Ho-Kyung Kim

Microstructure and elevated temperature behavior of a spray-atomized and co-deposited Ni3Al/SiC/TiB2 intermetallic matrix composite

Abstract This paper discusses preliminary results on the microstructure and elevated temperature creep rupture behavior of a spray-atomized and co-deposited Ni 3 Al composite reinforced with SiC and TiB 2 particulates. Microstructural studies suggest that SiC reacts extensively with Ni 3 Al, leading to the formation of other carbide phases, e.g. Cr 3 C 2 . A well-defined interfacial reaction zone, consisting of two distinct layers, was observed at the Ni 3 Al/TiB 2 interface. The layer I consisted of a chromium segregated Ni 3 Ti phase and the diffusion of nickel was suggested to be the rate control step in this interfacial reaction. The layer II was a nickel-rich Al 3 Ni-type phase; the formation of it may be related to the decomposition of the TiB 2 and the ensuing diffusion of boron. Regarding the elevated temperature behavior of the hot-extruded materials, the monolithic Ni 3 Al exhibited longer creep rupture lifetimes compared with that for the Ni 3 Al/SiC/TiB 2 intermetallic matrix composite. This behavior was attributed to the formation of cavities around small carbide particles, resulting from the decomposition of SiC particulates.

High-temperature rupture of microstructurally unstable 304 stainless steel under uniaxial and triaxial stress states

Specimens of 304 stainless steel were tested to failure under two different stress states, uniaxial tension using smooth bar specimens and triaxial tension using notched bar specimens. The tests were conducted at a temperature that gives rise to carbide particle growth which, in turn, leads to microstructural softening. Rupture times are compared for uniaxial and triaxial stress states with respect to multiaxial stress parameters that are directly related to physical mechanisms. The success of the parameters is judged according to how well the rupture times of notched specimens can be predicted using the rupture data for specimens under uniaxial tension. The data indicate that the rupture time is not governed by deformation processes, despite evidence for substantial softening by particle coarsening. The results further suggest that the creep rupture process is dominated by cavitation that is coupled with localized shear deformation along the inclined grain boundaries.

Effect of stress reduction ratio on the creep behavior of an Al-5wt.Ag alloy

Abstract Uninterrupted and stress reduction tests were conducted on an Al-5wt.%Ag alloy in the temperature range of 640–873 K and at a normalized stress range extending from 10−5 to 3×10−4. The experimental data in uninterrupted creep tests show that the creep behavior of Al-5wt.%Ag is similar to that reported for pure aluminum and that under the present experimental conditions, the alloy behaves as a metal class alloy (class II). A comparison between the stress exponent n determined from uninterrupted and the stress exponent n ∗ inferred from stress reduction experiments indicates the presence of two types of behavior, depending on the temperature: normal and mixed behavior. The normal behavior exists at 873 K and is characterized by close correspondence between n and n ∗ for all stress reduction ratios used. The mixed behavior is noted at 780 and 640 K and is manifested by the presence of a reproducible difference between n and n ∗ for lower values of the stress reduction ratio; for the higher values, n ∗ = n . The discrepancy between n and n ∗ in the region of mixed behavior is discussed in the light of results reported for the creep behavior of aluminum after stress reductions.

Elevated-temperature deformation mechanisms in Ni3Al

Abstract The creep behavior of single crystalline Ni 3 Al(Ta,B) was investigated over the temperature range 760–1115 °C under compressive stresses ranging from 30 to 700 MPa. The results reveal two different deformation mechanisms as a function of applied stress and temperature. For region I, a stress exponent of 3.2, an inverse primary creep behavior, a creep transient after a stress reduction where the initial creep rate is faster than the minimum creep rate at the reduced stress and dislocation substructure consisting of homogeneously distributed curved dislocations suggests that the dominant deformation mechanism is viscous dislocation glide. For region II, a stress exponent of 4.3, a normal primary creep behavior, a creep transient after a stress reduction where the initial creep rate is lower than the steady-state creep rate at the reduced stress and evidence for subgrain formation suggests that the dominant deformation mechanism is dislocation climb. It is believed that viscous dislocation glide in Ni 3 Al(Ta,B) is controlled by interdiffusion, whereas climb is controlled by Al lattice diffusion.

High temperature deformation and fracture mechanisms in a dendritic Ni3Al alloy

Abstract The mechanisms that control high temperature deformation and rupture were studied in a Ni 3 Al alloy that was thermo-mechanically treated to produce a non-porous dendritic grain structure. Comparisons of data corresponding to the dendritic grain morphology with that for the equiaxed grain structures indicate that the dendritic morphology results in significantly lower creep rates as well as substantially greater times to rupture. Comparison of the data with numerical calculations suggests that this difference in creep strength is due to an inherent resistance to grain boundary sliding by the dendritic grain structure. A constrained cavity growth model was adapted based on microstructural observations to account for cavitation within the dendritic microstructure. The success of the model indicates that rupture time is primarily determined by constrained cavity growth on isolated dendrite boundary segments.

Mechanisms of intergranular cavity growth in Ni3Al (Zr, B)

Abstract A technique based on hydrogen embrittlement has been used to study the morphology and growth characteristics of grain boundary cavities in a Ni3Al alloy. The technique facilitates brittle intergranular fracture of specimens previously tested at elevated temperatures. This capability makes it possible to distinguish cavities that have formed by cavity coalescence and also determine cavity shape in three dimensions. Contrary to earlier reports of crack-like cavity growth, the results indicate that the shapes of individually growing cavities in Ni3Al are consistent with quasi-equilibrium cavity growth theory. Cavity size measurements have also been compared with predictions of both quasi-equilibrium and crack-like cavity growth models. These results also support the finding that cavity growth in Ni3Al occurs in a quasi-equilibrium manner. The observations further suggest that the formation of crack-like cavities is primarily due to coalescence and, therefore, not representative of the growth mechanism of individual cavities.

Primary dendrite arm spacings and tip radii in directionally solidified Ni3Al

Abstract A nickel aluminide intermetallic material was directionally solidified under various growth rates utilizing a modified Bridgman apparatus. The primary dendrite arm spacing and dendrite tip radius of curvature (π) were measured as functions of the growth rate. The experimentally observed critical growth rate for the cellular-dendritic transition was in excellent agreement with the value predicted by the Kurz and Fisher model. The discrepancy between the present data and the results anticipated from this model is discussed in terms of the assumptions involved in the derivation of the model.

Analysis of Cable Supported Structure Considering Cable Sliding

The goal of this study is to develop a 3-dimensional elastic cable finite element which considers the sliding effect and uses the geometric nonlinear cable finite element based on elastic catenary theory. In this study, two types of sliding were considered: the roller sliding condition without friction and the frictional sliding condition. These were formulated to derive the nodal force vectors and tangential stiffness matrices. To validate the proposed 3-dimensional cable sliding element, experiments were conducted for both sliding conditions, and compared to calculations of the amount of sliding and displacement at the loading point. Overall calculations using the 3-dimensional cable sliding model were in very good agreement with the measured values.