Materials Science Forum Vols. 654-656

Abstract: Coarse aluminum surfaces were prepared by etching method, the wettability of NH4Cl -70wt%H2O solution on the surfaces was investigated, and then the substrates were undercooled and immerged into NH4Cl-70wt%H2O solution to trigger nucleation of NH4Cl crystals. Experimental results indicate that the aluminum surface with step-like micro-morphology shows a transformation from strong wettability to non-wettability during its exposure in the atmosphere, in the same time, nucleation density on the strongly wetting surface is significantly higher than that of the non-wetting surface. A hypothesis is proposed to explain the relationship between the wettability and the nucleation density: the capillary effect helps the solution infiltrate into the structures of the surface with strong wettability and drive the air out of the surface structures, so nucleation is effectively promoted by the sharp corners and hollows of the coarse surface; conversely, the air bubbles trapped in the non-wetting surface structures decrease the number of the effective nucleation sites, and leads to a higher nucleation energy barrier, thus the nucleation density is decreased.

Abstract: Directionally solidified (DS) oxide eutectic in situ composites are attracting increasing attention because of their unique properties and potential applications to high temperature structural materials, optical or electronic devices. Among the alumina-based eutectic composites, DS Al2O3/Er3Al5O12(EAG) eutectic is considered to be promising candidate for use as selective emitter at high temperature. In this work, eutectic in situ composites of Al2O3/EAG rods having smooth surface and full density are successfully prepared by directional solidification using the laser zone remelting method, aiming to investigate the growth characteristic of this novel binary eutectic under high temperature gradient. The microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The Al2O3/EAG eutectic presents a very fine irregular network structure consisting of only -Al2O3 and Er3Al5O12 phases without grain boundaries and amorphous phases between interfaces. The eutectic interphase spacing is strongly dependent on the laser scanning rate, rapidly decreasing at the sub-micron levels for the samples grown at high rate. Furthermore, the microstructural formation and evolution of the composite are analyzed and discussed.

Abstract: At normal solidification conditions, in-situ composites of a Ni-24.8%Nb hypereutectic alloy can be produced at growth velocities below 5μm/s, with a thermal gradient of 180K/cm, and this low productivity remarkably restricts the application of this kind of in-situ composites. In this paper, we proposed an approach that employs an abrupt growth velocity to make the in-situ composites grow stably out of the coupled zone. In-situ composites of the Ni-24.8%Nb hypereutectic alloy were obtained at a growth velocity of 100μm/s and the productivity was greatly improved. This value is in the same order magnitude imposed on the single-crystal superalloys. The compression strengths were investigated on different microstructures involving the coupled eutectics and non-coupled eutectics. The results showed that the crack distribution and extension were mainly localized in primary Ni3Nb dendrites in the non-coupled eutectics, and that in-situ composites with the entirely coupled eutectics have improved mechanical properties and different deformation behaviors.

Abstract: The diffusion coefficient D decides the diffusion length of solute boundary and plays a key role in the microstructure selection. This paper examines quantitatively the contribution of diffusion coefficient to the eutectic instability and amorphorization ability. The maximum growth velocity Vmax and the maximum undercooling Tmax as functions of activation energy Q in strong liquids are deduced theoretically based on eutectic growth model by separating Q from D. It reveals that the larger the Q, the smaller the Tmax and Vmax, which shows the same tendency as experimental values in some Al-based alloys and glass formers. This indicates that it is the sluggish movement of atoms that makes the transition from eutectic to others structural morphologies, even to amorphous phase, occur at smaller interface growth velocity or undercooling, which is the main contribution of the diffusion coefficient to the amorphorization ability.

Abstract: The Lipton Glicksman Kurz (LGK) growth model is commonly used to predict growth rates for equiaxed dendrites in solidifying mushy zones. However, the original LGK method treats an isolated dendrite growing in an infinite volume of liquid. In an equiaxed mushy zone, with multiple nucleation events, thermal and solutal interactions take place between the equiaxed dendrites. A modified version of the LGK model was developed that allows for measurement of the solute build-up ahead of the dendrites. To investigate the validity of the model, comparisons are made with results obtained from in-situ synchrotron X-ray videomicroscopy of solidification in a Bridgman furnace of an Al-12wt.%Ge alloy inoculated with Al-Ti-B grain refiner. Comparisons between the original LGK and modified LGK models are presented for discussion. The modified LGK model shows realistic tip temperature trends.

Abstract: This paper presents a combined experimental and modelling approach to understand dendrite fragmentation of atomised metal alloy droplets during deposition in spray forming, and to study quantitatively the relationship between this dendrite fragmentation behavior and subsequent microstructural evolution. A Gleeble 3500 physical simulator was used to create controlled thermal shock conditions in solid-liquid mixtures of Ni superalloy IN718 atomised powders, which simulated the environment of droplet deposition during the twin-atomiser spray forming of large diameter IN718 alloy billets at BIAM. The experiments were complemented by phase field modelling studies at Oxford. Experiment and modelling supported the hypothesis that the characteristic equiaxed spray formed microstructure depends critically upon the rapid remelting and thermal shock of fine-scale dendrites in solid particles in the spray to provide a high density of embryonic grains.

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

Abstract: Sn-Cu-Ni alloy solders have attracted considerable attention from electronic packaging manufacturers and suppliers owing to silver- and lead-free feature and low-cost advantage of the solders. However, there is still a lack of in-depth understanding on composition optimization and microstructure control of the solders. In the present study, the influences of the addition of a minute amount of mixed rare earth La-Ce, in the range of 0.05-0.50wt%, on melting characteristics, undercooling behavior and solidification microstructure evolution of Sn-0.7Cu-0.05Ni-xMRE (x=0.05, 0.10, 0.25, 0.50wt%) alloy solders were investigated. The results show that the addition of rare-earth La-Ce has brought about an obvious decrease of the undercooling for Sn-Cu-Ni series of solders, consequently the growth of primary solidification phase of Cu(Ni)-Sn type intermetallic compound has been suppressed and the β-Sn phase has exhibited a microstructural transition from dendritic grain to equiaxed grain with increasing the amount of La-Ce added to the solder.

Abstract: In this paper, the Cu-Fe alloys are fabricated by solidifying with and without a 1.0 Tesla horizontal magnetic field and they are drawn to composite wires under different drawing ratios, then their further strength and conductivity are investigated. The results show that, when the drawing ratio is lower, the strength of the Cu-Fe composites pre-solidified with a horizontal magnetic field is lower, which is caused by the coarser solidification microstructures induced by the injected magnetic field. However, the increase ratio in the strength of the Cu-Fe composites, which is plotted as a function of the Fe content and the drawing ratio, is higher in the case of the imposed magnetic field. It indicates that it is more efficient applying the magnetic field to fabricate Cu-Fe composites with high Fe contents and larger drawing ratio. A quantitative relationship is fitted to predict the influence of the Fe content on the strength of the Cu-Fe composites wires. On the other hand, the conductivity of the Cu-Fe composite wires is decreased with increasing Fe content. The injected magnetic field has no effect on the conductivity.

Abstract: Pb-free solders based on near-eutectic Sn-0.7Cu-xNi alloys provide excellent solderability during wave soldering with cost advantages compared to Ag-containing alternatives. However, there is only limited knowledge of the solidification mechanisms in this alloy system and, furthermore, the ternary Sn-Cu-Ni phase diagram is not yet fully established. In this study, unidirectional solidification has been conducted in a Bridgman furnace using both binary alloys from the Sn-Cu6Sn5 system and ternary Sn-rich Sn-Cu-Ni alloys. The influence of Ni additions on the solidification mechanisms is assessed by comparing the microstructures of the ternary and binary alloys. The results are used to discuss the contrasting Sn-Cu-Ni phase diagrams reported in the literature. The results demonstrate the complex phase relations in the Sn-Cu alloy system, and the important role of trace amounts of various solute elements.