High‐Temperature Compressive Behavior of Refractory Alumina–Niobium Composite Material

Abstract

The development of coarse‐grained refractory composites combining refractory ceramics with refractory metals provides new approaches for high‐temperature applications. In particular, coarse‐grained Nb‐Al2O3 composites are widely interested due to their promising functional properties such as high thermal shock resistance, low shrinkage and good electrical conductivity. In order to identify and release the potential of these materials in advanced applications, their mechanical behavior should be evaluated. This study presents room‐ and high‐temperature compression behavior up to 1500 °C of Nb‐Al2O3 refractory composites manufactured via castable and extrusion technology. The influences of production method, particle size, open porosity as well as temperature and strain rate are discussed. For comparison, pure niobium and alumina specimens are used as reference materials. The results indicate that Nb‐Al2O3 refractory composites show high ductility at high temperatures. This behavior is also verified with microstructural investigations by scanning electron microscope (SEM).

Coarse‐Grained Refractory Composite Castables Based on Alumina and Niobium

“…The difference in the yield stresses between coarse‐ and fine‐grained materials can be related to different porosities. [ 17 ]…”

“…However, in contrast, the porous material can be used to reduce internal stress due to its ability to show strain values up to 25%. [17] For this fpurpose, it is planned to investigate layered materials based on layers of alumina and niobium-alumina composite, respectively. (b) Figure 18.…”

“…True stress-true strain curves of: a) fine-grained and b) coarse-grained specimens under compressive load for four temperatures depicted from. [17] Table 7. Yield stresses or rupture stress (in MPa) obtained at a strain rate of 7.5 Â 10 À4 s À1 for the fine-w43 and the coarse-grained composites.…”

Nanoscale Oxide Formation at α‐Al2O3–Nb Interfaces

“…The development of composites based on refractory ceramics and refractory metals (RM), [ 1–5 ] thus, aims at taking advantage of the benefits of ceramics, but also at exploiting the ductility and electrical conductivity of the RM. [ 5–8 ] The electrical conductivity offers a path to strongly reduce the thermal gradients through resistive heating of the parts before service. Among ceramics and metals, the combination of body‐centered‐cubic Nb (Nb bcc ) and (hexagonal) α‐Al 2 O 3 is particularly promising due to their similar thermal expansion behavior ( α Nb = 8.0 to 10.3 × 10 −6 K −1 [ 9 ] and = 9.3 to 11.2 × 10 −6 K −1 [ 10 ] from 527 to 1827 °C) and, thus, low‐thermal misfit stresses during fabrication and service.…”

“…Besides the well‐known embrittlement of bcc metals by already low levels of interstitial solutes, [ 16,17 ] the local oxidation of Nb and the resulting lattice strain due to changing volumes will introduce additional interfaces and lower the strength of the material. [ 18 ] Indeed, Günay et al [ 7 ] investigated room‐ and high‐temperature compression behavior of Nb–Al 2 O 3 composites and observed crack initiation at regions where NbO formed. Besides these consequences for the mechanical behavior, protecting the metallic component of the composite from oxidation also infers sufficient electrical conductivity for the resistive heating of the parts.…”