4 phase formation that replaces metastable NiSn4 at Cu levels above 0.3wt%.