Effect of V-addition and Re-addition On the Creep-properties of Gamma/gamma'-alpha Eutectic Composites

Alloying a ternary monovariant Ni-Al-Mo eutectic alloy with vanadium and rhenium has been investigated as a means to increase the creep strength at elevated temperature. The creep properties of directionally solidified eutectic gamma/gamma'-alpha alloys with nominal composition Ni-31Mo-6A1, Ni-32Mo-6Al-1.4V and Ni-30Mo-6A1-1.6V-1.2Re (wt.%) have been determined using strain rate change test. The tests were conducted under vacuum at temperatures between 1223 and 1323 K. Comparison of the stress-strain rate behaviour to that for directionally solidified Ni-Al-Mo eutectic alloy has revealed that the additions of vanadium and rhenium increase the steady-state stress at given creep rate and temperature by 11 to 20%. This improvement is higher at higher temperatures and lower creep rates. The steady-state stress has been found to depend strongly on creep rate and temperature. The measured power law exponents varied between 6.7 and 7.9 over the studied temperature range. The activation energies for creep were calculated to range between 471 and 517 kJmol(-1). The ductility of these eutectic composites was high, typically ranging from 40 to 60% reduction of area and from 15 to 38% elongation.

Solvent-driven electron trapping and mass transport in reduced graphites to access perfect graphene

Herein, we report on a significant discovery, namely, the quantitative discharging of reduced graphite forms, such as graphite intercalation compounds, graphenide dispersions and graphenides deposited on surfaces with the simple solvent benzonitrile. Because of its comparatively low reduction potential, benzonitrile is reduced during this process to the radical anion, which exhibits a red colour and serves as a reporter molecule for the quantitative determination of negative charges on the carbon sheets. Moreover, this discovery reveals a very fundamental physical–chemical phenomenon, namely a quantitative solvent reduction induced and electrostatically driven mass transport of K+ ions from the graphite intercalation compounds into the liquid. The simple treatment of dispersed graphenides suspended on silica substrates with benzonitrile leads to the clean conversion to graphene. This unprecedented procedure represents a rather mild, scalable and inexpensive method for graphene production surpassing previous wet-chemical approaches

Stainless-steel Fiber-reinforced Aluminum-matrix Composites Processed By Squeeze Casting - Relationship Between Processing Conditions and Interfacial Microstructure

This work investigates the influence of some processing parameters on the extent of interfacial reaction in squeeze cast aluminium matrix composites reinforced with 12 mu m diameter, continuous stainless steel fibres. The average thickness of the reaction layer at fibre/matrix interfaces was measured by image analysis. When casting was made in a die at room temperature, the thickness of the reaction layer was affected on a distance of several mm from the lateral surface or from the bottom of the preform. The results indicate that the major part of the reaction occurs before solidification of the liquid metal. The control of the extent of interfacial reaction can be achieved through optimization of both infiltration parameters and features of the preform such as the volume fraction of the fibres

On the interaction mechanisms between sources of hydrogen and Al-Si coated or bare high strength steels during the hot stamping process

The economic and ecological challenges of the automotive industry coupled to safety considerations involve lightweight design based on the development of new ultra high strength materials. The hot stamping process of Al-Si coated steels ensures these strength levels and, moreover, allows an easy forming of the material without the necessity of a controlled atmosphere. At high temperatures, the coating is hydrogen-permeable, while at low temperatures (below 120°C), it becomes tight to hydrogen. In this context, the aim of this work is to understand how hydrogen enters in the material during the hot stamping process with respect to different atmospheric sources. An important aspect of the problem is the continuous evolution of the material during the hot stamping process. At ambient temperature, the coating is indeed a solid Al-Si alloy, which has weakly reacted with iron during deposition. As temperature increases, this coating first melts and then forms ternary intermetallic compounds Al-Fe-Si. These different subsystems of the material have been studied and compared to bare steel. The influence of the different hydrogen sources on bare and coated steels were also scrutinised: water vapour, which is probably the active source in the industrial process and dihydrogen, a more fundamental study case. Heavy water vapour (D2O) is also used to ensure a controlled atmosphere in the furnace, while it allows to avoid misinterpretation that could arise with the dissociation of water into hydrogen

Subcritical crack growth in freestanding silicon nitride and silicon dioxide thin films

Introduction/Purpose: Thin film materials are the main building blocks in many fields of application like flexible electronics, microelectromechanical /nanoelectromechanical systems (MEMS/NEMS) and functional coatings. However, the determination of the intrinsic fracture toughness of these materials, which is the key property controlling the failure resistance remains a challenge. Methods: In this work, a new on-chip testing method for freestanding films is developed. The technique consists of two long actuators beams pulling on a notched specimen by exploiting the residual stress inside the actuators. The residual stress upon release by chemical etching leads to the actuator contraction, hence pulling on the central notched specimen. A crack is initiated at the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value. This crack arrest measurement avoids the problem of introducing a sufficiently sharp precrack. Further subcritical crack growth can be investigated by this fracture on-chip testing technique through repeating crack length measurement over time, without monopolizing any test equipment. Results: The understanding of the mechanisms causing the subcritical crack growth is crucial to define the reliability of devices made with thin films that are showing time-dependent failure. Low-pressure chemical vapor deposition (LPCVD) silicon nitride (SiN) and silicon dioxide (SiO2) films deposited by electron beam-evaporation technique were studied with a variety of thicknesses. The specimens were tested in laboratory air and dry nitrogen environments under various temperature conditions. Conclusions: Based on both experimental data and finite element simulation results (FE), the stress intensity factor (K) and the crack velocity (v); K-v curve in different environments is determined, following classical exponential law

Temporal Variability of the Bioaerosol Background at a Subway Station: Concentration Level, Size Distribution, and Diversity of Airborne Bacteria

Naturally occurring bioaerosol environments may present a challenge to biological detection-identification-monitoring (BIODIM) systems aiming at rapid and reliable warning of bioterrorism incidents. One way to improve the operational performance of BIODIM systems is to increase our understanding of relevant bioaerosol backgrounds. Subway stations are enclosed public environments which may be regarded as potential bioterrorism targets. This study provides novel information concerning the temporal variability of the concentration level, size distribution, and diversity of airborne bacteria in a Norwegian subway station. Three different air samplers were used during a 72-h sampling campaign in February 2011. The results suggested that the airborne bacterial environment was stable between days and seasons, while the intraday variability was found to be substantial, although often following a consistent diurnal pattern. The bacterial levels ranged from not detected to 10(3) CFU m(−3) and generally showed increased levels during the daytime compared to the nighttime levels, as well as during rush hours compared to non-rush hours. The airborne bacterial levels showed rapid temporal variation (up to 270-fold) on some occasions, both consistent and inconsistent with the diurnal profile. Airborne bacterium-containing particles were distributed between different sizes for particles of >1.1 μm, although ∼50% were between 1.1 and 3.3 μm. Anthropogenic activities (mainly passengers) were demonstrated as major sources of airborne bacteria and predominantly contributed 1.1- to 3.3-μm bacterium-containing particles. Our findings contribute to the development of realistic testing and evaluation schemes for BIODIM equipment by providing information that may be used to simulate operational bioaerosol backgrounds during controlled aerosol chamber-based challenge tests with biological threat agents