Frank S. Biancaniello

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.

The formation of ordered ω-related phases in alloys of composition Ti4Al3Nb

During cooling of an alloy of composition Ti4Al3Nb from a B2 phase field above 1100°C, a metastable trigonal (P3m1) ω-related phase, designated ω″, forms along with small amounts of D019 and L10 phases. The ω″ phase exhibits partial collapse of 111 planes and reordering relative to its B2 parent. An apparently equilibrium low temperature phase with the B82 structure was found after 26 days of annealing at 700°C. Both ω″ and B82 structures were verified by means of transmission electron microscopy and by single crystal X-ray diffraction. The latter permitted detailed analysis of the collapse parameters and site occupancies. The observed transformation path, B2(Pm3m)→ω″(P3m1)→B82 (P63/mmc), occurs in two steps. The first involveds a subgroup transition during cooling that is primarily displacive with reordering consistent with the trigonal symmetry imposed by the ω-collapse. The second involves a supergroup transition during prolonged annealing that is primarily replacive and constitutes a chemical disordering. The direct equilibrium transformation, B2→B82, without the formation of an intermediate trigonal phase, can only occur by a reconstructive transformation.

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.

Solutal convection induced macrosegregation and the dendrite to composite transition in off-eutectic alloys

The effect of solute gradient induced convection during vertical solidification on the macrosegregation of Pb-rich Pb-Sn off-eutectic alloys is determined experimentally as a function of composition and growth rate. In many cases macrosegregation is sufficient to prevent the plane front solidification of the alloy. The transition from dendritic to composite structure is found to occur when the composition of the solid is close enough to the eutectic composition to satisfy a stability criterion based onGL/V (liquid temperature gradient/growth rate). A vertical or horizontal magnetic field of 0.1 T (1 kilogauss) does not reduce macrosegregation, but downward solidification (liquid below solid) virtually eliminates macrosegregation in small (∼3 mm) diameter samples.

Correlation between ultrasonic and hardness measurements in aged aluminum alloy 2024

Abstract Sound wave velocity, ultrasonic attenuation, eddy current and hardness measurements have been carried out on precipitation-hardening aluminum alloy 2024 subjected to a series of different pre-aging heat treatments prior to processing to T4, T351 and T851 tempers. For each temper the maximum hardness was found to correspond to a particular value of sound velocity. These results were correlated with electron microscopy observations of the microstructure. Ultrasonic attenuation was found to decrease consistently as hardness increased. Pre-aging at 350°C was found to induce the most rapid initial reduction in hardness, and corresponding changes in sound velocity and ultrasonic attenuation. This investigation has demonstrated the feasibility of ultrasonic techniques for non-destructive evaluation and characterization of age-hardened aluminum alloys.

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.

Rapid solidification and ordering of B2 and L21 phases in the NiAlNiTi system

Abstract Evidence is presented for the direct solidification of the B2 phase in the NiAlNiTi system at some compositions where the L21 phase is stable at the melting point. Subsequent continuous ordering produces the equilibrium phase. The metastable continuous ordering curve is used to discuss this result.

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).