R. J. Schaefer

The Effect of Rapid Solidification Velocity on the Microstructure of Ag-Cu Alloys

Electron beam solidification passes have been performed on a series of Ag-Cu alloys between 1 wt pct Cu and the eutectic composition (28.1 wt pct Cu) at speeds between 1.5 and 400 cm per second. At low growth rates conventional dendritic or eutectic structures are obtained. The maximum growth rate of eutectic structure is 2.5 cm per second. At high growth rates microsegregation-free single phase structures are obtained for all compositions. The velocity required to produce this structure increases with composition for dilute alloys and agrees with the theory of absolute stability of a planar liquid-solid interface with equilibrium partitioning. For alloys between 15 and 28 wt pct Cu, the velocity required to produce the microsegregation-free extended solid solution decreases with composition and is related to nonequilibrium trapping of solute at the liquid solid interface. At intermediate growth rates for alloys with 9 wt pct Cu or greater, a structure consisting of alternating bands of cellular and cell-free material is obtained. The bands form approximately parallel to the local interface.

Icosahedral and decagonal phase formation in Al-Mn alloys

The solidification conditions leading to the formation of the icosahedral phase in Al-Mn alloys have been investigated, using samples prepared by melt spinning and electron beam surface melting. It is found that the icosahedral phase can grow with a range of compositions, but that it grows in competition with another metastable phase which is decagonal. Both of these phases can displace the equilibrium intermetallic phases by nucleating ahead of them in the melt when the solidification velocity is greater than a few centimeters per second. The relative abundance of the icosahedral and decagonal phases varies with composition and solidification rate. Icosahedral crystals in electron beam melt trails are often about 25 μm in diameter, and they grow dendritically along a preferred crystallographic direction.

Stable and metastable phase equilibria in the Al-Mn system

The aim of the present investigation was resolution of certain obscure features of the Al-Mn phase diagram. The experimental approach was guided by assessment of the previous literature and modeling of the thermodynamics of the system. It has been shown that two phases of approximate stoichiometry “Al4Mn” (λ and μ) are present in stable equilibrium, λ forming by a peritectoid reaction at 693 ± 2 °C. The liquidus and stable equilibrium invariant reactions as proposed by Goedecke and Koester have been verified. A map has been made of the successive nonequilibrium phase transformations of as-splat-quenched alloys. Finally, the thermodynamic calculation of the phase diagram allows interpretation of complex reaction sequences during cooling in terms of a catalogue of all the metastable invariant reactions involving (Al), Al6Mn, λ, μ, ϕ, and Al11Mn4 phases.

Microstructural evolution in lead-free solder alloys: Part II. Directionally solidified Sn-Ag-Cu, Sn-Cu and Sn-Ag

The tin-silver-copper eutectic is a three-phase eutectic consisting of Ag 3 Sn plates and Cu 6 Sn 5 rods in a (Sn) matrix. It was thought that the two phases would coarsen independently. Directionally solidified ternary eutectic and binary eutectic samples were isothermally annealed. Coarsening of the Cu 6 Sn 5 rods in the binary and ternary eutectics had activation energies of 73 ′ 3 and 82 ′ 4 kJmol - 1 , respectively. This indicates volume copper diffusion is the rate controlling mechanism in both. The Ag 3 Sn plates break down and then coarsen. The activation energies for the plate breakdown process were 35 ′ 3 and 38 ′ 3 kJmol - 1 for the binary and ternary samples respectively. This indicates that tin diffusion along the Ag 3 Sn/(Sn) interfaces is the most likely the rate-controlling mechanism. The rate-controlling mechanisms for Cu 6 Sn 5 coarsening and Ag 3 Sn plate breakdown are the same in the ternary and binary systems, indicating that the phases evolve microstructurally independently of one another in the ternary eutectic.

Convection-induced distortion of a solid-liquid interface

Measurements of convective flow fields and solid-liquid interface shapes during the solidification of a pure and a slightly alloyed transparent material reveal that the convective transport of solute can cause a macroscopic depresssion to develop in the solid-liquid interface. This effect occurs under conditions close to those which are predicted to produce morphological instability of a planar interface. A cellular or dendritic microstructure later develops within the interface depression. The convection is attributed to the effect of radial temperature gradients in the crystal growth apparatus.

Precipitation in rapidly solidified Al-Mn alloys

Precipitation at 450 °C was studied in melt-spun ribbons containing up to 15 wt pct Mn in solid solution in Al. The as-spun ribbons were microsegregation-free at compositions up to 5 wt pct Mn, but in more concentrated alloys a cellular microstructure was present. Upon annealing, four precipitate phases are observed, some of them being found preferentially on cell boundaries and others being found within the cells. Al6Mn, G, and the Gℍ phase can coexist for long times at 450 °C, but the G phase appears to be slightly more stable. A less stable T phase was detected in Al-5 wt pct Mn foils following short annealing periods. The supersaturation of the Al matrix can persist for many hours in alloys containing up to 3 wt pct Mn, but is essentially gone after 1 hour in alloys with 5 wt pct Mn or more.

Rapidly solidifed Al-Cr alloys: structure and decomposition behaviour

Melt-spun ribbons of aluminium containing up to 15 wt% chromium were examined in the as-spun condition and after annealing. The more concentrated alloys contained multi-phase spherulites embedded in an α-Al matrix: chemical microanalysis showed the average composition of the spherulite core to be 22 ± 2 wt% chromium. The kinetics of precipitation at grain boundaries and within the matrix were determined by TEM and X-ray diffraction. Three very similar Al-Cr intermetallic phases are present under equilibrium conditions, but most of the precipitates in the melt-spun ribbons could be identified as Al7Cr.

Microsegregation in rapidly solidified Ag-15wt%Cu alloys

Spacings and composition profiles of cellular structures formed in Ag-15wt%Cu alloys at growth rates between 0.1 and 18 cm/s are measured. Cells of the Ag-rich phase occur with intercellular regions composed of eutectic or the Cu-rich phase. At the highest rates the cell spacings exceed the characteristic diffusion length D/V (ratio of liquid diffusion coefficient to growth rate) by a factor of ten. The rate of increase of the average cell composition with growth velocity is larger than predicted by existing diffusion models of the cell tip. Increases in cell compositions beyond the Ag metastable solidus retrograde are accounted for quantitatively by the use of non-equilibrium interface conditions (solute trapping).

On the Formation of Dispersoids during Rapid Solidification of an AI-Fe-Ni Alloy

Examination of the effect of rapid solidification velocity on the microstructure of Al-3.7 wt pct Ni-1.5 wt pct Fe has revealed a new mechanism for the formation of discrete second phase particles in rapidly solidified alloys. Cellular growth of α-Al occurs with the intercellular phase, Al9(Fe, Ni2 in two distinct morphologies. At low velocity (<50 cm/sec) the phase is continuous in the growth direction while at higher velocity discrete rounded particles are observed. Analysis of the orientation relationship and the number of variants which exists between phases leads to the proposal of a mechanism where liquid droplets are deposited by the grooves of a moving cellular interface. These droplets solidify subsequently to form the rounded second phase particles.

Amorphous phase formation in Al 70 Si 17 Fe 13 alloy

The alloy Al 70 Si 17 Fe 13 was subjected to a range of rapid solidification conditions and the resulting microstructures were evaluated. It was found that when solidification was sufficiently rapid to bypass the formation of primary intermetallic phases, the alloy consisted of spherical regions of amorphous (or microquasicrystalline) material surrounded by a crystalline phase(s). This microstructure is interpreted as the result of solidification of the amorphous phase from the melt by a first-order transformation. The structure of the amorphous phase is different from that of a liquid (or usual metallic glass).