A strategy for quality and uncertainty assessment of long-term eddy-covariance measurements

Aluminium–silicon irregular eutectic alloy was melted in a graphite crucible in vacuum. This alloy was directionally solidified at a constant growth rate, V (8.3m/s) and at different temperature gradients, G (2.0–7.8 K/mm) and also with a constant G (7.8 K/mm) and different V (8.3–498.7m/s) in the Bridgman type directional solidification furnace. The interflake spacings, λi, were measured from both transverse and longitudinal sections of the specimens. The relationships between interflake spacings, λi and solidification parameters V and G were obtained by linear regression analysis method. The variations of interflake spacings, λi with V, G and undercoolings, � Ti, were investigated and the relationships between them were examined. Operating parameters φ and η which reflect the spacing adjustment mechanism were obtained.

Interflake spacings and undercoolings in Al–Si irregular eutectic alloy

The combined and separate effects of microstructural scale and silicon phase morphology on the mechanical properties of Al–Si eutectic alloys are investigated here. The Bridgman-type gradient-zone directional solidification method is employed to produce as-cast structures characteristic of the full range of practical (i.e. casting) growth velocities, and the corresponding mechanical properties are characterized by uniaxial tension testing. The results are analyzed in light of previously reported microstructural changes associated with the flake to fiber or “quench modification” transition [1] . Both tensile strength and elongation were found to increase with solidification rate. Application of the Nan–Clarke [2] micromechanical analysis to the Al–Si composite structure, incorporating the strengthening effects of reinforcement-induced dislocations, suggests that the decreasing microstructural scale alone is sufficient to account for the increase in tensile strength with solidification rate. However, the flake to fiber transition was found to have a particular relevance with regard to the fracture behavior of the alloy, increasing tensile elongation and decreasing the overall variability of tensile properties. A maximum in elongation was observed at approximately 600 μm/s, corresponding to the upper threshold of the flake to fiber transition associated with complete disappearance of the flake morphology and dominance of the fibrous structure. These results emphasize the importance of understanding and controlling the flake to fiber transition that occurs with increasing solidification rate in Al–Si eutectic alloys.

The effect of the flake to fiber transition in silicon morphology on the tensile properties of Al–Si eutectic alloys

Variation of microindentation hardness with solidification and microstructure parameters in the Al based alloys

Hardness and tensile property measurements made on directionally solidified Al-Si eutectic alloys show that definite but different hardness-growth velocity and hardness-silicon interparticle spacing relationships exist for alloys with different silicon eutectic phase morphologies. Tensile property measurements show that hardness and 0.2% proof stress follow silicon interparticle spacing relationships of the same form. It is suggested that hardness can only be used to measure proof stress when the eutectic structure displays a single silicon eutectic phase morphology.

Hardness and mechanical property relationships in directionally solidified aluminium-silicon eutectic alloys with different silicon morphologies

The effect of antimony additions on the hardness and tensile properties of Al–Si eutectic alloy has been examined during directional solidification. Antimony, provided that its concentration in the melt was maintained at ~0·1 wt-%, improved the mechanical properties of the eutectics by almost 20–25%, all effects being additive to those that can be gained through increasing the growth velocity of the samples. Mechanical properties were found to correlate directly with the spacing between the silicon particles of the eutectic phase of the solidification structure, a Hall–Petch type relationship existing between the yield strength and silicon particle spacing.MST/653

A. I. Telli

Papers

Effect of antimony additions on hardness and tensile properties of directionally solidified Al–Si eutectic alloy