Non-isothermal crystallisation kinetics of self-assembled polyvinylalcohol/silica nano-composite

A novel polyvinylalcohol/silica (PVA/SiO₂) nano-composite is prepared with the self-assembly monolayer (SAM) technique. The SiO₂ nano-particles are homogenously distributed throughout the PVA matrixes as nano-clusters with an average diameter ranged from 15 to 240 nm depending on the SiO₂ contents. Using differential scanning calorimetry (DSC), the non-isothermal crystallisation behaviour and kinetics of the PVA/SiO₂ nano-composites are investigated and compared to those of the pure PVA. There are strong dependences of the degree of crystallinity (X_c), peak crystallisation temperature (T_p), half time of crystallisation (t_1/2), and Ozawa exponent (m) on the SiO₂ content and cooling rate. The crystallisation activation energy (E) calculated with the Kissinger model is markedly lower when a small amount of SiO₂ is added, then gradually increases and finally becomes higher than that of the pure PVA when there is more than 10% SiO₂ in the composite.

Consistent model predictions for isothermal cure kinetics investigation of high performance epoxy prepregs

An accurate kinetics model is essential for understanding the curing mechanism and predicting the end properties of polymer materials. Graphite/epoxy AS4/ 8552 prepreg is a recent high-performance thermosetting composite modified with thermoplastic, which is being used in the manufacture of aircraft and military structures. The isothermal cures of this system along with another thermoplastic toughened high-performance prepreg, the T800H/3900-2 system, were investigated by real-time Fourier transform infrared (FTIR) spectroscopy. The cure rate was quantitatively analyzed based on the concentration profiles of both the epoxy and primary amine groups. Three autocatalytic models were used to determine kinetics parameters for both composite systems. The model which utilizes an empirical term, the final relative conversion (at different isothermal curing temperatures), describes the experimental data of both systems more satisfactorily than the model which applies a diffusion factor. The modeling results suggest that the curing of epoxy within both prepregs can be assumed to be a second order process.

The kinetics of two-stage formation of TiAl3 in multilayered Ti/Al foils prepared by accumulative roll bonding

The solid-state reactions of Ti/Al multilayered samples produced by Accumulative Roll Bonding (ARB) have been investigated using differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. The kinetics of the formation of the intermetallic compound TiAl3 was highlighted. Experimental evidence and analysis of the data shows that, there was a two-stage process in the formation of TiAl₃ in the ARB Ti/Al reactive multilayered samples. Calorimetric and microstructural analyses also suggest that the interdiffusion of Al and Ti which led to solid solutions preceded the formation of intermetallic compounds. Despite the apparent chaos in the thickness of the ARB multilayered samples, the distribution of layer spacing did not become broad enough to lose the main features of the double exothermal behaviour. Isothermal DSC shows a larger Avarami constant in ARB Ti/Al multilayered structures than was found in Ti/Al thin films. A modified model based on thin films was set up to describe the kinetic characteristics of the formation of the intermetallic compound TiAl₃ in ARB samples.

Investigation of nonisothermal crystallization behaviors of poly and silica nanocomposites using differential scanning calorimetry

Nonisothermal crystallization behaviors of PVA and poly (vinyl alcohol) and Silica (PVA/SiO2) nanocomposites prepared via a self-assembly monolayer (SAM) technique are investigated in this study. Differential scanning calorimetry (DSC) is used to measure the crystallization temperature and enthalpy of PVA and nanocomposites in nitrogen at various cooling rate. The results show that the degree of crystallinity of PVA and nanocomposites decreases when the SiO₂ content increases but increases with an increasing cooling rate. The peak crystallization temperature decreases with an increasing cooling rate.

Effect of isothermal dissolution and re-precipitation of austenite on the mechanical properties of duplex structures

The hot deformation behaviour of a duplex ferritic/austenitic stainless steel was studied after different deformation conditions. The results showed a strange and interesting behaviour in the strength of the material during post-deformation studies. For most deformation conditions, the flow stress of the material was un-expectedly increased after annealing of deformed structures. This phenomenon implied that microstructural hardening occurred in the material during the interpass annealing rather than the expected softening. Also, an interesting change was observed where the morphology of the austenite phase changed from stringers or layers of austenite to a widmanstätten structure. The microstructural studies suggest that the austenite was dissolved and re-precipitated during the annealing process and the hardening was mostly associated with the change in the morphology of austenite.

Effect of silicon and prior deformation of austenite on isothermal transformation in low carbon steels

Isothermal transformation (TTT) behavior of the low carbon steels with two Si contents (0.50 wt pct and 1.35 wt pct) was investigated with and without the prior deformation. The results show that Si and the prior deformation of the austenite have significant effiects on the transformation of the ferrite and bainite. The addition of Si refines the ferrite grains, accelerates the polygonal ferrite transformation and the formation of M/A constituents, leading to the improvement of the strength. The ferrite grains formed under the prior deformation of the austenite become more homogeneous and refined. However, the influence of deformation on the tensile strength of both steels is dependent on the isothermal temperatures. Thermodynamic calculation indicates that Si and prior deformation reduce the incubation time of both ferrite and bainite transformation, but the effiect is weakened by the decrease of the isothermal temperatures.

Textures and compressive properties of ferromagnetic shape-memory alloy Ni48Mn25Ga22Co5 prepared by isothermal forging process

A ferromagnetic shape-memory alloy Ni₄₈Mn₂₅Ga₂₂Co₅ was prepared by the induction melting and isothermal forging process. Dynamic recrystallization occurs during the isothermal forging. The deformation texture was studied by the neutron diffraction technique. The main texture components consist of (110)[112] and (001)[100], which suggested that in-plane plastic flow anisotropy should be expected in the as-forged condition. The uniaxial compression fracture strain in the forged alloy reaches over 9.5%. The final room-temperature fracture of the polycrystalline Ni₄₈Mn₂₅Ga₂₂Co₅ is controlled mainly by intergranular mode.