The microstructure of rapidly solidified AlFe alloys subjected to laser surface treatment

Using traverse rates of between 0.1 and 6.0 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. Three types of microstructures were observed. At low growth rates, cellular/dendritic structures were obtained. At high growth rates, a banded structure was formed which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. At the lower concentrations, precipitate-free structures were obtained at very high solidification rates. A recent model, for dendritic growth under rapid solidification conditions, was compared with the experimental results and a good correlation was found. It was shown that, at rates close to the limit of absolute stability, steady-state planar-front growth was not the preferred growth morphology; but rather a banded structure. It was only at much higher rates that a fully precipitation-free structure, probably involving plane-front growth, developed.     

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.     

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.     

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.     

Unique metallic glass formability and ultra-high tensile strength in AlNiFeGd alloys

The metallic glass formability of aluminum-rich AlNiFeGd alloys has been systematically investigated. The critical cooling rate required to form an amorphous state in this system is generally low, and comparable to that of some of the best metallic glass formers, such as PdCuSi. Amorphous ribbons up to 0.25 mm thick can easily be produced by the single-roller melt-spinning technique. Tensile strengths as high as 1280 MPa and Young's modulus of 75 GPa have been obtained. Bulk amorphous alloys with good mechanical properties are optimized in Al₈₅Ni₆Fe₃Gd₆. DSC and DTA studies reveal that the glass formability is unique for Al-based alloys because the reduced glass temperature T_rg for AlNiFeGd can be as low as 0.44. This is much lower than conventional theory would suggest for easy glass forming systems. A mechanism for the unusual glass formability is suggested.     

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.     

C-C interaction energy in FeC alloys

A new approach has been suggested for the determination of the C-C interaction energy, the partial molar enthalpy and nonconfigurational entropy of carbon in FeC austenite and ferrite from available activity data. By application of the values obtained through the approach to the calculation of the FeC phase diagram, the results in the equilibrium region are in very good agreement with experiment. From the scattered and limited experimental activity data, the C-C interaction energy obtained through the present approach should be more reasonable than that through previous efforts. Further analysis indicates, however, that up to now the activity data on carbon in ferrite are not accurate enough for obtaining the C-C interaction energy in ferrite with clear physical significance.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

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.

     

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.