Role of vacancies in coprecipitation of δ′ – and S–phasesin Al–Li–Cu–Mg alloys

Abstract

Precipitation reactions in a series of Al–Li–Cu–Mg alloys, encompassing the composition ranges of the commercial Lital A and Lital B materials, have been studied to determine the role of solute and vacancy concentrations on homogeneous S-phase (Al₂CuMg) precipitation. Homogeneous precipitation of S-phase only occurs when a critical combination of free vacancy and copper and magnesium supersaturation is achieved. Lithium reduces the free-vacancy concentration by strong binding between vacancies and lithium atoms or atom clusters. Precipitation of δ′ (Al₃ Li) releases the bound vacancies and thereby promotes S-phase formation and, in extreme cases, vacancy condensation, producing dislocation loops.

MST/350     

Corrosion and Protection of Sintered Metal Parts

Abstract

An analysis of the mechanism of corrosion and protection of sintered powder metal parts reveals that the role of surface pores is an unusual and incompletely understood characteristic. Electrochemical principles can be enunciated and interpreted in the context of pores, which may or may not be interconnected throughout the structure, and models can be proposed on the basis of well-known metallographic features. In these models the main bulk surface may be assumed to be covered with an oxide film and acts as cathode with respect to the pores themselves which are relatively anodic and therefore corrosion sites. Any protective mechanism must therefore be primarily concerned with the elimination of pores by sealing, several processes being available for this stage. Subsequent coating may then be directed towards galvanic protection rather than the micro-throwing power problems of filling the pores.     

New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Abstract

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.

The precipitation of β-FeC₂O₄.2H₂O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.

Acicular shaping of the β-FeC₂O₄.2H₂O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.     

Process and compressive properties of porous nickel materials

Abstract

Compressive properties are investigated for the porous Ni materials processing by innovated powder metallurgical (PM) method. The porous Ni materials first show a short elastic region, then a long and oblique stress yield region within the strain range of about 10–50%, and finally, a densification region where the stress increases rapidly.     

Discussion: Manufacturing and Quality Assurance

Abstract

A comprehensive review is presented of packaging and transport needs for the US Department of Energy (DOE) into the twenty-first century. Although the study considered all of the materials to be transported by or on behalf of DOE, this paper focuses on the transport of radioactive materials. Current DOE radioactive material transport activities were established, and projections for the future were then made. The results of the assessment indicate that DOE can expect to experience a rapid increase in the shipment of radioactive and other hazardous materials during the late 1990s and even greater increases on into the first decades of the twenty-first century. This projected increase, which results from planned extensive clean-up and remediation of facilities and sites, indicates that DOE must be prepared to accommodate these shipping needs relative to human resources, packaging, systems interfacing, logistics, regulatory compliance, training, and operations.