P. Van Mourik

On precipitation in rapidly solidified aluminium-silicon alloys

The precipitation of silicon in rapidly solidified AlSi alloys was studied. For alloys with 2.4 and 11.0 wt % Si (2.3 and 10.3 at % Si, respectively) the lattice parameters of the Alrich and of the Si-rich phases were measured after ageing at 397,425 and 448 K. For alloys with 2.6 and 13.0 wt % Si crystallite sizes and lattice strains were determined by analysis of the X-ray diffraction line broadening. After ageing the lattice parameters of the Al-rich and the Si-rich phases were influenced by the difference in thermal expansion between both phases. After correction for this effect the amount of silicon dissolved in the Al-rich phase was estimated as a function of ageing time. Quenched-in (excess) vacancies influenced the precipitation kinetics. Activation energies for precipitation appeared to depend on the extent of transformation. Further, quenched-in vacancies caused anomalous maxima in the lattice parameter curves. The behaviour of the lattice microstrains on ageing was explained as a result of the disappearance of stresses due to quenching and the introduction and subsequent dissipation of stresses due to precipitation. After completed precipitation stresses due to the difference in thermal expansion between both phases still exist at room temperature.

Cooling and heating rate dependence of precipitation in an Al-Cu alloy

Differential scanning calorimetry and X-ray diffraction were used to study the cooling and heating rate dependence of precipitation in an Al-1.66 at.% Cu alloy. After homogenizing, cooling at a rate of 22 K min?1 (SC22) is sufficient to retain all copper in solid solution. GP-zone formation during subsequent heat treatment is hindered; this is ascribed to an insufficient number of (excess) vacancies. After a water quench (WQ) a large number of GP zones are formed during subsequent storage at room temperature for 1 h. The heat content of the GP-zone dissolution effect can quantitatively be described in terms of the heat of precipitation of GP I zones and the solid solubilities as derived from the GP I zone solvus. The heat content of the combined ??-/?-phase precipitation effect appeared to be proportional to the number of copper atoms precipitated, yielding an average value for the heat of copper precipitation of 36 kJ mol?1 copper. The activation energy for ??-phase formation is 0.75 eV for SC22 specimens and 1.10 ± 0.10 eV for WQ specimens. The differences in reported activation energies for ??-phase formation are discussed in terms of mobility of dissolve atoms (related to the vacancy concentration), interfacial energy and direction of growth (normal or perpendicular to the ??-phase plate).

Unusual lattice parameters in two-phase systems after annealing

Abstract The lattice parameter observed at room temperature for the aluminium-rich phase in fully precipitated AlSi alloys has been found to be significantly greater than the expected equilibrium value. This effect increased with silicon content and with annealing temperature. Two possible causes were considered: (i) the difference between the atomic volumes for silicon in the precipitates and in the aluminium matrix; and (ii) the difference between the thermal expansion coefficients of the matrix and the precipitates. A quantitative description is based on the theory of Eshelby developed originally for the case of elastic distortions in a crystal by point imperfections. Distortions due to cause (i) vanished during the anneal. For cause (ii) the theoretical prediction obtained agreed fairly well with the experimental data. The validity of the Eshelby model in this case was discussed. For precipitation studies where the change in lattice parameter is followed, a procedure is proposed to obtain correct soli...

Structural inhomogeneities of AlSi alloys rapidly quenched from the melt

Hypo- and hyper-eutectic AlSi alloys were rapidly quenched from the melt using the melt-spinning technique with two spinning velocities. Structural differences between the wheel (chill) and upper sides of the melt-spun ribbons were investigated by optical and scanning electron microscopy and X-ray diffraction methods (texture- and size-strain analyses). The Al-rich phase of the hypo-eutectic alloys was textured. The textures observed from both sides of the ribbons were different; in neither case was it of fibre type. For the larger spinning velocity applied, the structural imperfection of the wheel side was larger than that of the upper side for both the Al-rich and the Si-rich phases.

A Calorimetric Study of Precipitation in an Al-Cu Alloy with Silicon Particles

The precipitation of copper and silicon from the Al-rich matrix in an Al-1.3 at. pct Cu-19.1 at. pct Si alloy was investigated by differential scanning calorimetry (DSC). Both as-extruded (AE) and extruded and solution treated and quenched (solid-quenched, SQ) specimens were studied. The DSC curves of the SQ specimens showed two exothermic effects, A and B. Effect A corresponded to the simultaneous precipitation of silicon and copper, whereas effect B was caused by the transition from the state with the intermediate copper-containing phase, θ’, to the state with the equilibrium copper-containing phase,θ. The heat contents of effect A and B could quantitatively be described in terms of solid solubilities before and after precipitation and the heats of precipitation of the phases involved. From this description, it was derived that for heating rates ≤20 K/min, copper precipitated as theθ′ phase, while for heating rates ≥40 K/min, copper precipitated mainly as theθ phase. In SQ specimens, Guinier-Preston (GP)-zone formation occurred during aging at room temperature with a rate approximately 104 times slower than in the corresponding binary Al-Cu alloy. For the AE specimens, it was found that during extrusion, precipitation of copper and silicon proceeded to a large extent. However, from DSC experiments and from hardness measurements as a function of aging time at 453 K, it was deduced that copper precipitation had not finished during extrusion. The hardness increase as observed during aging directly after extrusion was interpreted to be due to formation of the semicoherent intermediateθ’ phase.

Lattice Parameter and Hardness Variatons resulting from Precipitation and Misfit accommodation in a Particle reinforced Al-Si-Cu-Mg Alloy

Precipitation processes occurring in ageing of AI-20at.%Si-l.5at.%Cu-l.lat.%Mg (ASCM) alloys with and without AI203 particle reinforcement were studied using X-ray diffraction and hardness measurements. During ageing at 453 K after solid quenching, the Q phase (AlsCu2MgsSi6) precipitates first, followed by the 0 phase (AI2Cu). An optimized heat treatment for the ASCM alloys consists of a short solution treatment at 779 K, followed by water quenching and ageing for 4 h at 453 K. After the peak in hardness has occurred, continued ageing decreases the hardness. This overageing results from coarsening of the Q phase. After heat treatment, the lattice parameters were affected by the misfit effects caused by the difference in the coefficients of thermal expansion between the Al-rich phase, Si-rich phase and Al203 particles if present. These misfits still existed at room temperature after precipitation was completed. A model assuming ideal plastic behaviour of the matrix describes fairly well the cooling-induced changes in the Si-rich phase lattice parameter. For the cooling-induced changes in the Al-rich phase lattice parameter, there remains a difference between model predictions and measurements.

Precipitation in liquid-quenched Al-Mg alloys; a study by use of X-ray diffraction line shift and line broadening

The precipitation process in liquid-quenched alloys of aluminium with 3.2, 10.5 and 16.7 at% Mg on ageing at 404, 450 and 492 K was investigated by X-ray diffraction analysis. It was found that only β′-phase precipitation occurred. During ageing, the average aluminium-matrix lattice parameter was determined from the line position, while a measure for the lattice spacing variations was deduced from line broadening. The β′-phase precipitation was found to occur inhomogeneously; for one alloy, splitting of diffraction lines was observed. The (asymmetrical) line-broadening changes observed during the β′-phase precipitation could consistently be conceived as dominated by the inhomogeneous nature of the β′-phase precipitation process. The (symmetrical) line broadening retained after completed precipitation was ascribed to the volume misfit between the precipitated β′-phase particles and the aluminium-matrix. Analysis of the aluminium-matrix lattice parameter changes with ageing time revealed that the β′-phase precipitation rate is governed by the magnesium volume diffusion.

Kinetics of precipitation and of relaxation of precipitation-induced stresses in aluminium-silicon alloys

Liquid quenching as well as solid quenching of aluminium-silicon alloys yields an aluminiumrich matrix supersaturated with silicon. Ageing of quenched alloys implies silicon precipitation (cf. phase diagram in Ref. i). Since the atomic volume of silicon as precipitated (diamond structure) is much larger than as dissolved in the Al-rich matrix, large stresses will develop in the Al-rich phase on ageing. This is clearly demonstrated by the behaviour of the Al-rich phase lattice parameter, aAl, as a function of ageing time, t, cf. Fig. 1 (2, 3). Silicon precipitation from the Al-rich phase causes an overall increase of the Al-rich phase lattice parameter, but precipitation-induced stresses and their subsequent relaxation cause a hump, AaXl(tmax), (3). This hump occurs at much shorter times for liquid-quenched (LQ) than for solid-quenched (SQ) alloys (compare Fig. la to Fig. ib). For the LQ alloys the determination and interpretation of ~Xl(tmax) is hardly possible because of the interwoven effects On the Al-rich phase lattice parameter of silicon precipitation, precipitation-induced stresses and relaxation stresses. In this letter the silicon precipitation and subsequent relaxation of precipitation-induced stresses in solid-quenched AISi alloys are investigated by measurement of the Al-rich phase lattice parameter as a function of temperature and time of ageing.

On misfit-induced lattice spacing variations in two-phase alloys : The case of cooling-induced microstrains in the Al-matrix of fully aged AlSi alloys

Analyse par diffraction de RX des phenomenes aux interfaces particules de Si precipites/matrice dans les alliages Al-0 a 18,2% at. Si vieilli a 447 pendant 1841 heures

Interaction between precipitating phases in quenched Al-Cu-Si alloys

The precipitation in two solution-treated Al-Cu-Si alloys, one with and one without free silicon particles, was studied by differential scanning calorimetry, X-ray diffraction and hardness measurements. In both alloys, θ′-phase and Si-phase precipitation processes proceed simultaneously. As in binary Al-Si and Al-Cu alloys aged at the same temperature, precipitation occurs at clearly separate time intervals; this finding suggests an interaction between the two precipitation processes. It is shown that the interaction of stress fields around newly formed precipitates can result in a significant decrease of the nucleation barrier and thus can explain the synchronous precipitation. The activation energy of the combined precipitation reaction is 0.95±0.06 eV.