Volume 98, Issue 3

Microstructure and texture evolution of an AlMg1Mn0.14 alloy after Equal-Channel Angular Pressing (ECAP) and subsequent heat treatment were investigated. Initially the material was subjected to up to 8 passes with a 90° die angle using routes A and B c . The deformed and annealed states were characterised by crystallographic texture analysis, Electron Back Scatter Diffraction, Transmission Electron Microscopy, optical microscopy and microhardness tests. The recorded texture and microstructure changes were compared to the behaviour of specimens produced by conventional cold rolling. It was shown that the observed discontinuous change in texture, microstructure and microhardness has to be attributed to discontinuous recrystallisation, which proceeded faster for route B c samples. Furthermore, it was found that ECAP deformed material is more stable against recrystallisation than cold rolled material.

Grain refinement and texture evolution during equal channel angular pressing (ECAP) by route “C” are modelled by the visco-plastic self-consistent (VPSC) model incorporating a disclination-based grain subdivision criterion. Simple shear and analytical fan models are used to describe the deformation history during ECAP. Textures calculated after the first four passes of ECAP by both the standard VPSC and the VPSC/disclination models are compared to available experimental data that show a better predictive capacity of the model incorporating the disclination subdivision criterion.

In the present work, the possibility of obtaining high strength nanostructured aluminium alloys by hydrostatic extrusion has been shown. Billets of 2017 aluminium alloy were subjected to hydrostatic extrusion either immediately after water quenching or after subsequent ageing. The results have shown that processing by hydrostatic extrusion offers a possibility of grain refinement down to the nanometre scale in the age-hardenable aluminium alloy. However, the effectiveness of the process depends on its initial microstructure. The most promising results are obtained if the alloy is deformed in the as-quenched condition. The hydrostatically extruded aluminium alloy exhibits very high tensile strength with reasonable ductility. The stability of mechanical properties over a wide range of temperatures (from cryogenic to well above the ambient) is discussed. The results obtained are compared to similar 2XXX aluminium alloys processed by other severe plastic deformation methods.

The plastic flow of a copper billet calculated using 2D and 3D finite element modeling are compared during a single extrusion of equal-channel angular pressing. Both models predict deformation heterogeneity irrespective of material behavior, die geometry and friction conditions. On the symmetry plane where the numerical approaches can be compared, their strain distributions differ, especially as the processing conditions deviate from the ideal conditions. Overall the three-dimensional finite element model calculates lower plastic strains than the two-dimensional model.

Deformation twins in an ultrafine grained commercial Al – Mg – Si alloy with nonequilibrium grain boundaries processed by equal channel angular pressing at room temperature have been observed by means of a transmission electron microscope and a high-resolution transmission electron microscope. It was found that deformation twins formed frequently at the intersections of several grain bands where the critical stress level is reached locally and a change in the dominant slip system may occur due to the significant plastic deformation. Deformation twinning was also observed to occur via partial dislocations emission from grain boundaries and grain boundary junctions in the ultrafine grained alloy. Twins and stacking faults formed by Shockley partial dislocations dissociated from 0° screw dislocations were characterized and analyzed. Three twinning mechanisms suggested by molecular dynamic simulations were identified and their relevance was discussed.

The flow stress behaviour of commercially available AA6063-AlMg0.5Si0.4 with ultrafine-grain size is investigated over a wide range of strain and strain rates under uniaxial compression. The ultrafine-grained microstructure is achieved by equal channel angular extrusion and characterised by grain sizes well below 1 μm. Results of quasi-static loading show that the ultrafine-grained states behave in an elastic – nearly perfect plastic manner with significantly reduced strain hardening capacity. When compared to the coarse grained counterpart, no change in strain rate sensitivity was measured for the material after two extrusions. In contrast to that, after eight extrusions the material shows significantly increased strain rate sensitivity especially in the range of highest rates above 10 2 s −1 .

Twist extrusion is a recently developed method of severe plastic deformation that principally uses torsion to generate high strains in a repeatedly extruded billet, while maintaining a constant cross-section. The method thus results in a deformation gradient across the billet section. To date, the grain refinement behaviour of alloys deformed by twist extrusion have only been investigated qualitatively. In this study high resolution electron backscattered diffraction has been used to quantify the deformation structures produced by twist extrusion, in a model, single-phase, Al-0.13 % Mg alloy, with the aim of investigating the potential of the technique for producing ultra-fine grained metals, as well as the homogeneity of the microstructure and strain distribution in processed billets.

Ternary mixtures of Ni – Li 2 CO 3 – Fe 2 O 3 have been used as starting materials in order to prepare binary NiO – LiFeO 2 solid solutions to be used as cathodic materal for molten carbonate fuel cells. By a combination of thermoanalytical techniques (Thermogravimetry and differential scanning calorimetry) and diffractometric techniques (X-ray powder diffraction) the present work deals with the study of the role played by thermal and mechanical treatment of the starting mixtures on the preparation of the final solid solutions.

In this study, fatigue performance and fracture behavior of interstitial-free, low carbon and phosphorus-added low carbon steels have been investigated by plane bending fatigue tests at room temperature in air. For all steels, fatigue tests were carried out at a frequency of 1000 cycles per minute, however, a limited number of specimens of low carbon steel were also tested at frequencies of 500 and 1500 cycles per minute. After all mechanical tests, fracture surfaces were cut and examined using scanning electron microscopy to identify the fracture features. At any applied stress level, low carbon steel showed better fatigue performance than interstitial-free steel, and phosphorus addition further improved the fatigue life of low carbon steel. Fractographic observations on fatigue fracture surfaces revealed transgranular fracture for low carbon steel and intergranular fracture for interstitial-free steel. Interestingly, addition of 0.12 wt.% of phosphorus in low carbon steel could not change the dominant transgranular fracture behavior of the steel.

Fe – Nd – P layers were prepared by electroless deposition. Discussed in this paper is the influence of the rare earth element Nd on the deposition rate, mixed potential, anti-corrosion performance, structure, and surface morphology of the deposit. The results show that the deposition rate increases with the addition of Nd and the increment can be as high as 30 %. The addition of Nd also improves the homogeneity of the deposit, eliminates pores, and reduces the particle size. Electroless Fe – P deposits incorporated with Nd also exhibit significantly improved anti-corrosion performance, especially in alkaline media.

At the start of directional solidification a complex interaction between the movement of the solidification front, the concentration pile-up ahead of the front and the temperature gradient at the solid – liquid interface takes place. Experiments with binary Al-Cu1.3 wt.% were performed in this regime in a Bridgman – Stockbarger type gradient furnace allowing solidification with a planar solid – liquid interface. The time-dependent velocity and temperature profiles were measured during the experiments, whereas the concentration profiles were analysed from the processed samples. The experimental results were compared to predictions from numerical simulations based on the model of Coriell et al. Good agreement is obtained in the description of time-dependent diffusive, planar growth in the initial transient.

A comparison of phases and the microstructures of textured Sr 0.4 Ba 0.6 Nb 2 O 6 ceramics without and with V 2 O 5 addition was carried out using transmission and analytical electron microscopy. The dopant was not found in Sr 0.4 Ba 0.6 Nb 2 O 6 grains but in the inter-granular regions, including both grain boundary and second phase. Grain boundaries in the sample without V 2 O 5 are clean, but those in the V 2 O 5 -doped sample exhibit a variety of structures, with and without an amorphous film of variable thickness. In general the vanadium excess follows the same trend of film width, while the Sr excess is generally negative and the Ba excess is positive, suggesting that the amorphous films at grain boundaries are rich in V and Ba and deficient in Nb and Sr. The second phase was determined as Ba 3 V 4 O 13 phase containing small amounts of Sr and Nb. Such V- and Ba-rich inter-granular liquids promoted the densification and texturing, and affected the dielectric constant at low frequency but not the Curie temperature.

Semi-solid processing has been combined with die-casting for the magnesium alloy AZ91D. The microstructure of the semi-solid diecast AZ91D has been examined by optical microscopy and scanning electron microscopy. No pores were visible under optical microscopy, indicating a very low porosity. Mechanical properties of the semi-solid diecast AZ91D alloy have also been tested. High ultimate tensile strength and elongation have been achieved due to the low porosity and the fine microstructure. The fracture surfaces have been investigated by scanning electron microscopy in order to reveal the fracture mechanism. A typical inter-granular fracture surface was observed, indicating that the alloy fails in a brittle manner. The vertical-section microstructure of the fractured sample has indicated that the crack mainly propagates along the grain boundaries through the brittle fracture of the eutectic phase or the interface decohesion of the primary Mg phase and the eutectic phases.

Taguchi's design of experiment method and statistical F-test were used to further optimize design parameters for as-fabricated resonance frequency of tuning forks for use in Qualcomm ® mobile station modem (MSM)-3000 TM series central processing unit chips of code division multiple access, personal communication system, and global system for mobile communication systems. After much trial and error, finite element method modeling and Taguchi's design of experiment method enabled selection of essential design parameters to effectively manufacture tuning fork test samples using photolithography with subsequent photoresist spray coating. As-fabricated resonance frequency values of a tuning fork blank thus manufactured remain almost the same prior to final frequency adjustment of the mounted crystals, regardless of pressure change from ambient to the designated packaging pressure of 6 × 10 −5 Torr. The tuning fork test samples thus fabricated were characterized and analyzed using Taguchi's design of experiment method and statistical F-test to evaluate the sensitivity of as-fabricated resonance frequency for various design parameters fror ambient to the designated packaging pressure of 6 × 10 −5 Torr. As-fabricated resonance frequency is most sensitive to tine width (x) and variance analysis results showed that only x and y (tine length) are significant design parameters and no design parameter significantly affects the as-fabricated resonance frequency change with the vacuum level of the package: the as-fabricated resonance frequency change with vacuum level turned out to be independent of design parameter contributions.