Size effects in materials due to microstructural and dimensional constraints: a comparative review

Advanced fission and future fusion energy will require new high-performance structural alloys with outstanding properties that are sustained under long-term service in ultrasevere environments, including neutron damage producing up to 200 atomic displacements per atom and, for fusion, 2000 appm of He. Following a brief description of irradiation damage and damage resistance, we focus on an emerging class of nanostructured ferritic alloys (NFAs) that show promise for meeting these challenges. NFAs contain an ultrahigh density of Y-Ti-O-enriched dispersion-strengthening nanofeatures (NFs) that, along with fine grains and high dislocation densities, provide remarkably high tensile, creep, and fatigue strength. The NFs are stable under irradiation up to 800°C and trap He in fine-scale bubbles, suppressing void swelling and fast fracture embrittlement at lower temperatures and creep rupture embrittlement at high temperatures. The current state of the development and understanding of NFAs is described, along wi...

Recent Developments in Irradiation-Resistant Steels

Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems

http://hotmetals.ms.northwestern.edu/refs/files/AM02-Al-Sc_creep.pdf

Precipitation strengthening at ambient and elevated temperatures of heat-treatable Al(Sc) alloys

Materials issues in microelectromechanical systems (MEMS)

The strongly stress-sensitive and temperature-dependent creep behaviour of dispersion strengthened materials cannot be described satisfactorily by current creep laws. In this paper a new creep equation is developed which considers as the rate-controlling event the thermally activated detachment of dislocations from dispersoid particles exerting an attractive force. The approach is motivated by recent TEM observations and theoretical calculations which strongly suggest that the “classical” view, according to which particles merely force dislocations to climb around them, is inadequate. The creep equation is applied to a dispersion-strengthened superalloy, two aluminium alloys and bubble-strengthened tungsten. Practical conclusions, regarding the optimum dispersoid size and alloy development, are drawn.

https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/23870/1/ea199001.pdf

A new model-based creep equation for dispersion strengthened materials

https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/24004/1/ea198812.pdf

The kinetics of dislocation climb over hard particles—II. Effects of an attractive particle-dislocation interaction

https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/24005/1/ea198811.pdf

The kinetics of dislocation climb over hard particles—I. Climb without attractive particle-dislocation interaction

The mechanical properties of dispersion-strengthened aluminum alloys, with various dispersoid types, volume fractions, and grain structures, were investigated in conjunction with systematic microstructural examinations. New theoretical concepts, based on thermally activated dislocation detachment from dispersoid particles, were used to analyze the creep behavior. A particularly strong dispersoid-dislocation interaction was identified as reason for the excellent creep properties of carbide dispersion-strengthened aluminum. Oxide particles (Al2O3,MgO) seem to exert a weaker interaction force and are therefore less efficient strengtheners. Although fine crystalline in the as-extruded condition, all alloys are remarkably resistant against diffusional creep. It is demonstrated that this behavior can be consistently understood by extending the concept developed for the interaction between bulk dislocations and dispersoids to grain boundary dislocations.

/pdf/microstructure-and-creep-properties-of-dispersion-43z7s3zpaz.pdf

Microstructure and creep properties of dispersion-strengthened aluminum alloys

In the prodn. of a casting (20) in a vacuum chamber (2), a mould (12) with molten alloy is transferred, through a hole (7) in a separating baffle (3), from a heating chamber (4) into a cooling chamber (5) in which the alloy solidifies in an oriented manner. The method is characterised by the fact that below the baffle(3) the mould is additionally cooled from outside by means of a gas flow. Also claimed is an appts. for implementation of the method. The appts. is characterised by the presence of means for producing and directing a gas flow in the cooling chamber below the baffle (3). The gas used for cooling the mould is an inert gas such as, in particular, argon or helium. The gas is introduced into the cooling chamber (5) after entry of the bottom of the mould (12). The gas is directed in the direction of the mould surface, and is then removed from the vacuum chamber (2). The gas is cooled, filtered and again introduced into the cooling chamber.

Joachim Rösler

Papers

A new model-based creep equation for dispersion strengthened materials

The kinetics of dislocation climb over hard particles—II. Effects of an attractive particle-dislocation interaction

The kinetics of dislocation climb over hard particles—I. Climb without attractive particle-dislocation interaction

Microstructure and creep properties of dispersion-strengthened aluminum alloys

Process for casting a directionally solidified article and apparatus for carrying out this process