Metastable Transition Phase Research Articles

Yttrium effect on 475 °C brittleness of Fe–13Cr–6Al–2Mo-0.5Nb-0.15Zr alloy

The effect of yttrium on the 475 °C brittleness and microstructure stability of Fe–13Cr–6Al–2Mo-0.5Nb-0.15Zr alloy after long-term aging at 475 °C has been investigated. At the early stage of aging, the nanoscale metastable transition phase precipitates in large quantities. After aging for 10,000 h, the nanophase in the Y-free alloy has completely disappeared, and only a great deal of granular Laves phases are left, while a large number of nanoscale phases still exist in the grain of Y-containing alloys. This is because the addition of yttrium combines with vacancies, hinders the diffusion of Laves phase-forming elements, and has the result that the transformation of nanoscale phases Fe2(Mo0.3Nb0.7)Al to Laves phase is inhibited during the aging process of the Y-containing alloys. During aging, precipitation-free zones (PFZ) are formed in the surrounding area where the Laves phase and grain boundaries exist. The elongation of Y-containing alloys decreases with yttrium content, but it is still higher than that of Y-free alloy. The addition of yttrium improves the aging plasticity and weakens the degree of 475 °C brittleness of the alloy, and the effectiveness is diminished with the yttrium. This plays a key role in the further study of Fe–Cr–Al alloys for fuel cladding materials.

Healing the cosmological constant problem during inflation through a unified quasi-quintessence matter field

We heal the cosmological constant problem by means of a cancellation mechanism that adopts a phase transition during which quantum fluctuations are eliminated. To this purpose, we propose that a generalized scalar (dark) matter field with a non-vanishing pressure term can remove the vacuum energy contribution, if its corresponding thermodynamics is written in terms of a quasi-quintessence representation. In such a picture, pressure differs from quintessence as it shows a zero kinetic contribution. Using this field, we investigate a metastable transition phase, in which the Universe naturally passes through an inflationary phase. To reach this target, we single out a double exponential potential, describing the metastable inflationary dynamics by considering suitable boundaries and thermodynamic conditions. We analyze stability investigating saddle, stable and unstable points and we thus predict a chaotic inflation that mimics the Starobinsky exponential potential. Consequently, the role of the proposed dark matter field is investigated throughout the overall Universe evolution. To do so, we provide a physical explanation on unifying the dark sector with inflation by healing the cosmological constant problem.

Low-Temperature Reactive Sintered Porous Mg-Al-Zn Alloy Foams

By using carbamide granules as the space holder, Mg alloy foams with interconnected pore structures were synthesized by reactive sintering of a mixture of Mg, Al and Zn powders. The effect of Zn/Al on the microstructural evolution and compressive strength of porous Mg-9Al-xZn (x = 1, 5) alloy foams was investigated. The phase diagram simulation approach was used to determine the sintering temperature. The analysis results show that the formation of binary secondary phases or intermetallic compounds is a crucial factor in achieving bonding strength for the porous Mg alloy foams. The intermetallic compounds were formed by solid-state diffusion between the metal powder elements. Mg17Al12 intermetallics was the most stable compound formed in the cell walls of porous Mg alloy foams. The addition of Zn influences the solubility and stability of the intermetallic compound. Thermodynamic calculations show that Mg17Al12 was preferentially formed in the cell walls owing to its high negative enthalpy energy. Moreover, various metastable transition phases may exist in the microstructures, especially in the porous Mg-9Al-5Zn alloy foam. The intermetallic phases act as reinforcing phases, combined with grain refinement, significantly increasing the strength of the foam. At the given relative density of 0.42, the porous Mg-9Al-5Zn alloy foam exhibits the highest yield strength of 9.0 MPa, which is 23% higher than the strength of the porous Mg-9Al-1Zn alloy foam.

Effect of the Carburizing Layer on the Morphology of Chromium Carbides

Low carbon steel substrates were face-hardened by cementing, after which thin layers of chromium were deposited electrolytically on these substrates. After deposition, the samples were exposed to isothermal annealing at a temperature of 950°C. The characterization of the thin layers was carried out by means of optical microscopy and interferometry using the Vickers microhardness test. The obtained results allowed establishing the phase shift kinetics (under the effect of the cementing layer) in thin layers of chromium, which are transformed into chromium carbide when passing through metastable transition phases. These transformations were due to diffusion of the carbon atoms coming from the layer of cementing, germination and growth in solid phase. This fact has been examined taking into account the annealing temperature, the lattice parameter evolution and the deposited chromium layer morphology. As to mechanical properties, it was established that the micro-hardness depends on the phase shift evolution

Phase stability of γ′-Ni2Cr and α-Cr in the Ni-Cr binary

The stability of the γ′-Ni2Cr long-range order (LRO) phase present on the Ni-Cr binary phase diagram was examined in a Ni-55Cr (wt. %) alloy. It is shown that the γ′-Ni2Cr phase is metastable, and that the two-phase γ-Ni + α-Cr phase field extends to room temperature. The as-cast alloy consisted of a typical dendritic microstructure, with Cr-rich primary α-Cr dendrites and a Cr-lean, γ-Ni interdendritic. Annealing the as-cast alloy at 500 °C for 1000 h (γ′-Ni2Cr + α-Cr phase field) resulted in pockets of α-Cr and γ-Ni + γ′-Ni2Cr at the dendritic/interdendritic interface, with little change to the interdendritic γ-Ni microstructure. Annealing at 900 °C for 4 h (γ-Ni + α-Cr phase field) resulted in the interdendritic precipitation of α-Cr, both as a continuous and discontinuous precipitate. Annealing at 900 °C for 4 h followed by 500 °C for 1000 h resulted in the interdendritic precipitation of α-Cr at 900 °C, but no precipitation of γ′-Ni2Cr during subsequent annealing at 500 °C. This behavior suggests that the γ′-Ni2Cr phase is not the equilibrium phase, but rather a metastable transition phase. Furthermore, it is speculated that this phase in more complex alloys (i.e. Alloy 690) is also metastable, and its metastability should be considered in applications involving long-term, high temperature exposures.

The dehydroxylation of serpentine group minerals

The thermal transformation, stability field, and reaction kinetics of serpentine minerals (antigorite, chrysotile, and lizardite) have been studied to draw a comprehensive model for their dehydroxylation and recrystallization reactions. In situ X-ray powder diffraction (XRPD) and kinetic studies were combined with transmission electron microscopy (TEM) observations to describe the mechanisms of dehydroxylation and later high-temperature crystallization. During dehydroxylation, a metastable transition phase with a characteristic peak around 9 A was observed in antigorite and, to a minor extent, in lizardite. Rietveld refinements confirmed that the 9 A phase actually possesses a talc-like structure. The appearance of this phase is controlled by structure and kinetic factors. The kinetic parameters and reaction mechanism for lizardite and antigorite dehydroxylation in air at ambient pressure were calculated using the Avrami models and compared to those of chrysotile. For both lizardite and antigorite, the kinetics of dehydroxylation is controlled by diffusion. Apparent activation energy of the reaction in the temperature range 612–708 °C was 221 and 255 kJ/mol for lizardite and antigorite, respectively. The reaction sequences of chrysotile, lizardite, and antigorite leading to the formation of stable high-temperature products (i.e., forsterite and enstatite) are described taking into account previous topotactic and dissolution-recrystallization models.

Zeolitization of a Phonolitic Ash Flow by Groundwater in the Laach Volcanic Area, Eifel, Germany

AbstractField and experimental studies were performed to understand the formation conditions of the Nettetal zeolite deposit, Laach volcanic area, Germany. This deposit shows pronounced small- (cm) and large-scale (tens of meters) variations of zeolitization, despite the same phonolitic precursor glass throughout the occurrence. Zeolitization of the pyroclastic ash flow is restricted to three distinct layers that are 0.15 to 10 m thick and separated by fresh ash. The glassy matrix is altered to chabazite, phillipsite, analcime and K-feldspar in various combinations, whereas the pumice clasts are altered predominantly to chabazite. Mass changes during zeolite formation appear to be small, and Ca enrichment in chabazite and phillipsite may have occurred after their formation by cation exchange.The zeolites and zeolite assemblages observed in the Nettetal deposit were experimentally reproduced by reacting the phonolitic glass at 100–200°C with distilled water and 0.01 M alkaline solutions as well as with varying solid/liquid ratios and grain-sizes. Chabazite and phillipsite represented metastable transition phases with respect to analcime and K-feldspar. A high solid/liquid ratio accelerated the conversion of glass to zeolites.None of the classic models of zeolite formation is fully applicable to the Nettetal deposit. The most probable environment for zeolitization in this deposit is the stagnant fringe water zone immediately above the groundwater table. In this zone, representing a relatively closed system, favorable solution compositions for zeolite formation could have been developed rather quickly by glass-water interaction, which is not possible within the more thoroughly flushed deeper parts of the groundwater system. The three distinct zeolite layers are probably the result of temporarily changing groundwater levels.

Preparation of alumina fibre mats by a sol–gel spinning technique

High alumina fibre mats (with 2 and 4 wt% silica) having a web-like structure have been fabricated directly during synthesis by a sol–gel technique. Points of fusion in gel fibre mats helped in the formation of a web-like fibrous body having reasonable strength and very little dust formation after calcination. Incorporation of silica as an additive, extended the stability range of the metastable transition phases of alumina, thereby increasing the strength of the alumina fibres and of the mats in general. Strong and resilient fibrous preforms could be obtained by a single step sol–gel method followed by thermal treatment.

Effect of the boron:rare-earth ratio on metastable phase formation in Nd2Fe14B-based nanocomposites

Magnetic nanocomposite alloys developed from amorphous precursors by annealing often exhibit metastable transition phases that ultimately control the kinetics of the nucleation and growth of the desired RE2Fe14B-type phase. It is thus important to understand the effect of starting composition on the formation of the metastable transition phases found in the nanocomposite system. The relationships between composition and the nature of metastable phase formation in the low-boron content melt-spun nanocomposite alloy Nd2[Co0.06(Fe1−xCrx)0.94]23.2B1.48 (0⩽x<0.09) were studied and compared with results of other authors. The crystallization sequence undertaken by the quenched samples during annealing was followed by differential thermal analysis and x-ray diffraction. Differences in the nominal starting boron:RE compositional ratio of the alloy may produce significant changes in the nature and stability of the metastable phases present in the system. In general, a high compositional ratio (B:RE≈1–4) promotes the initial formation of Fe3B and complex boron-rich intermetallic phases such as the cubic Nd2Fe23B3, the hexagonal NdFe12B6 and the cubic Y3Fe62B14-type phases in the quenched material, while a low compositional ratio (B:RE<0.5) is favors the formation of α-Fe and a 2-17-type phase. It is suggested that the focus of further studies done to optimize the performance of these nanocomposite alloys should be on the effect that alloying additions have upon the metastable phases, and not on the final equilibrium phases.

Carbide precipitation in γ-TiAl alloys

Carbide precipitation in gamma-TiAl is observed and found to involve the formation of coherent, rod-shaped perovskite precipitates as a metastable transition phase. At temperatures above 750 deg C, the formation of plate-shaped H-phase particles on dislocations and grain boundaries is accompanied by dissolution of the perovskite precipitate dispersions. This temperature regime includes possible service temperatures for gamma-TiAl-based alloys. 10 refs.

The effect of ageing temperature on the fracture toughness of an 8090 Al-Li alloy

It is well established that during plastic deformation of AI-Li alloys, the coherent 6' precipitates are sheared by dislocations. The resultant planar slip associated with 6' shearing leads to heavy localized dislocation pile-ups at grain boundaries. Suresh et al. [1] suggested that a major contribution to the fracture process in peak-aged and overaged alloys arises from intergranular failure promoted by microvoids formed around grain boundary precipitates. This process in enhanced by slip planarity, and Duva et al. [2] proposed that slip becomes more localized as the size of the coherent ageing precipitate increases. The size and distribution of the 6' phase in the AI-Li alloy 8090 will depend on the ageing temperature selected, and in the present work the fracture toughness of this alloy was investigated in specimens aged to the same hardness level at progressively lower ageing temperatures. A 90 mm thick 8090 plate supplied by Messrs Alcan Plate Ltd had the composition shown in Table I. It had been solution-treated at 545 °C before being cold-water quenched and stretched 7% before ageing. Samples 10 mm × 15 mm x 15 mm were cut from the mid-section of the plate and ageing was carried out at 130 and 150 °C in an air furnace and at 170 and 190 °C in an oil bath. The average value of five hardness indentations was recorded, and the surface of each specimen was ground before testing in order to remove any lithium-depleted layer. The object was to examine the variation of toughness with ageing temperature in material of constant hardness. The hardness selected was that achieved in 32 h at 170 °C, which is commonly commercially employed. The ageing times to achieve this hardness were 320 h at 130 °C, 78 h at 150 °C, 32 h at 170 °C and 8.3 h at 190 °C. Both K~ and crack opening displacement (COD) tests were conducted, and toughness specimens were also aged for 8 and 24 h at 170 °C, so that the variation of toughness with ageing time at constant temperature could be examined. K,~ tests were conducted to American Society for Testing and Materials standards on compact tension (CT) specimens of 20 mm thickness, and COD tests on specimens of 6.35 mm thickness. All specimens were machined so that they were strained in the longitudinal direction of the original plate, with the notch in the transverse direction. The change of hardness with ageing time at the four temperatures is shown in Fig. la to d. As expected, peak hardness was achieved at increasingly longer ageing times for increasingly lower ageing temperatures. There was also the expected tendency for the peak hardness to be greater the lower the ageing temperature was. ~ though the duration of ageing at 130 °C was insufficient to achieve a peak value, it is noteworthy that plateaux are present in the curve, which may reflect the formation of precursor phases to the 6' precipitate. These data appear to be consistent with those reported by Firrao et aL [3] for a ternary A1-Li-Cu, who similarly reported a maximum hardness achieved after ageing at 150 °C in comparison with the other ageing temperatures studied. It is obvious, however, that ageing to peak hardness at 150 °C would not be commercially viable in view of the protracted time involved. However, ageing at a temperature between 150 and 170 °C may produce a superior combination of strength and toughness within an economically viable ageing time. The values of KIo or K o obtained are shown in Table II. The COD values obtained on specimens after the different heat treatments are shown in Fig. 2, and it is apparent that the specimens aged at lower temperatures and for shorter times have higher toughnesses. From both sets of data it is apparent that the greatest toughness change occurs between ageing temperatures of 150 and 170 °C, which again suggests that an ageing treatment between these two temperatures would produce optimum properties. The materials showing higher toughnesses correspond to ageing treatments that lead to smaller 6' precipitate sizes. This in turn will result in a less planar slip distribution [2], and hence greater plasticity. It may thus be concluded that, first, greater peak hardnesses are achieved at lower ageing temperatures. Secondly, the form of the ageing curve at 130 °C suggests that other metastable transition phases precede the formation of the 6' precipitate. Thirdly, as the ageing time is increased (at a given temperature) the toughness decreases. Fourthly, if specimens are aged to the same hardness at different ageing temperatures, then the toughness (as measured by both Kic and COD) will be higher at

Phase transformations in a uranium + 14 at.% niobium alloy

After quenching or quenching and aging, uranium-niobium alloys near the monotectoid composition exhibit a wide range of mechanical properties including the curious shape memory effect (SME). In this study, the transformation of the elevated temperature b.c.c. phase during both continuous cooling and isothermal aging was investigated by dilatometry and X-ray diffraction on a 14 at.% alloy. Two transformation stages were detected during rapid cooling from 1075 K to room temperature. The end product was the metastable transition phase. α″, which had a monoclinic unit cell geometry and was thought to transform martensitically according to the reaction sequence γ(b.c.c.) → γ°(tetragonal) → α″(monoclinic). For the fastest quenching rate. M s was 435K and M F approximately 340 K. The M s was found to vary significantly with cooling rate, increasing by 120 K when the cooling rate was reduced from 243 K/s to 2.1 K/s. The reverse transformation temperatures, A s and A F were heating rate dependent and increased as the rate was decreased. The γ to α″ transition could also be accomplished isothermally by aging at a temperature 165 K above M s . Indirect evidence was presented to suggest that the supersaturated parent phase may be modified by solute atom precipitation during aging or slow cooling thus promoting the time-dependent transformation to α″ at temperatures above M s .

Crystal structures of transition phases formed in U/16.60 at% Nb/5.64 at% Zr alloys

Single-crystal X-ray diffraction studies of γ-quenched and aged alloys of uranium with 16.60 at% (7.5 wt%) niobium and 5.64 at% (2.5 wt%) zirconium reveal two metastable transition phases whose structures are derived from small atom displacements from some of the sites occupied in the A2 γ structure. The phases appear to be identical with those previously reported as having chemically ordered structures in γ-quenched uranium-molybdenum and uranium-niobium alloys. The first transition structure, γ s , is formed on γ quenching; it is cubic, space group I4̄3 m, with a unit cell parameter about twice that of the γ phase. It may be formally described as eight unit cells of the γ structure in which the body-centered atoms have moved in correlated 〈100〉 directions. On aging at 150 or 350 °C, these movements are further correlated along a particular [001] direction, producing the second transition structure, γ 0; it is tetragonal, space group P4/ nmm, with an atom arrangement similar to that reported for β-neptunium. No evidence was found to suggest significant chemical order in either transition structure.

Role of Fluxing Agents in Thermal Alteration of Sandstones

Abstract Rock may undergo great changes in physical properties when heated to high temperatures and then cooled. The temperature and intensity of reactions causing rock alterations can be controlled by introducing certain chemicals during heat treatment. Three typical outcrop sandstone samples were saturated with common salt solutions, then heated to several maximum temperatures. After cooling, it was found that permeabilities had increased much more for salt-saturated samples than for samples not saturated with salt but heated to the same temperatures. This was only true, however, for samples heated above the melting points of the particular salts. Potassium chloride was particularly effective with Banders sandstone. Samples saturated with potassium chloride solution and heated to 900C showed an 11-fold increase in permeability. Samples without potassium chloride but heated to the same temperature showed only, a 2.5 fold increase in permeability. In application, it seems possible that injecting chemicals into the formations from a wellbore followed by applying intensive borehole heating might promote reactions which would greatly improve permeability of the formations. Introduction Great changes in the physical properties of rocks which have been heated to temperatures in the range of 600 to 800C have been reported earlier. Permeability increases of 50 per cent, and equivalent decreases in sonic velocity and breaking strength have been observed. Although there might be some suppression of certain reactions responsible for these changes when rock samples are heated under simulated reservoir pressure conditions, recent work has shown that these are more than offset by the increased importance of other reactions at high pressures. The reactions considered responsible for alteration of rock properties by heating include differential thermal expansion, dehydration, phase changes and dissociation of mineral constituents. Ceramists have long known that temperatures at which reactions occur can be raised or lowered by the presence of certain impurities in the system. Thus the possibility exists that by saturating rocks with appropriate solutions, desired reactions might be accelerated and unwanted reactions might be suppressed when the rock is heated. Purpose of the present work was to investigate the effects of several common salts on the thermal alteration of sandstones. Types of reactions which might occur are reviewed and the findings of a number of thermochemical alteration tests are presented. THERMOCHEMICAL ALTERATION OF SANDSTONE MINERALS The most abundant constituent of most sandstones is quartz. Quartz can exist in at least eight different forms, but in the temperature range of immediate interest (to 900C) only two forms are of importance-alpha quartz below 573C and beta quartz above this temperature. Although the alpha-beta inversion of quartz is very important in thermal alteration of rocks, it is not particularly important here because the inversion temperature cannot be changed significantly by the presence of impurities or by the application of pressure. The transition temperature of quartz to tridymite is 867C, but the transition is very sluggish. Tridymite is considered to be a metastable transition phase between two stable phases: quartz-cristobalite. However, the transition temperature for tridymite to cristobalite is 1,470C. The quartz-tridymite-cristobalite transition can be accelerated substantially by the presence of fluxing agents. Finely divided calcium, magnesium and titanium oxides accelerate the conversion while Al 0 may retard it. A mixture of NaCl and CsCl reduces the temperature for cristobalite formation by as much as 420C.Feldspars constitute the second most important mineral in sandstones. The melting points of feldspars range between 1,000 and 1,700C depending upon the variety. No information available indicated any important role of fluxing agents in the thermal alteration of this group of minerals. Carbonate minerals are subject to dissociation at temperatures within the range of the present investigation. The dissociation of magnesite can start at a temperature of 373C, but the reaction is sluggish and might not occur until a temperature of 500C or -higher is reached. Dolomite dissociates in two stages at 500 and 890C, whereas calcite dissociation temperature is about 885C.Because CO2 is released in the dissociation of carbonates, all such reactions are somewhat dependent upon CO2 partial pressure. In the absence of CO2 in the surrounding atmosphere, the dissociation of calcite starts at 500C. However, when 1 atm of CO2 surrounds the sample, the dissociation does not start until 900C. The other carbonates are apparently much less sensitive to a change in partial pressure of CO2.The aragonite-calcite transformation can occur anywhere within the temperature range of 387 to 488C, depending upon the presence of impurities such as barium, strontium, lead and perhaps zinc. The differential thermal analysis (DTA) curves of both magnesite and dolomite vary with the presence of impurities. JPT P. 585ˆ