This paper investigates the β-phase coarsening behaviour during isothermal heat treatment of free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) coatings prepared by high velocity oxy-fuel (HVOF) thermal spraying. The microstructure of the coatings was characterised using scanning electron microscopy with energy dispersive X-ray (EDX) analysis and electron backscatter diffraction (EBSD). It comprises a two phase structure of fcc γ-Ni matrix and bcc β-NiAl precipitates. The volume fraction of the γ-Ni and the β-NiAl phases were measured to be around 70% and 30% respectively, with grain sizes varying largely from 0.5 to 2 µm for both phases. Isothermal heat treatments of the freestanding coatings were carried out at 1100 C for times up to 250 h. The β-phase coarsening behaviour during isothermal heat treatments was analysed by quantitative metallography. It is shown that the coarsening behaviour of β phase in the CoNiCrAlY alloy followed the classical Lifshitz-Slyozov-Wagner (LSW) theory of Ostwald ripening. By incorporating a dimensionless factor which correlates with volume fraction of the β phase, a modified LSW model coupled with formulaic interfacial energy and effective diffusion coefficient of the 2 CoNiCrAlY alloy was utilised to interpret the coarsening behaviour of the β phase. The coarsening rate coefficient obtained from the modified LSW model shows good agreement with the corresponding experimental result.
The cast Al-Mg2Si metal matrix composite was prepared by metal model casting process with rare earth element Nd as the modificator. The effects of modification duration and remelting times on microstructure and mechanical properties of the composite were investicated by optical microscope (OM) and electronic universal testing machine. The results show that, after introducing a proper amount of Nd, both primary and eutectic Mg2Si in the Al-18 wt.%Mg2Si composite were well modified. The morphology of primary Mg2Si is changed from irregular or dendritic to polyhedral shape and the morphology of the eutectic Mg2Si phase is altered from flake-like to very short fibrous or dot-like. Moreover, the effect is of long-lasting and remelting properties. After the composite is modified for 300 min and remelted by 6 times, its primary and eutectic Mg2Si structures are still in modification state of small block and slices, and the tensile properties of the composite are not significantly affected.
In this paper, hypoeutectic Al-Si matrix composite reinforced by 5wt. % Mg2 Si were prepared under different pouring temperature (superheat) and electromagnetic stirring (EMS), and the effects of pouring temperature and EMS on the morphology a nd size distribution of primary α-Al, (α-Al+Mg2Si) eutectic cell and eutectic Mg2Si are investigated. The experimental results show that the low superheat + EMS process not only refines the grain structure of the primary α-Al and (α-Al+Mg2Si) eutectic cell, an d promotes the formation of more non-dendritic structure of the phases, but also refine s the eutectic Mg2Si and changes the nucleation mode of (α-Al+Mg2Si) binary eutectic from the nucleation on primary α-Al under conventional casting to the one on nuclean t particles in the interdendritic liquid under low superheat + EMS.
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