Probable changes in mean and extreme precipitation in East Africa are estimated from general circulation models (GCMs) prepared for the Intergovernmental Panel on Climate Change Fourth Assessment ...Report (AR4). Bayesian statistics are used to derive the relative weights assigned to each member in the multimodel ensemble. There is substantial evidence in support of a positive shift of the whole rainfall distribution in East Africa during the wet seasons. The models give indications for an increase in mean precipitation rates and intensity of high rainfall events but for less severe droughts. Upward precipitation trends are projected from early this (twenty first) century. As in the observations, a statistically significant link between sea surface temperature gradients in the tropical Indian Ocean and short rains (October–December) in East Africa is simulated in the GCMs. Furthermore, most models project a differential warming of the Indian Ocean during boreal autumn. This is favorable for an increase in the probability of positive Indian Ocean zonal mode events, which have been associated with anomalously strong short rains in East Africa. On top of the general increase in rainfall in the tropics due to thermodynamic effects, a change in the structure of the Eastern Hemisphere Walker circulation is consistent with an increase in East Africa precipitation relative to other regions within the same latitudinal belt. A notable feature of this change is a weakening of the climatological subsidence over eastern Kenya. East Africa is shown to be a region in which a coherent projection of future precipitation change can be made, supported by physical arguments. Although the rate of change is still uncertain, almost all results point to a wetter climate with more intense wet seasons and less severe droughts.
The study focuses on the analysis of extreme precipitation events of the present and future climate over southern Africa. Parametric and non-parametric approaches are used to identify and analyse ...these extreme events in data from the Coordinated Regional Climate Downscaling Experiment (CORDEX) models. The performance of the global climate model (GCM) forced regional climate model (RCM) simulations shows that the models are able to capture the observed climatological spatial patterns of the extreme precipitation. It is also shown that the downscaling of the present climate are able to add value to the performance of GCMs over some areas depending on the metric used. The added value over GCMs justifies the additional computational effort of RCM simulation for the generation of relevant climate information for regional application. In the climate projections for the end of twenty-first Century (2069–2098) relative to the reference period (1976–2005), annual total precipitation is projected to decrease while the maximum number of consecutive dry days increases. Maximum 5-day precipitation amounts and 95th percentile of precipitation are also projected to increase significantly in the tropical and sub-tropical regions of southern Africa and decrease in the extra-tropical region. There are indications that rainfall intensity is likely to increase. This does not equate to an increase in total rainfall, but suggests that when it does rain, the intensity is likely to be greater. These changes are magnified under the RCP8.5 when compared with the RCP4.5 and are consistent with previous studies based on GCMs over the region.
The incessant quest in fabricating enhanced ceramic materials for use in aerospace, chemical plants, as a cutting tool, and other industrial applications has opened the way for the fabrication of ...ceramic-based composites with sintering additives which have been experimented to influence sinterability, microstructure, densification, and mechanical properties. The current research practices for the consolidation of ceramic matrix composite (CMC) have been in the utilization of metallic and non-metallic additives as a reinforcement for the ceramic matrix. The use of additives has a promising influence in ensuring the achievement of good microstructures and excellent properties. The use of metallic additives enhances the sinterability of CMC but it has a debilitating effect on its intrinsic mechanical properties, especially at high-temperature applications. Hence, its uses in a high-temperature application environment under high impact load are limited. Thus, the types and amount of additives to be added to a ceramic-based matrix composite depends on the type of application and properties desired to be achieved from the composites. One of the critical issues that have affected the properties of CMC is the type of powder metallurgy (PM) used for consolidation. PM has been experimented with to be efficient in manufacturing ceramic-based composites. Although, past review works have pinpointed diverse PM methods, viz, hot press, pressureless sintering, hot isostatic press, and spark plasma sintering (SPS), for manufacturing ceramics-based composites. Amidst these diverse methods, SPS has progressively been applied for the consolidation of ceramics, owning to its possibility of achieving a good sintered compact in a relatively short time with enhanced properties. This review focuses on the synthesis of TiC reinforced with sintering additives, with more attention on carbides as sintering additives. Carbide additives have the potential to improve microstructure, densification, and mechanical properties. In addition, future works on the consolidation and characterization of TiC are included in this review.
Modern day technology demands consistent and frequent advancements in materials for specialized applications. Nickel aluminide is one of such materials with distinctive properties that make them ...particularly suitable for high temperature applications especially in aerospace industries. However, the lack of ambient temperature ductility of this intermetallic has greatly restricted its applicability in service. In this review, the various efforts of researchers in solving this major limitation of nickel aluminides is evaluated and summarized, with particular emphasis on reinforcement types and processing methods that have been explored over the years.
Porous titanium samples were sintered at four different temperatures (500, 550, 600, and 650 °C), by following the spark plasma sintering (SPS)/space holder (SH) route. The aim was to study the ...effect of sintering temperature on the structural integrity and space holder (NaCl) removal, as well as airflow resistance. The porous titanium was characterized by X-ray diffraction (XRD) and scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy (EDS). SEM images of the porous samples showed an appropriate range of pore sizes above 100 μm and interconnectivity. Fracture surfaces reveal the formation and growth of sintering necks between adjacent particles which becomes clearer with increasing temperature. The EDS and XRD analysis showed that there are no contaminations or inclusions of NaCl in the sintered porous samples. The most efficient leaching parameters, which are adequate to completely remove the space holder, were by immersion in hot water, for at least 5 h (five cycles). An increase in sintering temperature significantly reduces the material porosity level, and consequently increases the airflow resistance through the material.
Based on the need to fabricate new or modified materials that possess demanding mechanical and wear properties that are needed for operation in extreme environmental conditions, it is necessary to ...understand the fundamentals of such materials based on their physical metallurgy and fabrication techniques to match up with industrial growth. The unique properties of titanium and its alloys have prompted research into ways of improving their utilization as a potential candidate for applications in extreme conditions. A way to improve the mechanical properties of titanium and its alloy is to develop a titanium matrix composite by adding ceramic reinforcements and choosing the appropriate fabrication route and parameters. The development of titanium matrix composites, the fabrication methods, and the incorporation of reported research works are discussed in this review. This serves to give a wide range of understanding into the development of titanium matrix composites.
The present work focused on the densification behaviour and heat treatment effect on the mechanical properties of spark plasma sintered Ni–Fe–Al–Cr alloys. Five initial alloys were prepared by mixing ...of elemental compositions of each alloy followed by sintering. The five alloys, Ni–30Fe–20Cr, Ni–30Fe–15Cr–5Al, Ni–30Fe–10Cr–10Al, Ni–30Fe–5Cr–15Al, and Ni–30Fe–20Al, were sintered at sintering temperature of 950 °C, pressure of 50 MPa, heating rate of 150 °C/min, and at a holding time of 10 min and heat-treated. Microstructure and mechanical properties of the sintered alloys were investigated prior to and after heat treatment by scanning electron microscopy (SEM) and a Future-tech Vickers microhardness tester respectively. Results showed that the relative density of the alloys increases as the aluminium content increases. The microhardness increases at 800 °C for 2 h ageing time and decreases after prolonged ageing. Therefore, optimum strengthening of sintered nickel alloys can be obtainable within short duration of ageing as prolonged results in adverse hardness values.
Abstract
This study investigates the effect of using ethanol as the process control agent during the wet ball milling of niobium (Nb). Dried nanocrystal Nb powders, of high purity, with particle ...sizes, ranging from 8.5 to 14.3 nm, were synthesized by ball milling. Commercial Nb powder of particle sizes of − 44 µm was employed by using the planetary ball mill equipped with stainless still vials with still balls in ethanol. A ball-to-powder mass ratio of 10:1 was used at a rotation speed of 400 rpm, an interval of 15 min with an interval break of 5 s, and a milling time of 10 h. The powder was dried in vacutec at a temperature of 100 °C, using a speed of 15 rpm in the vacuum of 250 mbar at a time of approximately 653 min. The crystal phase of the dried powders was analyzed using X-ray diffraction (XRD) with CuK
ɑ
radiation, and by modification of the Scherrer equation, a single crystallite size of 11.85 nm was obtained. The morphology of the particles was observed using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The XRD results show that the pure crystal sizes in nanometre (nm), which decreases as the 2θ and the full width at half maximum (FWHM) increases.
The influence of the environment on the properties of TiAl alloys is a concern for elevated temperature applications. One element that stands out in improving the resistance of TiAl alloys against ...high-temperature oxidation is Nb. Nb promotes the activities of Al and favors the formation of a more protective Al
2
O
3
surface oxide which subsequently stymies oxygen diffusion to the alloy. Ta is an equal replacement of Nb. Thus, this study aims to evaluate the effect of 0.8, 4, and 8 at. % Ta additions on the oxidation resistance of spark plasma sintered Ti-46.5Al (at. %) alloy. Isothermal oxidation tests were carried out for 360 and 100 h at 1123 and 1273 K respectively. Surface oxides examination include scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray diffractometry. The results show that 4 and 8 at. % Ta additions significantly support the formation of an interconnected, non-porous Al
2
O
3
layer at the metal-oxide interface. The continuous non-porous Al
2
O
3
layer serves as a diffusion barrier which leads to superior oxidation resistance of the TiAl alloys.
The availability of lower-cost titanium with unique properties required for engineering applications such as in automobile, aerospace and biomedical has created a renewed interest in titanium and its ...alloys. However, poor sinterability due to high affinity of titanium for oxygen results in the production of less dense sintered product. While this may be of benefits in biomedical applications, it is deleterious to engineering applications such as in aviation parts that require high fatigue performance. In order to overcome this challenge, this study synthesized and consolidated Ti–2Ni and Ti–10Ni binary alloys using spark plasma sintering technique. Ti (Gd 1) and Ni elemental powders were mixed together in a TF2 Turbula mixer, at the speed of 101 rpm for 8 h. Sintering was conducted at 850, 1100 and 1200 °C under vacuum using the spark plasma sintering system. The applied pressure, heating rate and holding time were maintained at 50 MPa, 100 °C/min and 10 min respectively. Sample characterization was carried out with the aid of X-ray diffractometer and scanning electron microscope, equipped with energy-dispersive X-ray spectroscopy. The density and hardness of the sintered alloys were obtained using the Archimedes technique and Vickers microhardness tester respectively. The results showed that the relative density and hardness increased with increasing sintering temperature for the sintered Ti–2Ni and Ti–10Ni alloys. Generally, it was observed that increasing the sintering temperature and nickel addition improved the sinterability and densification of the sintered alloy.
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