Primary Intermetallic Particles Research Articles

Characterization of intermetallic particles in a Mg–8Zn–4Al–0.5Ca (wt%) casting alloy

The crystal structure and thermal stability of intermetallic particles in the as-cast microstructures of a Mg–8Zn–4Al–0.5Ca (wt%) casting alloy have been characterized using transmission electron microscopy and electron diffraction. It is found that these particles have an average composition of Mg6Zn9Al4Ca and a primitive cubic structure, with space group and a lattice parameter of 1.43 ± 0.18 nm. The proposed structure is distinctly different from that of the equilibrium T phase (Mg32(Al,Zn)49), which is often reported to form in alloys of similar compositions. These primary intermetallic particles are thermally stable at temperatures close to the melting point of the alloy. They are still detectable in samples subjected to isothermal heat treatments at 325°C for up to 720 h.

A eutectoid phase transformation for the primary intermetallic particle from Al m(Fe,Mn) to Al 3(Fe,Mn) in AA5182 alloy

The size and morphology evolution of the primary intermetallic particles in a DC-cast AA5182 alloy during homogenization at 470 and 520 °C has been studied. A eutectoid phase transformation of the primary particles Al m (Fe,Mn) → Al 3(Fe,Mn) + Al is observed by field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM). After the transformation, Al m (Fe,Mn) particles become a lamellar mixture of Al 3(Fe,Mn) and Al, which is believed to be beneficial to the break-up of the large primary particles during hot rolling. The transformation mechanism and the transformation kinetics have been studied. During isothermal treatment at 520 °C, the fractional transformation rate is mainly controlled by nucleation. At 470 °C, the phase transformation is controlled by both nucleation and growth of Al 3(Fe,Mn) in Al m (Fe,Mn) phase.

Characterization of quasicrystalline primary intermetallic particles in Mg?8wt%Zn?4wt%Al casting alloy

The quasicrystal structure of primary intermetallic particles found in the ascast microstructure of a permanent mould-cast Mg-8wt%Zn-4wt%Al alloy has been characterized using transmission electron microscopy. Most primary intermetallic particles have an average composition of Mg

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

AbstractA method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2 intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al13Cr2 primary intermetallic particles. The solidification of these powders is discussed in terms of so...

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2 intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al13Cr2 primary intermetallic particles. The solidification of these powders is discussed in terms of solidification front velocity controlled by external heat flow and by the initial undercooling. Particles less than 10 μm in diameter undercool significantly before solidification. Segregation free microstructures occur in the fine <1 μm) particles, where the solidification front velocity exceeds the absolute stability velocity.MST/1247a

Hydrogen bubbles in embrittled Al-Zn-Mg alloys

Al-Zn-Mg alloys become embrittled during exposure to moist environments due to hydrogen penetration of grain boundaries. The result of this hydrogen penetration due to surface reaction with water vapour of both bulk specimens and electron-transparent “thin foils”, has been studied at high resolution in the JEM 100 C transmission electron microscope as a function of alloy composition and ageing treatment. In bulk specimens of alloys solution-heated, water-quenched, and aged in water-vapour-saturated air at 70° C, the hydrogen is in the form of a mobile atomic species which is transformed to bubbles of molecular hydrogen under the action of the electron beam. However, in electron-transparent specimens of aged alloys after exposure to water vapour the accumulated hydrogen is observed directly as bubbles. These bubbles take the form of hexagonal lenses bounded by {111} planes, and are associated with grain-boundary precipitates, particularly in over-aged microstructures, and with primary intermetallic particles in alloys containing sparingly soluble transition elements. The consequence of the observed hydrogen penetration of grain boundaries in promoting environmental debilitation of mechanical properties and stress-corrosion cracking of Al-Zn-Mg alloys is discussed.