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.     

Solidification paths of TiTaAl alloys

Microstructure evolution during solidification and high temperature phase equilibria were investigated for TiTaAl alloys in the vicinity of the 50 at.%Al isoconcentrate. Examination of dendrite morphologies and segregation profiles were used to deduce the phase sequencing during solidification and the boundaries of the relevant liquidi surfaces. In situ high-temperature X-ray diffraction and isothermal annealing experiments were conducted to determine the phases present at elevated temperatures and coupled with extensive characterization by transmission electron microscopy to elucidate the solid state transformations during cooling of the solidification microstructure. For approximately equiatomic TiAl, the primary phase selected from the liquid was α for the leaner Ta concentrations (< 7% Ta), as in the binary alloys of equivalent Al content, but changed to β with increasing Ta levels (> 10 %). With increasing Al and Ta the α liquidus penetrates deeply into the ternary. The interdendritic segregate was always γ. Dendrites of the β-forming alloys were heavily segregated leading to different microconstituents in the core and bulk dendrite regions during post-solidification cooling. In alloys with < 15 % Ta, (α₂ + γ_t) lath formed in the dendrite bulk due to the decomposition of α, with σ precipitating in the core (> 10% Ta). With increasing Ta levels the lath is gradually replaced by polycrystalline γ which grows into the dendrite bulk, and the core decomposes into a lamellar (γ + σ) microstructure from the decomposition of σ. The γ segregate does not transform further.     

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.     

Kinetics of leaching of a low-grade FeNiCuCo matte in ferric sulphate solution

A study of the dissolution of a low-grade FeNiCuCo matte in acid ferric sulphate solution is reported. This matte contained a number of different phases, and a detailed quantitative mineralogical analysis of the matte was used as a basis for mathematical modelling of the leaching process. The mathematical model assumed that the matte particles were leached by a shrinking-particle mechanism, and that the surface reaction was rate-limiting and electrochemical in nature. The experimental results verified these assumptions, and showed that the redox potential of the solution best approximated the potential of a matte particle during leaching.     

Thermodynamics of CoBH ternary solid solutions

Solubility isobars for H in CoB binary solvents containing up to 0.1 at.% B have been determined in the temperature range 920–1430 K. The solubility data yield values for the partial molar thermodynamic functions of the dissolved H-atoms which manifest a strong long-range HB repulsive interactions.     

The thermodynamics of NiCuH solid solutions

Solubility isobars at a pressure of 1.01 × 10⁵ N/m² have been measured in the temperature range of 714–1430 K for H in NiCu solid solution matrix alloys in the entire range of compositions from NiH to CuH. The solubility data provide values for the partial molar enthalpy and excess entropy of the dissolved H-atoms. The thermodynamic parameters have been shown to be consistent with a statistical model in which the energy levels of the interstitial atoms are a function of the local atomic configuration of the sites in which they are located.     

Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO

NiAl and NiAlSi alloys were internally oxidized at temperatures of 1073–1273 K by the Rhines Pack method. For the NiAl alloy, the oxidation process follows parabolic law and the oxidation front was flat with severe integranular oxidation occurring at 1073 K and extensive grain boundary sliding at 1273 K. As for NiAlSi alloys, the oxidation rate increased with increase of Si content at 1073 K but the rate decreased at higher temperatures due to total or partial continuous oxide layer formation at the internal oxidation front. The depth of intergranular oxidation was also greatly reduced. For all samples, nickel was found to be transported out to the surface with the amount proportional to the Si content. Lattice diffusion (Nabarro-Herring creep) was believed to be the main cause for nickel transport in the NiAl alloy while dislocation pipe diffusion is the mechanism for NiAlSi alloys.     

Ageing kinetics of a silicon carbide reinforced AlZnMgCu alloy

The ageing kinetics of a composite of an AlZnMgCu powder alloy (“CW67”) combined with a varied volume fraction of particulate silicon carbide were investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). DSC revealed that the maximum rate of precipitation of the metastable η′ phase was substantially lower for CW67/SiC/20p than for the unreinforced alloy or CW67/SiC/10p. TEM of isothermally aged material revealed differences between the unreinforced alloy and composites in respect of precipitate size and morphology. We conclude that SiC additions, by dint of additional dislocations generated during quenching, can affect the ageing of CW67 either by accelerating the nucleation of precipitates or by accelerating precipitate growth. The ageing rate in CW67/SiC/20p was increased by accelerating both the nucleation of precipitates and growth, whereas the ageing in CW67/SiC/10p was enhanced by accelerating precipitate growth only.     

Study of the new Frank-Kasper phases in AlLiCuMg alloys

A new group of Frank-Kasper phases has been observed in AlLiCuMg alloys using high resolution electron microscopy (HREM). The Y phase, which has a face-centre cubic structure with a = 2.0 nm, occurs in grain boundaries during precipitation heat treatment of supersaturated solid solution. Considerable amount of intrinsic faults, extrinsic faults and microtwins were observed in the Y phase. A new domain denominated as D intergrowth with Y phase along {111} planes has been found. By combination of Y phase and D domain, new domains can be constructed.     

The coarsening of lamellae in Ti48Al2Mn2Nb

The thermal stability of as-cast Ti48Al2Mn2Nb alloys has been investigated as a function of time at temperatures of 1200, 1350 and 1420°C. The resultant structures have been characterised using optical microscopy, electron probe microanalysis and transmission electron microscopy. The as-cast structure consists mainly of lamellae of α₂ and γ. Annealing at 1350 and at 1420°C resulted in the continuous and discontinuous coarsening of the primary lamellae. Discontinuous coarsening, which often originates at the sample surface, results in branched irregular secondary lamellae. The average interlamellar spacing of the secondary lamellae is up to a thousand times larger than the primary interlamellar spacing. The extremum principles of maximum velocity and maximum rate of entropy production, used to predict the secondary interlamellar spacing, are inconsistent with the experimental observations. The theoretical predictions of the velocity of the discontinuous coarsening by Livingston and Cahn are used to estimate the value of grain boundary diffusivity at 1350°C. The temperature dependence of discontinuous coarsening is weak, in contrast to the strong effect of lamellar orientation on the rate of discontinuous coarsening.     

The thermodynamics of PdAgH ternary solid solutions

The solubility of hydrogen in PdAgH ternary solutions in equilibrium with H₂ gas at atmospheric pressure has been measured in the temperature range 625–1111 K and in PdAg “binary solvents” containing up to 93 at.% of Ag. Concomitant elastic measurements have provided data which enable the partial thermodynamic functions of the H-atoms deduced from the solubility measurements to be converted so as to refer to a hypothetical PdAg lattice of constant specific volume. The resulting “volume corrected” functions have been discussed in terms of the cell model for ternary solutions and have been shown to vary with temperature and Ag-concentration in a manner in accord with this model.