Volume 109, Issue 12

Besides impurities in magnesium alloys containing aluminum, the microstructure also plays a role in the corrosion properties of the alloy. By targeted manipulation of grain size, secondary dendrite arm spacings and segregation in terms of amount and position, the corrosion properties are expected to be improved. For this, experiments were carried out by casting alloys with 0, 3, 6, 9, and 12 % aluminum into a mold with different applied cooling rates. The samples were analyzed regarding microstructure and composition, and the grain size and secondary dendrite arm spacings, as a function of aluminum content and cooling rate, were modeled. The results show a decrease in grain size and secondary dendrite arm spacings with an increased cooling rate. The segregated β phase was predominantly situated at the grain boundaries as divorced eutectic and in lamellar form. The assumed influences on the corrosion properties will be examined in Part 2.

During uniaxial compression tests of a twin-roll cast AZ31 alloy, the orientation induced anisotropy only lay in the initial strain range, which was attributed to the different twinning systems that were involved. Regardless of the initial orientation, contraction twinning always played an important role at higher strain range, especially with higher strain rate and lower temperature. At temperature higher than 300 °C, dynamic recrystallization would dominate. A single modified Arrhenius equation ∊˙=[sinh⁡(ασ)]nexp⁡(−Q/RT)$\dot = {\left[ {\sinh \left( {\alpha \sigma } \right)} \right]^n}\exp \left( { - Q/RT} \right)$ was used to describe the flow behavior for all experimental conditions. The n value was found to be higher at lower temperature when compression twinning was prevalent. Meanwhile the deformation activation energy was somewhat lower, which was attributed to the twinning induced softening effect.

The Al–Si–Cu alloys, which are widely used in automotive powertrains, exhibit limited high-temperature strength properties; the high diffusivity of the main strengthening elements Cu and Mg in α-Al at temperatures between 200 and 300 °C is a dominating factor in alloy softening. In this study, effects of slow-diffusing elements (Zr and V) and heat treatment on the microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy were investigated. Majorities of both Zr and V were retained inside the α-Al matrix during solidification. T6 heat treatment induced the solid-state precipitation of multiple, nano-sized particles in α-Al grain interiors. Unlike Cu/Mg-rich strengthening precipitates that form during aging, the Zr/V-rich precipitates formed during solution heat treatment, which indicates high potential for high-temperature strengthening in Al–Si alloys via transition metal addition. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in the base alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside α-Al grains. These thermally more stable precipitates were credited for the enhanced high-temperature strength properties of Al-7Si-3Cu-0.3Mg alloy by ∼20 %.

The effect of the heat treatment on the microsturucture and the mechanical response of a commercial Al–Zn–Mg–Cu alloy (AA7075) was studied. The influence of annealing on the hardening and plastic properties of Al–Zn-Mg–Cu alloy was investigated by indentation and tensile tests. The microstructural evolution of the alloy was examined by meant of optical, scanning and transmission electron microscopies. After various ageing states, the Al–Zn–Mg–Cu alloy showed significant changes in microstructure and mechanical behaviour. After quenching, the microstructure was characterized by high ductility. Following ageing at 140 °C, corresponding to the maximum hardness value, the alloy reveals a high volume fraction of small η′ precipitates. The minimum hardening is recorded after ageing at 350 °C. The radical change in the microstructure of the alloy influences its mechanical behaviour, especially in hardening and plasticity.

The microstructural evolution during semi-solid processing and thixoformability of a 7075 alloy prepared by conventional and new modified strain induced melt activation (SIMA) processes were comparatively investigated in this paper. The semi-solid slurries were thixoformed at 600 °C, at which temperature the solid fraction was estimated to be 0.8. The coarsening process of the semi-solid samples was described using Lifshitz–Slyozov–Wagner theory and the effect of pre-deformation on the coarsening kinetics of the solid particles was discussed. The coarsening rate constant of the new modified SIMA sample showed a remarkable decrease compared to that of the conventional SIMA sample. Microstructural and mechanical investigations indicated that the sample with a near-equiaxed microstructure deforms through the plastic deformation of solid grains mechanism. However, the sliding of solid grains and flow of liquid incorporating solid grains mechanisms were dominant in the sample with a globular microstructure. Also, it was observed that the yield and ultimate strengths and hardness of the sample prepared by the new modified SIMA process after thixoforming and T6 heat treatment increased by about 15 %, 10 % and 25 % respectively, compared to those of the conventional SIMA sample.

Hot deformation behavior of the SP-700 alloy is studied through continuous and interrupted hot torsion testing carried out at 850 and 1 000 °C at different pass-strains and inter-pass times. The flow curves of SP-700 alloy showed the discontinuous yield stress, which is followed by the steady state region. The dominant mechanism of softening in hot deformation at 1 000 °C was dynamic recrystallization (DRX), whereas at 850 °C serration and tanglement of the grain boundaries were observed. Nevertheless, the microstructure of the sample twisted at 850 °C indicated the occurrence of DRX and the formation of fine grains in the vicinity of grain boundaries and triple points by the bulging mechanism. With an increase in the pass-strain of interrupted torsion at 1 000 °C, due to the increase in driving force for nucleation and growth of new grains, the kinetics of static restoration increased. However, due to the completion of DRX, the kinetics rate of static restoration at steady state strain ( ∊ s ) was lower than that of the former state. The β → α phase transformation at 850 °C was found to be another factor contributing in fractional softening.

The present research evaluates different weight percentages of nano-B 4 C incorporated into Ti-6Al-4V through a powder metallurgy technique. Nano-B 4 C weight percentage was set at 0 %, 5 %, and 10 % with a particle size of ≤100 nm. Cylindrical preforms with different initial preform densities were prepared using a suitable die and punch assembly. Further, the preforms were sintered in a muffle furnace with argon atmosphere at a temperature of 1 100 °C for a holding period of 1 hr. Cold deformation experiments were carried out using a 1 000 kN hydraulic press; incremental loading steps of 5 kN were applied on the preform until the first visible cracks appeared on the free surfaces. The experimental results have shown that the powder metallurgy composite with 10 % nano-B 4 C demonstrates higher densification properties such as axial stress, axial strain, hoop stress, hydrostatic stress, and Poisson's ratio.

This study focuses on development of Nickel–Yttira Stabilized Zirconia (Ni/YSZ) based anode material by solid-state synthesis using polyvinyl alcohol (PVA) in varying weight concentrations (3 wt.% and 6 wt.%) both as a binder and as a pore-former. Scanning electron microscopy and X-ray diffraction based structural and morphological analyses were performed and the synthesis of a homogeneous NiO/YSZ phase was confirmed after sintering at 1 000 °C. Microscopic study investigating effects of PVA concentration upon pore density in pellets pressed under the same conditions was conducted. Thermo-gravimetric/differential thermal analysis of the powders pre-dried at around 120 °C was carried out up to 1 000 °C to investigate the phase change and the thermal decomposition behavior. Findings on reduction of NiO/YSZ in a mixed atmosphere of H 2 /Ar at 550 °C are also discussed in detail. The results demonstrated that areal porosity of 12 % and 24 % was achieved with 3 wt.% and 6 wt.% of PVA, respectively.

Ni–Cr–Fe alloy samples with different porosities were prepared via the Kirkendall effect, with a reactive synthesis of Ni, Cr, and Fe elemental powders. The microstructure, surface morphology and the compositions were analyzed with X-ray diffraction and scanning electron microscopy. The corrosion behavior was explored in a 2 mol L −1 KOH solution using electrochemical methods and an immersion test. The polarization resistance indicated that the free corrosion potential and the lower corrosion current of the porous Ni–Cr–Fe alloy were −0.18528 V and 3.5998 × 10 −7 A cm −2 . The immersion test showed that after 25 days of immersion, the weight change of porous Ni–Cr–Fe alloy was 0.132 %, which was less than the porous Ni (0.346 %). X-ray photoelectron spectroscopy results revealed the corrosion products of the porous Ni–Cr–Fe alloy and the porous Ni. The corrosion mechanism of the porous Ni–Cr–Fe alloy was discussed.

In this study, alumina ceramics were produced with high density and nanograin size without additives from nano-size γ-Al 2 O 3 . The effects of sintering pressure (3 GPa and 7 GPa) and temperature (600 °C and 1 200 °C) on the grain size of sintered samples were studied. It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase. Pressure-assisted densification decreases grain size, so that nano-scale grains can be achieved in the sintered structure. The effect of various sintering conditions on grain growth in alumina was investigated during high-pressure sintering for better understanding of pressure and temperature effects on microstructural properties.