Nils Ryum

Precipitation of dispersoids containing Mn and/or Cr in Al-Mg-Si-alloys

Abstract The formation of dispersoids containing Mn and/or Cr in Al–Mg–Si alloys was studied metallographically and by means of electrical resistivity measurements. Special emphasis was placed on the nucleation mechanisms. For alloys containing Mn and/or Cr it was shown that during the heating of the as-cast alloys to 580°C (which is a typical homogenization temperature) and at a heating rate of 3 K min −1 , an intermediate phase, referred to as the ‘u-phase’ nucleated on the β′-Mg 2 Si needles. The phase has a hexagonal unit cell with a =0.670 nm, c =0.808 nm. The precipitates were rod-shaped and were lined up in the [100] directions in the Al lattice. The phase was found to be rich in Mn and/or Cr. With continued annealing, dispersoids containing Mn and Mn+Cr nucleated heterogeneously on the ‘u-phase’ precipitates before these precipitates dissolved. The nucleation sites for the dispersoids with Cr were not conclusively identified.

The development of microstructures in AlLi alloys

Abstract The nucleation, growth and coarsening of precipitates of an AlLi alloy were investigated. The equilibrium phase AlLi nucleates on grain boundaries, while the coherent and metastable Al3Li nucleates homogeneously within the matrix grains. Isothermal aging resulted in the growth of Al3Li precipitates. The growth process was dependent on the cube root of the time. Together with the growth of the matrix precipitates, a preferential coarsening of the AlLi precipitates on the grain boundaries occurred. To accommodate the accelerated growth in the grain boundary regions a precipitate-free zone (PFZ) developed. The change in the PFZ width was a function of the square root of the time. The AlLi solvus was redetermined metallographically at three temperatures, the values corresponding closely to the AlLi solvus values given in the literature. From the coarsening experiments the product of the coefficient of diffusion and the interfacial free energy of the precipitates was found. The coefficient of diffusion was determined independently by measuring the PFZ growth rate.

Cyclic deformation of [001] aluminium single crystals

Abstract Cyclic deformation in symmetric push-pull of pure aluminium single crystals with [001] orientation has been carried out at constant plastic strain amplitudes within the range 10−4–10−3. The cyclic stress response curves are characterised by rapid hardening, softening and secondary hardening without saturation of the stress at any strain level. Fatigue failure by traditional crack growth was never observed, but the specimens failed by a complicated plastic collapse mechanism. Traditional slip lines were seldom observed. Instead, very characteristic deformation patterns with a cord-like appearance at low strain amplitudes, and a tweed-like appearance at higher strain amplitudes developed. In the collapse state, slip lines clustered in macrobands were also formed. The dislocation structure was studied by TEM and found to consist of {100} dislocation walls and elongated cells. In addition, spherical subcells were observed on the dislocation walls.

Microstructure and crystallographic orientation relationship in directionally solidified Mg–Mg17Al12-eutectic

Abstract The microstructure and crystallographic orientation relationship of the Mg- and the Mg 17 Al 12 -phase in a directionally solidified Mg–Al-eutectic have been study in the light microscope and in the scanning electron microscope equipped with an electron back scattering facility. The effects of the addition of small amounts of Sr on the microstructure were also studied. The Mg–Al-eutectic solidified with a lamellar morphology where the relationship λ 2 V =constant was obeyed for growth rates V −3 cm/s and lamellar spacings λ ≥2.5 μm. The crystallographic orientation relationship is: (1101) Mg‖(101) Mg 17 Al 12 , [1120] Mg‖[111] Mg 17 Al 12 . The interface plane was found to lie ∼10° from the (101)plane. The two phases of the eutectic grew with the angular discrepancy ( θ ND ) to the [1120]- and the [110]-directions. The angle θ ND was found to decrease with increasing growth rate. The microstructure and crystallographic orientation relationship seemed unaffected by Sr addition.

Cyclic deformation and fracture of pure aluminium polycrystals

Abstract Cyclic deformation and fatigue failure at room temperature of pure aluminium polycrystals, with grain size in the range 60–500 μm, have been investigated and compared with results obtained from aluminium single crystals, as well as with results from copper single and polycrystals. The cycling was carried at constant plastic strain with amplitudes in the range 3.2 × 10 −5 -4.3 × 10 −4 . A hardening-softening-secondary hardening was observed at all levels of strains. The maxima and minima in the cyclic stress response (csr)-curve were displaced to higher levels with increasing plastic strain amplitudes. The csr-curves were also shifted to higher levels with reduced grain sizes. The slip patterns consisted of slip bands clustered in macrobands, but also the deformation patterns characteristic of cycled [001] aluminium single crystal were frequently observed. Fracture occurred at all plastic strain levels and at all grain sizes. A Coffin-Manson relationship was established. The first fatigue cracks were formed along grain boundaries at a very early stage of the cyclic deformation. Crack propagation occurred mostly along the grain boundaries.

Precipitation on stationary and non-stationary grain boundaries in an Al-3.5 wt.% Cu alloy

Abstract The heterogeneous nucleation of second-phase precipitates at the grain boundaries in an Al-3.50 wt.% Cu alloy at low undercoolings has been investigated. By differing the heat treatment procedure, nucleation at stationary and non-stationary boundaries could be compared. It was found that non-stationary boundaries provide the more effective nucleation sites. It has recently been proposed [1], as an explanation for the observation that non-equilibrium boundaries have a greater ability to incorporate run-in dislocations than equilibrium boundaries, that the structures of these are different. As nucleation is known to be sensitive to grain boundary structure, the present result supports this view.

On the validation of the Monte Carlo technique in simulation of grain growth in small, two-dimensional systems

The kinetics of grain growth in real systems is influenced by several unknown factors, making a theoretical treatment very difficult. Idealized grain growth, assuming all grain boundaries to have the same energy and mobility (mobility M = k/ρ, where k is a constant and ρ is grain boundary curvature) can be treated theoretically, but the results obtained can only be compared to numerical grain growth simulations, as ideal grain growth scarcely exists in nature. The validity of the simulation techniques thus becomes of great importance. In the present investigation computer simulations of grain growth in two dimensions using Monte Carlo simulations and the grain boundary tracking technique have been investigated and compared in small grain systems, making it possible to follow the evolution of each grain in the system.

Cyclic Hardening and Softening of [001] Al Single Crystals

The cyclic deformation of aluminium single crystals of different crystallographic orientations has recently been studied in detail by Vorren and Ryum (1,2). During cycling at constant plastic strain amplitude a hardening-softening-hardening sequence was generally observed in the plastic strain range 10-4 < ep < 10-3. The slip pattern were characterized as persistent ma cwbands traversing through the single crystals, and in which the plastic deformation was localized. It was shown that the volume fraction of ma crobands increased with increasing cumulative strain at a particular strain level. The crystallography of the ma crobands was also determined.