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Abstract: During Pb-free soldering, βSn often requires a high nucleation undercooling and there is an ongoing effort to develop nucleation catalysts. It is shown here that NiSn4, PdSn4 and PtSn4 are heterogeneous nucleants for βSn, reducing the nucleation undercooling to ∼4K when these intermetallics are present either in the bulk solder or as the interfacial layer. Nucleation catalysis occurs by βSn nucleating on the (008) facet of XSn4 crystals with an orientation relationship (OR) (100)Sn||(008)XSn4 and [001]Sn||[100]XSn4 where there is a planar lattice match of ∼5%. This OR is also the origin of well-aligned lamellar βSn–XSn4 eutectic morphologies, even though the eutectics contain less than 2vol.% of faceted NiSn4, PdSn4 or PtSn4. [Copyright &y& Elsevier]

Abstract: Metastable intermetallics such as NiSn4 occasionally form in solder joints on Ni substrates. Here it is shown that impurity Fe or soldering to Fe–42Ni substrates promotes metastable NiSn4 during the solidification of Sn-rich Sn–Ni alloys. FeSn2 crystals act as heterogeneous nucleation sites for NiSn4, with nucleation occurring on the (001) facet of FeSn2 where the lattice mismatch between (001)FeSn2 and (001)NiSn4 is only ∼2%. The size of active FeSn2 nuclei was 2–10 μm and smaller particles or large, porous FeSn2 particles were ineffective nuclei for primary NiSn4. The use of high purity Sn–Ni alloys, where FeSn2 does not form, is shown to prevent primary NiSn4 formation and promote Sn–Ni3Sn4 eutectic growth. However, metastable Sn–NiSn4 eutectic grew at all Fe levels studied and both Sn–NiSn4 and Sn–Ni3Sn4 eutectics coexisted in high purity samples. Thus, metastable NiSn4 formation is enhanced by nucleation on FeSn2 and also depends on competitive growth between the Sn–NiSn4 and Sn–Ni3Sn4 eutectics. [Copyright &y& Elsevier]

Abstract: The growth of NiSn4 is studied during the solidification of Sn–Ni alloys containing 0–0.45wt%Ni. Where past research has found metastable NiSn4 after solid state ageing of Ni–Sn couples, it is shown here that NiSn4 also forms during solidification as both a primary and a eutectic phase over a wide range of solidification conditions. The NiSn4 phase has crystal structure isomorphous to PtSn4 (oC20) and no evidence is found for NiSn4 being stable at solidification temperatures. Primary NiSn4 crystal growth was promoted by high cooling rates and a comparison of growth ledges on the facets of primary NiSn4 and Ni3Sn4 crystals suggests easier interface attachment kinetics for NiSn4. The Sn–NiSn4 eutectic grew under all solidification conditions used, with an orientation relationship that produces a low lattice disregistry at the interface plane and the same NiSn4 growth direction as that selected during free growth of primary NiSn4. In hyper-eutectic compositions, both the Sn–NiSn4 and Sn–Ni3Sn4 eutectics grew with the latter growing from primary Ni3Sn4 crystals. [Copyright &y& Elsevier]

Cu6Sn5 is an important intermetallic compound used in lead-free soldering and anode materials for Li-ion batteries. Cu6Sn5 exists in at least two crystal structures, with a transformation from hexagonal η-Cu6Sn5 at temperatures higher than ∼186°C to monoclinic η′-Cu6Sn5 at lower temperatures. Using variable temperature synchrotron X-ray diffraction analysis, the kinetics of the η–η′ transformation are investigated and a time–temperature transformation diagram is partially developed. The significance of the results in terms of phase and dimensional stability in solder joints are discussed. [Copyright &y& Elsevier]

Abstract: Deformation mechanisms in equiaxed, partially solid Al–15wt.% 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. [Copyright &y& Elsevier]

The dynamics of globule motion are studied in situ during the deformation of globular semi-solid steel (Fe–2C–0.9Mn–0.5Si in mass%). Samples with globular morphology and 55–65% effective solid fraction were deformed in direct shear at a shear rate of ∼10−3 s−1 during time-resolved synchrotron X-ray imaging. Globule rearrangement involving dilation was the main deformation mechanism. Agglomeration is shown to significantly influence rearrangement by increasing the effective size of structural units. [Copyright &y& Elsevier]

Abstract: The dynamics of off-eutectic solidification are studied in two Pb-free solders using synchrotron X-ray radiography and post mortem scanning electron microscopy. The early stages of growth revealed the onset of coupled growth and phase competition. In Sn–0.7wt.% Cu, the βSn–Cu6Sn5 eutectic then grew with Cu6Sn5 rods leading the βSn phase, with a range of rod spacings and local interface curvatures. However, these nonfaceted–faceted eutectic features were exhibited much more weakly than in the Al–Si and Fe–C eutectics, despite primary Cu6Sn5 crystals exhibiting marked faceting. In Sn–0.7wt.% Cu–0.06wt.% Ni, primary intermetallic laths grew ahead of a rod-like βSn–(Cu,Ni)6Sn5 univariant eutectic front. The univariant eutectic had a narrow freezing range and a similar spacing to the invariant eutectic at the same velocity. Composition measurements suggest that the univariant eutectic groove descends in the direction of the Sn+Cu6Sn5 binary eutectic point. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 (d sb) in HPDCs have been developed. The w/d sb 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/d sb ratio decreases with increasing distance from the casting center in a cross-section containing multiple bands. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The influence of trace level Ni additions on the eutectic solidification mode of Sn–0.7Cu has been studied using continuous torque experiments during solidification. The solid fraction at which resistance to paddle rotation at the thermal centre of the sample occurs is related to the spatial distribution of solid during solidification. The results indicate that a transition in solidification mode occurs in the range 0–300ppm Ni. Growth occurs antiparallel to heat flow from near the mould walls in the Ni-free alloy, while equiaxed growth from distributed centres dominates in alloys containing at least 300ppm Ni. [Copyright &y& Elsevier]

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 Al 8 Mn 5 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 Al 8 Mn 5 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 Al 8 Mn 5 particles are twinned, consisting of four orientations related by ∼90° rotations around three common < 1 1 ¯ 02 > , which is discussed in terms of the pseudo-cubic <100> axes of the Al 8 Mn 5 rhombohedral gamma brass. Al 8 Mn 5 grew as equiaxed particles that are explained by polyhedron models based on {100}, {110} and {112} facets of the pseudo-cubic Al 8 Mn 5 cell. The results indicate that, at low Fe:Mn ratio, most impurity Fe is dissolved in B2 particles that are encapsulated by low-Fe Al 8 Mn 5 which may be important for corrosion resistance. [ABSTRACT FROM AUTHOR]

In the current generation of 3D electronic packaging, multiple reflows are often required during soldering. In addition, electronic packages may be subjected to additional solder rework or other heating processes. This paper investigates the effects of multiple reflow cycles on TiO 2 reinforced Sn–0.7Cu solder fabricated by a powder metallurgy microwave sintering technique. Compared to TiO 2 -free equivalents, a relative suppression of the Cu 6 Sn 5 phase, both as primary crystals and as an interfacial layer was observed. The likely mechanism relates to the TiO 2 nanoparticles promoting nucleation and decreasing the amount of time that liquid is in contact with the interfacial layer. The TiO 2 particles appear to stabilise the interfacial Cu 6 Sn 5 layer and result in a more planar morphology. The suppression of Cu 6 Sn 5 results in TiO 2 reinforced solder joints having a higher shear strength after multiple reflow cycles compared to Sn–0.7Cu solder joints. [ABSTRACT FROM AUTHOR]

Abstract: The particulate discrete element method (DEM) is used to simulate crystal rearrangement during equiaxed solidification of Al alloys for a range of morphologies across the globular to equiaxed-dendritic transition. It is shown that DEM is able to capture the key experimental results reported in past dendrite coherency studies, namely: the marked increase in resistance to shear at a critical solid fraction, the influence of crystal morphology on dendrite coherency, and that dendrite coherency marks the onset of dilatancy. Dendrite coherency is shown to be the lowest solid fraction at which long-range interconnectivity in the force chain network develops during shear. It is further found that dendrite coherency depends on both the internal solid fraction within dendrite envelopes and the mean shape of envelopes at coherency. The potential to extend DEM to the simulation of mushy-zone mechanics in casting problems is then discussed. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The development of shear bands in partially solid alloys is investigated during the deformation of a magnesium alloy at ∼45% solid during equiaxed solidification. Results show that shear bands initiate after reaching the peak stress and then increase in thickness during strain softening from ∼10 to ∼16 mean grains thick. Once shear bands become fully developed, shear continues at approximately constant stress and volume. [ABSTRACT FROM AUTHOR]

The faceted growth of primary Al 13 Fe 4 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. [ABSTRACT FROM AUTHOR]

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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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 Al 3 Ti on TiB 2 , and by the pushing and engulfment of TiB 2 by growing Al 3 Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB 2 crystal, solidifying multiple small Al 3 Ti crystals on one (0001) facet of TiB 2 , 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 TiB 2 particles being pushed and engulfed by Al 3 Ti 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 TiB 2 particles on Al 3 Ti facets during pushing. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The influence of Ni on the size and twinning of primary Cu 6 Sn 5 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 Cu 6 Sn 5 if the alloy is fully melted. However, for peak temperatures ≤250 °C relevant to industrial soldering, primary Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 liquidus slope and on the lattice parameters of (Cu,Ni) 6 Sn 5. Graphical abstract Image 1 Highlights • Ni solute itself is shown to be ineffective in refining primary Cu 6 Sn 5. • The size of primary Cu 6 Sn 5 after solidification strongly depends on the original size in the original solder ball/paste before melting. • Growth twinning occurs in primary Cu 6 Sn 5 and is promoted by Ni additions. • The changing lattice parameters of Cu 6 Sn 5 by Ni additions facilitate coexisting { 10 1 ¯ 2 } and { 10 1 ¯ 1 } twin planes. [ABSTRACT FROM AUTHOR]

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 (αCoSn 3 , IrSn 4 , PtSn 4 , PdSn 4 ) the ORs represent relatively simple atomic matches. ORs with lower potency nucleants (Cu 6 Sn 5 , Ag 3 Sn, Ni 3 Sn 4 ) 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 Cu 6 Sn 5 , Ag 3 Sn, Ni 3 Sn 4 ; 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. [ABSTRACT FROM AUTHOR]

Metastable Sn-NiSn 4 eutectic can form in electronic solder joints and is a reliability concern. Here the competition between stable Sn-Ni 3 Sn 4 and metastable Sn-NiSn 4 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-Ni 3 Sn 4 eutectic has a highly irregular morphology with diverging and converging rods because monoclinic Ni 3 Sn 4 grows only along [010] without a reproducible orientation relationship (OR) with Sn. The metastable Sn-NiSn 4 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. [ABSTRACT FROM AUTHOR]

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 Cu 6 Sn 5 is explored. [ABSTRACT FROM AUTHOR]

The growth mechanisms of primary Cu 6 Sn 5 are studied in Sn-Cu alloys and solder joints by combining EBSD, FIB-tomography and synchrotron radiography. With increasing cooling rate and Cu content, Cu 6 Sn 5 crystals developed from faceted hexagonal rods to grooved rods, in-plane branched faceted crystals and, finally, to nonfaceted dendrites. This range of growth morphologies has been rationalised into a kinetic microstructure map. Cu 6 Sn 5 hexagonal rods grew along [0001] bounded by { 10 1 ¯ 0 } facets and Cu 6 Sn 5 dendrites branched along <405> in the { 10 1 ¯ 0 } planes. The faceted to nonfaceted transition indicates a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature. Finally, it is shown that the full range of Cu 6 Sn 5 morphologies that grew for different composition and cooling rate combinations in bulk alloys can be engineered to grow in solder joints made with a single composition (Sn-0.7 wt%Cu/Cu) by altering the peak temperature and the cooling rate. [ABSTRACT FROM AUTHOR]

Large Ag 3 Sn 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 Ag 3 Sn 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 Ag 3 Sn 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 Ag 3 Sn nucleation. This is shown to be due to constitutional supercooling generated by Ag solute rejection into the liquid ahead of the Cu 6 Sn 5 reaction layer, with additional contributions from geometrical catalysis in the grooves between Cu 6 Sn 5 scallops and heterogeneous nucleation of Ag 3 Sn on Cu 6 Sn 5. The relative ease of Ag 3 Sn nucleation on the Cu 6 Sn 5 reaction layer is responsible for the large plates often reported in electronic solder joints. [Display omitted] [ABSTRACT FROM AUTHOR]

The study of mushy-zone deformation in steels is important for limiting defect formation in continuous casting. Here, we use in situ synchrotron radiography to quantify the shear deformation mechanisms of an 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 (Reynolds' 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. [ABSTRACT FROM AUTHOR]

A Cu 6 Sn 5 layer is an integral part of many electronic interconnections. Here we show that, although primary Cu 6 Sn 5 is not a potent nucleant for Sn, the Cu 6 Sn 5 layer plays a key role in Sn nucleation and microstructure formation in solder joints. Combining thermal analysis, FIB-tomography and EBSD, we show that conical cavities are present between the scallops of the Cu 6 Sn 5 layer that act as geometric nucleation sites for Sn, that Sn grows from the Cu 6 Sn 5 layer, and that reproducible nucleation orientation relationships (ORs) exist between Cu 6 Sn 5 and Sn. With these ORs, a near-random distribution of Sn orientations is predicted from joint to joint even for Cu 6 Sn 5 layers with a strong [0001] fibre texture. It is shown that the nucleation undercooling is strongly affected by manipulation of the Cu 6 Sn 5 layer shape, and that it is possible to prevent nucleation on the Cu 6 Sn 5 layer by adding more potent nucleants. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The pre-existing porosity behaviour of globular semi-solid Al–Cu alloys during uniaxial compression is quantified during time-resolved synchrotron tomography at ∼64% and ∼93% solid. At ∼64% solid, both tortuous and round pores exist before deformation, respectively persisting and closing during compression, while at ∼93% solid, pre-existing tortuous pores open under compressive axial strain. It is demonstrated that pore behaviour reflects the immediate grain neighbourhood where the semi-solid alloy acts as a granular material, and that pore opening results from shear-induced dilation during uniaxial compression. [ABSTRACT FROM AUTHOR]

Abstract: The granular micromechanics of semi-solid steel at ∼80% solid are studied by synchrotron radiography. A particulate soil mechanics approach to image analysis shows that deformation occurs by the translation and rotation of quasi-rigid grains under the action of contact forces, and that the changes in directional fabric and grain–grain contacts occur by mechanisms similar to those of highly compacted soils including “locked sands”. Grain-scale phenomena are then linked to the macroscopic displacement and strain fields and it is shown that shear-induced dilation is a fundamental response at both the grain and macro scales. Based on this, recommendations are made on future rheology experiments. [Copyright &y& Elsevier]

Abstract: The technique of trace element doping to modify the solidification mechanism of faceted/non-faceted eutectics has been applied to the Mg–Mg2Ni alloy system. It is demonstrated that the micro- and nano-structure of cast hypoeutectic Mg–Mg2Ni 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 6.8wt.% under the absorption parameters of 350°C and 1MPa for 24min and subsequent desorption at 0.2MPa for 24min 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. [Copyright &y& Elsevier]