Factors that Enhance the Resistance of High-Strength Alloys to Environmental Degradation
H. K. Birnbaum,* A.-M. Alvarez, D. Lillig
National Aeronautics and Space Administration, NAS 2-852

This program is applying our knowledge of the mechanisms of hydrogen embrittlement of metals to develop an understanding of the factors which strengthen alloys against environmental attack. The systems studied include high-strength aluminum alloys, beta titanium alloys, and the high-alloy nickel-based alloys. The experimental techniques used include mechanical property tests, electron microscopy, in situ environmental cell TEM, secondary ion mass spectrometry, and Auger spectroscopy. In most of the above alloys the studies include the effects of solutes and hydrogen on grain boundary fracture.

Fracture Mechanisms of Particulate-reinforced Metal Matrix Composites
J. K. Shang,* G. Liu
University of Illinois

The fracture mechanisms of particulate-reinforced metal-matrix composites are examined in several aluminum- and titanium-matrix composites at room and elevated temperatures. Most composite systems show a low-temperature fracture behavior controlled primarily by the reinforcing particles, with the matrix playing an inactive role in resisting the crack growth. The fracture properties of these composites are characterized by low fracture toughness, relatively flat R-curves, and weak dependence on the composition and microstructure of the matrix alloy. It is shown that a more efficient use of the matrix plasticity, which makes the fracture process more matrix-dominated, can lead to significant improvement in the fracture properties of these composites.

Hydrogen Effects on Intermetallics
I. M. Robertson,* H. K. Birnbaum,* D. Lillig
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The effects of hydrogen on the structure and properties of intermetallics (nickel and iron aluminides) are being studied using the environmental cell TEM and other analytical techniques. To understand the role of the environment on the lack of ductility in polycrystalline Ni3Al, the effect of boron on the distribution of deuterium around grain boundaries is being mapped by using SIMS.

Hydrogen Effects on the Properties of Metals
H. K. Birnbaum,* I. M. Robertson*
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The interactions of gaseous hydrogen environments on the properties of metal systems is being studied directly by using an environmental cell TEM. Use of this microscope permits gas-solid interactions to be studied in real time and at high spatial resolution. The focus of the program is primarily on the effects of aggressive environments on the mechanical behavior. Specifically, we are investigating the effect of hydrogen on the stress field of dislocations and on the stacking-fault energy by monitoring the change in dislocation node spacing in the presence and absence of gas.

Material Behavior under Near-Failure Stresses
Y. Q. Sun,* N. Yang, X. M. Gu
National Science Foundation, DMR 98-74854 CAR

This research project studies the effect of high stresses on the intrinsic properties of solids. A particular emphasis is on the collective formation of crystal defects in the presence of high stresses and the implications of these special defect formation mechanisms to the brittle failure of materials and the transition to ductility. The experiments involve testing dislocation-free crystals in tension and probing the defect content dynamically with electrical resistivity and x-ray diffraction. The collective nucleation of dislocations is modeled with analytical methods and computer simulations. The materials studied include TiAl and NiAl intermetallics, Si, and metal whiskers.

Mechanisms of High-Temperature Crack Growth in Ceramic-Matrix Composites
J. K. Shang,* D. Yao
National Science Foundation, MSS 91-11141

The ability of ceramic materials to withstand high-temperature and hostile environments offers great prospects for potential major improvements in the design performance of high-temperature components in chemical processing, power generation, and industrial waste recovery applications. Their use as structural materials, however, has been limited primarily because of their poor fracture toughness and lack of damage tolerance. The purpose of this program is to examine the fundamental micromechanisms of high-temperature subcritical crack growth in three classes of ceramic-matrix composites chosen to reflect different primary room temperature toughening mechanisms, namely, crack deflection, crack trapping, and crack bridging.

Strengthening of g/a2 TiAl-based Alloys
Y. Q. Sun*
University of Illinois

g/a2 two-phase alloys based on intermetallic compound TiAl are high-strength, low-density materials for applications at high temperatures, e.g., as turbine and compressor blades in aircraft engines. This research investigates (1) the relationship between microstructure and the mechanical properties such as yield strength, creep resistance, and toughness and (2) the control of microstructures by means of heat treatment and mechanical working. The experiments involve microscopy characterization of microstructure over the nanometer to micrometer range and mechanical property tests. The relationship between the microstructure and mechanical properties is modeled, based on the dislocation pile-up mechanism and the continuum theory of dislocations.

Transient High Strain-Rate Plasticity and Plastic Instability in Crystals
Y. Q. Sun,* N. Yang, X. M. Gu
National Science Foundation, DMR 98-74854 CAR; UES Inc.

The plastic deformation of crystalline materials is spatially and temporally inhomogeneous. Spatial inhomogeneity is characterized by locally intensive dislocation activity in slip bands and kink bands. Temporal inhomogeneity is characterized by precipitous onset and stoppage of plastic deformation and the transient in between, as in a sharp yield drop or serrated flow. This project is an experimental research to investigate the transient plastic strain-rate in the deformation of crystalline materials and to discover the underling dislocation mechanisms. The experimental approach is from the perspective in which the onset of plasticity is viewed to be inherently unstable.