Showing papers in "Materials Science Forum in 1996"

Fundamental Aspects of Bulk Metallic Glass Formation in Multicomponent Alloys

Abstract: During the past several years, a number of multicomponent alloy families have been investigated in which the liquid alloys form metallic glass at cooling rates below 1000 K/s and as low as 1 K/s or less. These ``bulk`` metallic glass forming alloys have been cast from the melt into glass samples with the smallest dimension ranging from millimeters to centimeters. The undercooled liquid alloys show remarkable resistance to crystallization permitting studies of thermal and physical properties, the glass transition, and crystallization behavior of the melt over previously inaccessible temperatures in the deeply undercooled region. One group of these alloys, which includes the Zr-Ti-Ni-Cu-Be and Zr-Ti-Ni-Cu systems, has been extensively investigated in our laboratory. Results of atomic diffusion and viscosity measurements, crystallization behavior and TTT-diagrams, and studies of liquid phase separation in the deeply undercooled liquid have been carried out and will be discussed. Engineering properties and potential applications of these bulk glasses will be briefly mentioned. (orig.)

Mechanical Properties of Nanophase Materials

Abstract: It has become possible in recent years to synthesize new materials under controlled conditions with constituent structures on a nanometer size scale (below 100 nm). These novel nanophase materials have grain-size dependent mechanical properties that are significantly different than those of their coarser-grained counterparts. For example, nanophase metals are much stronger and apparently less ductile than conventional metals, while nanophase ceramics are more ductile and more easily formed than conventional ceramics. The observed mechanical property changes are related to grain size limitations and/or the large percentage of atoms in grain boundary environments; they can also be affected by such features as flaw populations, strains and impurity levels that can result from differing synthesis and processing methods. An overview of what is presently known about the mechanical properties of nanophase materials, including both metals and ceramics, is presented. Some possible atomic mechanisms responsible for the observed behavior in these materials are considered in light of their unique structures.

Influence of Hydrogen on the Stacking-Fault Energy of an Austenitic Stainless Steel

Abstract: The effect of hydrogen on the stacking-fault energy has been evaluated in a 310 austenitic stainless steel. The stacking-fault energy was calculated from measurements of the radius of curvature of dislocation nodes, which were imaged using weak-beam conditions. Preliminary results indicate that the presence of 40 torr of hydrogen gas in a transmission electron microscope equipped with an environmental cell, causes an increase in node spacing which corresponds to about a 19% reduction in the stacking-fault energy. (orig.)

The composition of fine-scale precipitates in Al-Mg-Si alloys

Abstract: The compositions of intermediate precipitates in Al-Mg-Si alloys have been examined using atom probe field ion microscopy (APFIM), energy dispersive electron spectroscopy (EDS) and parallel electron energy loss spectroscopy (PEELS). It was found that the Mg:Si ratios in small, equiaxed precipitates, β" precipitates and β' precipitates were close to 1.1 (atomic). The Mg:Si ratio in a B' precipitate was approximately 0.9. The implications of these results to the understanding of the mechanical properties of Al-Mg-Si alloys are discussed.