Imperial College London

DrChristopherGourlay

Faculty of EngineeringDepartment of Materials

Reader in Metallurgy
 
 
 
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Contact

 

+44 (0)20 7594 8707c.gourlay

 
 
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Location

 

301DBessemer BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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133 results found

Feng S, Cui Y, Liotti E, Lui A, Gourlay CM, Grant PSet al., 2020, In-situ X-ray radiography of twinned crystal growth of primary Al13Fe4, SCRIPTA MATERIALIA, Vol: 184, Pages: 57-62, ISSN: 1359-6462

Journal article

Su TC, O'Sullivan C, Yasuda H, Gourlay CMet al., 2020, Rheological transitions in semi-solid alloys: in-situ imaging and LBM-DEM simulations, Acta Materialia, Vol: 191, Pages: 24-42, ISSN: 1359-6454

Rheological transitions from suspension flow to granular deformation and shear cracking are investigated in equiaxed-globular semi-solid alloys by combining synchrotron radiography experiments with coupled lattice Boltzmann method, discrete element method (LBM-DEM) simulations. The experiments enabled a deformation mechanism map to be plotted as a function of solid fraction and shear rate, including a rate dependence for the transition from net-contraction to net-dilation, and for the initiation of shear cracking. The LBM-DEM simulations are in quantitative agreement with the experiments, both in terms of the strain fields in individual experiments and the deformation mechanism map from all experiments. The simulations are used to explore the factors affecting the shear rate dependence of the volumetric strain and transitions. The simulations further show that shear cracking is caused by a local liquid pressure drop due to unfed dilatancy, and the cracking location and its solid fraction and shear rate dependence were reproduced in the simulations using a criterion that cracking occurs when the local liquid pressure drops below a critical value.

Journal article

Zeng G, Shuai SS, Zhu XZ, Ji SX, Xian JW, Gourlay CMet al., 2020, Al8Mn5 in high-pressure Die Cast AZ91: twinning, morphology and size distributions, Metallurgical and Materials Transactions A, Vol: 51, Pages: 2523-2535, ISSN: 1073-5623

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.

Journal article

Cui Y, King DJM, Horsfield AP, Gourlay CMet al., 2020, Solidification orientation relationships between Al3Ti and TiB2, Acta Materialia, Vol: 186, Pages: 149-161, ISSN: 1359-6454

Orientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.

Journal article

Pham M-S, Dovgyy B, Hooper P, Christopher G, Alessandro Pet al., 2020, The role of side-branching in microstructure development in laser powder-bed fusion, Nature Communications, Vol: 11, ISSN: 2041-1723

In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (i.e. 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67 rotation between layers.

Journal article

Gu T, Xu Y, Gourlay CM, Britton TBet al., 2020, In-situ electron backscatter diffraction of thermal cycling in a single grain Cu/Sn-3Ag-0.5Cu/Cu solder joint, Scripta Materialia, Vol: 175, Pages: 55-60, ISSN: 1359-6462

The heterogeneous evolution of microstructure in a single grain Cu/SAC305/Cu solder joint is investigated using in-situ thermal cycling combined with electron backscatter diffraction (EBSD). Local deformation due to thermal expansion mismatch results in heterogeneous lattice rotation within the joint, localised towards the corners. This deformation is induced by the constraint and the coefficient of thermal expansion (CTE) mismatch between the β-Sn, Cu6Sn5 and Cu at interfaces. The formation of subgrains with continuous increase in misorientation is revealed during deformation, implying the accumulation of plastic slip at the strain-localised regions and the activation of slip systems (110)[11]/2 and (0)[111]/2.

Journal article

Belyakov SA, Arfaei B, Johnson C, Howell K, Coyle R, Gourlay Cet al., Phase stability and solid solubility in high reliability Pb-free solders containing Bi, Sb or In, SMTA International

Conference paper

Belyakov SA, Sweatman K, Akaiwa T, Nishimura T, Gourlay Cet al., Precipitation of (Bi) and SbSn phases in next-generation Pb-free solders, SMTA International

Conference paper

Peng L, Zeng G, Su TC, Yasuda H, Nogita K, Gourlay CMet al., 2019, Al8Mn5 particle settling and interactions with oxide films in liquid AZ91 magnesium alloys, JOM, Vol: 71, Pages: 2235-2244, ISSN: 1047-4838

Al8Mn5 particles form as primary solidification phases in Mg-Al-based alloys and are important for ensuring adequate corrosion resistance. However, excessive Al8Mn5 formation above the α-Mg liquidus temperature can lead to sludge formation, and the clustering of Al8Mn5 particles on oxide films can generate deleterious casting defects. Here, we use analytical SEM and real-time synchrotron x-ray radiography to study Al8Mn5 particle settling, the formation of a sludge layer, and the mechanism of Al8Mn5 clustering on oxides in AZ91 containing two Fe levels. It is shown that settling Al8Mn5 can become trapped in entrained oxide and that new Al8Mn5 particles also seem to nucleate on oxide films. On cooling, these Al8Mn5 particles continue to grow, creating large Al8Mn5 clusters.

Journal article

Ma ZL, Shang H, Daszki AA, Belyakov SA, Gourlay CMet al., 2019, Mechanisms of beta-Sn nucleation and microstructure evolution in Sn-Ag-Cu solders containing titanium, Journal of Alloys and Compounds, Vol: 777, Pages: 1357-1366, ISSN: 0925-8388

The mechanisms by which Ti additions catalyse the nucleation of β-Sn are studied in 550 μm Sn-3Ag-0.5Cu (wt%) solder balls and joints on Cu and Ni substrates. It is shown that at least two new intermetallic compounds (IMCs), Ti2Sn3 and (Ti,Fe,Cu)Sn2, form as a result of a 0.2 wt% Ti addition. The nucleation potential of each IMC was studied by electron backscatter diffraction (EBSD) of tin droplets solidified on the cross sectioned facets of each IMC. It is found that reproducible orientation relationships (ORs) form only between β-Sn and Ti2Sn3 and that the two ORs generate good atomic matching between the Sn atoms in Ti2Sn3 and the closest packed plane in β-Sn, {100}. β-Sn cyclic twinning occurred in droplets on Ti2Sn3 where the twinning axis 〈100〉Sn was always parallel with the lowest disregistry direction in the ORs. In solder balls and joints, the Ti2Sn3 addition triggered up to 12 independent β-Sn grains, whereas Ti-free SAC305 always solidified with one independent grain.

Journal article

Su T-C, O'Sullivan C, Nagira T, Yasuda H, Gourlay Cet al., 2019, Semi-solid deformation of Al-Cu alloys: A quantitative comparison between real-time imaging and coupled LBM-DEM simulations, Acta Materialia, Vol: 163, Pages: 208-225, ISSN: 1359-6454

Semi-solid alloys are deformed in a wide range of casting processes; an improved understanding and modelling capability is required to minimise defect formation and optimise productivity. Here we combine thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation at 40–70% solid with 2D coupled lattice Boltzmann method - discrete element method (LBM-DEM) simulations. The simulations quantitatively capture the key features of the in-situ experiments, including (i) the local contraction and dilation of the grain assembly during shear deformation; (ii) the heterogeneous strain fields and localisation features; (iii) increases in local liquid pressure in regions where liquid was expelled from the free surface in the experiment; and (iv) decreases in liquid pressure in regions where surface menisci are sucked-in in experiments. The verified DEM simulations provide new insights into the role of initial solid fraction on the stress-deformation response and support the hypothesis that the behaviour of semi-solid alloys can be described using critical state soil mechanics.

Journal article

Belyakov SA, Nishimura T, Akaiwa T, Sweatman K, Nogita K, Gourlay CMet al., 2019, Role of Bi, Sb and In in microstructure formation and properties of Sn-0.7Cu-0.05Ni-X BGA interconnections, International Conference on Electronics Packaging (ICEP), Publisher: IEEE, Pages: 235-239

Conference paper

Belyakov S, Xian J, Zeng G, Sweatman K, Nishimura T, Akaiwa T, Gourlay Cet al., 2019, Precipitation and coarsening of bismuth plates in Sn-Ag-Cu-Bi and Sn-Cu-Ni-Bi solder joints, Journal of Materials Science: Materials in Electronics, Vol: 30, Pages: 378-390, ISSN: 0957-4522

In recent years there has been increased interest in Bi-containing solders to improve the reliability of electronics and drive down processing temperatures. When the Bi content is more than ~ 2 wt% in Sn–Ag–Cu–Bi and Sn–Cu–Ni–Bi solders, (Bi) phase forms by a non-equilibrium eutectic reaction and also precipitates in the solid state. Here, we study the development of bismuth plates during room temperature storage in a range of Bi-containing solders. It is shown that Bi plates precipitate with one of two (Bi)–βSn orientation relationships (ORs). During coarsening the OR with the better lattice match grows at the expense of the other, leading to a single OR. The plate coarsening kinetics scale with the cube root of holding time and are relatively rapid, with plates exceeding 1 µm in length within 1 day at room temperature. Prolonged room temperature storage of more than 1 year resulted in Bi accumulation in 5–20 µm (Bi) particles at βSn grain boundaries.

Journal article

Gu T, Gourlay C, Britton T, 2019, Evaluating creep deformation in controlled microstructures of Sn-3Ag-0.5Cu solder, Journal of Electronic Materials, Vol: 48, Pages: 107-121, ISSN: 0361-5235

The reliability of solder joints is affected significantly by thermomechanical properties such as creep and thermal fatigue. In this work, the creep of directionally solidified (DS) Sn-3Ag-0.5Cu wt.% (SAC305) dog-bone samples (gauge dimension: 10 × 2 × 1.5 mm) with a controlled <110> or <100> fibre texture is investigated under constant load testing (stress level: ∼ 30 MPa) at a range of temperatures from 20°C to 200°C. Tensile testing is performed and the secondary creep strain rate and the localised strain gradient are studied by two-dimensional optical digital image correlation (2-D DIC). The dominating creep mechanisms and their temperature dependence are explored at the microstructural scale using electron backscatter diffraction (EBSD), which enables the understanding of the microstructural heterogeneity of creep mechanisms at different strain levels, temperatures and strain rates. Formation of subgrains and the development of recrystallization are observed with increasing strain levels. Differences in the deformation of β-Sn in dendrites and in the eutectic regions containing Ag3Sn and Cu6Sn5 are studied and related to changes in local deformation mechanisms.

Journal article

Gourlay CM, Arfaei B, 2019, Advances in electronic interconnection materials, JOM, Vol: 71, Pages: 131-132, ISSN: 1047-4838

Journal article

Xian J, Mohd Salleh A, Belyakov S, Su T, Zeng G, Nogita K, Yasuda H, Gourlay Cet al., 2018, Influence of Ni on the refinement and twinning of primary Cu6Sn5 in Sn0.7Cu-0.05Ni, Intermetallics, Vol: 102, Pages: 34-45, ISSN: 0966-9795

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.

Journal article

Ma ZL, Belyakov SA, Xian JW, Nishimura T, Sweatman K, Gourlay CMet al., 2018, Controlling BGA joint microstructures using seed crystals, 7th Electronic System-Integration Technology Conference (ESTC), Publisher: IEEE

Conference paper

Hsu Y, Zeng G, Xian JW, Belyakov SA, Gourlay CMet al., 2018, Tailoring the Cu6Sn5 layer texture with Ni additions in Sn-Ag-Cu based solder joints, 7th Electronic System-Integration Technology Conference (ESTC), Publisher: IEEE

Conference paper

Piglione A, Dovgyy B, Liu C, Gourlay C, Hooper P, Pham Met al., 2018, Printability and microstructure of the CoCrFeMnNi high-entropy alloy fabricated by laser powder bed fusion, Materials Letters, Vol: 224, Pages: 22-25, ISSN: 0167-577X

The CoCrFeMnNi high-entropy alloy is a promising candidate for metal additive manufacturing. In this study, single-layer and multi-layer builds were produced by laser powder bed fusion to study microstructure formation in rapid cooling and its evolution during repeated metal deposition. CoCrFeMnNi showed good printability with high consolidation and uniform high hardness. It is shown that microstructure in the printed alloy is governed by epitaxial growth and competitive grain growth. As a consequence, a bi-directional scanning pattern without rotation in subsequent layers generates a dominant alternating sequence of two crystal orientations.

Journal article

Zeng G, Xian J, Gourlay C, 2018, Nucleation and growth crystallography of Al8Mn5 on B2-Al(Mn,Fe) in AZ91magnesium alloys, Acta Materialia, Vol: 153, Pages: 364-376, ISSN: 1359-6454

Manganese is widely added to Mg-Al-based alloys to form Al-(Mn,Fe) intermetallics that combat impurity Fe. Here EBSD, deep etching and FIB-tomography are combined to study the nucleation and growth crystallography of Al-Mn-Fe particles in Mg-9Al-0.7Zn-0.15Mn-0.002Fe (wt%) solidified at 1 K⋅s−1. It is shown that Al8Mn5 nucleates on B2-Al(Mn,Fe) particles and an incomplete peritectic transformation results in a Fe-richer B2-Al(Mn,Fe) core enveloped by a low-Fe Al8Mn5 shell. A reproducible orientation relationship (OR) is measured, and the close lattice match and OR are discussed in terms of the group-subgroup relationship between these phases. It is found that most rhombohedral Al8Mn5 particles are twinned, consisting of four orientations related by ∼90° rotations around three common , which is discussed in terms of the pseudo-cubic <100> axes of the Al8Mn5 rhombohedral gamma brass. Al8Mn5 grew as equiaxed particles that are explained by polyhedron models based on {100}, {110} and {112} facets of the pseudo-cubic Al8Mn5 cell. The results indicate that, at low Fe:Mn ratio, most impurity Fe is dissolved in B2 particles that are encapsulated by low-Fe Al8Mn5 which may be important for corrosion resistance.

Journal article

Belyakov SA, Nishimura T, Akaiwa T, Sweatman K, Nogita K, Gourlay CMet al., 2018, Optimization of Ni and Bi levels in Sn-0.7Cu-xNi-yBi solders for improved interconnection reliability, Pages: 211-215

© 2018 Japan Institute of Electronics Packaging. 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.

Conference paper

Ma Z, Xian J, Belyakov S, Gourlay Cet al., 2018, Nucleation and twinning in tin droplet solidification on single crystal intermetallic compounds, Acta Materialia, Vol: 150, Pages: 281-294, ISSN: 1359-6454

βSn nucleation is a key step in the formation of microstructure in electronic solder joints. Here, the heterogeneous nucleation of βSn is studied in undercooled tin droplets spread on the facets of various intermetallic compounds (IMCs). Nucleation undercoolings are measured in solidifying droplets and are linked to orientation relationships (ORs) measured by electron backscatter diffraction (EBSD). Preferred ORs developed on all IMCs studied. For the more potent nucleants (αCoSn3, IrSn4, PtSn4, PdSn4) the ORs represent relatively simple atomic matches. ORs with lower potency nucleants (Cu6Sn5, Ag3Sn, Ni3Sn4) had more complex atomic matches that are explored based on matching of the closest packed atomic rows. βSn solidification twinning is shown to be more complex than has been reported previously: both nucleation on an IMC facet and cyclic twinning of that grain occurred in many droplets on Cu6Sn5, Ag3Sn, Ni3Sn4; in all twinned droplets the <100>Sn twinning axis occurred along a direction on the IMC with the lowest linear atomic disregistry; and interrelated cyclic twins formed consisting of up to five rings of cyclic twins all related by shared <100>Sn axes.

Journal article

Hou N, Belyakov S, Pay L, Sugiyama A, Yasuda H, Gourlay Cet al., 2018, Competition between stable and metastable eutectic growth in Sn-Ni alloys, Acta Materialia, Vol: 149, Pages: 119-131, ISSN: 1359-6454

Metastable Sn-NiSn4 eutectic can form in electronic solder joints and is a reliability concern. Here the competition between stable Sn-Ni3Sn4 and metastable Sn-NiSn4 eutectic growth is studied during unidirectional solidification. The stable and metastable eutectic points are measured, the dynamics of eutectic growth and the transition between the two eutectics is investigated by synchrotron radiography, and the crystallography of eutectic growth is measured by EBSD. It is shown that the Sn-Ni3Sn4 eutectic has a highly irregular morphology with diverging and converging rods because monoclinic Ni3Sn4 grows only along [010] without a reproducible orientation relationship (OR) with Sn. The metastable Sn-NiSn4 eutectic has a more regular broken-lamellar morphology and a reproducible OR with Sn. The critical velocity between the two eutectics is less than 1 μm/s which is discussed in terms of the small temperature difference between the two eutectic points and the kinetic growth advantages of the metastable eutectic.

Journal article

Zeng G, Zhu X, Ji S, Gourlay CMet al., 2018, The morphology and distribution of Al<inf>8</inf>Mn<inf>5</inf>in high pressure die cast AM50 and AZ91, Pages: 137-144, ISSN: 2367-1181

© The Minerals, Metals & Materials Society 2018. The morphology and distribution of Al8Mn5is studied in AM50 and AZ91 produced by hot and cold chamber high pressure die casting (HPDC). It is found that, in HPDC, primary Al8Mn5particles take a wide range of morphologies within the same casting spanning from faceted polyhedra to weakly-faceted dendrites. These different morphologies exist across the whole cross-section without any clear trend in morphology versus radial position. A comparison with Al8Mn5in samples solidified at low cooling rate suggests that the larger polyhedral particles are externally solidified crystals (ESCs) that nucleate and grow in the shot chamber analogous to αMg ESCs, and that the dendritic Al8Mn5nucleated and grew at high cooling rate in the die cavity.

Conference paper

Xian JW, Mohd Salleh MAA, Zeng G, Belyakov SA, Yasuda H, Nogita K, Gourlay CMet al., 2018, Synchrotron radiography of Sn-0.7Cu-0.05Ni solder solidification, Pages: 66-71

© 2018 Trans Tech Publications, Switzerland. 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).

Conference paper

Ma ZL, Belyakov SA, Sweatman K, Nishimura T, Nishimura T, Gourlay Cet al., 2017, Harnessing heterogeneous nucleation to control tin orientations in electronic interconnections, Nature Communications, Vol: 8, ISSN: 2041-1723

While many aspects of electronics manufacturing are controlled with great precision, the nucleation of tin in solder joints is currently left to chance. This leads to a widely varying melt undercooling and different crystal orientations in each joint which results in a different resistance to electromigration, thermomechanical fatigue and other failure modes in each joint. Here we identify a family of nucleants for tin, prove their effectiveness using a novel droplet solidification technique, and demonstrate an approach to incorporate the nucleants into solder joints to control the orientation of the tin nucleation event. With this approach, it is possible to change tin nucleation from a stochastic to a deterministic process, and to generate single crystal joints with their c-axis orientation tailored to best combat a selected failure mode.

Journal article

Pandee P, Gourlay C, Belyakov S, Patakham U, Zeng G, Limmaneevichitr Cet al., 2017, AlSi2Sc2 intermetallic formation in Al-7Si-0.3Mg-xSc alloys and their effects on as-cast properties, Journal of Alloys and Compounds, Vol: 731, Pages: 1159-1170, ISSN: 0925-8388

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.

Journal article

Li Z, Belyakov S, Xian J, Gourlay Cet al., 2017, The influence of primary Cu6Sn5 size on the shear impact properties of Sn-Cu/Cu BGA Joints, Journal of Electronic Materials, Vol: 47, Pages: 84-95, ISSN: 0361-5235

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.

Journal article

Xian JW, Zeng G, Belyakov S, Gu Q, Nogita K, Gourlay Cet al., 2017, Anisotropic thermal expansion of Ni3Sn4, Ag3Sn, Cu3Sn, Cu6Sn5 and βSn, Intermetallics, Vol: 91, Pages: 50-64, ISSN: 0966-9795

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.

Journal article

Xian J, Belyakov S, Nogita K, Yasuda H, Gourlay Cet al., Faceted and Nonfaceted Growth of Cu6Sn5 Crystals, 6th Decennial International Conference on Solidification Processing

Conference paper

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