Wenchao Yang

Precipitate characteristics and selected area diffraction patterns of the β′ and Q′ precipitates in Al–Mg–Si–Cu alloys

In this study, the precipitate characteristics and selected area diffraction patterns (SADP) of the β ′ and Q ′ precipitates formed at the over-aged state of Al–Mg–Si–Cu alloys are systematically investigated by means of high-resolution transmission electron microscopy and transition matrix. The β ′ precipitates have two cross-sections, rectangle-shaped and round-shaped aligned with [0 0 1]Al direction, but only rectangle-shaped cross-section exists for Q ′ precipitates. And, both of them have 12 variants and orientations with Al matrix. However, there are only three different zone axes, [0 0 0 1]β ′, [1 4 5 0]β ′, and [5 4 1 0]β ′ for β ′ precipitates, and [0 0 0 1]Q ′, [1 4 5 0]Q ′, and [3 2 1 0]Q ′ for Q ′ precipitates, parallel to the [0 0 1]Al direction when they are precipitated from the Al matrix, respectively. Then, a new [0 0 1]Al SADP model, which superposes diffraction patterns of the β ′ and Q ′ precipitates, is established. Furthermore, some “cross-shaped” diffraction streaks appeared at over-aged state can be explained reasonably by this model, which is good in agreement with the experimental data.

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000 h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.

Melt superheating on the microstructure and mechanical properties of diecast Al-Mg-Si-Mn alloy

The application of aluminium alloys in automotive body structure is one of the main developments in recent years. The increase of the mechanical properties of the ductile die-casting is one of the most critical topics for the application. In this work, the effect of melt superheating on the morphology, size and distribution of α-Al phase and Fe-rich intermetallics, and on the mechanical properties of the Al-Mg-Si-Mn diecast alloy was investigated. The results showed that the refined microstructure could be obtained through melt superheating. The volume fraction of dendritic α-Al phase and the Fe-rich intermetallic phase formed in the shot sleeve was significantly reduced, resulting in the refined microstructure. Overall, the melt superheating could improve the mechanical properties of the yield strength, ultimate tensile strength, and elongation of the diecast Al-Mg-Si-Mn alloy. And, the Fe-rich intermetallic phase formed in the shot sleeve with the coarse compact morphology and formed in the die cavity with the fine compact particles were identified as the same α-Al12(Fe,Mn)3Si composition in the present experimental conditions, which was not affected by the melt superheating.

Formation and sedimentation of Fe-rich intermetallics in Al–Si–Cu–Fe alloy

The financial support from TSB (UK) under project No. 101172 is acknowledged. The authors also would like to thank the EPSRC (UK) and Jaguar Cars Ltd. (UK) for financial support under the grant for the EPSRC Centre - LiME.

Improvement of mechanical properties of HPDC A356 alloy through melt quenching process.

Melt quenched high pressure die casting (MQ-HPDC) is a new die casting process developed recently for improving the casting quality of the conventional HPDC process. In the MQ-HPDC process, an alloy melt with a specified dose and superheat is quenched by directly pouring the alloy melt into a preheated metallic container. The thermal mass and preheating temperature of the container is selected so that the alloy melt is quenched just below the alloy liquidus and heterogeneous nucleation takes place during the melt quenching. The quenched alloy melt is then fed immediately into the shot sleeve for component casting. In this paper we present the MQ-HPDC process and the resultant microstructures and mechanical properties of a MQ-HPDC A356 alloy.