Simulations of Creep in Ductile-Phase Toughened Nb₅Si₃/Nb In-Situ Composites

Abstract

The primary and steady-state creep behavior of ductile-phase toughened Nb5Si3/Nb in-situ composites has been simulated using analytical and finite element (FE) continuum techniques. The microstructure of these composites is complex, consisting of large, elongated primary dendrites of the ductile (Nb) solid-solution phase in a eutectoid matrix with the silicide as the continuous phase. This microstructure has been idealized to facilitate the modeling; the effects of these idealizations on the predicted composite creep rates are discussed. Further, it has been assumed that the intrinsic creep behavior of each phase within the composite is the same as that of the corresponding bulk material. Thus, the experimentally measured creep properties of the bulk Nb5Si3 and (Nb) phases have been analyzed to provide the required material constants in the creep constitutive equation. Model predictions of the steady-state composite creep rate have been compared with the experimental results for a Nb-10 at.% Si alloy. While accurate at low stress, the models underpredict the composite creep rate at large stresses because the composite stress exponent is underpredicted. In the case of primary creep, the models somewhat over-predict the composite creep strain but are reasonably accurate given uncertainties in the primary creep data. Finally, FE predictions of the tensile stress distributions within the composites have been shown to be qualitatively consistent with the cracking observed experimentally during tertiary creep.

The Effect of Silicide Volume Fraction on the Creep Behavior of Nb-Silicide Based In-Situ Composites

“…The Nb 5 Si 3 creep rates are much lower than those for the Nb-based solid solutions and are similar to those for the high-Si quaternary alloys. In order to develop a better understanding of the response of the composite to increasing silicide volume fraction and increasing stress, the constitutive behavior of the composite was simulated using the expressions provided by Henshall et al [8,9], where A σ is the applied stress, n is the stress exponent for the silicide (1.0), and m is the stress exponent for the Nb (2.9). s V and w V are the volume fractions of the silicide and Nb, respectively.…”

Niobium Silicide High Temperature In Situ Composites

“…Parametric modeling has indicated that the creep properties of these composites are insensitive to volume fraction of metallic phase (Henshall et al, 1995), although the experimental data do not completely support this proposal. The role of the volume fraction of silicide in controlling composite creep behavior is shown in Figure 14, which shows the effect of stress on secondary creep rate for a range of quaternary alloy composites with Si concentrations from 12 to 22%.…”