Characteristic Change In Slope Research Articles

Basic modelling of creep rupture in austenitic stainless steels

Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves.

Molecular dynamics simulations to calculate glass transition temperature and elastic constants of novel polyethers

Molecular dynamics simulations studies are carried out on hydroxyl terminated polyethers that are useful in energetic polymeric binder applications. Energetic polymers derived from oxetanes with heterocyclic side chains with different energetic substituents are designed and simulated under the ensembles of constant particle number, pressure, temperature (NPT) and constant particle number, volume, temperature (NVT). Specific volume of different amorphous polymeric models is predicted using NPT-MD simulations as a function of temperature. Plots of specific volume versus temperature exhibited a characteristic change in slope when amorphous systems change from glassy to rubbery state. Several material properties such as Young's, shear, and bulk modulus, Poisson's ratio, etc. are predicted from equilibrated structures and established the structure–property relations among designed polymers. Energetic performance parameters of these polymers are calculated and results reveal that the performance of the designed polymers is comparable to the benchmark energetic polymers like polyNIMMO, polyAMMO and polyBAMO. Overall, it is worthy remark that this molecular simulations study on novel energetic polyethers provides a good guidance on mastering the design principles and allows us to design novel polymers of tailored properties.

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.

The glass transition temperatures of amorphous trehalose–water mixtures and the mobility of water: an experimental and in silico study

Isothermal–isobaric molecular dynamics simulations are used to calculate the specific volume of models of trehalose and three amorphous trehalose–water mixtures (2.9%, 4.5% and 5.3% (w/w) water, respectively) as a function of temperature. Plots of specific volume versus temperature exhibit a characteristic change in slope when the amorphous systems change from the glassy to the rubbery state and the intersection of the two regression lines provides an estimate of the glass transition temperature T g. A comparison of the calculated and experimental T g values, as obtained from differential scanning calorimetry, shows that despite the predicted values being systematically higher (about 21–26 K), the trend and the incremental differences between the T g values have been computed correctly: T g 5.3 % ( w/w ) < T g 4.5 % ( w/w ) < T g 2.9 % ( w/w ) < T g 0.0 % ( w/w ) . The mobility of water has been investigated over temperature ranges covering the rubbery and the glassy phases of the trehalose–water mixtures by calculating the diffusion coefficients of water. The temperature dependence of the diffusion coefficient changes in the region of the glass transition and can be used as well to estimate T g values. The activation energies for water diffusion were found to be independent of the amount of water in amorphous trehalose.

Glass Transition Temperature of Glucose, Sucrose, and Trehalose: An Experimental and in Silico Study

Isothermal-isobaric molecular dynamics simulations are used to calculate the specific volume of models of different amorphous carbohydrates (glucose, sucrose, and trehalose) as a function of temperature. Plots of specific volume vs temperature exhibit a characteristic change in slope when the amorphous systems change from the glassy to the rubbery state. The intersection of the regression lines of data below (glassy state) and above (rubbery state) the change in slope provides the glass transition temperature (T(g)). These predicted glass transition temperatures are compared to experimental T(g) values as obtained from differential scanning calorimetry measurements. As expected, the predicted values are systematically higher than the experimental ones (about 12-34 K) as the cooling rates of the modeling methods are about a factor of 10(12) faster. Nevertheless, the calculated trend of T(g) values agrees exactly with the experimental trend: T(g)(glucose) < T(g)(sucrose) < T(g)(trehalose). Furthermore, the relative differences between the glass transition temperatures were also computed precisely, implying that atomistic molecular dynamics simulations can reproduce trends of T(g) values in amorphous carbohydrates with high quality.

Mn 5Si 3-type host-interstitial boron rare-earth metal silicide compounds RE5Si 3: Crystal structures, physical properties and theoretical considerations

A series of binary rare-earth metal silicides RE 5Si 3 and ternary boron-interstitial phases RE 5Si 3B x ( RE=Gd, Dy, Ho, Lu, and Y) adopting the Mn 5Si 3-type structure, have been prepared from the elemental components by arc melting. Boron “stuffed” phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6 3/ mcm, Z=2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6–1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and θ p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er 5Si 3B x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl–Klemm concept would predict a limiting composition RE 5Si 3B 0.6 for a valence compound and should then preclude the stoichiometric formula RE 5Si 3B. Density functional theory calculations carried out on some RE 5Si 3Z x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.

Structural chemistry, magnetism and electrical properties of binary Gd silicides and Ho 3Si 4

Binary gadolinium silicides were thoroughly investigated with respect to their structural chemistry and physical properties as well. Formation and structure types of all known binary phases were confirmed. In addition, a new binary compound Gd 3Si 4 belonging to the Ho 3Si 4 type was obtained through a sluggish peritectoidal reaction between the binary phases GdSi and GdSi 2− x . In addition, the limits of the homogeneity range of the GdSi 2− x phase (AlB 2 type) were also confirmed. Moreover, the crystal structures of Gd 5Si 3 (Mn 5Si 3 type) and GdSi (FeB type) were determined from X-ray single crystal data. The magnetic and electrical properties for most of the binary Gd–Si phases, encountered within this system, have been studied. The magnetic properties are characterised by the onset of magnetic ordering below 100 K. Gd 5Si 3 shows an antiferromagnetic transition at T N = 75 K, GdSi orders weakly ferromagnetically at T C = 78 K followed by an antiferromagnetic spin alignment at T N = 54 K, GdSi 2− x reveals also two consecutive transitions at T N = 54 K and T C = 35 K and GdSi 2 exhibits the same behaviour but at T N = 26 K and T C = 14 K. In addition, magnetic measurements were also performed for the binary silicide Ho 3Si 4 (antiferromagnetic behaviour, T N = 16 K), structurally related to Gd 3Si 4. The electrical resistivities for all silicides resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states.

Crystal structure and physical properties of ternary compounds RPt 3B, R=La, Pr, Nd

The crystal structure of a new series of ternary rare-earth platinum borides RPt 3B (R=La, Pr, Nd) has been studied by X-ray powder diffraction analyses from the ‘as-cast’ alloys. The tetragonal CePt 3B structure type, space group P4mm (No. 99), has been confirmed for all compounds. Rietveld refinements for the two compounds, PrPt 3B and NdPt 3B, were performed. LaPt 3B is a temperature-independent Pauli-type paramagnet from room temperature down to 4 K. PrPt 3B orders antiferromagnetically at T N=15 K followed by a ferromagnetic spin flip at T C=5 K, whereas NdPt 3B exhibits an antiferromagnetic spin alignment at a Néel temperature T N=7 K. The temperature dependence of the electrical resistivity, ρ( T), reflects the metallic character of these compounds. Furthermore the characteristic changes of slope of ρ( T) plots prove the magnetic transitions.

Physical properties of ternary compounds R 2Ni 2Pb (R=Y, Sm, Gd, Tb, Dy, Ho, Er and Tm)

We have synthesized samples of R 2Ni 2Pb (R=Y, Sm–Tm) ternary intermetallic compounds and measured the bulk magnetism and electric transport properties. These plumbides crystallize with Mn 2AlB 2 structure type (space group Cmmm, no. 65, Pearson code oC10). Y 2Ni 2Pb is a Pauli-type paramagnet, the temperature dependence of the electrical resistivity, ρ( T), resembles the shape of a pure nonmagnetic metal. The remaining alloys undergo ferromagnetic transitions at temperatures below 50 K. The ρ( T) plots decrease linearly upon lowering the temperature and reveal characteristic changes of slope at the ordering temperatures.

Structural chemistry, magnetism and electrical properties of binary Pr-silicides

Binary praseodymium silicides have been thoroughly investigated with respect to their crystal chemistry and physical properties. Formation and structure types of all known binary phases have been confirmed. Moreover, the crystal structure determination of a phase, previously reported to exist with the approximate composition Pr 3Si 4, was successfully performed from X-ray powder data on the basis of the isotypic orthorhombic Ho 3Si 4 type of structure. The magnetic properties of these Pr-silicides are characterized by the onset of ferromagnetic order below 110 K. Pr 5Si 3 ( T C=44 K) shows a canted spin structure. Pr 5Si 4 ( T C=40 K) and PrSi ( T C=51 K) reveal a weak aniosotropy. Whereas Pr 3Si 4 ( T C=105 K) exhibits a magnetic behavior as typically observed in narrow domain wall ferromagnets favoring easy axis anisotropy, the reduced saturation moment suggests a ferrimagnetic spin structure. PrSi 2− x ( x=0.12, GdSi 2 type) exhibits an antiferromagnetic transition at a Neél temperature T N=11 K followed by a spin reorientation towards a parallel spin alignment at 9 K. The electrical resistivities resemble a temperature dependence of metals, with characteristic changes of slope of the ρ( T) curves at the Curie temperatures. The ρ( T) plots follow ∼ T 2 relations as predicted for isotropic ferromagnets in the ordered state for the compound PrSi 1.33, only, whilst for the other compounds a ∼ T 2.8–3.7 dependency was observed.

Structural chemistry, magnetism and electrical properties of binary Nd silicides

Binary neodymium silicides were thoroughly investigated with respect to their crystal chemistry and physical properties. Formation and structure types of all known binary phases were confirmed. Moreover, the crystal structure determination of a phase previously reported to exist with approximate composition Nd 3Si 4 was successfully performed from X-ray powder data on the basis of the isotypic orthorhombic Ho 3Si 4-type structure. The magnetic properties of these Nd silicides are characterized by the onset of ferromagnetic order below 120 K. Nd 5Si 3 shows a canted spin structure, which might eventually also be the case for NdSi ( T C=45 K). Nd 5Si 4 ( T C=67 K) and NdSi 1.4 ( T C=75 K) exhibit magnetic behavior as typically observed in narrow domain wall ferromagnets favoring easy axis anisotropy. NdSi 2− x (I) ( T C=9 K) seems to adopt a more complex non-linear spin arrangement. NdSi 2− x (II) exhibits an antiferromagnetic transition at a Neél temperature of T N=10 K followed by a spin reorientation towards a parallel spin alignment at 9 K. The electrical resistivities resemble the temperature dependence of metals, with characteristic changes of slope of the ρ( T) curves at the Curie temperatures. The ρ( T) plots follow T 2 relations as predicted for isotropic ferromagnets, with the exception of NdSi 2− x (I) ( T 3.6, as usually observed for antiferromagnets).

Adapting the Self-Cycling Fermentor to Anoxic Conditions

The self-cycling fermentation (SCF) technique for cell cultivation is a useful tool for studying microorganisms. To date, this method has only been applied to aerobic systems, since dissolved oxygen has been used exclusively as the feedback control parameter, which defines this technique. To extend the range of redox environments to which the SCF can be applied, an alternate measurement is required. Transient oxidation reduction potential (ORP) and carbon dioxide evolution were screened as potential feedback control parameters in a denitrifying system. The characteristic change in slope of the transient ORP profile commonly referred to as the “nitrate break point” was found to be a useful indicator of complete removal of oxidized nitrogen only in the presence of significant quantities of nitrate. An inflection point occurring after the break point was found to be a more general indicator of oxidized nitrogen removal. A control strategy based upon real-time detection of the inflection point was found to re...