Doctor of Philosophy

dissertationNickel base superalloys exhibit superior high temperature mechanical properties required for aircraft engine components. It has been known that the processing of these alloys by the powder metallurgy route introduces inclusions inside the material. The presence of such inclusions often leads to competing failure modes in fatigue that is described by a step-wise or two distinct S-N curves involving both the surface and internally-initiated cracks, resulting in large uncertainties of fatigue life. A clear understanding of such behavior is yet to be established. The principal objective of this research is to examine the effect of inclusions on the extent of fatigue failure competition from surface and internal initiators at two different specimen test volumes. Experimental fatigue testing has been performed to explore how the presence of inclusions affects the competing fatigue failure modes. In addition, how the competing failure modes will behave with changes in the specimen size was also studied. Two groups of material each with two different specimen sizes were used in this study. It has been shown that the two crack initiation mechanisms occurred in the small unseeded René 88DT specimens tested at 650ºC over the stress range tested. Additionally, the fatigue lives were reduced with increase in specimen volume. All fatigue failures in seeded material occurred due to crack initiations from the seeded inclusions. In the fatigue life of seeded material, two competing and separate S-N curves were found in small test volume, whereas, in the large test volume, the regions were separated by a "step" in S-N curve. It has been found that the largest inclusion size observed in metallographic surfaces was smaller than the size determined from the fatigue failure origin. An analysis method based on extreme value statistics developed by Murakami was used to predict the largest size of inclusion in the test volume. The results of this study clearly show that competition for fatigue failure between surface and internal initiations is affected by inclusion content and specimen volume. Extreme value statistics is capable of predicting the largest inclusion that can start fatigue failure, even if the inclusion is not seen in microscopy

Experiments And Modeling Of Fatigue And Fracture Of Aluminum Alloys

In this work, understanding the microstructural effects of monotonic and cyclic failure of wrought 7075-T651 and cast A356 aluminum alloys were examined. In particular, the structure-property relations were quantified for the plasticity/damage model and two fatigue crack models. Several types of experiments were employed to adapt an internal state variable plasticity and damage model to the wrought alloy. The damage model was originally developed for cast alloys and thus, the model was modified to account for void nucleation, growth, and coalescence for a wrought alloy. In addition, fatigue experiments were employed to determine structure-property relations for the cast alloy. Based on microstructural analysis of the fracture surfaces, modifications to the microstructurally-based MultiStage fatigue model were implemented. Additionally, experimental fatigue crack results were used to calibrate FASTRAN, a fatigue life prediction code, to small fatigue-crack-growth behavior. Lastly, a set of experiments were employed to explore the damage history effect associated with cast and wrought alloys and to provide motivation for monotonic and fatigue modeling efforts

Microstructural mechanisms and advanced characterization of long and small fatigue crack growth in cast A356-T61 aluminum alloys

Fatigue crack growth-based design is a significant modern engineering consideration for the transportation sector, and its implementation requires accurate characterization and understanding of crack propagation mechanisms with respect to microstructure. To support this goal, long and small fatigue crack growth studies were conducted on widely used A356-T6 cast aluminum alloys in various microstructural conditions. Microstructural variations were created through processing and chemistry means in order to systematically investigate the individual and combined effects of the materials’ characteristic microstructural features on fatigue crack growth at all growth stages. Crack growth mechanisms and failure mode transitions are identified with respect to the eutectic Si morphology/distribution and grain structure by fractographic techniques and electron backscatter diffraction. Crack-microstructure interactions were investigated in depth across all crack sizes, and the respective roles of microstructural features were identified experimentally and further corroborated by numerical models. It is concluded that the eutectic Si phase enhances the alloys’ fatigue crack growth resistance in early growth stages (by transferring stresses off of the α-Al matrix), and progressively decreases due to damage localization. In later growth stages, the eutectic Si phase becomes increasingly detrimental to fatigue crack growth resistance because of its inherently low debonding strength and brittle fracture, as evidenced by the crack selectively following eutectic Si colonies

Influence of constituent particles on fracture of aluminum alloys under high-triaxiality loading

Single-edge notch bending tests are conducted to study the influence of constituent particles on the fracture resistance of aluminum alloys 6061, 6063, and 6110 under high-constraint loading conditions. The alloys are tested in the as-cast state after homogenization and artificial aging to temper T6. Each alloy type was delivered with two different volume fractions of constituent particles to enable a quantitative assessment of its impact on the toughness of these aluminum alloys. One variant corresponds to the commercial alloy, whereas the other variant is tailor made with an increased amount of constituent particles by adding Fe and Si to the commercial alloy. All alloys exhibit a dendritic structure with particles clustered at grain boundaries and dendrite arm boundaries. The increased content of constituent particles in the tailor-made alloys is shown to be purely detrimental for the toughness and reduces relevant fracture energy parameters by more than 50% in the alloys tested herein. In the plane-strain-dominated regions of the specimens where the stress triaxiality is highest, crack propagation was found to take place on grain boundaries and dendrite arm boundaries due to void nucleation, growth, and coalescence from the constituent particles. Differences in toughness between the alloys are primarily related to variations in the content, size, and spacing of the constituent particles. A comparison between the three different alloy types, i.e. 6061, 6063, and 6110, shows that strength affects the toughness, but it does not follow the commonly reported trade-off between strength and ductility.publishedVersio

An investigation of the plastic fracture of high strength steels

Three generally recognized stages of plastic fracture in high strength steels are considered in detail. These stages consist of void initiation, void growth, and void coalescence. A brief review of the existing literature on plastic fracture is included along with an outline of the experimental approach used in the investigation

THREE-DIMENSIONAL MICROSTRUCTURAL EFFECTS ON MULTI-SITE FATIGUE CRACK NUCLEATION BEHAVIORS OF HIGH STRENGTH ALUMINUM ALLOYS

An experimental method was further developed to quantify the anisotropy of multi-site fatigue crack initiation behaviors in high strength Al alloys by four-point bend fatigue testing under stress control. In this method, fatigue crack initiation sites (fatigue weak-links, FWLs) were measured on the sample surface at different cyclic stress levels. The FWL density in an alloy could be best described using a three-parameter Weibull function of stress, though other types of sigmoidal functions might also be used to quantify the relationship between FWL density and stress. The strength distribution of the FWLs was derived from the Weibull function determined by fitting the FWLs vs. stress curve experimentally obtained. As materials properties, the FWL density and strength distribution could be used to evaluate the fatigue crack nucleation behaviors of engineering alloys quantitatively and the alloy quality in terms of FWL density and strength distribution. In this work, the effects of environment, types of microstructural heterogeneities and loading direction on FWLs were all studied in detail in AA7075-T651, AA2026-T3511, and A713 Al alloys, etc. It was also found that FWLs should be quantified as a Weibull-type function of strain instead of stress, when the applied maximum cyclic stress exceeded the yield strength of the tested alloys. In this work, four-point bend fatigue tests were conducted on the L-T (Rolling-Transverse), L-S (Rolling-Short transverse) and T-S planes of an AA7075-T651 alloy plate, respectively, at room temperature, 20 Hz, R=0.1, in air. The FWL populations, measured on these surfaces, were a Weibull-type function of the applied maximum cyclic stress, from which FWL density and strength distribution could be determined. The alloy showed a significant anisotropy of FWLs with the weak-link density being 11 mm-2, 15 mm-2 and 4 mm-2 on the L-T, L-S and T-S planes, respectively. Fatigue cracks were predominantly initiated at Fe-containing particles on the L-T and L-S planes, but only at Si-bearing particles on the T-S plane, profoundly demonstrating that the pre-fractured Fe-containing particles were responsible for crack initiation on the L-T and L-S planes, since the pre-fracture of these particles due to extensive deformation in the L direction during the prior rolling operation could only promote crack initiation when the sample was cyclically stressed in the L direction on both the L-T and L-S planes. The fatigue strengths of the L-T, L-S and T-S planes of the AA7075 alloy were measured to be 243.6, 273.0 and 280.6 MPa, respectively. The differences in grain and particle structures between these planes were responsible for the anisotropy of fatigue strength and FWLs on these planes. Three types of fatigue cracks from particles, type-I: the micro-cracks in the particles could not propagate into the matrix, i.e., type-II: the micro-cracks were fully arrested soon after they propagated into the matrix, and type-III: the micro-cracks became long cracks, were observed in the AA7075-T651 alloy after fatigue testing. By cross-sectioning these three-types of particles using Focused Ion Beam (FIB), it was found that the thickness of the particles was the dominant factor controlling fatigue crack initiation at the particles, namely, the thicker a pre-fractured Fe-containing particle, the easier it became a type-III crack on the L-T and L-S planes. On the T-S plane, there were only types-I and III Si-bearing particles at which crack were initiated. The type-I particles were less than 6.5 μm in thickness and type-III particles were thicker than 8.3 μm. Cross-sectioning of these particles using FIB revealed that these particles all contained gas pores which promoted crack initiation at the particles because of higher stress concentration at the pores in connection with the particles. It was also found that fatigue cracks did not always follow the any specific crystallographic planes within each grain, based on the Electron Backscatter Diffraction (EBSD) measurement. Also, the grain orientation did not show a strong influence on crack initiation at particles which were located within the grains. The topography measurements with an Atomic Force Microscope (AFM) revealed that Fe-containing particles were protruded on the mechanically polished surface, while the Si-bearing particles were intruded on the surface, which was consistent with hardness measurements showing that Si-bearing particles were softer (4.030.92 GPa) than Fe-containing ones (8.9 0.87 GPa) in the alloy. To verify the 3-D effects of the pre-fractured particles on fatigue crack initiation in high strength Al alloys, rectangular micro-notches of three different types of dimensions were fabricated using FIB in the selected grains on the T-S planes of both AA2024-T351 and AA7075-T651 Al alloys, to mimic the three types of pre-fractured particles found in these alloys. Fatigue testing on these samples with the micro-notches verified that the wider and deeper the micro-notches, the easier fatigue cracks could be initiated at the notches. In the AA2024-T351 samples, cracks preferred to propagate along the {111} slip plane with the smallest twist angle and relatively large Schmid factor. These experimental data obtained in this work could pave a way to building a 3-D quantitative model for quantification of fatigue crack initiation behaviors by taking into account the driving force and resistance to short crack growth at the particles in the surface of these alloys

A MICROSTRUCTURE-BASED MODEL VALIDATED EXPERIMENTALLY FOR QUANTIFICATION OF SHORT FATIGUE CRACK GROWTH IN THREE-DIMENSIONS

Built on the recent successes in understanding the crystallographic mechanism for short fatigue crack (SFC) growth across a grain boundary (GB) and developing an experimental method to quantify the GB resistance against short crack growth, a microstructure-based model was developed in this study to simulate the growth behaviors of SFCs in 3-D, by taking into account both the driving force and resistance along at each point along the crack front in an alloy. It was found that the GB resistance was a Weibull function of the minimum twist angle of crack deflection at the boundary in AA2024-T3 Al alloys. In the digital microstructure used in the model, the resistance at each GB that the short crack interacted with could be calculated, as long as the orientations of grains and the crack were known. In the model, an influence function accounting for the overlapping effect of the resistance from the neighboring grain boundaries was proposed, allowing for calculation of the total resistance distribution along the crack front. In order to overcome the time consuming problem for the existing equations to derive the distribution of stress intensity factor along the crack front under cyclic loading, an analytical equation was proposed to quantify the stress intensity factor distribution along an irregular shape planar crack. By introducing two shape-dependent factors, the fractured area and the perimeter of the crack front, the newly proposed equation could readily and accurately derive the stress intensity factor distribution along the crack front that had large curvatures and singularities. Finally, a microscopic-scale Paris’ equation was proposed that took into account both the driving force, i.e., stress intensity factor range, and the total resistance to calculate the growth rate at each point along crack front. The model developed in this work was able to incorporate microstructure, such as grain size and shape, and texture into simulation of SFC growth in 3-D. It was capable of simulating all the anomalous growth behaviors of SFCs, such as the marked scatters in growth rate measurement, retardation and arrest at grain boundaries, and crack plane deflection at grain boundaries, etc. The model was used to simulate the growth behaviors of SFCs initiated from prefractured constituent particles in order to interpret the multi-site fatigue crack initiation observed in AA2024-T351 Al alloys. Three types of SFCs were observed initiating from these particles, namely, type-I non-propagating cracks; type-II cracks which were arrested soon after propagating into the matrix; and type-III propagating cracks. To quantitatively study the 3-D effects of particle geometry and micro-texture on the growth behaviors of micro-cracks in these particles, rectangular micro-notches with different dimensions were fabricated using focused ion beam in the selected grains on the T-S planes in AA2024-T351 Al alloys, to mimic the pre-fractured particles in these alloys. Knowing the notch dimensions or particle shape, grain orientation and GB geometry, the simulated crack growth behaviors were consistent with the experimental observations, and the model was able to verify that the three types of cracks evolved from these particles were mainly associated with the thickness and width of the pre-fractured particles, though the particle geometry and grain orientation could also affect the behaviors of fatigue crack initiation at the particles. When the widths of the particles were less than 15 μm, like in most high strength Al alloys, the simulated results confirmed that the crack type was only associated with the particle thickness, consistent with the experimental results in AA2024-T351 alloys with a strong rolling texture. The lives for the SFCs to reach 0.5 mm in length were quantified with the model in the AA2024 alloy, revealing that there was a bimodal distribution in the life spectrum calculated, with the longer life peak being related to larger twist angles of crack deflection at the first GB the cracks encountered and the shorter life peak being associated with small twist angles (\u3c 5°) at the first GB. The model further demonstrated the influence of grain structure on SFC growth by considering two different grain structures with the same initial short crack, namely, a layered grain structure with only the primary GBs perpendicular to the surface and the layered grains with both primary and secondary GBs. Depending on their positions and geometry, the secondary GBs could still exert a strong retarding effect on SFC growth on surface. The model was validated by matching to the growth rate measured on surface of a SFC in an AA8090 Al-Li alloy. Good consistency was achieved between the simulated and experimentally measured growth rates when both the primary and secondary GBs were considered in the model. The model developed in this study exhibits its potential applications to optimizing the microstructure and texture in alloys to enhance their fatigue resistance against fatigue crack growth, and to satisfactory life prediction of engineering alloys