H. Jones

Dry wear behaviour and its relation to microstructure of novel 6092 aluminium alloy–Ni3Al powder metallurgy composite

Abstract Novel aluminium alloy matrix composites reinforced by 15xa0vol.% Ni 3 Al intermetallic particles were prepared by a powder metallurgy route. The 6092 aluminium alloy was produced by gas atomisation, followed by blending with Ni 3 Al particles. Consolidation by extrusion at 515°C, with an extrusion ratio of 30:1, produced a uniform distribution of Ni 3 Al in the Al alloy matrix. The wear properties of the aluminium alloy–Ni 3 Al composites and the monoliths were examined by dry sliding against an M2 steel counterface at 0.94xa0m/s and a load in the range 42–140xa0N in a block-on-ring configuration. At 42 and 91xa0N, the composites offered superior wear resistance to the monoliths. Although, some surface deformation occurred at 42xa0N, the Ni 3 Al largely retained its original shape, allowing it to act as a load-supporting reinforcement. At the highest load (140xa0N), the monolith offered superior wear resistance to the composite. Gross plastic deformation was observed at the worn surface of the composite, resulting in severe fragmentation of the Ni 3 Al. A mechanically mixed layer (MML) was observed for both materials at all loads, the thickness of which increased from ∼25xa0μm at 42xa0N for both materials to ∼80xa0μm for the monolith and ∼40xa0μm for the composite at 140xa0N. Transmission electron microscopy (TEM) of MML identified a complex structure comprising an amorphous phase containing Al, O, a nanoscale Fe-, Al-, O-based phase, found mainly in the monolith, and in the composite, Ni 3 Al fragmented down to the nanometre scale. A linear relationship was found between the wear rate (mm 3 /m) and the depth of deformation, with a similar relationship being observed for both composite and monolith. The relationships between wear rate, surface structural changes and starting microstructure are discussed.

Formation of B2 antiphase domains in rapidly solidified Fe-Al alloys

Abstract Fe-Al-X (X = Cr, Mo) alloys in the composition range 50 to 80 at.% Fe, 0 to 20 at.% X were rapidly solidified by chill-block melt-spinning. Thin foil transmission electron microscopy of the rapidly solidified ribbons revealed that antiphase domains occur in the B2 structure in the limited composition range 22 to 37 at.% Al. The B2 antiphase domain size was found to increase with the increasing Al-content of the ribbon. The criterion for antiphase domain (APD) formation is investigated in terms of mechanisms for formation of the B2 structure.

Morphologies of primary Al3Fe in Bridgman solidification and TIG weld traversing of hypereutectic AlFe alloys

Abstract This study indicates that the final morphology of primary Al 3 Fe in Bridgman solidification and TIG weld traversing of Alue5f8Fe alloys is dependent on both the growth velocity and alloy concentration. With an increase in growth velocity, the morphology changes from clustered and microfaceted to being plate-like, finally becoming needle-like with or without a star-like transverse section. The initial morphology of plate-like primary Al 3 Fe is a needle-like dendritic from with a star-like transverse section behind the tip. A route for the formation of the plate-like primary Al 3 Fe from such needle-like dendrite tips is proposed.

Effect of ceramic particle size, melt superheat, impurities and alloy conditions on threshold pressure for infiltration on SiC powder compacts by aluminium-based melts

Abstract An instrumented heated pressure vessel has been used to determine the effect of some experimental variables on the pressurized infiltration of SiC powder preforms by a series of Al-based melts. Threshold pressure for infiltration of 2014 Al alloy decreased with increasing SiC particle size and increasing melt superheat. Decreased purity of unalloyed aluminium and addition of 4.2 wt.% Cu to 99.999% pure aluminium increased the threshold pressure while addition of an extra 1.0 wt.% Mg to 2014 alloy lowered it. Increasing the melt superheat of 2014 alloy reduced the incidence of entrapped porosity in the compacts which tended to increase in the direction of infiltration. There was evidence of partial dissolution of SiC into the liquid during infiltration. These findings are discussed in the context of present understanding of metal matrix composite formation via melt infiltration routes.

Modelling of growth and microstructure selection in rapid solidification : a progress report

Abstract The prediction and control of microstructures are key issues in the development of high performance materials by rapid solidification routes. The present contribution reviews progress on the following aspects: the conditions for forming a segregation-free solid; the characteristics of cellular and dendritic growth at high front velocities; and the derivation of microstructure selection diagrams. Points of agreement and disagreement between theory and experimental fact are highlighted and priorities for further investigation are identified.

Composition and solidification microstructure selection in the interdendritic matrix between primary Al3Fe dendrites in hypereutectic AlFe alloys

Abstract The composition and constitution of matrix microstructure between plate-like Al3Fe dendrites in Bridgman-grown hypereutectic Al Fe alloys has been determined as a function of alloy concentration C0 and growth velocity V in the ranges 2.5 Al3Fe to regular αAl AlxFe with increasing V, the transition velocity increasing from 0.1 to 0.2 mm/s for C0 values of 9.5 and 14 wt%Fe up to 0.34–1.0 mm/s for C0 values of 18.7–28.1 wt%Fe. This increased transition velocity, compared with that for αAl Al3Fe to αAl Al6Fe at lower concentration, is indicative of a lower formation temperature for the αAl AlxFe than the αAl Al6Fe eutectic.

Formation of microstructure in rapidly solidified materials and its effect on properties

Abstract The capability of rapid solidification to generate novel, refined microstructures containing extended equilibrium and non-equilibrium constituents has been known for decades, but it is only relatively recently that progress has been made in predicting the microstructural outcome of particular sets of rapid solidification conditions. The present contribution reviews the progress made which has involved advances both in theoretical modelling of phase formation and growth kinetics and in the formulation and execution of experimental studies in which the conditions applicable at the solidification front are controlled and known. Examples of some effects of rapidly solidified microstructure on properties of relevance to prospective applications are given.

Novel faulted structures in rapidly solidified Fe-37 at.% Al-15 at.% Mo alloy

Abstract Fe-37 at.% Al-15 at.% Mo has an equilibrium two-phase structure consisting of an ordered B2 (FeAl-based) matrix with Mo 3 Al as the second phase. Rapid solidification by chill-block melt-spinning suppresses the formation of the Mo 3 Al phase, giving instead single phase B2-FeAl supersaturated with Mo and containing a network of faults revealed by TEM. Imaging with B2 superlattice reflections revealed an anisotropic antiphase domain contrast while imaging with fundamental (b.c.c.) reflections showed typical stacking fault or thin precipitate contrast for the same faults. The faults were identified as a /2〈111〉{100} type B2 antiphase boundaries. It is proposed that the formation of such anisotropic and non-conservative antiphase boundaries is related to the retention of excess Mo within the B2 matrix by rapid solidification.

Beta-Eutectoid Decomposition in Rapidly Solidified Titanium-Nickel Alloys

The eutectoid reaction, β → α + Ti2Ni, has been observed in as-produced rapidly solidified ribbons and flakes of hypoeutectoid and near-eutectoid beta titanium-nickel alloys prepared by chill block melt spinning (CBMS), pendant drop melt extraction (PDME), and electron beam melting/splat quenching (EBSQ) processes. Microstructural characterization of these materials was carried out by scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. The occurrence of eutectoid decomposition in the rapidly solidified alloys was attributed to the breakaway of the ribbons or flakes (while still at an elevated temperature) from the quench wheel, resulting subsequently in a lower cooling rate. Fast quenching, as obtained in the hammer-and-anvil process, resulted in a martensitic structure free from products of eutectoid decomposition. The eutectoid morphology was nonlamellar in hypoeutectoid alloy ribbons, while a hitherto unreported lamellar eutectoid was observed in the near-eutectoid ribbons and flakes. The formation of this unusual lamellar eutectoid was rationalized in terms of the predominance of allotriomorphs of alpha phase and consequent availability of sufficiently mobile and maneuverable alpha/beta interphase boundaries in the fine-grained, rapidly solidified titanium-nickel alloys.

Characteristics of Al-Cr-Zr alloy powders made by confined nozzle atomisation

AbstractThe effects of superheat and powder particle size fraction on the characteristics of Al–5Cr–2Zr (wt-%) alloy confined nozzle atomised powder particulate has been investigated. The median powder particle size decreased from 62 to 38 μm with increase in superheat from 140 to 300 K for the atomising conditions studied, with a broadening of the size distribution and the increasing presence of large irregular powder particles at the lowest superheats. Essentially featureless α-Al based microstructures in splat caps and small sized powder particles were replaced by cellular α-Al with increasing amounts and sizes of primary L12Al3Zr and Al13Cr2 inclusions with increasing powder particle size. Refinement of the α-Al cell and intermetallic inclusion sizes in a fine compared with a coarse powder size fraction was consistent with the expected powder relationship between these variables. The decreased microhardness for coarser powder particle size fractions was attributed to the formation of increasingly coar...