Imperial College London


Faculty of EngineeringDepartment of Materials

Professor of Physical Metallurgy



+44 (0)20 7594 8707c.gourlay




301DBessemer BuildingSouth Kensington Campus





Publication Type

157 results found

Yang Y, Deng Y, Zhang R, Su Y, Liu S, Gourlay CM, Zeng Get al., 2023, Influence of β-Mg₁₇Al₁₂ and Al-Mn intermetallic compounds on the corrosion behaviour of cast and solution treated Mg-Al-Zn-Mn alloys, Corrosion Science, Vol: 222, ISSN: 0010-938X

This study aims to investigate the correlation between the microstructure and corrosion behaviour of AZ series alloys, using analytical scanning electron microscopy, X-ray computed tomography, hydrogen evolution test, weight loss method, electrochemical measurement, and observation of corroded morphology. The results indicate that a 3D interconnected eutectic network is effective in preventing localized corrosion in chloride solution. It is also revealed that solution treated Al-Mn intermetallics lead to filiform corrosion, rather than pitting corrosion, as seen τ-AlMn and Al8Mn5 particles. This is due to the potential difference reduction between the complete Al11Mn4 shell and the α-Mg matrix.

Journal article

Schwarz TM, Yu W, Zhan H, Gault B, Gourlay CM, McCarroll Iet al., 2023, Uncovering Ce-rich clusters and their role in precipitation strengthening of an AE44 alloy, Scripta Materialia, Vol: 232, ISSN: 1359-6462

The peak-aged (250°C for 4 h) magnesium-aluminum-rare earth (Mg-Al-RE) alloy AE44-2 produced using high-pressure die casting, exhibits a 10 % increase in yield strength attributed to the formation of nanoscale AlMn precipitates. Here, in the as cast state, we show by atom probe tomography Ce-rich clusters in the dendritic and not in the inter-dendritic α-Mg, despite a higher RE concentration. Their formation hence depends mainly on solidification kinetics and not on the concentration. The Ce-rich clusters within the dendritic region, act as heterogeneous nucleation sites for the formation of Al10Mn7RE2 nanoscale precipitates during the T5 heat treatment. No changes in the Al11RE3 lamella phase were observed, including to the concentration of segregated Si at the Al11RE3/α-Mg interface. Understanding the precipitate formation pathways, specifically the influence of low concentrations of Ce in the dendritic matrix, provides a pathway for tailoring mechanical properties by manipulating these clusters and consequently precipitation.

Journal article

Cui Y, Xian JW, Zois A, Marquardt K, Yasuda H, Gourlay CMet al., 2023, Nucleation and growth of Ag3Sn in Sn-Ag and Sn-Ag-Cu solder alloys, Acta Materialia, Vol: 249, Pages: 1-15, ISSN: 1359-6454

Large Ag3Sn plates in solder joints can affect the reliability of electronics, however, the factors affecting their nucleation and morphology are not well understood. Here, the faceted solidification of Ag3Sn was studied as a function of melt undercooling, revealing transitions from single crystal {001} plates to cyclic twinned plates and then to highly branched structures created by twin mediated branching. Real-time X-ray imaging proved that Ag3Sn cyclic twins come from a common point, indicating they initiate in the process of nucleation or in the very early stages of growth in the undercooled melt. Soldering to copper substrates significantly catalysed Ag3Sn nucleation. This is shown to be due to constitutional supercooling generated by Ag solute rejection into the liquid ahead of the Cu6Sn5 reaction layer, with additional contributions from geometrical catalysis in the grooves between Cu6Sn5 scallops and heterogeneous nucleation of Ag3Sn on Cu6Sn5. The relative ease of Ag3Sn nucleation on the Cu6Sn5 reaction layer is responsible for the large plates often reported in electronic solder joints.

Journal article

Lin CJ, Peng L, Xian JW, Li Q, Gourlay CMet al., 2023, Eutectic solidification in Mg-9Al-0.7Zn: from divorced to coupled growth, Journal of Alloys and Compounds, Vol: 938, Pages: 1-16, ISSN: 0925-8388

In hypoeutectic Mg-Al-based alloys, Mg17Al12 forms in the last stages of solidification by a eutectic reaction that is usually fully or partially divorced. Here we investigate the nucleation, growth and morphology of the Mg17Al12-containing eutectic in a multicomponent Mg-9Al-0.7Zn-0.2Mn alloy. Under all solidification conditions studied, Mg17Al12 formed an interconnected skeleton in 3D and Mg17Al12 orientation domains spanned multiple α-Mg grains, indicating that Mg17Al12 nucleation events are relatively infrequent. While most regions of α-Mg+Mg17Al12 had no preferred orientation relationship (OR), occasional eutectic regions had the Burgers OR that may be related to the nucleation of Mg17Al12 on α-Mg during solidification. It is shown that the degree of partially divorced growth depends on a simple geometrical measure for the space available for eutectic growth involving the dendrite arm spacing, the eutectic fraction and the eutectic spacing. We then demonstrate that fully coupled eutectic microstructures can be generated in this alloy by independently manipulating the magnesium dendrite solidification time and the eutectic cooling rate.

Journal article

Hou N, Xian J, Sugiyama A, Yasuda H, Gourlay Cet al., 2023, Ag3Sn morphology transitions during eutectic growth in Sn-Ag alloys, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 54, Pages: 909-927, ISSN: 1073-5623

Eutectic Ag3Sn can grow with a variety of morphologies depending on the solidification conditions and plays an important role in the performance of Pb-free solders. Here, we study morphology transitions in the β-Sn + Ag3Sn eutectic at increasing growth velocity using a combination of real-time X-ray imaging and analytical electron microscopy. Orthorhombic Ag3Sn grew as faceted plates or rods with [010] growth direction and (001) as the largest facet in all three eutectic growth morphologies: Irregular plate, broken-lamellar, and rod. Reproducible β-Sn + Ag3Sn orientation relationships formed for the latter two morphologies. The mechanisms of spacing adjustment and the reversible transition from plate to rod growth are studied for transient solidification after velocity changes. The transition from fully eutectic growth to β-Sn dendrites plus eutectic is reasonably reproduced using a competitive growth criterion. At the highest velocities, Ag3Sn rods developed perturbations whose break down into particles is discussed in terms of unsteady growth and pinch-off mechanisms.

Journal article

Xu Y, Gu T, Xian J, Giuliani F, Ben Britton T, Gourlay CM, Dunne FPEet al., 2022, Multi-scale plasticity homogenization of Sn–3Ag-0.5Cu: From β-Sn micropillars to polycrystals with intermetallics, Materials Science and Engineering: A, Vol: 855, Pages: 1-15, ISSN: 0921-5093

The mechanical properties of β-Sn single crystals have been systematically investigated using a combined methodology of micropillar tests and rate-dependent crystal plasticity modelling. The slip strength and rate sensitivity of several key slip systems within β-Sn single crystals have been determined. Consistency between the numerically predicted and experimentally observed slip traces has been shown for pillars oriented to activate single and double slip. Subsequently, the temperature-dependent, intermetallic-size-governing behaviour of a polycrystal β-Sn-rich alloy SAC305 (96.5Sn–3Ag-0.5Cu wt%) is predicted through a multi-scale homogenization approach, and the predicted temperature- and rate-sensitivity reproduce independent experimental results. The integrated experimental and numerical approaches provide mechanistic understanding and fundamental material properties of microstructure-sensitive behaviour of electronic solders subject to thermomechanical loading, including thermal fatigue.

Journal article

Xu Y, Xian J, Stoyanov S, Bailey C, Coyle R, Gourlay C, Dunne Fet al., 2022, A multi-scale approach to microstructure-sensitive thermal fatigue in solder joints, International Journal of Plasticity, Vol: 155, ISSN: 0749-6419

This paper presents a multi-scale modelling approach to investigate the underpinning mechanisms of microstructure-sensitive damage of single crystal Sn-3Ag-0.5Cu (wt%, SAC305) solder joints of a Ball Grid Array (BGA) board assembly subject to thermal cycling. The multi-scale scheme couples board-scale modelling at the continuum macro-scale and individual solder modelling at the crystal micro-scale. Systematic studies of tin crystal orientation and its role in fatigue damage have been compared to experimental observations. Crystallographic orientation is examined with respect to damage development, providing evidence-based optimal solder microstructural design for in-service thermomechanical fatigue.

Journal article

Deng Y, Zeng G, Xian J, Zhan H, Liu C, Gourlay CMet al., 2022, Eutectic intermetallic formation during solidification of a Mg-Sn-Al-Zn-Mn alloy, Materials Characterization, Vol: 186, Pages: 1-15, ISSN: 1044-5803

Multiple primary and eutectic intermetallic compounds (IMCs) often form during the solidification of multicomponent magnesium alloys. Here, we comprehensively explore eutectic intermetallic formation in Mg-4Sn-3Al-1.5Zn-0.6Mn (wt%) as-cast alloy, in comparison with Mg-3Al-1Zn-0.3Mn, to better understand Mg2Sn formation during complex multiphase solidification and how it affects the formation of other IMCs. Various IMCs formed at different stages of solidification: τ-AlMn and Al8Mn5 early during solidification and Mg2Sn, β-Mg17Al12, and τ-MgAlZn as late forming eutectic phases. Most Mg2Sn phase shared a faceted interface with β-Mg17Al12 and τ-MgAlZn grains, forming a fully divorced eutectic particle containing multiple phases. EBSD-based trace analysis in combination with XCT and TEM results revealed that β-Mg17Al12 and τ-MgAlZn phase grew adjacent to the {100} growth facets of Mg2Sn. Within multi-phase particles, seven types of reproducible orientation relationships (ORs) were measured between each intermetallic. The ORs between Mg2Sn and τ-MgAlZn were found to have the best lattice match among the observed phase pairs. For the defined ORs, Mg2Sn can be correlated with the icosahedral coordination in τ-MgAlZn.

Journal article

Peng L, Zeng G, Xian J, Gourlay CMet al., 2022, Al–Mn–Fe intermetallic formation in AZ91 magnesium alloys: Effects of impurity iron, Intermetallics, Vol: 142, Pages: 1-15, ISSN: 0966-9795

The influence of iron on the formation of Al–Mn–Fe intermetallic compounds (IMCs) has been investigated in the solidification of Mg–9Al-0.7Zn-0.2Mn (wt.%, AZ91) with iron contents ranging from ∼0.001 to > 0.01 wt.% Fe. Four Al–Mn–Fe IMCs formed depending on the Fe-content and location in the crucible: B2–Al(Fe,Mn), Al8Mn5, Al11Mn4 and, at the bottom of crucibles, Al5Fe2. The four IMCs nucleated and grew on one another, producing multiphase particles. These usually contained numerous orientations that were all interrelated through simple orientation relationships that are discussed in terms of the similarities between the IMC crystal structures. The iron content affected the IMC phase fractions and the multiphase particle morphology. At low iron content, the Fe-rich B2 phase was encapsulated by a low-Fe Al8Mn5 shell. With increasing iron content, the Fe-rich phases (B2 and Al5Fe2) gradually became in direct contact with the α-Mg. The threshold Fe:Mn content for adequate corrosion performance is found to correlate approximately to where B2–Al(Fe,Mn) first becomes exposed to the α-Mg matrix.

Journal article

Cui Y, Gourlay C, 2022, Growth twinning and morphology of Al45Cr7 and Al13Fe4, Journal of Alloys and Compounds, Vol: 893, Pages: 1-14, ISSN: 0925-8388

Twinned crystal growth is studied in primary Al45Cr7 and Al13Fe4 intermetallic compounds (IMCs) to explore how different twin types and twin variants affect the growth morphology during solidification. In both IMCs, the number of twin variants increased as the cooling rate increased and, by ~ 5 K/s, both IMCs formed cyclic twins with combined icosahedral (Al45Cr7) or decagonal (Al13Fe4) pseudosymmetry. The growth morphology depended on which twin variants were present. When all twin domains shared a common direction that was a rod growth direction in single crystals, twinning did not prevent crystals from growing as rods. This was the case for both IMCs at slow cooling rate and Al13Fe4 at all cooling rates. Cyclic twinning of Al13Fe4 generated many re-entrant corners but resulted in only a modest reduction in rod aspect ratio. In contrast, when twin domains in Al45Cr7 had common directions along multiple pseudo-i(2) axes, crystal growth transitioned from rod-like to a near-equiaxed morphology.

Journal article

Peng L, Zeng G, Wang D, Xian J, Ji S, Zhan H, Gourlay CMet al., 2022, Al-Mn intermetallics in high pressure die cast AZ91 and direct chill cast AZ80, Metals, Vol: 12, Pages: 1-18, ISSN: 2075-4701

Manganese-bearing intermetallic compounds (IMCs) are important for ensuring adequate corrosion performance of magnesium-aluminium alloys and can be deleterious to mechanical performance if they are large and/or form clusters. Here, we explore the formation of Al-Mn IMCs in Mg-9Al-0.7Zn-0.2Mn produced by two industrial casting processes, high-pressure die casting (HPDC) and direct chill (DC) casting. As Al8Mn5 starts forming above the α-Mg liquidus temperature in this alloy, we consider its formation during melt handling as well as during casting and heat treatment. In HPDC, we focus on sludge formation in the holding pot, partial solidification of IMCs in the shot chamber, and Al-Mn IMC solidification in the die cavity. In DC casting, we focus on interactions between Al-Mn IMCs and oxide films in the launder system, Al-Mn IMC solidification in the billet, and the partial transformation of Al8Mn5 into Al11Mn4 during solution heat treatment. The results show that minimising pre-solidification in the shot sleeve of HPDC and controlling pouring and filtration in DC casting are important for ensuring small Al-Mn intermetallic particles in these casting processes.

Journal article

Su TC, O'Sullivan C, Yasuda H, Gourlay CMet al., 2022, Understanding the rheological transitions in semi-solid alloys by a combined <i>In Situ</i> Imaging and granular micromechanics modeling approach, Solid State Phenomena, Vol: 327, Pages: 127-132, ISSN: 1012-0394

To gain better understanding of rheological transitions from suspension flow to granular deformation and shear cracking, this research conducted shear-deformation on globular semi-solid Al-Cu alloys to study the rheological behavior of semi-solid as a function of solid fraction (38% - 85%) and shear rate (10<jats:sup>-4</jats:sup> – 10<jats:sup>-1</jats:sup> s<jats:sup>-1</jats:sup>) under real-time synchrotron radiography observation. By analyzing 17 X-ray imaging datasets, we define three rheological transitions: (i) the critical solid fraction from a suspension to a loosely percolating assembly; (ii) from the net contraction of a loose assembly to the net dilation of a densely packed assembly, and (iii) to shear cracking at high solid fraction and shear rate. Inspired by in-situ observations of semi-solid deformation showing a disordered assembly of percolating crystals in partially-cohesive contact with liquid flow, we reproduced a two-phase sample using the coupled lattice Boltzmann method-discrete element method (LBM-DEM) simulation approach for granular micromechanical modeling. In DEM, each globular Al grain is represented by a discrete element, and the flow of interstitial liquid is solved by LBM. The LBM-DEM simulations show quantitative agreement of semi-solid strain localization with the experiments and are used to explore the components involved in the shear rate dependence of the transitions, and the role of liquid pressure on the initiation of shear cracking.

Journal article

Xian JW, Peng L, Zeng G, Wang D, Gourlay CMet al., 2021, Al11Mn4 formation on Al8Mn5 during the solidification and heat treatment of AZ-series magnesium alloys, Materialia, Vol: 19, Pages: 1-12, ISSN: 2589-1529

The crystallography and kinetics of Al11Mn4 formation on Al8Mn5 have been studied in magnesium alloys AZ80 / AZ91 and AZ31. During solidification, Al11Mn4 formation was promoted by low cooling rates where triclinic Al11Mn4 nucleated on rhombohedral Al8Mn5 particles with one of multiple related orientation relationships (ORs) and their variants. Al11Mn4 grew as (010) plates that were commonly twinned and the interrelationships amongst Al11Mn4 twins, Al8Mn5 twins and Al8Mn5-Al11Mn4 ORs are discussed. During solid state heat treatment at 410 °C, Al8Mn5 particles transformed into Al11Mn4 by a core-shell reaction with cracking in the Al11Mn4 shell. The solid-state reaction kinetics were consistent with interface reaction controlled growth. The results show that heat treatment can be used to tailor the Al-Mn compound in contact with the matrix (Mg) phase which may enable some control of corrosion performance.

Journal article

Tan XF, Belyakov S, Su TC, Gu Q, Liu S, McDonald S, Gourlay C, Yasuda H, Matsumura S, Nogita Ket al., 2021, Rapid fabrication of tin-copper anodes for lithium-ion battery applications, Journal of Alloys and Compounds, Vol: 867, ISSN: 0925-8388

The intermetallic Cu6Sn5 is ubiquitous in electronic interconnects where research has focused on controlling the size and distribution of this phase for improved performance. Cu6Sn5 also finds application as an anode material for advanced lithium-ion batteries. Cu6Sn5 anodes can be fabricated via an in-situ growth method involving the reaction between molten Sn and the Cu current collector. This manufacturing route offers some advantages over traditional anode fabrication however the process is slow, limiting its practical application. In this work we show the addition of 6 wt% Ni to the Cu current collector greatly accelerates the growth of (Cu,Ni)6Sn5 in Cu-xNi/Sn solid-melt couples, leading to a growth rate of up to 50x faster, reducing the processing time above 200 °C to less than 10 min. This research studies the dynamics of the formation of (Cu,Ni)6Sn5 between Cu-xNi alloys and liquid Sn through real-time observation using synchrotron X-ray imaging. The (Cu,Ni)6Sn5 growth dynamics are characterised, and the growth kinetics are analysed. Subsequently, the mechanism of the accelerated growth is investigated with electron backscatter diffraction and transmission electron microscopy. The results show the accelerated growth is due to the formation of η-(Cu,Ni)6Sn5 grains with two distinct Ni concentration ranges, leading to finer grains and spalling, which in turn facilitates the diffusion of Sn, enhancing the η-(Cu,Ni)6Sn5 formation kinetics.

Journal article

Altuhafi FN, O'Sullivan C, Sammonds P, Su T-C, Gourlay Cet al., 2021, Triaxial compression on semi-solid alloys, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 52, Pages: 2010-2023, ISSN: 1073-5623

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.

Journal article

Belyakov SA, Coyle RJ, Arfaei B, Xian JW, Gourlay CMet al., 2021, Microstructure and damage evolution during thermal cycling of Sn-Ag-Cu solders containing antimony, Journal of Electronic Materials, Vol: 50, Pages: 825-841, ISSN: 0361-5235

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.

Journal article

Daszki AA, Gourlay CM, 2021, On the 3-D shape of interlaced regions in Sn-3Ag-0.5Cu solder balls, Journal of Electronic Materials, Vol: 50, Pages: 808-817, ISSN: 0361-5235

The microstructure of Sn-Ag-Cu (SAC) solder joints plays an important role in the reliability of electronics, and interlaced twinning has been linked with improved performance. Here, we study the three-dimensional (3-D) shape of interlaced regions in Sn-3.0Ag-0.5Cu (SAC305) solder balls by combining serial sectioning with electron backscatter diffraction. In solder balls without large Ag3Sn plates, we show that the interlaced volume can be reasonably approximated as a hollow double cone with the common 〈100〉 twinning axis as the cone axis, and the 〈110〉 from all three twinned orientations making up the cone sides. This 3-D morphology can explain a range of partially interlaced morphologies in past work on 2-D cross-sections.

Journal article

Peng L, Zeng G, Lin CJ, Gourlay CMet al., 2021, Al2MgC2 and AlFe3C formation in AZ91 Mg alloy melted in Fe-C crucibles, Journal of Alloys and Compounds, Vol: 854, Pages: 1-13, ISSN: 0925-8388

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.

Journal article

Xu Y, Gu T, Xian J, Giuliani F, Britton T, Gourlay C, Dunne Fet al., 2021, Intermetallic size and morphology effects on creep rate of Sn-3Ag-0.5Cu solder, International Journal of Plasticity, Vol: 137, ISSN: 0749-6419

The creep behaviour of directionally solidified SAC305 (96.5Sn-3Ag-0.5Cu wt%) alloy has been investigated with integrated particle matrix composite (PMC) crystal plasticity modelling and quantitative experimental characterisation and test. In this manuscript, the mechanistic basis of creep rate dependence is shown to be influenced by plastic strain gradients, and the associated hardening due to geometrically necessary dislocation (GND) density. These gradients are created due to heterogenous deformation at the Sn phase and intermetallic compound (IMCs) boundaries. The size and distribution of IMCs is important, as finer and well dispersed IMCs leading to higher creep resistance and lower creep rates, and this agrees with experimental observations. This understanding has enabled the creation of a new microstructurally homogenized model which captures this mechanistic link between the GND hardening, the intermetallic size, and the corresponding creep rate. The homogenised model relates creep rates to the microstructure found within the solder alloy as they evolve in service, when ageing and coarsening kinetics are known.

Journal article

Gu T, Gourlay CM, Britton TB, 2021, The role of lengthscale in the creep of Sn-3Ag-0.5Cu solder microstructures, Journal of Electronic Materials, Vol: 50, Pages: 926-938, ISSN: 0361-5235

Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near-<110> orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing (λ2), eutectic intermetallic spacing (λe) and intermetallic compound (IMC) size, indicating a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature (TTM > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction and strain heterogeneity is identified between β-Sn in dendrites and β-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings; however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest λ2 and λe) is only localised next to IMCs without growth in size.

Journal article

Xian JW, Belyakov SA, Gourlay CM, 2020, Time-lapse imaging of Ag3Sn thermal coarsening in Sn-3Ag-0.5Cu solder joints, Journal of Electronic Materials, Vol: 50, Pages: 786-795, ISSN: 0361-5235

The coarsening of Ag3Sn particles occurs during the operation of joints and plays an important role in failure. Here, Ag3Sn coarsening is studied at 125°C in the eutectic regions of Sn-3Ag-0.5Cu/Cu solder joints by SEM-based time-lapse imaging. Using multi-step thresholding segmentation and image analysis, it is shown that coalescence of Ag3Sn particles is an important ripening process in addition to LSW-like Ostwald ripening. About 10% of the initial Ag3Sn particles coalesced during ageing, coalescence occurred uniformly across eutectic regions, and the scaled size distribution histograms contained large particles that can be best fit by the Takajo model of coalescence ripening. Similar macroscopic coarsening kinetics were measured between the surface and bulk Ag3Sn particles. Tracking of individual surface particles showed an interplay between the growth/shrinkage and coalescence of Ag3Sn.

Journal article

Gu T, Tong V, Gourlay C, Britton Tet al., 2020, In-situ study of creep in Sn-3Ag-0.5Cu solder, Acta Materialia, Vol: 196, Pages: 31-43, ISSN: 1359-6454

The creep behaviour and microstructural evolution of a Sn-3Ag-0.5Cu wt% sample with a columnar microstructure have been investigated through in-situ creep testing under constant stress of 30 MPa at ~298 K. This is important, as 298 K is high temperature within the solder system and in-situ observations of microstructure evolution confirm the mechanisms involved in deformation and ultimately failure of the material. The sample has been observed in-situ using repeat and automatic forescatter diode and auto electron backscatter diffraction imaging. During deformation, polygonisation and recrystallisation are observed heterogeneously with increasing strain, and these correlate with local lattice rotations near matrix-intermetallic compound interfaces. Recrystallised grains have either twin or special boundary relationships to their parent grains. The combination of these two imaging methods reveal grain 1 (loading direction, LD, 10.4° from [100]) deforms less than the neighbour grain 2 (LD 18.8° from [110]), with slip traces in the strain localised regions. In grain 2, (10)[001] slip system is observed and in grain 1 (10)[1]/2 and (110)[11]/2 slip systems are observed. Lattice orientation gradients build up with increasing plastic strain and near fracture recrystallisation is observed concurrent with fracture.

Journal article

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

The faceted growth of primary Al13Fe4 intermetallic compounds was studied using in-situ X-ray radiography in a solidifying Al-3Fe alloy. Microscopic twins were frequently observed in the growing intermetallics and were confirmed by post-solidification electron backscatter diffraction. A twin plane re-entrant growth mechanism was suggested, where repeated formation of re-entrant corners facilitated crystal growth along a preferential direction, forming elongated plates. In contrast, for intermetallics where this preferential growth was constrained by surrounding crystals, formation of layered twins perpendicular to the preferential direction was promoted and led to lower aspect ratios, known to be less deleterious to tensile properties.

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

Nishimura T, Sweatman K, Nishimura T, Gourlay C, Ma Z, Belyakov Set al., 2019, Solder joint and bonding method, WO2019009427A1

Provided are a solder joint in which βSn grains are oriented in a specific desired direction, the βSn grains having a desired structure, and a bonding method relating to the solder joint. Provided is a solder joint in which at least two copper substrates (2) are bonded to each other using a lead-free solder alloy including Sn. A solder ball (1) relating to the lead-free solder alloy comprises: one or a plurality of nucleation grains (4); and a single grain βSn of which a [001] direction is parallel with a facet plane of the nucleation grains (4), and which is crystal-oriented in a specific direction with respect to the copper substrates (2).


Belyakov SA, Arfaei B, Johnson C, Howell K, Coyle R, Gourlay Cet al., 2019, Phase stability and solid solubility in high reliability Pb-free solders containing Bi, Sb or In, SMTA International

Conference paper

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