In-Situ Studies of Mechanical Behavior [Liaw & Bourke (Group Leaders), Aurrecoechea, Choo, Clausen, Daymond, Dunand, Edwards, Holden, Hubbard, Jackson, Klarstrom, Pang, Payzant, Priesmeyer, Sankaran, Thompson, Ustundag, XL Wang, and Withers; 1 Postdoctor, 3 Graduate Students; 2 Undergraduate Students per year]

1.1       Fatigue: Using in-situ neutron-scattering technologies, the evolution of elastic strain and damage under cyclic loading in alloys and composites will be monitored during the different stages of fatigue behavior, i.e., initiation, propagation, and final failure. In particular, the evolution of phases [precipitation], texture, and intergranular stresses during cyclic fatigue will be characterized, and their influence on fatigue lifetime will be studied. Furthermore, thermo-mechanical fatigue tests will be conducted to simulate the combined effects of mechanical fatigue and thermal cycling experienced by high-temperature structural materials, such as gas turbine blades. Parametric studies of the effects of the simultaneous thermal and mechanical cyclic loading on the lattice responses will be examined. The interplay between the thermal and mechanical effects during the in-phase and out-of-phase cycles will be investigated and compared with the theoretical model. The deformation mechanisms at various stages “during a single fatigue cycle” may be carefully examined using the time-slicing approach where the data collection is synchronized with the cycling of the applied load so that diffraction patterns are recorded at different points of the load cycle simultaneously. Real-time measurements during cyclic conditions will require new capabilities in data collection, i.e., the time-slicing methodologies. Some experiments have been conducted in UK^[9], and ANSWER will facilitate further breakthroughs in this field by working together with HFIR and ILL [France], and also with LANSCE [SMARTS^[10] instrument], and ISIS [UK: ENGIN-X^[11] instrument]. This methodology will provide an in-depth understanding of fatigue damage, and lead to the development of new fatigue theory.

1.2       Creep Deformation: Major advances in in-situ studies on time-dependent processes are being made through the development of new instruments at LANSCE and at the ISIS facility. It is feasible to measure strain and phase evolution in the transient response to applied loads and/or temperatures. A time-dependent mechanical property at elevated temperatures, namely creep, will be studied^[12-15] with an unprecedented temporal/microstructural resolution. The time-resolved strain measurements [a few minutes] at the ISIS facility and at LANSCE will provide insights into deformation processes in the primary and tertiary creep regimes as well as the secondary creep regime. It is of particular interest to quantitatively study the load-partitioning in composites at elevated temperatures.^[12-14] The effects of reinforcement volume fraction, size, and shape on the high-temperature micromechanics of load partitioning and strengthening [or weakening] in the composites will be examined quantitatively. In particular, the effectiveness of the ‘compositing’ will be studied in-situ over a wide range of stresses and temperatures, representing different deformation mechanisms, such as diffusional or dislocational creep. For example, ANSWER will facilitate an international collaboration opportunity with a research group led by Prof. Phil Withers at Manchester University [UK], which is one of the most active and advanced groups in this field.

1.3       Polycrystalline Deformation, Twinning, and Texture:     Individual grains in a polycrystal have the same deformation mechanisms as single crystals. The inherent anisotropy in the thermal, elastic, and plastic properties causes inhomogeneous deformations among grains.^[16-34] Deformations of the grains are restrained by the surroundings, and the grains are left with internal stresses. These stresses have significant impacts on texture development, intergranular failure, stress corrosion cracking, dislocation pile-ups, and many other effects that are not dependent on the average level of stress, but rather on the local behavior and the distribution of stresses around the average. To date, most of the reported neutron measurements for the plasticity theory studies focus on single-phase materials under uniaxial loading. Systematic experiments will be performed to study the effects on strain heterogeneity at various levels of constraint conferred on grain families by hard second phases [i.e., composites], by different nearest neighbors [i.e., different levels of texture], and by various deformation modes, such as slip and twinning, under different loading conditions. The experimental results will provide quantitative information on deformation of polycrystalline materials at the microscopic level. Neutron sources equipped with in-situ loading capabilities [such as at LANSCE, US; ISIS, UK; ILL, France; and NRC, Canada] are particularly well suited for this kind of experiment.