Prediction of thermal cross-slip stress in magnesium alloys from a geometric interaction model

We develop a geometry-based model from first-principles data for the interaction of solutes with a prismatic screw dislocation core, and predict the thermally activated cross-slip stress above room temperature in Mg alloys. Electronic structure methods provide data for the change in prismatic stacking fault energy for different possible fault configurations for 29 different solutes. The direct solute-dislocation interaction energies for solutes that produce stable prismatic screw dislocation cores (K, Na, Sc and Ca) is correlated with stacking fault misfits. This geometric interaction model produces similar prediction errors for all 29 solutes. The model predicts alloys with cross-slip stresses lower than pure Mg for three previously considered solutes (K, Na and Sc) and three new solutes (Ca, Y and Zr). The model also qualitatively confirms the experimental observation that Mg-Li alloys have lower cross-slip stress than pure Mg. In particular, low concentrations of Y are predicted to significantly decrease the cross-slip stress in Mg.     

Addressing Sulfate-Induced Heave in Lime Treated Soils

Civil engineers are at times required to stabilize sulfate-bearing clay soils with calcium-based stabilizers. Deleterious heaving in these stabilized soils may result over time. This paper addresses critical questions regarding the consequences of treating sulfate laden soils with calcium-based stabilizers. The authors describe the nature (chemistry and structure) of the minerals (ettringite/thaumasite) blamed for deleterious reactions and explain why these structures may lead to damage. The writers also describe the mechanisms of the mineral growth, and the extent of mineral growth based on the amount of sulfate minerals present in the soil. The writers explain why the rate of ettringite growth in treated soils should not be expected to follow a controlled rate of ettringite development such as that which normally occurs in portland cement concrete. The writers compare the rate and degree of ettringite development in soils to the classical model of nucleation and growth typical of most crystal structures. Finally, the writers evaluate the role of soil mineralogy in controlling soil behavior at varying sulfate contents and verify the existence of a threshold level of soluble sulfates in soils that can trigger substantial ettringite growth.     

Maximizing Parallel Testing in an FM Receiver

High volume production environments create great challenges for production testing and verification of Radio Frequency (RF) devices. In this environment, much emphasis is put on parallel testing, or the ability to test multiple devices at the same time using a single tester. In order for this parallelism to become a reality, there is a need for production RF tests to be simplified and reduced to requiring only the simplest test stimulus and analysis. In this paper, we present a new method of measuring the performance of a Frequency Modulation (FM) receiver that requires only a continuous wave signal input in order to eliminate the more costly Signal-to-Noise Ratio test. Using this new technique we will then demonstrate how simplifying this test and adding frequency diversity enables testing of up to 8 devices in parallel using a single tester.     

Effect of Pre-coated Vs. Added Coupling Agents on the Vulcanization of Silica-filled Polyisoprene

Solid-state ¹³C NMR was utilized in analysis of the crosslink structure in silica-filled polyisoprene. Samples containing either a pre-coated coupling agent, a coupling agent added in the mixing process, or polyethylene glycol (PEG) were vulcanized and the type and density of crosslinking determined. The precoated coupling agent sample gave lower %swelling, lower extraction amounts, and a lower T ₂ relaxation than the mixed coupling agent, indicating increased crosslinking or increased filler –rubber interaction. The precoated also showed less cis –trans isomerization than the mixed, which may be due to its more uniform coating effect on the silica. The PEG sample yielded a ¹³C spectrum similar to an unfilled sample indicating decreased silica adsorption of vulcanization agents. Neither coupling agent significantly decreased the total sulfurization nor the percent monosulfidic crosslinks from that of a silica-filled sample without coupling agent.     

Impurities block the alpha to omega martensitic transformation in titanium

Impurities control phase stability and phase transformations in natural and man-made materials, from shape-memory alloys to steel to planetary cores. Experiments and empirical databases are still central to tuning the impurity effects. What is missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound. Pure titanium transforms from ductile alpha to brittle omega at 9 GPa, creating serious technological problems for beta-stabilized titanium alloys. Impurities in the titanium alloys A-70 and Ti-6Al-4V (wt%) suppress the transformation up to at least 35 GPa, increasing their technological utility as lightweight materials in aerospace applications. These and other empirical discoveries in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: the effect on the relative phase stability, and the energy barrier of the transformation. Ab initio methods calculate both changes due to impurities. We show that interstitial oxygen, nitrogen and carbon retard the transformation whereas substitutional aluminium and vanadium influence the transformation by changing the d-electron concentration. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti-6Al-4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations.     

Frequency-modulation mode-locking performance for four Nd(3+)-doped laser crystals

To determine an optimum host for simultaneous short-pulse-width and large-slope-efficiency generation, the performance of Nd:YAG, Nd:YLF, Nd:YVO(4), and Nd(3+):Sr(5)(VO(4))(3)F [Nd:strontium fluorovanadate (S-VAP)] was characterized under a variety of end-pumped and frequency-modulation mode-locking conditions. The slope efficiency, threshold pump power, and pulse width were recorded for each laser and compared with theory. A figure of merit was defined, yielding Nd:YLF and Nd:YVO(4) as the experimentally determined crystals of choice.     

Non-negative integer basis algorithms for linear equations with integer coefficients

A new class of non-negative integer basis algorithms for linear equations with integer coefficients is developed. Computer experiments with one of the new algorithms and comparisons with other non-negative integer basis algorithms are reported. When the total run times for collections of common simple examples typical of automated deduction applications are computed, the new algorithm has been found to be significantly faster than previous algorithms.     

Fracture behavior of as-cast pearlitic nodular iron

The fatigue crack propagation and fracture toughness behavior of an ASTM A536, as-cast, pearlitic nodular iron with a bull's-eye structure was studied. The material had an ultimate strength of 76 ksi, a yield strength of 59 ksi, and an elongation of 1.6%. Fracture toughness tests were conducted on compact tension specimens with thicknesses of 0.30, 0.50, 0.70, and 1.00 inches. The conditional fracture toughness, K_Q, was found to be insensitive to the specimen thickness in the above range and to have a value of approximately 30 ksi-in^1/2. The dependence of the fatigue crack growth rate, da/dN, on the stress intensity factor, δK, was determined for stress ratios of 0.1, 0.3, 0.5, and 0.7. At the same °K level, the da/dN rates were higher for the higher stress ratios. The parameters, C and n, of the Paris equation, da/dN=C(δK)ⁿ were determined for each stress ratio. Near-threshold tests were also conduced for the stress ratio of 0.1. The threshold stress intensity factor, δK_TH, was found to be in the range of 6.3 to 7.7 ksi-in^1/2. A statistical model was used to calculate the agreement of the results of two duplicate fatigue crack growth tests.     

Cross-situational consistency in causal attributions: Does attributional style exist?

Two studies evaluated the concept of an attributional style, as operationalized by the Attributional Style Questionnaire (ASQ). Study 1, with 1,333 undergraduates, examined the reliability and validity of the ASQ and analyzed the factor structure of the measure. Only weak evidence of a cross-situationally consistent attributional style was found. An attempt to identify Ss who tended to be very consistent in their causal attributions on the ASQ similarly did not provide strong support for the attibutional style concept. In Study 2, the relation between scores on the ASQ and causal attributions for actual negative events, as assessed by the Social Readjustment Rating Scale, were examined among 85 pregnant women. Attributional Style scores were poor predictors of actual causal attributions, and selecting highly consistent Ss did not improve the ASQ's predictive validity. Implications for the attributional style concept and an attributional analysis of depression are discussed. (46 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Three-dimensional reconstruction of a plant dictyosome from series of ultrathin sections using computer image processing

A single dictyosome from an actively secreting ovary gland cell of Aptenia cordifolia has been reconstructed in 3-D from a series of twenty-nine electron micrographs by computer image processing. The reconstruction is presented under different viewing angles in the form of shaded perspective displays. From these displays the entire dictyosome, surrounded by numerous vesicles, appears to be more a spherical than a flat body. The plate-like region of the dictyósome is demonstrated when only a portion of the electron micrographs is used for the image processing, leading to ‘cut-off’ displays. Since some upper planes were removed, such ‘cut-off’ displays revealed both tubular connections between cisternae of the dictyosome and the neighbouring endoplasmic reticulum as well as tubular continuities between adjacent Golgi cisternae within the same stack. Possible consequences of both types of interconnections on transport and processing of proteins and glycoproteins are discussed.