Volume 104, Issue 8

Recycled commercially pure Ti having different Fe and O contents was rotary swaged at 700°C in order to break down the coarse as-cast microstructures. After swaging, materials were annealed at various temperatures to study potential coarsening of the recrystallized α-grains. While marked grain growth occurred in material A with low Fe and O contents after annealing for 1 hour at temperatures as low as 500°C, the grain size in material B with higher Fe and O contents hardly increased even after a 1 hour exposure at temperatures as high as 800°C. Yield stress, tensile strengformatth and high cycle fatigue strength in both as-cast and annealed conditions of material B were significantly higher than in the corresponding conditions of material A, presumably owing to more marked solid solution hardening and fine grain size strengthening of material B. In addition, precipitation hardening by fine Fe containing oxide particles may also contribute to the higher strength level of material B. Shot peening and roller-burnishing were found to drastically enhance the high cycle fatigue strengths relative to the electrolytically polished baseline conditions.

An atomic mobility database for binary liquid phase in multicomponent Al–Cu–Fe–Mg–Mn–Ni–Si–Zn alloys was established based on critically reviewed experimental and theoretical diffusion data by using DICTRA (Diffusion Controlled TRAnsformation) software. The impurity diffusivities of the elements with limited experimental data are obtained by means of the least-squares method and semi-empirical correlations. Comprehensive comparisons between the calculated and measured diffusivities indicate that most of the reported diffusivities can be well reproduced by the currently obtained atomic mobilities. The reliability of this diffusivity database is further validated by comparing the simulated concentration profiles with the measured ones, as well as the measured main inter-diffusion coefficients of liquid Al–Cu–Zn alloys with the extrapolated ones from the present binary atomic mobility database. The approach is of general validity and applicable to establish mobility databases of other liquid alloys.

A constrained free energy model for an interfacial system of interacting layers is derived in the regular solution and Redlich–Kister formalisms. Composition and interfacial energies are solved using a model based on the minimisation of the total free energy of the system with a fixed interfacial area. As example cases iron–oxygen and copper–oxygen surfaces and liquid–liquid metal alloy interfaces are studied.

The machinability of two prehardened mould steels, one continuously-cooled and one quenched and tempered, is compared in terms of cutting temperature, cutting force and tool life. Both materials have a hardness of ∼40 HRC which is a typical hardness for prehardened mould steels. The results of machinability tests are related to mechanical properties and microstructural features, and the material removal rate is estimated for the materials. The continuously-cooled steel with bainitic structure shows higher machinability and possesses superior impact toughness and ductility compared to the quenched and tempered steel with martensitic structure. These properties are very important for plastic mould steels. The continuously-cooled bainitic steel also has the advantage of not requiring any costly quenching and tempering processes in connection with its production.

The paper reports mechanical properties of magnesium alloy AZ31 deformed at low and high strain rates. Material was prepared using squeeze casting technology. Compression tests were performed at an initial strain rate of 8.3 × 10 −5 s −1 at temperatures between room temperature and 300°C. Dynamic compression Hopkinson tests were carried out at room temperature with impact velocities ranging from 11.2 to 21.9 m s −1 . Transmission electron microscopy investigations showed significant dislocation and twins densities. Results are discussed considering the importance of the activation volume determined in the stress relaxation tests obtained at the low strain rate and, quite separately, the importance of adiabatic shear banding at high strain rates.

The purpose of this study is to evaluate the welding characteristics of joints of martensitic stainless steel to medium carbon steel using friction welding. Martensitic stainless steel and commercial medium carbon steel each of 12 mm diameter were used to fabricate the joints. The friction welding tests were carried out using a direct-drive friction welding machine, which was designed and manufactured for this purpose by us. After friction welding, in order to determine the microstructural changes that occurred, the interface regions of the welded specimens were examined by means of scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. Microhardness and tensile tests were conducted to determine the mechanical properties of the welded specimens. The experimental results indicated that martensitic stainless steel could be joined to AISI medium carbon steel using the friction welding technique and for achieving a weld with sufficient strength, the friction time has to be held as short as possible, while the rotational speed, friction and forging pressure have to be as high as possible.

Several mechanisms for porosity growth in single crystal nickel-based superalloys during homogenisation heat treatment have been proposed in the literature. They were carefully checked using different experimental methods, namely quantitative light microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and density measurements. It is shown that the main mechanism is the Kirkendall–Frenkel effect, i.e. generation of voids due to uncompensated efflux of Al atoms from dissolving γ/γ′-eutectic areas. The Al diffusion is supported by the afflux of vacancies from surrounding γ-matrix which results in porosity growth. This conclusion is confirmed by the estimation of the vacancy afflux towards the dissolving eutectic.

Thermal stability after recalescence of the single-phase supersaturated granular grains prepared in Co-15 at.%Cu immiscible alloy melt was investigated. As for undercooled Co–Cu alloy, a transition from dendritic to granular crystals occurred when Δ T ≥ Δ T ∗, which was induced by an abrupt change in the dendritic growth mechanism from solute-controlled to purely thermally-controlled growth. However, the recalescence effect led to grain growth and solute segregation of trapped Cu to grain boundaries. On this basis, a thermo-kinetic model applicable for nano-scale materials was extended to the system of micro-scale undercooled Co–Cu alloy. The model showed close agreement with the experimental results. It is concluded that Cu atoms captured by solute trapping re-segregated to grain boundaries, which was responsible for the reduction in grain boundary energy and improvement in thermal stability in undercooled Co–Cu alloy.

Mg-6Zn-5Ca-3Ce and Mg-6Zn-5Ca-3Ce-0.5La (wt.%) alloys in the form of rods were prepared by hot extrusion of rapidly solidified powders, produced via atomization of the alloy melt and subsequent splat-quenching between a pair of water-cooled copper rollers. The microstructures and phase constitutions of the alloys were investigated. The results showed that the thermally stable Mg x Zn y RE z phase with a small amount of Ca was present in the alloys, and its formation may be due to the dissolving of Ca in Mg x Zn y RE z phase. For the Mg-6Zn-5Ca-3Ce alloy, the grain size ranged from 350 nm to 1 μm. With an addition of 0.5 wt.% La, the microstructure of the alloy was refined, for which the grain size ranged from 300 nm to 500 nm, mainly resulting from the increase in both the number of RE -containing intermetallics and the melting point of the Mg x Zn y RE z phase with a small amount of Ca.

The objective of this work is to study the graphitization of diamonds due to the presence of iron. Composites of Fe-15 wt.% diamonds were processed by means of conventional powder metallurgy with the parameters of cold pressing at 350 MPa and sintering at 1100°C/360 min. The oxidative behavior of diamonds was analyzed using thermal analysis. The influence of sintering was analyzed through microstructural investigation by means of scanning electron microscopy and energy-dispersive X-ray spectroscopy. The absorption bands of the C–C bonds of the graphite and diamond structures were analyzed using infrared spectroscopy. The results show the intensity of graphitization in the presence of Fe, and that the temperature utilized in the experiments was appropriate.

In the present study, textured and highly oriented nano-structured ZnO films were synthesized via chemical bath deposition. The effects of solution temperature have been investigated. It is concluded that the solution temperature is crucial to the crystallography, morphology, electrical and optical behaviors of the ZnO films. X-ray diffraction studies and scanning electron microscopy observations revealed that the structures grown at 95°C had a large aspect ratio, a faster c -axis growth and better vertical orientation than those obtained at relatively lower temperature. The variations depending on solution temperature have been provisionally explained theoretically. Electrical resistivity and activation energies of the films decreased with increasing solution temperature. The variation was attributed to enhancement in the crystallographic structure with increasing growth temperature and to delocalized phonon states. Through the optical absorption spectra a red shift was observed and attributed to crystal defects, non-stochiometry that Zn +2 ions substitute oxygen vacancies and delocalized phonon states.