D. Eliezer

Magnesium Science, Technology and Applications

The science, technology and applications of magnesium alloys are reviewed. The very low density of magnesium in combination with excellent castability is leading to increased use, despite poor galvanic corrosion resistance and a higher cost than aluminum, especially in automotive applications. Even further expansion of the magnesium market should come from an expanded design base, a better understanding of the scientific “underpinning” of magnesium alloys, and development of cost-affordable cast and wrought products.

The influence of austenite stability on the hydrogen embrittlement and stress- corrosion cracking of stainless steel

A study has been made of the HE and SCC of a type 304 and a type 310 austenitic stainless steel, and the results correlated with the presence or absence of α′ martensite, determined by means of a ferrite detector. Hydrogen induced slow crack growth (SCG) was observed at room temperature when type 304 was stressed i) in 1 psig (∼105 N/m2) gaseous hydrogen, ii) after high temperature charging, and iii) while undergoing cathodic charging. The fracture surfaces corresponding to SCG were primarily transgranular and cleavage-like, and were found to be associated with α′. Conditions i) to iii) did not produce SCG in the type 310 steel, in which α′ martensite was not detected, nor did SCG occur when type 304 was stressed in gaseous hydrogen above the MD temperature (∼110°C). These observations indicated that the formation of the martensitic phase was a prerequisite for SCG under these test conditions. Stressing of type 310 while it was undergoing cathodic charging at room temperature was found to produce shallow, nonpropagating cracks, confirming earlier reports that austenite can be embrittled by hydrogen in the absence of α′. SCC occurred in both alloys in boiling aqueous MgCl2 (154°C) with no evidence for α′ formation. The results are discussed in terms of the mechanisms of HE and SCC.

Characteristics of hydrogen embrittlement, stress corrosion cracking and tempered martensite embrittlement in high-strength steels

Abstract Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diffraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel.

Positive effects of hydrogen in metals

Hydrogen is often present in metals as a result of production, fabrication and processing operations or service conditions. Thus, it can be regarded as an alloying element. Although, high hydrogen levels in metals can have a devastating effect on the mechanical properties, many positive effects can also be derived from its high solubility. The objective of this paper is to review some positive effects of hydrogen in metals. An emphasis will be made on enhancements in the processing properties due to hydrogen (thermohydrogen processing (THP)), though other uses of hydrogen, such as an energy storage device and in the electronics industry will also be presented.

An increase of the spall strength in aluminum, copper, and Metglas at strain rates larger than 107 s−1

Measurements of the dynamic spall strength in aluminum, copper, and Metglas shocked by a high-power laser to hundreds of kilobars pressure are reported. The strain rates in these experiments are of the order of 107 s−1, which cannot be reached in impact experiments. The free-surface velocity behavior associated with spallation is characterized by oscillations caused by the reverberations of the spall layer. An optically recording velocity interferometer system was developed to measure the free-surface velocity time history. This diagnostic method has the advantages of being a noninterfering system and produces a highly accurate continuous measurement in time. The spall strength was calculated from the free-surface velocity as a function of the strain rate. The results show a rapid increase in the spall strength, suggesting that a critical phenomenon occurs at strain rates ∼107 s−1, expressed by the sudden approach to the theoretical value of the spall strength.

Hydrogen-assisted processing of materials

Abstract Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.

An Overview of Hydrogen Interaction with Amorphous Alloys

Theories, experimental results and applications associated with hydrogen behavior in amorphous metals and alloys are reviewed. An emphasis is made on the potential use of these advanced materials for hydrogen storage technology. Therefore, several properties that are especially relevant for such applications are assessed. These include structural models for hydrogen occupancy, sorption characteristics, solubility, diffusion behavior and thermal stabilities. Hydrogen effects on the mechanical properties and fracture modes of glassy metals are also presented, and possible mechanisms of hydrogen embrittlement are discussed. Similarities and differences between hydrogen behavior in amorphous and crystalline metals and alloys are discussed in detail.

Experimental measurements of the strength of metals approaching the theoretical limit predicted by the equation of state

The approach to the ultimate strength of metals is determined experimentally. The ultimate strength of metals was calculated using a realistic wide-range equation of state. The strength of metals was measured using shock waves created in aluminum and copper foils with a short- (20–100 ps) pulse high-power laser. The strength of the materials was determined from the free-surface-velocity time history, which was measured with an optically recording velocity interferometer system. The strain rates in these experiments were in the range (1.5–5)×108 s−1.

Hydrogen effects on phase transformations in austenitic stainless steels

The effects of hydrogen and stress (strain) on the phase transitions of a variety of stainless steels (316, 321, 347) were investigated. Hydrogen was introduced by severe cathodic charging at room temperature. X-ray diffraction was employed to reveal the transformations occurring in thin surface layers. After charging expanded ∈ phase is always present,α′ martensite content increases during ageing and the final content depends on the stability of the austenite. The broadening of diffraction peaks of austenite after cathodic charging is caused by nonuniform distribution of hydrogen. The state of hydrogen distribution in the steel and the relationship between internal stresses, surface cracking and phase transition is discussed.

Environmental Behavior of Magnesium and Magnesium Alloysd

Because of their low density, magnesium (Mg) alloys exhibit higher specific strength compared to other metals and alloys and are of growing interest as struc tural materials in the a uto mo tive, aerospace and electronic industries. Magnesium, being one third lighter than an equal volume of Al, offers great possibilities to reduce the vehicle weight, which is essential to save energy. Further applications include e.g. computer and cellular telephone hou ings. Mg-housings are an attractive alternative to polymers with significant advantages in te rms o f e lectromagnetic shielding and recycling. However, Mg is very reactive to the environment, above all in the presence of water and oxygen. Corrosion and oxidation are major shortcomings of Mg parts and these are still poorly understood. Hence, a widespread industrial usage of Mg alloys will ultimately depend on their capability to maintain the o riginal performance during environmental exposure over extended periods of se rvice. Magnesium alloys for structural applications are processed by casting (die, sand and mold) or used as wrought products (extrusions, forgings, sheet and plate). In addition to the high specific strength, Mg alloys exhibit excellent die castability, superior machinability, good ductility and damping capacity. The most common recent die casting Mg alloy is the AZ91 D (high purity alloy containing typically 8.3-9. 7% Al + 0.35 1 % Zn + >0.13% Mn [ l]; unless otherwise indicated in the text additions to Mg are given in mass percentage). This alloy combine the above properties with go d corrosion resistance, comparable to that of Al casting a lloys. For applications requiring improved ductility and fracture toughness a series of die ca ting Mg alloys with reduced content of Al wa developed. AM50 and AM60 alloys (4.4-5.4 and 5 . .5-6.5% Al, respect ively, <0.22% Zn and >0.25% Mn) have found