Effects of Cr Addition on the Microstructure and Mechanical Properties of an Al-Si-Cu-Mg Alloy.

The effects of chromium (Cr) addition ranging 0.1-0.3 wt.% on the microstructure and mechanical properties of Al-7Si-4Cu-0.25Mg (wt.%) alloy have been investigated. The cast Cr-free alloy consisted of α-Al, eutectic Si, Q-Al5Mg8Cu2Si6 and θ-Al2Cu phases. Doping of Cr resulted in the appearance of a polyhedron-shaped α-Al13Cr4Si4 phase with a cubic structure. The Al13Cr4Si4 particles were found to embed with Al2Cu blocks and bring about size reduction for the Al2Cu blocks. The area fraction of Al13Cr4Si4 monotonously increased with Cr content. After T6 treatment, the Al2Cu blocks almost fully dissolved and transformed to θ'-Al2Cu precipitates in the Cr-containing alloys. TEM observation revealed relatively large-sized θ' precipitates attached to Al13Cr4Si4 dispersoids. The Cr-containing alloys showed impressive mechanical properties, with the peak strength up to 452 MPa at room temperature. The ductility exhibited an increasing trend with Cr content, but the strength dropped dramatically when the Cr content reached 0.3 wt.%. It is suggested that the strength contribution from the Al13Cr4Si4 phase is limited, especially at an elevated temperature.

The Dependence of Isothermal ω Precipitation on the Quenching Rate in a Metastable ß-Ti Alloy.

The precipitation behavior of the α strengthening phase in metastable ß-Ti alloys is highly dependent on heat treatment parameters such as quenching rate, heating rate and ageing temperature. In this paper we have investigated the influence of quenching rate on the formation of isothermal ω precipitates that have been regarded as potent nucleant sites for α precipitation. The results show that the ß-solutionized alloy contains a ß matrix with a layer structured morphology. Regular atomic movement of the (002)ß plane along the <002> direction was observed in the alloy. The increase in quenching rate refines the thickness of layers, subsequently influencing the nucleation and growth of isothermal ω precipitates after ageing treatment. The high quenching rate promotes the occurrence of ω precipitation, broadens the stage of ω precipitation and increases the number density of ω precipitates. Since the isothermal ω phase provides a heterogeneous nucleation site for α precipitates, it is inferred that the quenching rate may indirectly influence the mechanical properties of metastable ß-Ti alloy.