Low Quench Rates Research Articles

Critical Analysis of an FeP Empirical Potential Employed to Study the Fracture of Metallic Glasses.

An empirical potential that has been widely used to perform molecular dynamics studies on the fracture behavior of FeP metallic glasses is shown to exhibit spinodal decomposition in the composition range commonly studied. The phosphorous segregation induces a transition from ductility to brittleness. During brittle fracture the atomically sharp crack tip propagates along a percolating path with higher P concentration. This embrittlement is observed to occur over a wide range of chemical compositions, and toughness decreases linearly with the degree of compositional segregation over the entire regime studied. Stable glass forming alloys that can be quenched at low quench rates do not, as a rule, exhibit such thermodynamically unstable behavior near to or above their glass transition temperatures. Hence, the microstructures exhibited in these simulations are unlikely to reflect the actual microstructures or fracture behaviors of the glassy alloys they seek to elucidate.

Influence of grain structure on quench sensitivity relative to localized corrosion of high strength aluminum alloy

The influence of grain structure on quench sensitivity relative to localized corrosion of high strength aluminum alloy 7055 was investigated by electrochemical test, accelerated exfoliation corrosion test, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The decrease of quench rate led to lower corrosion resistance of both the homogenized and solution heat treated (HS) alloy with equiaxed grains and the hot-rolled and solution heat treated (HRS) alloy with elongated grains, but there was a higher increment in corrosion depth and corrosion current density and a higher decrement in corrosion potential for the latter alloy, which therefore exhibited higher quench sensitivity. It is because in this alloy the larger amount of (sub) grain boundaries led to a higher increment in the amount of quench-induced η phase and precipitates free zone at (sub) grain boundaries with the decrease of quench rate, and there was a larger increment in the content of Zn, Mg and Cu in the η phase at grain boundaries due to slow quenching. The presence of subgrain boundaries in the HRS alloy tended to increase corrosion resistance at high quench rates higher than about 630 °C/min but decrease it at lower quench rates.

Raman scattering as a probe of intermediate phases in glassy networks

AbstractBulk glass formation occurs over a very small part of phase space, and ‘good’ glasses (which form even at low quench rates ∼10 K/sec) select an even smaller part of that accessible phase space. An axiomatic theory provides the physical basis of glass formation, and identifies these sweet spots of glass formation with existence of rigid but stress‐free networks for which experimental evidence is rapidly emerging. Recently, theory and experiment have come together to show that these sweet spots of glass formation occur over a range of chemical compositions identified as ‘intermediate phases’ (IPs). These ranges appear to be controlled by elements of local and medium‐range molecular structures that form isostatically rigid networks. IP glasses possess nonhysteretic glass transitions (Tg's) that do not age much. Raman scattering has played a pivotal role in elucidating the molecular structure of glasses in general, and in identifying domains of IPs. Experiments reveal that these phases possess sharp phase boundaries and are characterized by an optical elasticity that varies with network mean coordination number, r, as power law. In this review, we provide examples in chalcogenide and oxide glass systems in which these phases along with optical elasticity power laws have been established. IP glasses represent self‐organized nanostructured functional materials optimized by nature. Copyright © 2007 John Wiley & Sons, Ltd.

Quench modelling of Al-7Si-Mg casting alloys

Quench factor analysis was applied to A356 and A357 aluminium casting alloys in order to model time-temperature-yield strength curves, to predict T6 yield strengths as a function of continuous cooling quench rate and to examine quench sensitivity. The predicted T6 yield strengths for given quench curves were typically within ± 10 MPa of experimentally measured yield strengths. It was found that A356 is more quench sensitive than A357 at high quench rates, whereas A357 is more sensitive at low quench rates. This is explained in terms of silicon clustering, which alters the matrix Mg/Si ratio and subsequent precipitation events. These observations were confirmed by differential scanning calorimetry.

Generation and termination of runaway electrons at major disruptions in JT-60U

The operation conditions to avoid runaway electron generation at the major disruption have been investigated in JT-60U tokamak plasmas. It has been found that runaway electrons are not observed for low Bt of ⩽ 2.2 T or low plasma current quench rates (Iγ ≡ -(dIp/dt)/Ip) of <50 s-1. Furthermore, they are not observed for low effective safety factors defined at the plasma edge (qeff) of ⩽ 2.5 even for high Iγ of 300-400 s-1, which is the case for uncontrolled disruptions accompanied by large plasma displacements (e.g., vertical displacement events (VDEs)). On the other hand, in controlled disruptions with small plasma shifts, qeff easily increases above 8, and runaway electrons are observed even for low current quench rates of 50-100 s-1. Furthermore, it has been found that in these position controlled disruptions the runaway current tail can rapidly decay even for zero or weakly positive plasma surface voltages. These observations of the avoidance and termination of runaway electrons suggest an anomalous loss mechanism for runaway electrons.

Magnetic properties of melt-spun ribbons of DyFe/sub 2/-B alloys

Magnetic properties of rapidly solidified ribbons of the alloys (Dy/sub 0.33/Fe/sub 0.67/)/sub 1-x/B/sub x/ with x=0, 0.05, 0.1 and 0.15 were investigated as a function of the quench rate during melt-spinning. At low quench rates where the crystalline microstructure has a single phase, room temperature magnetization at 15 kOe (M/sub 15/) is approximately 60 emu/g and is independent of the B content. The coercive force (H/sub c/), however, increases greatly with the addition of B by a factor of two. This may be due to the increase in magnetocrystalline anisotropy which is generated by a distortion of the highly symmetrical cubic lattice by the introduction of B. At high quench rates where the microstructure mainly consists of an amorphous phase, the values of M/sub 15/ and H/sub c/ are reduced greatly by amorphization and they decrease monotonically with increasing B content.

A generalized solidification model and microstructural verification for the Nd–Fe–B–Ti–C system processed by rapid solidification

For the Nd2Fe14B (2-14-1) compound, the optimal grain size should be smaller than the size of the single domain size of 150 nm. Transition metal carbides (TMC) also reduce the quench rate necessary to achieve the optimal or overquenched condition. This allows inert gas atomization (IGA) to produce viable magnetic materials. In this article we will demonstrate that optimal microstructure for the 2-14-1 can be produced by IGA with the addition of TiC. Moreover, a solidification model will be presented to show (1) how recalescence is a critical feature to the evolution of the microstructure in rapidly solidified materials and (2) the role TMC and other solute phases have on inhibiting grain growth so that lower quench rates can be employed.

Coordination chemistry of Ti(IV) in silicate glasses and melts: III. Glasses and melts from ambient to high temperatures

The local structural environment of Ti in five Na-, K-, and Ca-titanosilicate glass/melts with TiO 2 concentrations ranging from 2.7–30.5 wt% has been determined by in situ Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at temperatures ranging from 293–1650 K. In parallel, two Ti-model compounds (Ni 2.6Ti 0.7O 4 spinel and TiO 2 rutile) were studied under the same conditions to better understand the effects of temperature (anharmonicity) on the XAFS spectra. Temperature-induced anharmonicity was found to vary, largely as a function of the Ti-coordination, and increases significantly around Ti with increasing temperature when present as [6]Ti. In contrast, anharmonicity appears negligible around [4]Ti at temperatures below 1200 K. We predict that anharmonicity should be weak around [5]Ti as well. No clear evidence was found for a significant change in the average nearest-neighbor coordination environment of Ti in the Na- and K-titanosilicate glasses and melts that exhibit anomalous heat capacities variations just above their glass transition temperatures, T g (860–930 K). The small (predicted and measured) linear thermal expansion of the ( [5]TiO 2+) O bond in these systems at high temperature is expected to have an insignificant effect on the local environment of [5]Ti during the glass-to-supercooled liquid transition. In the most dilute Ti-glass studied (KS1; 2.7 wt% TiO 2), the local environment around [4]Ti (especially the second-neighbor alkalis) is relatively ordered at ambient temperature, but this order decreases dramatically above T g. Lower quench rates appear to favor [4]Ti over [5]Ti. The origin of the observed anomalous positive variations in heat capacities of these melts may be related to significant changes in the medium-range environment around Ti above T g including the disappearance of percolation domains involving interfaces between alkali-rich and network-former rich regions during structural relaxation at T g; these percolation domains are related to the dual structural role of Ti in silicate glass/melts (acting simultaneously as network former and network modifier).

Thin film morphology and crystal structure of the poly( p-phenylene terephthalamide)/sulphuric acid system

The morphology and crystal structure of poly( p-phenylene terephthalamide) (PPTA) thin films shear-cast from concentrated sulphuric acid solutions have been investigated, with emphasis on the effects of molecular weight, quench rate and coagulant. The films exhibit the banded optical texture typical of the relaxation patterns of liquid crystal polymers, except in the case of very thin films. On a finer scale, row-nucleated structures are observed for low quench rates and fibrillar structures for higher rates or direct coagulation experiments. The periodicity of the row-nucleated structure is related to the polymer chain length. In the case of delayed coagulation, a crystallosolvate structure is obtained which, upon subsequent crystallization, collapses into either of the two PPTA polymorphs depending on the extraction conditions. A polymorphic transformation scheme is proposed accordingly. The results are discussed in relation to fibre properties.

Vacancy concentrations in quenched binary alloys

The effect of solute atoms on quenched-in vacancy concentrations has been studied theoretically using a simple model for vacancy annihilation during quenching. The model has been developed in generalized terms, making the results applicable to a wide variety of experimental conditions. For high quench rates, the quenched-in vacancy concentration is limited by the equilibrium concentration of vacancies at the quench temperature and solute effects are small. For low quench rates, the quenched-in vacancy concentration is determined by vacancy losses during quenching and solute effects can be enormous. Quantitative definition of high and low quench rates depends on vacancy properties, quench temperature, solute concentration and other parameters. The calculated results are in excellent accord with experimental results for pure Au and pure Pt.

The effect of thermal history on the crystallization kinetics of a liquid-quenched metallic glass

The crystallization behaviour of a liquid quenched (Ni0.5Pd0.5)82P18 amorphous alloy is not markedly affected by the imposed heating conditions. This compares with the effect of the initial quench rate such that lower quench rates stabilize the glass by raising the crystallization temperature observed on continuous heating. This stabilization is not accompanied by gross changes in the morphologies of the crystallization products.

Quench rate effects in Al-Zn-Mg-Cu alloys

The effect of quench rate on alloys with a bcomposition of 6.75 pct Zn, 2.5 pct Mg, 1.2 pct Cu and various levels and combinations of the minor addition elements (MAE’s) Cr, Mn, and Zr is reported. The level of chromium and to a lesser extent the level of zirconium determine quench sensitivity. Slow quenching of chromium- or zirconium-bearing alloys led to the nucleation of (MgZn) plates or laths at MAE-bearing particles which had certain orientation relationships with the matrix. The density of the hardening precipitation (100 to 200A in diam) was found to be high in slowly quenched, sensitive alloys. Even when the yield strength had fallen to 50 pct of that of the fast-quenched material, a large fraction of the fine precipitate was still formed. A fracture mechanism is proposed to account for the magnitude of quench sensitivity in such materials. An embrittlement, at low quench rates, was found in alloys containing manganese or very low levels of MAE. This embrittlement was thought to be due to precipitation in the grain boundaries, and not to the width of the precipitate-free zone.