Multi-scale crack closure measurements with digital image correlation on Haynes 230

An experimental campaign was developed to study fatigue crack growth in Haynes 230, a Ni-based superalloy. The effects of crack closure were investigated with digital image correlation, by applying two different approaches. Initially, full field regression algorithms were applied to extract the effective stress intensity factor ranges from the singular displacement field measured at crack tips. Local closure measurements were then performed by considering crack flanks relative displacements. Two points virtual extensometers were applied in this phase. Experimental results were then compared to the reference da/dN –∆Keff curve: it was found that the correct estimation of crack opening levels shifts all the experimental points on the reference curve, showing that DIC can be successfully applied to measure crack closure effects

Multi-scale crack closure measurements with digital image correlation on Haynes 230

An experimental campaign was developed to study fatigue crack growth in Haynes 230, a Ni-based superalloy. The effects of crack closure were investigated with digital image correlation, by applying two different approaches. Initially, full field regression algorithms were applied to extract the effective stress intensity factor ranges from the singular displacement field measured at crack tips. Local closure measurements were then performed by considering crack flanks relative displacements. Two points virtual extensometers were applied in this phase. Experimental results were then compared to the reference da/dN –?Keff curve: it was found that the correct estimation of crack opening levels shifts all the experimental points on the reference curve, showing that DIC can be successfully applied to measure crack closure effects

Superelasticity in high strength heterophase single crystals of Ni51.0Ti36.5Hf12.5 alloy

The effect of precipitated disperse H-phase particles on the thermoelastic B2–B19′ martensitic transformation (MT) under compressive load has been studied on [001]-, [236]-, and [223]-oriented single crystals of Ni51.0Ti36.5Hf12.5 (at %) alloy in the initial (as-grown) state. It is established that, in Ni51.0Ti36.5Hf12.5 single crystals containing disperse H-phase particles with dimensions within 125–150 nm at a volume fraction of ~30%, neither the critical stresses of martensite formation nor superelasticity strain depend on the orientation. Fully reversible B2–B19′ MTs in Ni51.0Ti36.5Hf12.5 single crystals have been observed in tests at external axial stresses up to 1700 MPa and temperatures up to Tt ~ 373 K

Understanding Fundamental Material Degradation Processes in High Temperature Aggressive Chemomechanical Environments

The objective of this project is to develop a fundamental understanding of the mechanisms that limit materials durability for very high-temperature applications. Current design limitations are based on material strength and corrosion resistance. This project will characterize the interactions of high-temperature creep, fatigue, and environmental attack in structural metallic alloys of interest for the very high-temperature gas-cooled reactor (VHTR) or Next–Generation Nuclear Plant (NGNP) and for the associated thermo-chemical processing systems for hydrogen generation. Each of these degradation processes presents a major materials design challenge on its own, but in combination, they can act synergistically to rapidly degrade materials and limit component lives. This research and development effort will provide experimental results to characterize creep-fatigue-environment interactions and develop predictive models to define operation limits for high-temperature structural material applications. Researchers will study individually and in combination creep-fatigue-environmental attack processes in Alloys 617, 230, and 800H, as well as in an advanced Ni-Cr oxide dispersion strengthened steel (ODS) system. For comparison, the study will also examine basic degradation processes in nichrome (Ni-20Cr), which is a basis for most high-temperature structural materials, as well as many of the superalloys. These materials are selected to represent primary candidate alloys, one advanced developmental alloy that may have superior high-temperature durability, and one model system on which basic performance and modeling efforts can be based. The research program is presented in four parts, which all complement each other. The first three are primarily experimental in nature, and the last will tie the work together in a coordinated modeling effort. The sections are 1) dynamic creep-fatigue-environment process, 2) subcritical crack processes, 3) dynamic corrosion – crack initiation processes, and 4) modeling

High-temperature superelasticity of Ni50.6Ti24.4Hf25.0 shape memory alloy

Changes in transformation temperatures and high-temperature mechanical behavior of the new Ni50.6Ti34.4Hf25.0 alloy were determined after selected aging treatments. Differential scanning calorimetry was used to compare the homogenized, solution-treated and aged specimens for a wide combination of aging temperatures and times. Isothermal deformation experiments were conducted measuring reversible transformation strains with digital image correlation. Extraordinary superelasticity was found at test temperatures up to 300 °C for the specimen aged at 500 °C for 4 h

Final report DOE project, ''Origins of asymmetric stress-strain response in phase transformations,'' DEFG02-93ER143993

For the first time, experiments on NiTi under pressure loadings were conducted in Ref. (1). This work showed that the stress-strain response of NiTi is highly pressure sensitive and there was an asymmetry of tension and compression results. The results were obtained based on the special rig developed in (Ref. 2) by Sehitoglu and his students. Several experiments under pressure were also conducted on CuZnAl alloys with also pressure dependent response. accounted for variant-variant interaction and texture effects in the case of NiTi alloys (Ref. 3). It was found that the polycrystalline version of these materials has a strong texture due to the cold rolling process (Figure 4). Consequently, they almost behave as single crystals oriented in the [111] direction (Figure 3). We showed that if the texture effects are not accounted for the models give the incorrect trends when compared with experiments (Figure 5). Our work also showed that the evolution of the variants in tension is much more rapid compared to the compression case (Ref. 3). In the second year of the work, our attention focused exclusively on the deformation behavior of single crystals. Several key results were achieved with single crystals. Initially, we studied the role of aging treatment on tension compression asymmetry and crystal orientation dependence. It was shown that the orientation dependence of critical resolved shear stress is significant in the case of peak aged crystals while the orientation dependence decreases with overaging. A micro-mechanical model was developed to explain these trends based on the determination of the local shear stresses due to the precipitate on the 24 possible martensite variants (Figure 6). It was found that those variants that have high resolved shear stress due to external loading experience low local stresses due to the precipitate weakening the orientation dependence (Refs. 4-6). Overall the results and the model showed that the introduction of precipitates reduce the critical transformation stress in these materials and reduce the orientation dependence (Figure 7). It was also noted that overaging results in loss of Ni in the matrix resulting in decrease in strength levels. as a function of orientation and stress direction (Refs. 8-9). We made preliminary attempts at explaining the orientation dependence of the recoverable strains based on the C W formation and detwinning models (Table 2) but pointed out that further work is necessary in this area. The precipitates curtail the detwinning phenomenon and reduce the overall transformation strain levels in these alloys (Table 2). Finally, we, note the unusual amount of hardening under cyclic loading (Ref.9) with combined effects of twinning and slip. The strengthening in compression was found to be remarkably high (Figure 10). Further study of cyclic loading was put aside for the proposed research. The stress-strain results for NiTi were predicted with a micro-mechanical model, which in the third year of the research, our attention focused on cyclic loading and recoverable strains

Changes in State Variables at Elevated Temperatures

Two state variables, strength and internal (back) stress, were determined during cyclic deformation by rapid unloading-reloading experiments. The experiments were performed at different temperatures and strain rates, Considerable increases in strength at low temperatures (20°C–300°C) due to cyclic deformation and due to strain aging were measured. The variation of internal stresses at elevated temperatures (600°C) with strain rate was identified. Regions of high back stress rates were established on stress-back stress diagrams. High strain rates during unload-reload excursions are needed to minimize changes in back stress during measurements. The results (state variables) and the stress-strain response were compared to predictions based on a unified constitutive model. The capabilities of the model under cyclic loading, decrease in strength at high temperatures due to spheroidization, and increase in strength due to strain aging at lower temperatures, were found to be satisfactory. The extension of the unloading-reloading technique to multiaxial loading is outlined.</jats:p

Constraint Effect in Thermo-Mechanical Fatigue

Many structural members in service experience fluctuating loads and temperatures. Thermal stresses devlop on a contrained element if the temperature is cycled. Constraint exists in structural components due to temperature gradient, material anisotropy, and geometry effects. The equilibrium and compatibility equations determine the degree of constraint on the critcal component of the body. A two-bar structure is employed in this study to simulate different constraint conditions encountered in practice. The constraint types are termed (a) total, (b) partial, (c) over, and (d) notch constraint. Thermo-mechanical fatigue test results indicate that the fatigue lives decrease severely with increasing constraint, increasing temperature range and with the presence of notch or a defect.</jats:p