When could global warming reach 4°C?

The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) assessed a range of scenarios of future greenhouse-gas emissions without policies to specifically reduce emissions, and concluded that these would lead to an increase in global mean temperatures of between 1.6°C and 6.9°C by the end of the twenty-first century, relative to pre-industrial. While much political attention is focused on the potential for global warming of 2°C relative to pre-industrial, the AR4 projections clearly suggest that much greater levels of warming are possible by the end of the twenty-first century in the absence of mitigation. The centre of the range of AR4-projected global warming was approximately 4°C. The higher end of the projected warming was associated with the higher emissions scenarios and models, which included stronger carbon-cycle feedbacks. The highest emissions scenario considered in the AR4 (scenario A1FI) was not examined with complex general circulation models (GCMs) in the AR4, and similarly the uncertainties in climate-carbon-cycle feedbacks were not included in the main set of GCMs. Consequently, the projections of warming for A1FI and/or with different strengths of carbon-cycle feedbacks are often not included in a wider discussion of the AR4 conclusions. While it is still too early to say whether any particular scenario is being tracked by current emissions, A1FI is considered to be as plausible as other non-mitigation scenarios and cannot be ruled out. (A1FI is a part of the A1 family of scenarios, with 'FI' standing for 'fossil intensive'. This is sometimes erroneously written as A1F1, with number 1 instead of letter I.) This paper presents simulations of climate change with an ensemble of GCMs driven by the A1FI scenario, and also assesses the implications of carbon-cycle feedbacks for the climate-change projections. Using these GCM projections along with simple climate-model projections, including uncertainties in carbon-cycle feedbacks, and also comparing against other model projections from the IPCC, our best estimate is that the A1FI emissions scenario would lead to a warming of 4°C relative to pre-industrial during the 2070s. If carbon-cycle feedbacks are stronger, which appears less likely but still credible, then 4°C warming could be reached by the early 2060s in projections that are consistent with the IPCC's 'likely range'.     

Alumina formation and microstructural changes of aluminized CoNiCrAlY coating during high temperature exposure in the temperature range 925°C–1075°C

Abstract

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. In the present work the microstructural features and oxidation behavior of an aluminized Co-base MCrAlY-coating on a Ni-based superalloy have been investigated in the temperature range 925–1075 °C. Microstructural studies of the oxidized coatings by SEM/EBSD were complemented with numerical thermodynamic calculations using the software package ThermoCalc. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY. During high-temperature air exposure alumina based surface scales were formed but the oxidation induced Al depletion of the aluminized coating did not result in formation of the γ’-(Ni₃Al) phase. Rather, the subscale formation of Co/Cr-rich phases was observed and a direct transformation of β- into γ-Ni phase after longer times. It is expected that these subscale microstructural changes thus affect the alumina formation and growth as well as the critical aluminum depletion in a different manner as in the case of corresponding β-NiAl coatings, although a direct comparison between various coating systems was not possible on the basis of the present results.     

Water availability in +2°C and +4°C worlds

While the parties to the UNFCCC agreed in the December 2009 Copenhagen Accord that a 2°C global warming over pre-industrial levels should be avoided, current commitments on greenhouse gas emissions reductions from these same parties will lead to a 50?:?50 chance of warming greater than 3.5°C. Here, we evaluate the differences in impacts and adaptation issues for water resources in worlds corresponding to the policy objective (+2°C) and possible reality (+4°C). We simulate the differences in impacts on surface run-off and water resource availability using a global hydrological model driven by ensembles of climate models with global temperature increases of 2°C and 4°C. We combine these with UN-based population growth scenarios to explore the relative importance of population change and climate change for water availability. We find that the projected changes in global surface run-off from the ensemble show an increase in spatial coherence and magnitude for a +4°C world compared with a +2°C one. In a +2°C world, population growth in most large river basins tends to override climate change as a driver of water stress, while in a +4°C world, climate change becomes more dominant, even compensating for population effects where climate change increases run-off. However, in some basins where climate change has positive effects, the seasonality of surface run-off becomes increasingly amplified in a +4°C climate.     

Compositional changes in minor phases present in 12CrMoVNb steels during thermal exposure at 550°C and 600°C

Abstract

One of the techniques considered promising in the assessment of the time–temperature history of creep resistant power plant steels is based on studies of compositional changes occurring in the minor phases during service exposure. Chemical compositions of alloy carbides and/or nitrides precipitated during quality heat treatment of high alloy steels are expected to be very close to equilibrium. However, since in practice the operating temperatures for these steels are usually much lower than the tempering temperatures used in their original heat treatments, changes in the compositions of their minor phases are likely to occur during service exposure. These changes depend on the differences between the respective equilibrium compositions of the minor phases at the tempering and service temperatures. The greater the difference between these temperatures, the greater will be the expected changes in the compositions of the minor phases during exposure at the service temperature. Since these changes are dependent on the time and temperature of exposure, any minor phase, which is present in the microstructure in the as received condition, as well as after prolonged service exposure, represents a potential indicator of time–temperature history of the material. In this paper the results of quantitative microstructural studies are reported on a series of martensitic 12CrMoVNb steels in their as received heat treated condition and following long term exposure at 550 and 600°C. The results of minor phase composition changes are discussed in terms of the equilibrium minor phase compositions calculated using the MTDATA computer program and the SGTE database, and their potentials as in service time–temperature history indicators for the materials are evaluated.     

High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na₂SO₄–60% V₂O₅) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr₂O₃, NiO and spinel NiCr₂O₄ has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.     

R-curves of an yttria- and alumina-doped hot-pressed silicon nitride ceramic at 1200°C and room temperature

An energy approach has been utilized to measure theR-curves of an Y2O3~A3-doped hot-pressed silicon nitride ceramic at 1200C in an argon atmosphere in three-point bending. In order to evaluate theR-curves at 1200C, a low constant displacement rate of =5 m min^–1 was applied in cyclic loading to obtain the cyclic loading/ unloading-displacement curves during controlled-crack propagation. Propagated crack lengths were measured directly by a microscope and they were compared to compliance-calculated crack lengths. After digitizing the cyclic load-displacement and crack length-displacement curves, crack-resistance parameters,R-curves andK-curves, were calculated by computer. At 1200C this material behaved non-elastically and the crack parameters, obtained here, represent the non-elastic ones. For comparison, at room temperature, continuous loading was applied to obtain the load-displacement curves. At room temperature, linear-elastic fracture mechanics behaviour was observed.     

Low cycle fatigue behaviour of 15-15Ti at 20 and 550°C under vacuum

15-15Ti is one of the potential candidate materials for fuel rod cladding of lead-based reactors, and its fatigue properties have been investigated. Low cycle fatigue (LCF) tests of 15-15Ti have been carried out under total strain amplitudes ranging from ±0.3% to ±0.9% at 20 and 550°C in vacuum. The results show the LCF life decreases with the increase of temperature. Crack propagates more tortuously and the fatigue crack propagation (FCP) rate is lower when it is tested at 20°C compared to 550°C. In addition, the effect of twin boundaries (TBs) and stacking fault energy (SFE) on FCP rate is discussed in this paper.     

Behavior and failure of uniformly hydrided Zircaloy-4 fuel claddings between 25 °C and 480 °C under various stress states,including RIA loading conditions

The anisotropic plastic behavior and the fracture of as-received and hydrided Cold-Worked Stress Relieved Zircaloy-4 cladding tubes are investigated under thermal–mechanical loading conditions representative of Pellet–Clad Mechanical Interaction during Reactivity Initiated Accidents in Pressurized Water Reactors. In order to study the combined effects of temperature, hydrogen content, loading direction and stress state, Axial Tensile, Hoop Tensile, Expansion Due to Compression and hoop Plane Strain Tensile tests are performed at room temperature, 350 °C and 480 °C on the material containing various hydrogen contents up to 1200 wt. ppm (hydrides are circumferential and homogeneously distributed). These tests are combined with digital image correlation and metallographic and fractographic observations at different scales. The flow stress of the material decreases with increasing temperature. The material is either strengthened or softened by hydrogen depending on temperature and hydrogen content. Plastic anisotropy depends on temperature but not on hydrogen content. The ductility of the material decreases with increasing hydrogen content at room temperature due to damage nucleation by hydride cracking. The plastic strain that leads to hydride fracture at room temperature decreases with increasing hydrogen content. The influence of stress triaxiality on hydride cracking is negligible in the studied range. The influence of hydrogen on material ductility is negligible at 350 °C and 480 °C since hydrides do not crack at these temperatures. The ductility of the material increases with increasing temperature. The evolution of material ductility is associated with a change in both the macroscopic fracture mode of the specimens and the microscopic failure mechanisms.     

Agriculture and food systems in sub-Saharan Africa in a 4°C+ world

Agricultural development in sub-Saharan Africa faces daunting challenges, which climate change and increasing climate variability will compound in vulnerable areas. The impacts of a changing climate on agricultural production in a world that warms by 4°C or more are likely to be severe in places. The livelihoods of many croppers and livestock keepers in Africa are associated with diversity of options. The changes in crop and livestock production that are likely to result in a 4°C+ world will diminish the options available to most smallholders. In such a world, current crop and livestock varieties and agricultural practices will often be inadequate, and food security will be more difficult to achieve because of commodity price increases and local production shortfalls. While adaptation strategies exist, considerable institutional and policy support will be needed to implement them successfully on the scale required. Even in the 2°C+ world that appears inevitable, planning for and implementing successful adaptation strategies are critical if agricultural growth in the region is to occur, food security be achieved and household livelihoods be enhanced. As part of this effort, better understanding of the critical thresholds in global and African food systems requires urgent research.     

Photoemission studies of the vicinal SiC(100) 4° surface and the Cs/SiC(100) 4° interface

Photoemission studies of the electronic structure of the vicinal SiC(100) 4° surface, which was grown using a new substrate atom substitution method, and the Cs/SiC(100) 4° interface have been performed for the first time. The modification of spectra of the valence band and C 1s and Si 2p core levels in the process of formation of the Cs/SiC(100) 4° interface was analyzed. The suppression of the surface SiC state with a binding energy of 2.8 eV and the formation of a cesium-induced state with a binding energy of 10.5 eV were observed. The modification of the complex component structure in the spectrum of C 1s core level has been detected and examined for the first time. It was found that Cs adsorption on the vicinal SiC(100) 4° surface results in intercalation of graphene islands on SiC(100) 4° with Cs atoms.     

Effect of a temperature change from 20 to 50°C on the basic creep of HPC and HPFRC

This research was performed in order to study the basic creep of High Performance Concretes (HPC) under uniaxial compression at 20 and 50°C. The aim of this work is to contribute to a better understanding of the basic creep phenomena of HPC at moderate temperature and to provide experimental data which will be used in Thermo-Hydro-Mechanical models such as those necessary for the National project CEOS.FR (Sellier, Thermo hydro mechanical numerical modelling, invited paper at the CEOS International workshop on Control of cracking in R.C. structures: a major step towards serviceability, 2009). The article also presents the fitting of a model considering the effect of temperature via an Arrhenius law affecting its viscous modules (Sellier and Buffo-Lacarriere, Eur J Environ Civ Eng 10:1161–1182, 2009). The concretes are those envisioned for future storage structures of Intermediate Level Long-Life Nuclear Wastes. The research programme has been established with four HPC, two non fibrous and two fibrous; the kinetics and amplitude of basic creep under uniaxial compression are measured during several months at 50°C and compared to those obtained at 20°C for the same materials (Camps, PhD thesis, 2008). Experimental results show that the average creep at 50°C is about twice the creep at 20°C. Besides, results show that this amplification depends on the binder type; the sensitivity to the temperature rise is greater for blended cement based concretes than for OPC based ones. The creep increase due the temperature rise is higher for the HPC under study than for ordinary concretes inventoried in a literature survey. The creep amplitude of HPC seems correlated to their amount of secondary C–S–H. At last, the fitting of the model parameters on the experimental results shows that the values of activation energy are quite close to those obtained by other authors on ordinary concretes (Bazant et al., J Eng Mech ASCE 130(6): 691–699, 2004).     

Low-cycle fatigue strength under step loading of a Si3N4 + SiC nanocomposite at 1350°C

The low-cycle fatigue behaviour of a hot pressed silicon nitride/silicon carbide nanocomposite and a reference monolithic Si₃N₄ have been investigated in 4-point bending at 1350°C in air using stepwise loading. The nanocomposite was prepared using 20% of SiCN amorphous powder as an additive, together with 5% yttria, to crystalline -silicon nitride powder. Two types of specimen have been tested, with and without a sharp notch (notch tip radius 10 m) at applied loads from 50 N with steps of 25 N and from 50 N with steps of 50 N, respectively. Five cycles have been performed at all applied load levels with an amplitude of 50 N for both types of specimen. The deflection of the specimens has been recorded up to specimen failure. The failure load of the unnotched nanocomposite was significantly higher than that of the monolithic material whereas for the notched specimens only a small difference has been found between the failure loads of the monolithic and the composite. Notched specimens of both materials exhibited a similar size of the slow crack growth area at catastrophic fracture, whereas for unnotched specimens the size of the slow crack growth area was significantly larger for the monolithic ceramic. The nanocomposite exhibits higher fatigue strength due to its higher resistance against stress corrosion damage and stress corrosion crack growth.     

Comparison of the long-term strength of α- and α + β- titanium alloys under a layer of chlorides in the temperature range 200–380°C

Conclusions Chlorides have no effect up to a temperature of 300°C on the-Ti alloys studied, but the strength characteristics of the+-Ti alloys fall by 10% (on the basis of 2000 h). In the range 300–380°C the long-term strength under a layer of chlorides on a basis of 2000 h decreases by 2.5 times in the+-Ti alloys, but only by 20—30% in the-alloys. Under the influence of the prolonged action of chlorides (up to 2000 h), the+-Ti alloys suffer corrosion cracking at a lower stress level than do the-alloys. The deformation capacity of the-alloys falls by 2–4 times in the same temperature range, while in the + —alloys it falls practically to zero. A decrease in thickness of the specimen from 5 to 1.5 mm leads to a reduction in the long-term strength limit based on 2000 h by 1.3–1.4 times. A significant size-scale effect appears in- and+-Ti alloys which have a room-temperature yield point of not less than 70 kg/mm². The extent of the fall in long-term strength and deformation capacity with reduction in the thickness of the specimen is somewhat greater in+ —Ti alloys than in-alloys.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 11, No. 5, pp. 67–70, September–October, 1975.     

Limits of the oxidation resistance of several heat-resistant steels under isothermal and cyclic oxidation as well as under creep in air at 650°C

In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidation leads to Cr subsurface zone depletion the question arises as to how the oxidation behaviour is affected by the decrease in Cr concentration with oxidation time. Four ferritic heat-resistant commercial steels containing 9–12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650°C in air to determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for times up to 3000 hours. In addition to isothermal tests, thermal cycling tests and creep tests were also performed. Surprisingly large differences in oxidation behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9Cr steels while the other one was best. Thermal cycling improved the oxidation behaviour of the steels which was worse under isothermal conditions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal subsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical value which had been determined by model calculations. The tendency observed under isothermal conditions is enhanced by superimposed creep deformation. It is concluded from the results that growth stresses in the oxide scales combined with the actual Cr-concentration in the metal subsurface zone play a major role in oxidation resistance.     

Effect of Sc and Zr alloying on microstructure and precipitation evolution of as cast Al–B4C metal matrix composites

Abstract

Additions of Sc and Zr were introduced into Al–15 vol.-%B₄C composites, and eight experimental composites with different Sc and Zr levels were prepared via a conventional cast process. Optical microscopy, SEM and TEM were applied for observing the as cast microstructures, including the interfaces between the Al matrix and the B₄C as well as the evolution of the precipitates. It was found that Sc involved the interfacial reactions with B₄C that partially consumed the Sc. On the other hand, no major Zr reaction products were found in the interfaces, and the major part of Zr remained in the matrix for precipitation strengthening. The Sc addition yielded considerable precipitation strengthening in the as cast and peak aged conditions. The combination of Sc and Zr significantly enhanced the precipitation strengthening. Nanoscale precipitates Al₃Sc and Al₃(Sc,Zr) were found in the as cast microstructure and contributed to the significant increase of matrix hardness.     

Sintering of 6H(α)-SiC and 3C(β)-SiC powders with B4C and C additives

The sintering behavior of three fine industrial SiC powders (two 6H()-type and one 3C()-type) has been comparatively investigated. The powders were pressureless sintered with B₄C and C additives between 1950°C and 2250°C in a high temperature dilatometer with flowing Ar atmosphere. The densification and shrinkage rate curves, polytype content, and grain growth were correlated with physical and chemical characteristics of starting powders. One of 6H()-type powders presented good sinterability only after extensive milling, even though it presented small average particle size, narrow particle size distribution and high specific surface area. The main difference in densification behavior among powders was the narrower shrinkage rate curve of -SiC powder, with its maximum shifted to higher temperature. Grain growth and phase transformation simultaneously occurred. In -SiC, 6H polytype partially transformed to 4H. This transformation was favored by aluminum impurity and resulted in a microstructure with more elongated grains. In -SiC, 3C transformed mainly to 6H, 15R and 4H, introducing many stacking faults which resulted in elongated SiC grains.