Addition Of HF Research Articles

Mechanical behavior of bulk (ZrHf)TiCuNiAl amorphous alloys

Experimental investigation of thermodynamic behavior and mechanical properties of bulk Zr/Hf-based amorphous alloys revealed that crystallization and glass transition temperatures, hardness and modulus increased with increasing Hf content. It is postulated that the observed strengthening might result from the microstructural changes with addition of Hf in place of Zr.

Gas-phase ion chemistry and ion thermochemistry of phenyltrifluorosilane

The ion/molecule reactions of phenyltrifluorosilane with F −, CH 3O − and OH − and the thermochemical properties of the resulting ions were investigated by using a flowing afterglow—triple quadrupole mass spectrometer. The silane reacts with fluoride and methoxide ions by addition, whereas the reaction with hydroxide proceeds by addition and loss of HF to form the phenyldifluorosiloxide ion. Collision-induced dissociation (CID) of the adduct ions produces predominantly phenide ion, although rearrangement processes are also observed for the methoxide adduct. CID of the siloxide ion results in direct cleavage to form the SiF 2O − ion, in addition to products formed by rearrangement. We also report the formation of the hydroxide adduct of phenyltrifluorosilane, and provide the first confirmation that activated hydroxide adducts of silanes dissociate by loss of RH. The hydride adduct of phenyltrifluorosilane is generated indirectly by the addition of fluoride to phenyldifluorosilane. Bond dissociation energies and rearrangement barriers were obtained for all ions from the energy-resolved CID cross sections. From the measured bond dissociation energies, the fluoride, hydride and methoxide affinities of phenyltrifluorosilane are determined to be 79.4±3.3 and 64.0±3.2, and 76.6±5.9 kcal/mol, respectively, and the fluoride affinity of phenyldifluorosilane is found to be 72.3±4.4 kcal/mol. The hydride affinity (HA) of phenyltrifluorosilane is comparable to that of borane, suggesting that C 6H 5SiHF 3 − is a potential hydride reducing agent in the condensed phase.

Crystallization and mechanical behavior of (Hf, Zr)–Ti–Cu–Ni–Al metallic glasses

We have prepared a series of glass-forming alloys of composition (Hf x Zr 1− x ) 52.5Cu 17.9Ni 14.6Al 10Ti 5 and (Hf x Zr 1− x ) 57Cu 20Ni 8Al 10Ti 5 with x=0–1. The substitution of Hf for Zr reduces the glass-forming ability of these alloys; while all compositions of (Hf x Zr 1− x ) 52.5Cu 17.9Ni 14.6Al 10Ti 5 can be cast in bulk form, amorphous samples of the Hf-rich compositions ( x>0.8) of (Hf x Zr 1− x ) 57Cu 20Ni 8Al 10Ti 5 can only be produced by rapid solidification. The alloys show similar crystallization behavior, although we observe an intermediate phase in the Hf-rich glasses (Al 16Hf 6Ni 7) that is not observed in the Zr-rich glasses. The Young’s modulus and the flow strength under quasi-static uniaxial compression increase linearly with increasing Hf content. Under dynamic loading, the failure strength of all of the alloys tested decreases with increasing strain rate. The differences in mechanical behavior between the Zr- and Hf-rich glasses can be rationalized in terms of the higher melting point of Hf. The addition of Hf increases the average bond energy of the alloy, resulting in a higher modulus and increasing the activation energy for atomic motion by which plastic deformation occurs.

Magnetic property enhancement in Pr 2(FeCo) 14B/α-(FeCo)-based nanocomposite magnets through Hf, Ti or Al additions

The effects of various substitutions of M (M=Al, Ti, Hf) on the microstructure and magnetic properties of melt-spun nanocomposite Pr 8.5(Fe 0.9Co 0.1) 84.5M 1B 6 alloys have been investigated. The magnetic property improvement for Hf doping is discussed in terms of the grain refinement and of a more uniform nanostructure.

The effect of Hf content on the phase structures and electrochemical properties of V 2.1TiNi 0.5Hf x ( x=0–0.25) hydrogen storage alloys

The phase structures and electrochemical properties of the V 2.1TiNi 0.5Hf x ( x=0–0.25) alloys were investigated. It is found that the addition of Hf ( x≥0.05) into the V 2.1TiNi 0.5 alloy changes the secondary phase from the b.c.c. TiNi-based phase to a C14 Laves phase with hexagonal structure, in the form of a three-dimensional network along the grain boundaries of the main phase. The amount of the C14 Laves phase increases with increasing Hf content. The discharge capacity of the Hf added alloys remains high. The V 2.1TiNi 0.5Hf 0.05 alloy electrode has a discharge capacity of 444 mAh/g (discharge current: 25 mA/g), exceeding that of the V 2.1TiNi 0.5 alloy. Proper Hf content ( x≤0.12) is advantageous for improving the high-rate dischargeability of V 2.1TiNi 0.5Hf x alloys. The presence of Hf always leads to a degradation in cycle stability of the alloys, mainly due to the increase of the V content in the secondary phase and the increasing tendency to form cracks on the alloy surface in cycling.

The morphology of alumina scales formed on Fe-20Cr-4Al-S alloys with reactive element (Y, Hf) additions at 1273 K

The morphology of alumina scales formed on Fe–20Cr–4Al–S (185 ppm) alloys with reactive elements (10, 300, 800, and 3700 ppmY, 500, 900, 1900, and 4300 ppmHf) was studied in oxidizing atmospheres at 1273 K for 18, 1800 and 3600 ks by mass gain measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). No spalling of the oxide scales was observed on any of the alloys after any of the oxidation times. Mass gain decreased for alloys with small additions of reactive elements, and then increased with increasing reactive element content. Oxide scales on the 185 ppmS, the 500 and 900 ppmHf alloys showed a wavy morphology. On the other hand, oxide scales on the other alloys with reactive elements changed from small wavy to planar morphologies with increasing reactive elements after any of the oxidation times studied. Oxide scales formed on all of the alloys were recognized as mainly _-Al2O3. After oxidation for 3600 ks, Y3Al5O12 particles were observed for the 800 and the 3700 ppmY alloys, and HfO2 particles were also detected for the 1900 and the 4300 ppmHf alloys.

Impurity and surface condition effects on pore formation at Al2O3/FeAl interfaces

Interfacial pores are commonly observed at the scale/alloy interface in NiAl and FeAl alloys after high temperature oxidation. The pores are often an order of magnitude larger than the oxide grains, and many are formed during the initial stage of oxidation. In order to better understand the development of these pores, the effects of pre-oxidation surface condition and surface impurities on pore formation at the oxide/alloy interface in Fe–40at%Al are investigated. Surface conditions included polishing to 4000 grit with SiC and polishing to 1 µm with diamond suspension. Surface impurities included a number of nitrate salts dissolved in water. Pore number distribution in each case was determined and related to the treatment effects. Testing was also performed on an Hf-containing Fe–40Al to evaluate the effect of adding a reactive element. It was found that surface roughness and the presence of impurities, even residues from water drops, increased the number of interfacial pores. Hf addition made the alloy more resistant to pore formation, but large pores developed under heavy salt deposits.

Effect of hafnium on the castability of directionally solidified nickel-base superalloys

Abstract The effect of Hf additions on the solidification characteristics and castability of the Ni-base superalloy IN792 has been studied. A quenching technique was employed to freeze-in the microstructure during solidification. The crack length in tube-shaped demonstration castings was evaluated. Hf additions were found to reduce the hot-tearing susceptibility significantly. Hf also caused a more gradual decrease of the liquid fraction with temperature in the last stages of solidification, which was observed to improve castability in other alloys previously and which can be correlated with lower strains and strain rates in the vulnerable period where hot tears form. It was also found that the γ/γ′ rosette structure, often referred to as γ/γ′ eutectic in the literature, actually forms by a discontinuous precipitation mechanism from the solid. The amount of γ/γ′ observed in the interdendritic area, therefore, does not reflect the amount of remaining liquid in the last stages of solidification.

Hydrogen bonding and catalysis of solvolysis of 4-methoxybenzyl fluoride.

Values of k(o) = 8.0 x 10(-3) s(-1) and k(H) = 2.5 x 10(-2) M(-1) s(-1), respectively, were determined for the spontaneous and the acid-catalyzed cleavage of 4-methoxybenzyl fluoride (1-F) to form the 4-methoxybenzyl carbocation (1+). Values of k(F) = 1.8 x 10(7) M(-1) s(-1) and k(HF) = 7.2 x 10(4) M(-1) s(-1) were determined for addition of F- and HF to 1+ for reaction in the microscopic reverse direction. Evidence is presented that the reversible addition of HF to 1+ to give 1-F + H+ proceeds by a concerted reaction mechanism. The relatively small 250-fold difference between the reactivities of fluoride ion and neutral HF toward 1+ is attributed to the tendency of the strong aqueous solvation of F- to decrease its nucleophilic reactivity and to the advantage for the concerted compared with the usual stepwise pathway for addition of HF. There is no significant stabilization of the transition state for cleavage of 1-F from general acid catalysis by 0.80 M cyanoacetate buffer at pH 1.7. The estimated 3 kcal/mol larger Marcus intrinsic barrier for heterolytic cleavage of 1-F than for cleavage of 1-Cl is attributed to a lag in the development at the transition state of the ca. 30 kcal/mol greater stabilizing solvation of the product ion F- compared with Cl-. The decrease in the electronegativity of X along the series X = F, OH, Cl is accompanied by a ca. 10(10)-fold increase in the carbon basicity compared with the proton basicity of X-.

Influence of digestion procedures on the determination of rare earth elements in peat and plant samples by USN-ICP-MS

Analytical procedures for the determination of rare earth elements (REE) in 250 mg aliquots of difficult-to-digest peat and plant matrices by ICP-MS were developed. Three different pressurised digestion approaches were tested for this purpose, namely (i) closed vessel acid digestion on a hot-plate at 180 °C, (ii) digestion in a microwave high pressure autoclave at a temperature of 240 °C and (iii) high pressure ashing (HPA) at a temperature of 320 °C. Acid mixtures for digestion contained concentrated nitric acid (3–5 ml) alone or additions of hydrofluoric acid (HF) or tetrafluoroboric acid (HBF4) at volumes of 0.05–1.0 ml. The selection of appropriate volumes of HF or HBF4 was identified as a critical step in obtaining accurate results. For several reasons, HBF4 is preferred in comparison with the normally used addition of HF for the destruction of siliceous matter in the samples investigated. The optimum acid mixture consisted of 3 ml of HNO3 and 0.1 ml of HBF4. High sample throughput (40 samples simultaneously in about 2 h) favours the microwave autoclave over the other two digestion systems. An ultrasonic nebuliser (USN) with membrane desolvation used for sample introduction reduced the spectral interferences originating from oxide formation of lighter REE and Ba to a negligible extent. Internal standardisation with Rh and Re proved to be essential for obtaining correct results. In this way, all REE could be reliably quantified by USN-ICP-MS without applying any mathematical correction equations. The accuracy of the optimised procedures was assessed by the determination of REE in digests of the certified reference material GBW 07602 Bush Branches and Leaves and of the candidate reference material CRM 670 Aquatic Plant. The developed analytical procedures were applied to the determination of REE in two different peat matrices. Results for these peat samples obtained by USN-ICP-MS showed good agreement with INAA values. Strong fractionation of REE caused by the addition of HF or HBF4 in excess, known as lanthanide contraction, could be experimentally established, except for europium, which revealed a different behaviour.

Electroanalytical methods for determination of the metal content and acetic-acid-available metal fractions in soils.

The pseudo-total and available arsenic, cadmium, and lead content of soils have been determined by stripping voltammetry with a hanging-mercury-drop electrode and by atomic absorption spectrometry with electrothermal atomization. For determination of pseudo-total metals microwave digestion with a mixture of HNO3 and HClO4, with and without addition of HF, was investigated. The single-extraction procedure with 0.43 mol L-1 CH3COOH, proposed by BCR, was used to assess the availability of metals in soils. The results obtained were validated by analysis of a certified reference material.

Magnetic anisotropy induced in Nd 16Fe 76B 8 by Hf additions

Remanence, coercivity and maximum energy product ( BH) max of Nd 16Fe 76− x Hf x B 8 ( x=0, 0.1, 0.2) magnets processed under different hydrogenation-disproportionation-desorption-recombination (HDDR) conditions, were studied. Vibrating sample magnetometry results showed that Hf-doped materials develop an important degree of anisotropy, especially for the case of solid-HDDR treatments at 800°C and 850°C, with the largest effect at 850°C. Maximum values of remanence and coercivity were observed for Hf-added samples S-HD at 850°C, and 900°C, respectively. The highest ( BH) max value was also observed in S-HD 900°C Hf-added samples. These results are discussed in terms of the expected microstructure of the intermediate HD and final HDDR processed powders.

Probing Archean lithosphere using the Lu–Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth’s mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites (<1100°C), which represent the shallow Somerset lithosphere, are characterized by higher 176Lu/ 177Hf ratios (0.03–0.05) and Hf isotopic values ( 176Hf/ 177Hf (0.1Ga)=0.28296–0.28419) than the deep-seated high-temperature peridotites (>1100°C; 0.004–0.03, 0.28265–0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu–Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ∼150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/ 177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the Somerset low-temperature peridotites indicate that part of the mantle root beneath the North American craton is characterized by a more radiogenic Hf isotope signature than that estimated for a typical ‘depleted’ mantle.

The Effects of Substitutional Additions on Creep Behavior of Tetragonal and Hexagonal Nb-Silicides

ABSTRACTNb-silicide based in-situ composites combine a ductile Nb-based solid solution with high-strength silicides, and they show great promise for aircraft engine applications. The Nb-silicide controls the high-temperature creep behavior of the composite. Previous work has shown that the silicide composition has an important effect on the creep rate, with particular attention on the role of Ti and Hf additions. The aim of the present study is to understand the effects of the substitutional elements on the stability of the silicide phase, ordering in the crystal lattice, including the hP16-tI32 transition, and the creep behavior of the monolithic phases. To pursue this goal monolithic alloys with a range of compositions were prepared and the creep rates were measured at temperatures of 1100–1350°C. The stress sensitivities of the creep rates of the monolithic phases were also determined.

Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Hafnium isotopic compositions have been determined on a suite of INTRODUCTION calc-alkaline and high-alumina-olivine tholeiitic lavas from the The geochemical and isotopic variability of mantle wedge Lassen region of California and are used, in conjunction with peridotites beneath continental volcanic arcs is poorly previously published mineralogical, geochemical, and isotopic data, constrained. This is a result of the fact that most of what to constrain their petrogenesis. Positive correlation between Hf is known about the geochemistry of the mantle wedge is values and geochemical indices of the modern subduction component inferred from the composition of basaltic arc lavas, which, indicates that the isotopic compositions of the calc-alkaline lavas although derived from the wedge, are affected by variable record addition of radiogenic Hf from the subducted slab. However, contributions from the subducting slab and continental the addition of the modern subduction component increases the Hf crust (e.g. Gill, 1981; Tatsumi et al., 1986; Hawkesworth values of most calc-alkaline lavas by <0·5 units over estimates of et al., 1993, 1994). Despite these limitations there is non-subduction enriched peridotites of the mantle wedge. The Lu–Hf significant evidence to suggest that mantle wedge periisotopic systematics of the Lassen lavas suggest that the calc-alkaline dotites are heterogeneous in composition and mineralogy. magmas have equilibrated with garnet at some point in their history, For example, it has been postulated that the compositions whereas the tholeiitic magmas have not. These observations require of mantle peridotites beneath the Cascade arc vary from the two lava types to be derived from different sources. The isotopic compositions that are similar to sources for mid-ocean variability of the Lassen lavas cannot be produced by mixing mantle ridge basalt (MORB), Hawaiian ocean-island basalt sources inferred to be present in the eastern–central Pacific and (OIB), and high-alumina-olivine tholeiites (HAOT) from western USA with a modern subduction component. Instead, the the Basin and Range province (Hughes, 1990; Leeman isotopic variability is consistent with mixing of a depleted mantle et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne source, a more fertile mantle source enriched by an ancient subduction & Borg, 1997). The restite mineralogy of the source component, and a modern subduction component. regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model

Effect of NiAl microcrystalline coating on the high-temperature oxidation behavior of NiAl-28Cr-5Mo-1Hf

The effect of an NiAl microcrystalline coating prepared by magnetron sputtering on the high-temperature oxidation behavior of NiAl-28Cr-5Mo-1Hf was investigated in static air at 1000-1150degreesC. The additions of Cr, Mo, and Hf changed the single beta-phase structure into a multiphase structure [beta-NiAl, alpha-Cr(Mo), and Heusler phase]. The NiAl-28Cr-5Mo-1Hf alloy formed a nonprotective mixed scale of Al2O3 + Cr2O3 + HfO2 and exhibited relatively large weight gains. The large weight gains were attributed to extensive internal oxidation. The sputtered NiAl microcrystalline coating remarkably improved the oxidation resistance of NiAl-28Cr-5Mo-1Hf due to the formation of a compact and adherent Al2O3 scale at all test temperatures. It was found that the theta- alpha-Al2O3 transformation caused the anomalous behavior of the oxidation-kinetics cures of the NiAl microcrystalline coating in the temperature range 1000-1150degreesC. A change in the morphology of scales occurred with the transformation.

Microstructure and Mechanical Properties of Sol-Gel Derived Mullite Containing Needle-Like Grains

Synthesis of mullite whiskers in F− atmosphere was carried out by means of vapour-phase reaction. A mullite precursor sol was prepared by mixing aluminium nitrate and ethyl silicate in propanol medium. The addition of HF triggers the gelation of mullite. The powders was uniaxialiy pressed into bars and was heat-treated at 1100°C in an airtight container. During this process, mullite whiskers were formed and they were used as growth centres for needle like grains. The sapmles when sintered at 1600°C for 4 h in air, showed needle morphology and the fracture toughness was 2.3 MPa.m½.

Site occupancy of ternary additions to B2 alloys

In this broad-based survey study, the substitutional site preference of ternary alloying additions to B2 compounds (stable at room temperature and 50/50 composition) is determined using the Bozzolo–Ferrante–Smith (BFS) method for alloys. The method is applied to Ni, Al, Ti, Cr, Cu, Co, Fe, Ta, Hf, Mo, Nb, W, V and Ru additions to NiAl, FeAl, CoAl, CoFe, CoHf, CoTi, FeTi, RuAl, RuSi, RuHf, RuTi, and RuZr. The results are compared, when available, to experimental data and other theoretical results.

Kinetics of Cerium(IV) Extraction from H 2SO 4–HF Medium with Cyanex 923

Studies of the extraction kinetics of cerium(IV) from H 2SO 4–HF solutions with Cyanex 923 in n-heptane have been carried out using a constant interfacial area cell with laminar flow. The experimental hydrodynamic conditions were chosen so that the contribution of diffusion to the measured rate of reaction was minimized. The data were analyzed in terms of pseudo-first order constants. The results were compared with those of the system without HF. It was concluded that the addition of HF reduces the activation energy for the forward rate from 46.2 to 36.5 kJ mol −1 while it has an opposite effect on the activation energy for the reverse process(the activation energy increased from 23.3 to 90.8 kJ mol −1). Thus, HF can accelerate the rate of cerium(IV) extraction. At the same time, the extraction rate is controlled by a mixed chemical reaction–diffusion rather than by a chemical reaction alone. A rate equation has also been obtained.

Optimization of toughness and strength in multiphase intermetallics

We have examined the effects of a fine dispersion of precipitates in the matrix phase of two multiphase NiAl-based alloys on strength and toughness. The first system is a directionally solidified Ni–30Fe–23Al alloy composed of a B2 matrix reinforced with a ductile fcc-based second phase. Spinodal decomposition leads to fine-scale bcc precipitates within the B2 phase, resulting in a 50% increase in room-temperature strength, but with reduced ductility and toughness compared to similar alloys without the strengthening precipitates. The increase in strength limits matrix plasticity prior to cleavage crack initiation, but some slip transfer still occurs from the fcc-based phase to the (B2+bcc) matrix. A mixed dendritic and lamellar microstructure also contributes to lower toughness. The second system is a directionally solidified NiAl–31Cr–3Mo eutectic composed of a B2-NiAl matrix reinforced with a Cr(Mo) phase. Small additions of Hf and Si to this material result in the precipitation of a fine cuboidal G-phase in the NiAl matrix. Reduced toughness in this modified alloy relative to unalloyed NiAl–Cr(Mo) is attributed to the lack of plasticity in the precipitate-strengthened matrix and partial loss of the aligned lamellar microstructure by Hf and Si alloying. Observations of the deformation and fracture mechanisms in these alloys are used as a basis to discuss microstructural design of multiphase intermetallics with optimized strength and toughness.