This work investigated the impact of cyclic heat treatment (CHT) on the microstructure of cast peritectic solidifying titanium aluminide (TiAl) alloy 4822 (Ti-47.2Al-1.83Nb-1.83Cr at.%) in the HIP and homogenised (HH) condition, and its subsequent effect on forgeability.The study adds to the growing body of research around CHT for TiAl alloys by applying industrially relevant induction heating to conduct the five cycles to the single α phase temperatures (1370°C) necessary for grain refinement. Two different cooling rates were explored from each cycle: air cooling (ACCHT) and a controlled furnace-like (FCCHT) cooling rate. The resulting microstructures and HH material were assessed in terms of their morphologies and subsequent primary compression behaviour under uniaxial isothermal conditions. The resulting post-compression microstructures were analysed to assess lamellar grain size and content as well as phase and dynamic recrystallised fraction. This was followed by secondary compression to determine the strain rate sensitivity of each material condition.The FCCHT process returns a homogeneous refined fully lamellar microstructure compared to that of HH. The ACCHT process returned a more heterogeneous microstructure, largely consisting of lamellar and feathery γ (γf) at differing fractions across the piece, depending on cooling rate.The compression results show that both ACCHT and FCCHT proved beneficial with primary compression at 1100 °C and 0.001 s-1, by presenting a microstructure to primary compression that returns a successful outcome, compared to the instability seen with primary compression of non-cyclic heat-treated HH material. Primary compression of ACCHT material generated higher volumes of dynamic recrystallised material compared to the HH material, as well as the lowest lamellar content across the work piece. This and other factors led to successful secondary compression combined with the highest strain rate sensitivity of 0.25.
The adsorption of water by the graphene surfaces of multi-wall carbon nanotubes (MWCNTs) in either the untreated (4.3 atom% oxygen) or oxidised (22.3 atom% oxygen) surface states has been studied. Different concentrations of surface oxygen groups, which have been directly measured using XPS, give rise to distinctly different shapes of water adsorption isotherms. Those from the untreated materials follow the pressure axis which lends them a Type III character in the BDDT classification. However, since they display a clear point of inflection at the lowest pressure, they are strictly speaking Type II isotherms but indicative of relatively few polar interactions and weak water adsorptivity. In sharp contrast, the isotherms from the oxidised MWCNTs are typically Type II and are characterised by a marked positive curvature in their low pressure region due to the increased numbers of specific interactions occurring between water molecules and the polar surface oxygen groups. The water adsorption data were modelled by the equation of D'Arcy and Watt with a direct correlation being observed between the surface polarity parameters (a mL and a 0 ) and also a s (the limiting water uptake) and the surface oxygen levels of the MWCNTs. The difference in polar surface energy was confirmed by measurements of the calorimetric enthalpies of immersion in water (Δh i ), which were −54 mJ/m 2 for the untreated and −192 mJ/m 2 for the oxidised materials. These values also reflect the difference in the integral net enthalpies of adsorption for the two hydrophilic surfaces: a value of ca. −35 mJ/m 2 being obtained for an oxygen-free (hydrophobic) surface. Water adsorption on these hydrophilic graphene surfaces was shown to occur by specific hydrogen bonding and was therefore strongly dependent on the numbers of oxygen-containing polar surface sites. This behaviour is well known for other types of porous and non-porous carbon materials and is also predicted for carbon nanotubes by molecular simulation studies. The work described herein therefore provides early experimental confirmation of the quantitative role of surface oxygen chemistry in determining the water adsorption character of MWCNT graphene surfaces; it also validates previous simulation studies.
The objective of this research was to improve the forging outcome of peritectic solidifying cast titanium aluminide (TiAl) 4822 alloy (Ti-48Al-2Nb-2Cr at.%) in hot isostatic pressed and homogenised (HH) condition using cyclic induction heat treatment (CHT). This study adds to research around CHT for TiAl alloys by applying industrially relevant induction heating to conduct five heating cycles at the single αphase temperatures (1370 °C) necessary for grain refinement. Two cooling rates were explored in each cycle, air cooling (ACCHT) and controlled furnace-like cooling (FCCHT), without returning to room temperature. Samples were assessed at each stage in terms of their morphologies, lamellar grain size and content, as well as phase and dynamic recrystallised fraction, and subsequent primary and secondary compression behaviour with uniaxial isothermal compression. The FCCHT process resulted in a homogeneously refined fully lamellar microstructure, and ACCHT, in a heterogeneous microstructure consisting of lamellar and feathery γ (γf) at differing fractions across the piece, depending on the cooling rate compared with HH. The results show that CHT improved forging outcomes for both compression stages investigated, resulting in uniform compression samples with higher volumes of dynamic recrystallised material compared with the instability seen with the compression of HH material.
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