The solute-nanostructures formed in the primary α-Al grains of a Semi-Solid Metal cast Al-7Si-0.6Mg alloy (F357) during ageing at 180°C, and the age-hardening response of the alloy have been systematically investigated using transmission electron microscopy, atom probe tomography and hardness testing. A 120-h natural pre-ageing led to the formation of solute clusters and GP zones. The natural pre-ageing slowed down the precipitation kinetics six-fold during 1 h ageing at 180°C, but this effect diminished after 4 h when the sample reached the same hardness as that without the pre-ageing treatment. It reduced the number density of β″-needles to approximately half of that formed in samples without the treatment, and postponed the peak hardness occurrence to 4 h, 4 times that of the as-quenched sample. A hardness plateau developed in the as-quenched sample between 1 h and 4 h ageing corresponds to the growth of 2 the β″ precipitates and a significant concurrent decrease of solute clusters and GP zones. The average Mg:Si ratio of early solute clusters is < 0.7 while that of GP zones changes from 0.8 to 0.9 with increasing in their size, and that of β″-needles increases from 0.9 to 1.2. β″-needles, GP zones and solute clusters are important strengthening solute nanostructures of the alloy. The partitioning of solutes and precipitation kinetics of the alloy are discussed in detail.

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The heat treatment response of semisolid metal high pressure die cast Al-7Si-Mg alloys A356 and F357 was studied and compared. It was found that the heat treatment behaviour of alloy F357 is influenced markedly by the stability of the Mg containing p phase. This phase, which dissolves in alloy A356 during solution treatment, persists in F357 and decreases the amount of magnesium in solid solution. This is the likely origin of the decrease in the aging response of the F357 alloy. The tensile properties (yield strength and ultimate tensile strength) of the alloys correlate much better with the Mg concentration in solid solution than with the bulk Mg content of the alloys. The recently developed shortened T6 heat treatment cycles for rheocast A356 were tested on alloy F357. The optimum artificial aging treatment was determined to be 180uC for 4 h, regardless of the prior natural aging period.

The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for dendritic casting alloys. These heat treatments are not necessarily the optimum treatments for non-dendritic microstructures. For rheocast alloy A356, it is shown that natural aging prior to artificial aging causes the time-to-peak-hardness to be longer compared to the time when only artificial aging is used. Furthermore, a hardness plateau is maintained during artificial aging at 180oC between 1 and 5 hours without any prior natural aging. A natural aging period as short as 1 hour results in a hardness peak (rather than a plateau) to be reached during artificial aging after 4 hours at 180oC.

The γ-TiAl intermetallic alloy doped with Nb, Cr, and Si metals has been developed.It has emerged that the ternary Ti-48Al-2Nb and quinary Ti-48Al-2Nb-0.7Cr-0.3Si alloys yielded the best mechanical and cyclic oxidation properties. In particular, the quinary Ti-48Al-2Nb-0.7Cr-0.3Si alloy rapidly developed the protective stable Al 2 O 3 oxide doped with nitrogen and titanium oxynitride during cyclic oxidation. This showed a lower hardness when compared to the other alloys after cyclic oxidation.The oxide layer proved to have a good adhesive relationship with the parent metal.The alloys microstructures were analysed with the scanning electron microscopy (SEM). The sample hardness test were conducted by Vickers hardness tester. Research Highlights• Ti-48Al, Ti-48Al-2Nb, Ti-48Al-2Nb-0.7Cr and Ti-48Al-2Nb-0.7Cr-0.3Si were developed.• Ti-48Al-2Nb and Ti-48Al-2Nb-0.7Cr-0.3Si has excellent mechanical properties• Ti-48Al-2Nb and Ti-48Al-2Nb-0.7Cr-0.3Si revealed best cyclic oxidation resistance.• Formation of oxynitrides mixed with Al 2 O 3 /TiO 2 was detected.for scientists and engineers aiming to minimise the effect of both oxidation and oxygen embrittlement when these materials are in applications. One of the γ-TiAl intermetallic competitive material for high temperature applications are NiAl alloys [1-3]. Different approaches have been adopted to resolve the high temperature oxidation, leading to the development of coatings to prevent detrimental environmental effect on these alloys [4-6].The heat and creep-resistant intermetallic substrates deposited with nanocrystalline γ-TiAl intermetallic TiAl-based coatings micro-alloyed with Cr atoms induced very good oxidation resistance at 1173 K [4]. Furthermore, TiAl 3 /Al 2 O 3 composite powders were prepared by high energy ball milling and subsequent heat-treatment. The TiAl 3 /Al 2 O 3 powders deposited on γ-TiAl intermetallic and decreased the oxidation rate of γ-TiAl substrate [7] and Cr 2 AlC coatings with better Al rich oxide scale [8]. The laser aided manufacturing of γ/β-Al 2 O 3 coating on NiCrAlY coatings proved to withstand the operating temperature of 1200°C for ten cycles [9]. Additionally, another method of improving the properties of γ-TiAl alloys is to modify their chemical composition with alloying elements. For example, doping of traditional high temperature alloys such as β-NiAl to improve cyclic oxidation have been investigated [10-12]. Also, attempts to improve the oxidation resistance of Ti-Al intermetallic phase based alloys showed positive results [13-17]. Ding et al, [18] further investigated Nb-doped TiAl for cyclic deformation and microstructure evolution of high Nb containing TiAl alloy during high temperature low cycle fatigue. Our current investigation focus on the innovative alloying elements to produce the binary, ternary, quaternary and quinary alloys by addition of the Nb, Cr and Si γ-Ti-Al alloys. The arc button melting performed under vacuum was employed to manufacture the four γ-TiAl based alloys of nominal composition binary Ti-48Al (at. ...