Banding phenomena in AlFe alloys subjected to laser surface treatment

Using beam rates of between 0.005 and 18.0 m/s, laser-melted tracks were produced on AlFe alloy samples containing between 0.25 and 8.0 wt% Fe. The local solidification rates were measured by taking a longitudinal section through the centre of the laser trace, and the corresponding microstructures were studied quantitatively using transmission electron microscopy. Two different banded structures were observed: one at slow scanning rates (low-velocity bands) and another at high growth rates (high-velocity bands). The low-velocity bands were shown to depend essentially upon conditions prevailing at the surface, and were attributed to convection (Marangoni) effects. For all of the alloys there was a critical growth rate, at which the cellular-dendritic structure was replaced by a high-velocity banded structure which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. The structure of the dark bands was similar to that of the cellular-dendritic structure prior to the onset of the banded structure. Diffraction patterns from the light bands exhibited spots only of the f.c.c. α-Al solid solution and microanalyses showed that, within a light band, the concentration was uniform and equal to the nominal concentration of the alloy. With increasing growth rate, the widths of the dark bands decreased continuously and, since the overall spacing of the bands remained constant, this finally led to the disappearance of the dark bands. A completely precipitation-free was then observed at very high growth rates in the more dilute alloys. A phenomenological model, based upon periodic instabilities of the growth rate, was proposed in order to explain the origin of the high-velocity banded structure.     

The coupled zone of rapidly solidified AlSi alloys in laser treatment

Laser-melted tracks were produced on AlSi samples containing between 15.5 and 26 wt% Si with the resultant solidification rates being measured by taking a longitudinal section through the centre of the laser trace. The Al-rich boundary of the coupled zone, i.e.the growth rate-concentration limit at which the transition from fibrous AlSi eutectic to α-Al dendrites plus interdendritic eutectic takes place, has been experimentally determined for concentrations of Si varying from 15.5 to 20 wt%. Supposing that the growing structure, for a given growth rate, is the one having the higher growth temperature, good agreement is found with the more recent microstructural growth models when kinetic effects are taken into account. For concentrations of Si higher than 20 wt%, primary Si crystals imbedded in equiaxed eutectic grains are observed which replace columnar eutectic and dendritic growth.     

The microstructures of rapidly solidified hyper-eutectic AlBe alloys

Several alloys of hyper-eutectic composition (in the range 4.4–20.0 at% Be) have been rapidly solidified by melt-spinning in an He environment. For low solute compositions, the microstructure consists of cells elongated in the transverse direction of the ribbons. The intercellular regions consist of a refined dispersion of Be particles whereas the cells themselves (the intracellular regions) contain a small supersaturation of Be. As the solute concentration is increased the microstructure at first becomes more refined, consisting of a dispersion of Be particles (≈ 50 Å in diameter with an interparticle spacing of ≈ 200 å) in an Al matrix. At higher concentrations, a bimodal distribution of Be particles is present in the Al matrix, the finer precipitates being similar in appearance to those described above, and the coarser particles being ∼0.1–0.15 μm in diameter. These various microstructures may be most satisfactorily described on the basis of a set of metastable phase transformations involving a monotectic reaction. In this way, the refined distribution of Be particles is a product of this reaction, whereas the larger particles are thought to form during a pro-monotectic transition.     

A TEM study on the microstructure of rapidly solidified CuCo alloys

The present paper reports TEM investigation on the detailed microstructure and its formation of rapidly solidified CuCo alloys. Results show that the previously observed f.c.c. α cobalt phases in some of the rapidly solidified CuCo alloys are precipitates formed when the ribbon loses its intimate contact with the substrate wheel. The solid solubility of cobalt in copper could further be extended beyond the previously reported value by taking appropriate measures during melt spinning. The f.c.c. precipitates with twins are parallel and coherent with the matrix.     

Enhanced decomposition of rapidly solidified microstructures in AlFeMo and TiAlEr alloys by plastic deformation and applied stress

In this work, the effects of plastic deformation and applied stress on enhanced decomposition of rapidly solidified microstructures in Al8Fe2Mo and TiEr alloys were examined. The results indicate that there is little effect of either plastic deformation or applied stress on decomposition of the Zone A microstructure in Al8Fe2Mo. It was concluded that decomposition of the microstructure during low temperature extrusion must be a result of adiabatic heat generated during the process. The effect of plastic deformation and applied stress on microstructural degradation of erbia-strengthened TiEr alloys was found to be significant, the extent of coarsening as identified by average particle size increasing as a result of plastic deformation and with increased applied stress. Recrystallization was observed to reduce these effects. The difference in enhanced decomposition behavior between these two microstructures was concluded to be a result of the mechanism of microstructural degradation and atomic transport.     

Microstructures and mechanical properties of rapidly solidified CuCr alloys

Rapidly-quenched CuCr alloys have been prepared by melt-spinning and the microstructures and mechanical properties examined as a function of alloy content and subsequent annealing treatment. Mechanical properties have been successfully measured by tensile testing on the ribbon samples and it is shown that this method is more suitable than hardness testing for a proper evaluation of the materials. An alloy containing 2% Cr has been prepared in the totally solid-solution state, whilst a 5% Cr alloy contains large chromium particles distributed uniformly throughout the ribbon and fine-grain-sized CuCr solution. These large chromium particles are deduced to arise by a uniform, primary-solidification mode prior to, and independent of, the nucleation and growth of the copper from the lower ribbon surface. The increased solid solubility obtained allows extensive precipitation and strengthening following ageing: the primary chromium particles play an important role both in strengthening and in restricting grain coarsening.     

Rapidly solidified AlCu alloys—I. experimental determination of the microstructure selection map

Laser rapid solidification experiments have been performed on AlCu alloys of hypereutectic composition 36, 40 and 44 wt% Cu. By taking thin foils from the surface of the laser traces it has been possible to study the resulting growth morphologies using TEM; the microstructural orientation allowing the morphologies to be correlated with their local growth velocities. Microstructural orientation allowing velocity range 0.01–2.0 ms⁻¹ have been studied, and eutectic (both with and without oscillatory) instabilities), dendritic, cellular, banded and planar front growth have all been observed. By combining these results with earlier observations made on alloys with lower Cu concentrations, it has been possible to produce a microstructure selection map for AlCu alloys. The map correlates microstructure to growth velocity and composition in the ranges 0.01–2.0 ms⁻¹ and 0–44 wt% Cu, i.e. the major part of the binary AlAl₂Cu eutectic (0–54 wt% Cu). As well as providing an interesting overview of solidification structures, several features of this map can be used to gain information on the AlCu phase diagram.     

Precipitation in a rapidly solidified and aged NiAlMo alloy

The early stages of decomposition of a highly supersaturated nickel-base alloy have been studied using transmission electron microscopy, scanning transmission electron microscopy and X-ray diffraction. The material was produced as a metastable solid solution by chill-block melt-spinning. On ageing the material exhibited a number of decomposition products appearing in series or concomitantly. Some of the decomposition products of this alloy, Ni₄Mo, Ni₃Mo (D0₂₂) and Ni₂Mo, are related to those found in NiMo binary alloys. α-Mo formed during solidification was distinguished from that formed by precipitation in the solid state by orientation relationships.     

Microstructural characteristics of quasicrystalline phases in alloys of AlCuFeCo

An investigation of the structural characteristics of quasicrystalline phases obtained in quaternary alloys is carried out. The coexistence between the quasicrystalline phases phases is analyzed. HREM images and diffraction patterns which correspond to the decagonal and icosahedral phases show pronouned deviations from the perfect decagonal and icosahedral symmetries. These effects are more pronounced when the specimen is annealed. Both kind of quasicrystalline phases show planar faults and dislocation-type of defects. Planar faults show the same image contrast features as in the crystalline case. Evidence is presented based on image contrast characteristics of two different types of planar faults. Dislocations in these phases show no evidence of spliting into partials.     

The formation of polytwinned structures in FePt and FePd alloys

• 1.1. The polytwinned structures in the FePt and FePd alloys derived from a nucleation and growth process in which the coherent ordered regions form aligned particle arrangements under the influence of the elastic strain energy. The impingement and coalescence of the ordered particles to form twin related structural domains give rise to a high density of APB's within the twin plates.
• 2.2. The nuclei form as disks along the {110} planes of the cubic matrix and these nuclei may merge with an order parameter or c/a ratio less than the equilibrium value but as the strain energy of the system relaxes the c/a ratio approaches equilibrium.
• 3.3. The polytwinned structure undergo a coarsening process under the mutual influence of the strain and surface energies analogous to “discontinuous coarsening” of lamellar two-phase aggregates resulting from cellular phase separation.

     

Partial modification in unidirectionally solidified AlSi eutectic alloys

Previous investigations on partially modified cast AlSi alloys have indicated that the transition from an unmodified to a modified eutectic structure occurs discretely and is apparently associated with a critical level of impurity. The present work extends the partial modification study to unidirectionally solidified samples in order to more clearly define the role of impurity level and solidification rate. The values of strontium concentration and growth rate leading to the different types of AlSi structure (i.e. unmodified, partially modified, and fully modified) have been determined. Partially modified samples are produced at relatively low growth rates (less than 25 μm/s) when the strontium level lies between 0.004 and 0.014 wt%. Their structures consist of discrete regions of flake and fibrous silicon, the distribution of which is dependent on the growth rate and the segregation characteristics of the strontium modifier. The results support the argument that impurity modification is due to the adsorption of the impurity element onto the silicon phase during growth.     

A study of work hardening in austenitic FeMnC and FeMnAlC alloys

Two austenitic FeMnAlC alloys with aluminium contents of 0 and 2.7 wt% were strained in tension between 193 and 823 K. Serrated stress-strain curves, inverse strain-rate dependence of flow stress, and high work hardening exhibited in particular temperature ranges for both alloys were characteristic of dynamic strain aging. The apparent activation energy for the onset of serration increased from 14.4 to 22.3 kcal/mol due to the addition of 2.7 wt% Al. It was found that the high work-hardening rate cannot be attributed to strain-induced deformation twinning when serrated stress-strain curves occurred. From the evidence of the present study and the known effect of aluminium on the diffusivity and activity of carbon in austenitic high-manganese steel, it is suggested that dynamic strain aging is the major cause of work hardening within the intermediate temperature range from 298 to 493 K for 0 wt% Al and 393 to 593 K for 2.7 wt% Al in the present austenitic FeMnAlC alloys.     

The martensite transformation in FeNiC alloys

The effect of austenite defect structure upon the sub-zero martensite burst transformation temperature in FeNiC has been investigated using a combination of optical and electron microscopy, differential scanning calorimetry and microhardness testing. In the absence of a change in composition or dislocation density, the martensite start transformation temperature (M_s) was found to be determined by the grain size of the austenite. Above a grain size of 150 μm, M_s was found to be independent of grain size, but below 150 μm, the transformation temperature was strongly depressed by up to approximately 50 K at a grain size of 10 μm. For any given grain size, an increase in the dislocation density from that typical of a fully recrystallised specimen, i.e. approximately 10¹⁰ lines m⁻², to that of approximately 10¹⁵ lines m⁻² raised M_s by approximately 15 K. The depression of M_s and reduction in the initial burst size of the transformation with decreasing grain size was found to be related to the observation that a fine grain size results in a heterogeneous transformation restricted to a few small pockets of grains. The depression of M_s in the fine grained alloy is consistent with a segregation of active martensite nuclei into a few small grains, a suppression of the autocatalytic stimulation of martensite plates between adjacent grains, and a possible reduction in the number of martensite nuclei.