L. Katgerman

Mechanical properties in the semi-solid state and hot tearing of aluminium alloys

This review represents a comprehensive coverage of results reported in the literature over last 50 years on the methods of studying hot tearing and mechanical properties of semi-solid aluminium alloys; the mechanical properties of these alloys in the semi-solid state; and hot tearing criteria. While compiling this review, the authors attempted to include in it all available sources including quite a few works never published in English before. The review consists of three parts. The first part introduces the reader to the phenomenon of hot tearing. The second part describes different techniques for testing metallic alloys in the semi-solid state and summarizes reported results on strength and ductility of semi-solid model and commercial aluminium alloys. The third part describes the methods for assessing hot tearing susceptibility of aluminium alloys, gives the results on hot cracking of various aluminium alloys and discusses different hot tearing criteria.

The effect of heat treatment on the structure and abrasive wear resistance of autocatalytic NiP and NiP–SiC coatings

Abstract A systematic study on the relationships between the structure and abrasive wear resistance of autocatalytic nickel–phosphorus coatings (particle-free and SiC composite) with different phosphorus contents (i.e. 2.5–10.2 wt.% P) and under different thermal treatments (i.e. 300, 400 and 500°C) has been performed. The phase structure, composition and properties of the coatings could be controlled by changing the phosphorus content of the nickel–phosphorus matrix and by performing thermal treatments. The improvement in abrasive wear behaviour of the nanocrystalline (i.e. ≤6.0 wt.% P) coatings with heat treatment temperature up to 400°C was related to (i) the formation of a metastable equilibrium phase and (ii) precipitation of Ni 3 P compound. At higher thermal treatments (500°C), a change in the deformation mechanisms (Orowan mechanism) determined by the coarsening of Ni 3 P precipitates was associated with the decrease in abrasive wear resistance of the coatings. In addition, for the NiP–SiC coatings after annealing at 500°C, Ni 3 Si was formed and the adhesion between the reinforcement and the matrix was enhanced.

Characterization of Al-Si-alloys rapidly quenched from the melt

Aluminium-silicon alloys with compositions in the range 0 at% to 33.9 at % Si were rapidly quenched from the melt at cooling rates between 106 and 107 K sec−1 using the melt-spinning technique. The resulting ribbons were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction methods. Metastable solid solubilities of silicon in aluminium were determined from lattice parameter and DSC data. The values found were strongly dependent on specimen thickness and a maximum of about 5 at % Si was reached for an alloy composition of 15 at % Sl (maximal equilibrium solid solubility of silicon in aluminium is 1.58 at % Si). Discrepancies between published values of metastable silicon solid solubities were related to the interpretation of the lattice parameter data. Alloy composition was shown to determine the lattice parameter of the silicon-rich phase. The crystallite sizes and the lattice distortions in the aluminium-rich and silicon-rich phases were determined by X-ray diffraction line profile analysis. From the aluminiumrich phase only strain broadening was observed whereas the silicon-rich phase gave rise to both size and strain broadening. The origin of the lattice strains was discussed. Changes in solidification behaviour are reflected in the structure parameters measured.

Tensile behaviour of semi-solid industrial aluminium alloys AA3104 and AA5182

Tensile tests with as-cast industrial aluminium alloys AA3104 and AA5182 were carried out in the semi-solid state to determine the constitutive behaviour at the final stage of solidification during direct chill casting. The behaviour is dominated by the solid network but the geometry of the liquid determines how much of the solid network contributes to the strength. Using a geometrical model, the fraction of liquid which covers the grain boundaries is calculated. This value rises steeply with increasing liquid fraction, which should be taken into account in any modelling of tensile behaviour of semi-solid material. A simple modification of a standard creep law, which takes into account the geometry of the liquid film, provides a continuous description of the constitutive behaviour of these alloys from the creep regime into the semi-solid state.

Solid-state reactions in low-phosphorus autocatalytic NiP–SiC coatings

Abstract Composite NiP–SiC coatings with a nanocrystalline nickel matrix produced by autocatalytic deposition were subjected to phase transformations by isochronal and isothermal heating. Isochronal heating to 700°C (heating rate 10°C min−1) revealed three exothermic effects (reactions) occurring in the coatings. X-Ray diffraction showed that only the second and third thermal effects were associated with phase transformations. The first peak was associated with chemical and structural relaxation and a slight grain growth of the matrix. The second reaction was attributed to the nucleation and growth of Ni3P precipitates, while the third was related to the complete dissolution of SiC particles in the matrix with the formation of Ni3Si and carbon. A similar trend in the phase formation sequence was observed by isothermal heating. However, the formation of nickel silicides at the SiC/matrix interface occurred at lower temperatures (i.e. 500°C for 1 h). The formation of silicides appeared to be governed by the diffusion of nickel atoms into the SiC lattice, as indicated by transmission electron microscopy.

On the formation of the stircast structure

The conditions of solidification in a stirred bulk liquid are investigated to explain the non-dendritic microstructure of stircast alloys. A model of stirred solidification is presented, which allows a comparison of the solidification behaviour of metal alloys and organic analogues. This shows that nucleation and growth of solid in e bulk liquid is facilitated under the influence of stirring, provided the Prandtl-number is greater than unity. It is shown further that the solute gradient ahead of the solid-liquid interface of floating particles in a bulk liquid is reduced by the fluid flow. Combined with the thermal properties, and in analogy with the constitutional supercooling criterion, A is shown that solid growth in metals is likely to be cellular in an early stage of the solidification, In contrast, in stirred organic analogues, the solidification is dendritic in the early stage.

Voltage transients and morphology of AlSi(Cu) anodic oxide layers formed in H2SO4 at low temperature

Abstract Anodic oxidation of three different aluminum substrates (i.e. Al, AlSi10 and AlSi10Cu3) in 2.25 M H2SO4, at 0 °C, for 50 min and using three different current densities (i.e. 3.0, 4.2 and 6.0 A dm−2) was performed in this study. The aim was to examine the growth of thick oxide layers over the macroscopic surface bearing large (1–20 μm) second phases with different oxidation rates relative to aluminum. The results revealed different voltage transients for the three different compositions during the stages of barrier layer growth and steady-state pore growth. In addition, the morphology of the oxide layers, as determined by optical microscopy, scanning electron microscopy (cross-section) and laser scanning confocal microscopy (surface) indicated: (i) thickening of the oxide layers with current density and time regardless of the substrate composition; (ii) entrapment of silicon in the growing layers (for the binary and ternary substrates), associated with a non-uniform thickness, a scalloped substrate/oxide interface and surface cracks; and (iii) appearance of flaws like features from the commencement of anodizing (AlSi10Cu3) and after a certain anodizing time (AlSi10) associated with enhanced oxygen evolution and (partial) oxidation of second phases.

Vickers microhardness of AlSi(Cu) anodic oxide layers formed in H2SO4 at low temperature

Abstract The effects of substrate composition and anodizing conditions (i.e. current density and anodizing time) on the Vickers microhardness of anodic oxide layers formed in 2.25 M H 2 SO 4 at 0 °C on Al, AlSi10 and AlSi10Cu3 permanent mold cast substrates were assessed. The microhardness of the substrates and anodic oxide layers was determined on the polished cross-sections using a 25-g load. The results indicated that the hardest substrate, i.e. AlSi10Cu3 resulted in anodic oxide layers with lowest average microhardness (e.g. 222±78 HV 0.025 at 4.2 A dm −2 and 50 min) followed by AlSi10 (489±63 HV 0.025 ) and Al (541±28 HV 0.025 ). Increasing the current density from 3.0 to 6.0 A dm −2 showed no improvement in average microhardness measured at half layer thickness. Anodizing times exceeding 25 min resulted in a decreased average microhardness for the binary and ternary compositions. Variations of microhardness across the layer thickness were encountered for all substrates anodized for 50 min. Their extent, however, depended on substrate composition and layer thickness. The main differences in layers’ microhardness could be associated with the defects generated by silicon and copper containing constituent particles during anodizing. Their presence hinders reliable microhardness measurements.

The structure of stircast Al-6Cu

A series of batch-type stirring experiments have been performed to investigate the morphological changes in the growth of the primary solid phase in Al-6Cu(Al-6 wt% Cu), as a function of the cooling rate and the rotational speed of the stirrer. The experiments show, that the cell-spacings of primary particles in stircast microstructures increase as a result of stirring, when compared to secondary dendrite arm spacings in the unstirred alloy. This result can be explained regarding heat transport during solidification. It is suggested that the solid-liquid interface of floating crystals in bulk metal liquids is cellular.

Experimental study of ordering kinetics in aluminum alloys during solidification

Abstract The microscopic structure and crystallization behavior of liquid Al and Al–0.3Ti–0.02B (wt.%) are studied by time-resolved neutron diffraction measurements during the liquid–solid phase transformation for continuous cooling. A specially developed furnace insert was used to obtain a temperature stability of 40 mK in the vicinity of the solidification temperature of T o =933 K. The evolution of the static structure factor S ( Q ) has been monitored during the liquid to solid phase transformation as a function of the cooling rate. The evolution of the liquid fraction f L during the transformation is determined from the value of the first peak in the liquid structure factor. The evolution of the solid volume fraction f S =1− f L is analyzed in terms of the Johnson–Mehl–Avrami model. The Avrami exponent n is found to change for pure Al as well as for Al–0.3Ti–0.02B alloy with cooling rate and the rate constant k decreases by an order of magnitude for Al–0.3Ti–0.02B alloy compared to pure Al. Anomalous temporal oscillations were observed in the Bragg peak intensity of the solid grains during the solidification of the Al–0.3Ti–0.02B alloy.