High temperature oxidation / creep deformation behavior of a diffusion barrier coated
Hastelloy-X alloy, with large grain size ~500μm, was investigated at 970°C in air with external
tensile stress of 22.5, 27.5, 32, and 40MPa. The diffusion barrier coating formed on Hastelloy-X
consisted of a duplex structure with an inner diffusion barrier layer of Re-Cr-Ni alloy, and an outer
oxidation resistant layer of β-NiAl. Un coated bare Hastelloy-X alloy with same grain size was also
examined under the same conditions for comparison. The composition of the as-coated diffusion
barrier coating was (15~21)Ni, (33~37)Cr, (30~33)Re, (11~15)Mo, and (9~14)Fe. This composition
corresponds to σ-phase in the Ni-Cr-Re ternary system, which is known as a topologically close
packed, TCP phase. The composition of this diffusion barrier layer did not change during the
experiment. The oxide scales formed after creep testing on the coated and un-coated alloy surfaces
were needle-like θ-Al2O3, and Cr2O3 with small amount of FeCr2O4, respectively. Grain boundary
oxidation was also found in the subsurface region of the un-coated alloy. The Al2O3 scale exhibited
severe spallation, and many cracks were formed perpendicular to the stress direction. However, no
spallation or cracks were observed in the Cr2O3. The creep rupture times for the diffusion barrier
coated alloy were about 1.5 times longer than those for bare alloy at all creep stress conditions. The
fracture surface after rupture indicates that fracture occurred along alloy grain boundaries in both the
coated and un-coated alloy substrate. Many cavities and cracks were observed within the diffusion
barrier coated alloy substrate. These cavities and cracks tended to propagate from the substrate
toward the diffusion barrier layer, and then stopped at the Re-Cr-Ni / β-NiAl interface. Cracks formed
in the un-coated alloy initiated at the tip of grain boundary oxides, and propagated into alloy
substrate. However no major cavities were observed inside the alloy substrate. The stress index, n, for
both specimens was about 6, and this indicates that the deformation mechanism of both samples was
dislocation creep. These results suggest that the Re-Cr-Ni diffusion barrier layer acts as a barrier
against the movement of dislocations at the interface with the alloy surface.
Effect of Grain Size on the High Temperature Creep Deformation of Austenitic Heat Resisting Steel with a Large Amount of Precipitates
